WO2000049450A9

WO2000049450A9 - Optical circulator

Info

Publication number: WO2000049450A9
Application number: PCT/US2000/004660
Authority: WO
Filing date: 2000-02-22
Publication date: 2001-08-23

Abstract

An optical circulator having a minimum number of optical circulators, resulting in a lowered insertion loss, and a simplified structure for ease of assembly is presented. The optical circulator includes two birefringent crystals (403, 406) separated by a non-reciprocal rotator section (404, 405). Optical circulators according to this invention exhibit reduced polarization mode dispersion. Additionally, the optical path through the optical circulators are free of epoxy, reducing insertion loss. Additionally, the optical circulator includes alignment prisms (402, 407) which insure that light beams traveling through the circulator are parallel with the optical axis of the circulator. The components of the optical circulator can be manufactured in such a way that there is a reduced need for matching individual components. Additionally, assembly is faster because there is no need to control the linear separation between components.

Description

OPTICAL CIRCULATOR

BACKGROUND

1. FIELD OF THE INVENTION

The invention relates to devices in optical communications networks and, more particularly, to optical circulators.

2. RELATED ART

Optical devices, especially passive optical devices, are in demand for the advancement of optical communications networks. In particular, optical circulators that are compact and efficient are in demand for optical networks. Although circulators are well known in microwave communication systems, circulator designs that have generally been employed in microwave communication systems cannot be employed in optical networks.

Optical circulators are passive, non-reciprocal devices that are used in several applications, such as bidirectional transmission systems for multiplexing the forward and reverse paths of an optical signal. Another application is to combine optical circulators with fiber gratings to build devices for wavelength division multiplexing (WDM), including optical add/drop multiplexers (OADMs). These devices must meet critical performance parameters, including low insertion loss. Insertion loss refers to the differences in power between light coupled into the optical circulator and light that exits the optical circulator. Insertion loss in optical circulators is largely due to absorption and scattering of light in the various elements of the circulator and to incomplete polarization separation.

Figure 1 shows an optical circulator similar to that described in U.S. Patent 4,650,289 to Kuwahara. Optical circulator 100 includes polarizer prisms 101 and 102, mirrors 103 and 104, Faraday rotators 105 and 106, and optical rotator elements 107 and 108. A randomly polarized light beam incident on polarizer prism 101, for example, is separated by polarization so that light polarized out of the page of Figure 1 is directed towards mirror 103 and light polarized in the page of Figure 1 is directed towards mirror 104. The first light beam, having a polarization that is out of the page of Figure 1, is reflected by mirror 103. The polarization of the first light beam is rotated by 45° in optical rotator element 107 and by another 45° at Faraday rotator 105, resulting in a light beam that is polarized in the page of Figure 1.

Similarly, the light from polarizer prism 101 that is polarized in the page of Figure 1 is rotated 90° by the combination of optical rotator element 108 and Faraday rotator 106, reflected from mirror 104, and enters polarizer prism 102 with a polarization that is out of the page of Figure

- i - 1. The separated light beams are recombined at port B of polarizer prism 102. The polarizations of the light beams shown in Figure 1 are consistent for light propagating from Part A to Part B. Similarly, it can be shown that light incident at port B exits the optical circulator at port C, light incident at port C exits the optical polarizer at port D and light incident at port D exits at the optical circulator port A.

The optical circulator described above is deficient in that insertion loss, cross-talk, and return loss is unacceptably high for many communications purposes. Another deficiency is that the circulator is limited to utilizing four-ports or fewer. Also, the physical size and shape of these types of circulators (i.e., having large lateral dimensions and access ports on all sides) makes implementation of large numbers of circulators inconvenient for optical network switching stations. Additionally, optical circulator 100 includes high-cost components, such as polarization prisms 101 and 102, causing the circulator to be expensive.

Figure 2 shows an optical circulator 200 similar to that described in U.S. Patent Number 5,204,771 to Koga. Optical circulator 200 includes birefringent crystal plates 201, 202 and 203. Birefringent plate 201 is cut such that a randomly polarized light beam incident on the crystal is separated into two orthogonally polarized light beams by deflecting a light beam having a first polarization while leaving substantially undeflected a light beam having an orthogonal second polarization. In Figure 2, a light beam incident at port A is separated into a Y-polarized beam and a X-polarized beam (with reference to the coordinate system shown by axis 210). In birefringent plate 201, the X-polarized beam is deflected while the Y polarized beam is not. Birefringent plate 202 is cut such that a X-polarized beam is undeflected but a Y- polarized beam is deflected in a direction that is nearly orthogonal to the direction of deflection that was experienced in birefringent crystal 201. Birefringent crystal 203 is cut identically with birefringent crystal 201. Rotator 204 rotates the polarized light beam traveling in the +Z direction clockwise with respect to the +Z direction. Rotator 205 rotates the polarized light beam traveling in the +Z direction counterclockwise with respect to the +Z direction. Faraday rotator 206 is arranged to rotate the polarization clockwise with respect to the direction of travel of the light beam. Rotator 206 is disposed between birefringent crystal 201 and 202. Similarly, rotator 207, which rotates the polarization of the light beam traveling in the +Z direction counter clockwise by 45°, rotator 208, which rotates the polarization of the light beam traveling in the +Z direction clockwise by 45°, and Faraday rotator 209 which rotates the polarization of the light beam clockwise with respect to the direction of travel of the light beam is disposed between birefringent crystals 202 and 203. A randomly polarized beam incident at port A on birefringent crystal 201 is split by polarization into a first beam, which is polarized in the Y direction, and a second beam, which is polarized in the X direction. The first beam passes through birefringent crystal 201 substantially undeflected. The polarization of the first beam is rotated by 45° clockwise in rotator 204 and another 45° clockwise in Faraday rotator 206 so that it is polarized in the X direction when it enters birefringent crystal 202. The first beam is again substantially undeflected by birefringent crystal 202. The polarization of the first beam is rotated counterclockwise by 45° at rotator 207 and clockwise by 45° at Faraday rotator 209. The first beam, therefore, enters birefringent crystal 203 as an X-polarized beam and is deflected by birefringent crystal 203.

The second beam, polarized in the X direction, is deflected by birefringent crystal 201. The polarization of the second beam is rotated 45° counterclockwise by rotator 205 and 45° clockwise by Faraday rotator 206 and therefore the second beam enters birefringent crystal 202 as an X-polarized beam and is undeflected. The polarization of the second beam is rotated 45° clockwise by rotator 208 and another 45° clockwise by Faraday rotator 209 and therefore the second beam enters birefringent crystal 203 as a Y-polarized beam. The second beam is undeflected by birefringent crystal 203 and rejoins the first beam to exit the circulator at port B. The lengths of birefringent crystals 201 and 203 (i.e., the physical dimension of the crystal along the direction of light propagation) are nearly the same so that the two beams separated by birefringent crystal 201 are rejoined at a birefringent crystal 203. A similar argument shows that light entering optical circulator 200 at port B exits at port C.

The above circulator is, however, difficult and costly to produce. The optical components need to be precisely matched, contributing to the difficulty and cost of producing the components. Additionally, the number of components, and associated optical surfaces, adds to the insertion loss of the circulator.

Figure 3A shows an optical circulator 300 similar to that described in U.S. Patent No. 5,574,596 to Cheng. Optical circulator 300 includes birefringent crystals 301, 304, 305 and 306 and Faraday rotators 302 and 303. As an example, Figure 3B shows a surface projection of the polarized light beam passing from port A to port B of optical circulator 300. For convenience, each of surfaces 307 through 312 shown in Figure 3B is first divided into top and bottom areas and then further divided into a left, left-middle, right-middle and right areas. Therefore, clockwise, each of surfaces 307 through 312 has top-left, top-left-middle, top right- middle, top right, bottom right, bottom right-middle, bottom left-middle and bottom left areas. Further, a light beam is depicted in Figure 3B as a circle with the polarizations of the light beam depicted as lines inside the circle.

A light beam enters port A at the bottom-left area of surface 307 and is separated by birefringent crystal 301 into two beams having orthogonal polarizations. A first beam is substantially undeflected by birefringent crystal 301 and appears at surface 308 in the bottom left area. A second beam, having the orthogonal polarization, is deflected such that it appears at surface 308 in the top left-middle area. The polarization of each of the first and second beams is rotated counterclockwise (with respect to a direction along the propagation direction of the beams) by 45° in Faraday rotator 302, as is shown at surface 309 in Figure 3B. Therefore, at surface 309 the first beam remains in the lower left area and the second beam remains in the upper left-middle area. The first beam is deflected laterally by birefringent crystal 306 while the second beam is substantially undeflected by birefringent crystal 305. Therefore, at surface 310 the second beam remains in the top left-middle area while the first beam is deflected to the bottom right-middle area. The polarization of each of the first and second beams is rotated by 45° clockwise by Faraday rotator 303, as is shown at surface 311 in Figure 3B. The first beam is then shifted to appear at the top left-middle area at surface 312 to join the second beam by birefringent crystal 304. The right and right-middle areas are necessary for light beams propogating between other ports of the circulator. As is shown by Figures 3 A and 3B, the optical paths between the two polarizations are not the same, resulting in a polarization mode dispersion (PMD) problem. Additionally, circulators of this type are commonly constructed with epoxy between optical components. Epoxy in the beam path contributes to absorption and scattering losses and therefore increase the insertion loss of the circulator. There is a need for optical circulators that have lower insertion losses, less polarization mode dispersion, and less cross-talk. There is also a need for optical circulators that are easier to align, and therefore cheaper to produce. Also, there is a need for optical circulators having a minimum number of components to produce so that it is easier and cheaper to produce. Additionally, there is a need for optical circulators having multiple circulation ports.

SUMMARY OF THE INVENTION

According to the invention, an optical circulator having a minimum number of optical surfaces is disclosed. One embodiment of the optical circulator includes two birefringent crystals, two non-reciprocal rotator sections, and at least one alignment prism. An alignment prism assists in the coupling of light beams from two optical fibers (i.e., two optical ports) through a collimator and into the circulator and reduces the difficulties, and expense, of aligning the crystal. In an embodiment where each of the optical fibers are terminated with a microlens collimator, the alignment prism is not necessary and can be omitted.

A first birefringent crystal accepts one light beam from each of the two optical fibers and separates each of the respective light beams into two light beams having orthogonal polarization. Some embodiments of the invention include alignment prisms which, along with allowing for easier and faster optical alignment, also arrange that light beams propagating between components all travel along parallel paths, when they are not being specifically deflected by one of the birefringent crystals. In embodiments where optical fibers are terminated with microlens collimators, the optical fibers are arranged such that light beams from the optical fibers travel along parallel paths. Therefore, there is no need to control the spacing between components, reducing the alignment time and expense of manufacturing the optical circulator. Furthermore, the symmetry of the optical paths of two polarizations from the same light beam is maintained so that polarization mode dispersion effects are minimized. Additionally, during construction care is taken to insure that no materials other than the optical components are in the optical path of the circulator in order to minimize insertion losses.

In one embodiment of the invention, the two birefringent crystals are cut substantially identical so as to nearly maximize the walk-off distance between two orthogonal polarizations of light. Additionally, the crystals are cut such that the extraordinary beam is deflected in a convenient direction. Additionally, the crystals are long enough to completely separate the ordinary (substantially non-deflected) and extraordinary (deflected) beams. In one embodiment of the invention, the two non-reciprocal rotator sections are substantially identical, but differently oriented in the optical circulator, so that production arrangements need to be made for only one component.

In yet another embodiment of the invention, the two non-reciprocal rotator sections share one birefringent crystal sandwiched between a first set of non-reciprocal rotators and a second set of non-reciprocal rotators. In one embodiment, the birefringent crystal of the two non-reciprocal rotator sections is cut substantially identical with the two birefringent crystals, but is differently oriented, so that only one type of birefringent crystal is produced and three crystals of identical cuts are utilized in the resulting circulator. Having only one cut of birefringent crystal allows for ease of manufacture and relieves much of the necessity of matching birefringent crystals to one another for use in the optical circulator.

In one embodiment of the invention, the components of the optical circulator are arranged such that all of the ports of the optical circulator are coplanar. The cuts of the birefringent crystals in this embodiment are arranged such that the parts of the resultant optical circulator are in a plane parallel with a mounting plane. The mounting plane is an external plane, such as a silicon substrate where the components of the circulator are mounted.

In another embodiment of the invention, an optical circulator has a minimum number of optical surfaces. The minimum optical circulator includes a coupling prism, a mirror assembly, a first birefringent crystal, a non-reciprocal rotator that does not include a birefringent crystal, and a second birefringent crystal. The cuts of the birefringent crystals are arranged such that the optical path lengths of the extraordinary beam is substantially that of the ordinary beam, even though their geometrical paths will differ.

According to the present invention, a method of assembling an optical circulator into an optical circulator package includes producing and cutting birefringent crystals having appropriate crystallographic orientations; coating the optical surfaces of the birefringent crystals such that reflections of light at the operating wavelengths of the circulator are minimized; assembling non-reciprocal rotator sections from a birefringent crystal and at least one non-reciprocal rotator (such as Faraday rotator); assembling the optical components into a central position; aligning the central portion with collimators to form a circulator; and sealing the circulator into the circulator package. The above outlined procedure (discussed further in detail below) provides for quick and inexpensive assembly of optical circulators without the time-consuming need of matching every crystal and without over-concern with alignments of one component with another.

In another embodiment, an optical circulator package includes mounting a coplanar optical circulator onto a silicon substrate by producing and cutting birefringent crystals having appropriate crystallographic orientations; coating the optical surfaces of the birefringent crystals such that reflection of light at the operating wavelengths of the circulator are minimized; assembling non-reciprocal rotator sections from a birefringent crystal and at least one non-reciprocal rotator (such as a Faraday rotator); forming a set of V-grooves on a substrate; forming a base on the substrate, the base being aligned with the set of V-grooves; bonding the birefringent crystals and the non-reciprocal rotator sections to the base; mounting collimators, which can be optical fibers terminated with microlens collimators, in each of the set of V-grooves; and aligning the collimators with the birefringent crystals and the non- reciprocal rotator sections.

These embodiments of the invention and others are further discussed below with reference to the following figures.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic diagram of a prior art four-port optical circulator. Figure 2 shows a schematic diagram of a previously known linear type optical circulator.

Figures 3 A shows a schematic diagram of a previously known linear type optical circulator.

Figure 3B shows a diagram of the polarization states and positions of light beams at various points in the optical circulator of Figure 3 A for propagation of light between ports A and B.

Figure 4A shows an optical circulator according to the present invention. Figure 4B shows the polariazation states of light beams at various points in the optical circulator of Figure 4 A for propagation of light from port A to port B.

Figure 4C shows the polarization states of light beams at various points in the optical circulator of Figure 4 A for propagation of light from port B to port C.

Figure 4D shows the polarization states of light beams at various points in the optical circulator of Figure 4 A for propagation of light from port C to port D. Figure 4E shows the polarization states of light beams at various points in the optical circulator of Figure 4 A for propagation of light that enters port D.

Figure 4F shows a schematic diagram of a circulator having four ports.

Figure 5 A shows a combination of a collimator and a coupling prism according to the present invention.

Figure 5B shows a plan view of a collimator according to the present invention.

Figure 5C shows a cross-sectional view of a collimator according to the present invention.

Figure 5D shows an example of an alignment prism according to the present invention. Figures 5E through 5G show embodiments of lenses at the termination of optical fibers.

Figure 5H shows an optical fiber terminated with a microlens collimator.

Figure 6 A shows an example of the cut of birefringent crystals according to the present invention.

Figure 6B shows double refraction through a birefringent crystal. Figures 7 A and 7B show embodiments of non-reciprocal devices according to the present invention.

Figures 8 A and 8B show embodiments of non-reciprocal elements according to the present invention.

Figures 9 A and 9B show embodiments of non-reciprocal elements according to the present invention.

Figure 10A shows a projection of the components of one embodiment of an optical circulator according to the present invention.

Figures 10B-10E show crystal orientation and cuts for one embodiment of an optical circulator according to the present invention. Figures 10F-10H show crystal orientation and cuts for an embodiment of an optical circulator according to the present invention.

Figures 1 IA and 1 IB show a coplanar embodiment of an optical circulator according to the present invention.

Figures 12A through 12D show an embodiment of an optical circulator according to the present invention.

Figure 13 shows schematically an embodiment of a circulator package according to the present invention.

Figures 14A and 14B show coatings of optical surfaces according to the present invention. Figures 15 A through 15E show an embodiment of non-reciprocal rotator elements according to the present invention and illustrate its assembly.

Figures 16A through 16D show an embodiment of an optical component assembly of assembly according to the present invention and illustrate construction of the assembly. Figures 17A through 17D show an embodiment of an optical component assembly according to the present invention and illustrate construction of the assembly.

Figure 18 A through 18D show an embodiment of a collimator assembly and a collimator prism assembly according to the present invention and illustrate their construction. Figures 19A and 19B illustrate alignment and final assembly of a component assembly.

Figures 20A through 20C show an embodiment of a four-port coplanar optical circulator.

Figures 21 A through 21 F illustrate production of a coplanar circulator.

In the above figures, elements or components having the same or similar functions are assigned the same identification number.

DETAILED DESCRIPTION OF THE INVENTION

Figure 4A shows a schematic diagram of an optical circulator according to the present invention. Optical circulator 400 includes birefringent crystals 403 and 406 and non-reciprocal rotator elements 404 and 405. Additionally, some embodiments of optical circulator 400 include coupling prisms 402 and 407 and collimators 401 and 408.

Figure 4F shows a schematic symbol for an optical circulator 450 such as optical circulator 400 of Figure 4A. Optical circulator 450 is a four-port circulator where light entering at port A exits at port B, light entering at port B exits at port C, and light entering at port C exits at port D.

Optical circulator 400 of Figure 4A is illustrated as a four port circulator where light entering at port D is lost. Embodiments of optical circulators according to the present invention can have any number of ports greater than two (2). Three and four-port circulators, however, are illustrated here because of their more common practical application to optical networks.

In Figure 4 A, collimator 401 collimates and focuses light carried on optical fibers corresponding to ports A and C onto coupling prism 402. In some embodiments, collimator 401 can be a group of optical fibers terminated with a microlens collimator where each optical fiber is arranged to focus light direction on birefringent crystals 403 and 406. Each optical fiber corresponds to one of the ports of circulator 400.

Collimator 401 has an inside surface 401a, which is polished and can be coated with an anti-reflection coating. In most embodiments of the invention, collimator 401 accommodates the insertion of two optical fibers. However, in general the number of optical fibers that can be accommodated in collimated 401 is restricted only by the lens aperture of collimators 401 and 408. Coupling prism 402, having a first surface 402a and a second surface 402b, allows for easier coupling of light from the optical fibers corresponding to ports A and C into optical circulator 400. Coupling prism 402 has angled surfaces 402c at surface 402a, which are arranged such that light beams at surface 402b are substantially parallel and light beams at surface 401a are coupled into the appropriate optical fiber in collimator 401. As such, surface 402a includes an angled surface 402c for each fiber accommodated by collimator 401.

Alignment prism 402 has an apex 402d at the intersection between adjacent angled surfaces 402c. For ease of discussion, a coordinate system axis 410 is defined such that the Y direction aligns with apex 402d and the +Z direction is the optical axis of optical circulator 400, i.e. the direction of light propagation between ports A and B. Light beams at collimator 401 are randomly polarized. Therefore, a light beam at collimator 401 can be considered a combination of a first light beam component linearly polarized in a first direction, and a second light beam component linearly polarized in a second direction, where the second direction is orthogonal to the first direction. Birefringent crystal 403 deflects light polarized in the first direction while not substantially deflecting light polarized in the second direction, i.e., the light beam having polarization along the second direction is the ordinary beam in birefringent crystal 403 while the light beam having polarization along the first direction is the extraordinary beam in birefringent crystal 403. Although in general birefringent crystal 403 can be oriented to deflect the extraordinary beam in any direction, in the embodiment shown in Figure 4A birefringent crystal 403 is oriented to deflect extraordinary light beams out of the X-Z plane. Therefore, in birefringent crystal 403. a light beam polarized in the first direction is deflected out of the X-Z plane in a negative Y direction if traveling in a XL direction and is deflected out of the X-Z plane in a positive Y direction if traveling in a -Z direction (as indicated by axis 410). As such, birefringent crystal 403 is a reversible component (light beams traveling in one direction will retrace their path if diverted to travel in the opposite direction). Birefringent crystal 403 has a first surface 403a and a second surface 403b, each of which can be coated with an anti- reflecting film in order to reduce reflection. Non-reciprocal rotary element 404 in Figure 4A rotates the polarization of light polarized in the second direction and traveling in the +Z direction by about 90° (with reference to the +Z direction) such that the light beam exits rotary element 404 polarized in the first direction. Light polarized in the first direction and traveling in the +Z direction through non- reciprocal rotary element 404 is rotated by about 90° such that it becomes polarized in the second direction. In general non-reciprocal rotator 404 can rotate the polarization in any fashion such that an ordinary light beam in birefringent crystal 403 is an extraordinary light beam in birefringent crystal 406.

Additionally, light that enters non-reciprocal rotary element 404 having the first polarization is deflected in the X-Z plane in rotary element 404. Light that enters rotary element 404 in the +Z direction and having the second polarization is substantially undeflected.

Light polarized in the first direction and traveling in the -Z direction is deflected in the X-Z plane and the polarization is rotated into the first direction. Light traveling in the -Z direction and polarized in the second direction is undeflected and the polarization is rotated into the second direction. Non-reciprocal rotary element 404 has a first surface 404a and a second surface 404b, each of which can be coated with an anti-reflecting film in order to reduce reflections.

Non-reciprocal rotary element 405 operates in reverse fashion to non-reciprocal rotary element 404. For example, the output beam from rotary element 405 for a light beam traveling in the +Z direction through rotary element 405 is the same as a light beam having the same polarization traveling in the -Z direction through rotary element 405. Non-reciprocal rotary element 405 has a first surface 405a and a second surface 405b, each of which can be coated with anti-reflecting film in order to reduce reflections. In most embodiments, non-reciprocal rotary element 405 is identical with non-reciprocal rotary element 404 rotated by about 180° in the X-Z plane.

Birefringent crystal 406 is aligned such that light beams that were separated in birefringent crystal 403 are recombined in birefringent crystal 406. Similarly, light beams that are separated in birefringent crystal 406 are recombined in birefringent crystal 403.

In most embodiments, birefringent crystal 406 has the same properties as birefringent crystal 403. Therefore, the extraordinary beam in birefringent crystal 406 is identically polarized with the extraordinary beam of birefringent crystal 403 and the deflection of the extraordinary beam in birefringent crystal 406 is identical with that of the extraordinary beam in birefringent crystal 403. Birefringent crystal 406 has a first surface 406a and a second surface 406b, each of which can be coated with anti-reflecting film in order to reduce reflection. Coupling prism 407, also, is substantially the same as coupling prism 402 and couples optical circulator 400 into collimator 408. Coupling prism 407 has a first surface 407a and a second surface 407b that can be coated with anti-reflection film. Collimator 408 has an inside surface 408a. Ports A and C are at collimator 401 and ports B and D are at collimator 408.

The lengths of birefringent crystals 403 and 406 are substantially the same so that the amount of deflection experienced by a light beam in each crystal is the same. In some embodiments, the material utilized to form birefringent crystal 406 is not the same as the material utilized to form birefringent crystal 403. In that case, birefringent crystal 406 and birefringent crystal 403 are each arranged such that the "walk-off (how far the deflected light beam is ultimately deflected from the undeflected beam) distances are the same and oriented appropriately for use in optical circulator 400. Light beams that are separated in one of birefringent crystals 403 or 406 are, therefore, recombined in birefringent crystals 406 or 403 respectively. Utilizing different materials for birefringent crystals 403 and 406, however, may result in an effective optical path difference between light beams of differing polarization, causing an undesirable polarization mode dispersion of an optical signal propagation through optical circulator 400.

Figure 4B shows schematically an example of a light beam traveling from port A to port B through an embodiment of optical circulator 400 of Figure 4A. The polarization of the light beam at surface 401a, port A, is random. In Figure 4B, the random polarization is shown as a combination of a first beam having a first polarization and a second beam having an orthogonal second polarization. The light beam from port A is incident on surface 402a (Figure 4A) of coupling prism 402. In Figure 4B, the polarization at surface 402b of coupling prism 402 (showing the light beam upon exit from coupling prism 402) is again random, as shown. Prism 402 arranges that a light beam entering optical circulator 400 at port A is parallel with the Z direction (see axis 410 of Figure A). The light beam at surface 402b is propagating in the top- left portion of optical circulator 400 (as viewed along the +Z direction). For convenience, the cross section (along the Z-axis) of optical circulator 400 is segregated into portions. The portions, for the present discussion, are labeled as they appear in the +Z direction (see axis 410 of Figure A). Therefore, the six portions shown in Figures 4B through 4E are labeled as follows: top-left, top-middle, top-right, bottom-left, bottom-middle and bottom-right. Other embodiments of optical circulators can require different definitions of the portions. For example, an optical circulator having more than four ports will require more than six portions to explain. Alternatively, an optical circulator that includes only ports A and B only requires four (4) portions for discussion. Additionally, optical circulators that have different deflections of polarized beams will have different definitions of the portions. In the embodiment of optical circulator shown in Figures 4B through 4E, birefringent crystals 403 and 406 deflect the extraordinary beams in a direction (the "walk-off direction) of about 45° from the Y direction in the X-Y plane (see axis 410). The walk-off directions that occur in non-reciprocal rotators 404 and 405 occur in the X-Z plane. The walk-off distance (i.e., how far in the X-Y plane the extraordinary beam) is deflected from the ordinary beam is a function of the Z-direction length of the crystal. Figures 4B through 4E show the polarization states and locations on surfaces of selected components of optical circulator 400. Axis 410 indicates the orientation of the surface, and the polarization of the light beams, in each of Figures 4B through 4E. In general, the actual polarization of the ordinary and extraordinary beams is determined by the crystallographic orientation of the birefringent crystal (i.e., the relation between the crystallographic axis of the crystal with axis 410).

The solid vertical line shown on surface 402b in Figure 4B indicates the apex 402d of alignment prism 402. As is shown in Figure 4B, randomly polarized light entering optical circulator 400 at port A exits in the top-left portion of surface 402b of prism 402. Additionally, surfaces 401a and 408a in Figures 4B through 4E are expanded to depict coupling of lights into optical fibers at the corresponding ports of circulator 400. Light beams associated with adjoining ports of the circulator can occupy overlapping regions of the surface of collimators 401 and 408.

The light beam then passes through birefringent crystal 403. Figure 4B shows the polarization and location of the separated light beams at surface 403b, upon exiting from birefringent crystal 403. The second light beam, which is polarized along a second direction, is undeflected and remains in the top-left portion of surface 403b (i.e., the second polarization direction is the polarization of the ordinary light beam in crystal 403). The first light beam polarized along the first direction is the extraordinary beam in birefringent crystal 403 and is deflected in about a downward 45° direction to appear in the bottom-middle portion of surface 405b. Therefore, the first light beam having the first polarization is incident on non-reciprocal rotator 405 while the second light beam having the second polarization is incident on non- reciprocal rotator 404. As shown in Figure 4B, the polarization of the second light beam is rotated by about 90° counterclockwise (with respect to the +Z direction) into the first polarization direction and the polarization of the first light beam is rotated by about 90° clockwise into the second polarization direction. Neither light beam is deflected in non-reciprocal rotators 404 and 405 and therefore the second light beam, which now has the first polarization, is in the top-left portion of surface 404b and the first light beam, which now has the second polarization, is in the bottom-middle portion of surface 405b. Birefringent crystal 406 is arranged to operate in the same fashion as birefringent crystal 403 and therefore the first light beam passes through birefringent crystal 406 substantially undeflected while the second light beam, because of its polarization, is deflected into the lower-middle portion of optical circulator 400 to recombine with the first light beam. The recombined, randomly polarized, beam passes through coupling prism 407 and is coupled into port B of collimator 408. The solid vertical line on surface 407b of Figure 4B depicts the apex of coupling prism 407. Notice that coupling prism 407 is shifted in the -X direction (see Figure 4A), with respect to coupling prism 401, by the width of one position. The polarizations and locations of the light beam at surfaces 407b and 408a are shown in Figure 4B.

Walk-off directions of less than 45° from +Y in birefringent crystals 403 and 406 can result in optical circulators being wider because the position width will be greater. A walk-off direction that is less than 45° from +Y can result in an unattainable separation of beams for separate ports, resulting in increased cross-talk. In one embodiment, the width of one portion is the diameter of light beams inputted to the ports of optical circulator 400.

Figure 4C shows the propagation of a light beam that enters optical circulator 400 at port B. The light beam at surface 408a is randomly polarized. It is also randomly polarized, and positioned in the lower-middle portion of surface 407a, as it exits coupling prism 407. As before, the solid vertical line on surface 407a depicts apex 407d of coupling prism 407.

Birefringent crystal 406 separates the light beam into a first light beam polarized along the first direction, which is deflected into the upper-left portion at surface 406a, and a second light beam polarized in the second direction, which remains substantially undeflected to appear in the lower-middle portion of surface 406a. The first light beam travels through non-reciprocal rotator 404 and exits with polarization rotated by about 90° clockwise (as viewed in the +Z direction) into the second polarization direction. The first light beam is also deflected in the -X direction so that it exits rotator 404 in the upper-middle portion of surface 404a. The second light beam is rotated by about 90° counterclockwise (with respect to the +Z direction) and is also laterally deflected into the lower-right portion of surface 405a. The second light beam is deflected into the top-middle portion to rejoin the first light beam in birefringent crystal 403. The recombined, randomly polarized light beam, therefore, reappears in the top-middle portion of surface 403 a. The light beam then travels through coupling prism 402 to exit optical circulator 400 at port C, as is shown by randomly polarized light in the top-middle portion of surface 402a and in port C of surface 401a.

Figure 4D shows a light beam traveling from port C to port D in optical circulator 400. The light beam enters at port C in collimator 401 and is coupled into the upper right portion of optical circulator 400 by coupling prism 402. In Figure 4D, the incoming light is randomly polarized at port C as shown at surface 401a. At surface 402b, the light beam remains randomly polarized and is located in the top-middle portion of surface 402b. Birefringent crystal 403 separates the light beam by polarization into a first beam having a first polarization that appears in the bottom-right portion of surface 403b and a second beam having a second polarization that appears in the top-middle portion of surface 403b. The first beam passes through non-reciprocal rotator 405 and is still positioned in the bottom-right portion of surface 405b, but its polarization is rotated to have the second polarization. The second beam passes through non-reciprocal rotator 404 and is positioned in the top-middle portion of surface 404b, but its polarization has been rotated to have the first polarization, as is shown in surface 404b. Birefringent crystal 406 recombines the first beam with the second beam with the resultant beam positioned in the bottom-right portion of surface 406b, as is shown. Coupling prism 407 couples the combined beam into collimator 408 at port D. Therefore, the light beam appears in the bottom-right portion of surface 407b and at port D of surface 408a.

Figure 4E shows transmission of light through a five-port optical circulator 400 that enters optical circulator 400 at port D. The randomly polarized light beam appears at port D on surface 408a and in the bottom-right portion of surface 407a. Birefringent crystal 406 separates the light beams such that the first beam having a first polarization appears in the top-middle portion of surface 406a and the second beam having a second polarization, orthogonal to the first polarization, appears in the bottom-right portion of surface 406a. Non-reciprocal rotator 404a rotates the polarization of the first beam and deflects the first beam such that the first beam, now having the second polarization, appears in the top-right portion of surface 404a.

In a four-port embodiment, the second beam is shifted out of optical circulator 400 by non-reciprocal rotator 405 and lost. In an embodiment having more than four ports, non- reciprocal rotator 405 is wide enough to include an extra portion to the right of the bottom-right portion. In one embodiment, because the bottom-left portion of non-reciprocal rotator 405 is never utilized, an extra portion can be obtained by shifting rotator 405 in the -X direction by one portion width.

If the second beam is not lost, then it is shifted by birefringent crystal 403 (which also must be broad enough to accommodate the second beam) to recombine with the first beam in the top-right portion of surface 403 a. Coupling prism 402 couples the light beam into the fifth port, a port E, of surface 401a. As such, the combined light beam appears in the top-right portion of surface 402a. Embodiments of the invention can have any number of ports greater than two. Figure 4A shows optical circulator 400 having four ports and Figure 4E shows an extension of optical circulator 400 having five ports. An optical circulator having two ports, for example, could be only two portions wide in the X-direction. An optical circulator having any number of ports would be as wide in the X-direction as necessary in order to accommodate the desired number of ports. The maximum number of ports attainable is limited only by the available sizes of birefringent crystals 403 and 406 and rotators 404 and 405, which must be wide enough to accommodate all of the light beams from all of the ports.

Figure 5 A shows a combination of collimator 501 and alignment prism 502 such as the combination of collimator 401 (Figure 4A) and alignment prism 402 or the combination of collimator 408 and alignment prism 407. Collimator 501 includes housing 510, fiber receiving portion 511, and transmission portion 512. Fiber receiving portion 511 is typically a material that is transparent at the operational wavelength of the optical circulator. Fiber receiving portion 511 has a fiber access 515 in which at least one optical fiber is inserted. Embodiments may include any number of optical fibers, restricted only by the lens aperture of collimator 501. In some embodiments, optical fibers, such as optical fiber 550, may be inserted as spacers. Spacer fiber 550 can assist in separating beams 519 and 520 by changing the angle θ, resulting in a shorter length 1.

In many embodiments the operating wavelengths are about 40 nm (or even wider) around 1550 nm or around 1310, although optical circulators according to the present invention can operate in any wavelength ranges, including from the far infrared to the deep ultraviolet regions.

In Figure 5A, optical fibers 503 and 504 are inserted into fiber access 515. Fibers 503 and 504 are terminated at surface 514. Transmission portion 512 is a solid piece of material that is transparent and acting as a lens at the operating wavelengths of the optical circulator. Transparent portion 512 and fiber receiving portion 511 are separated such that there is a gap between optical fibers 503 and 504 and surface 513 of transmission portion 512. Surface 513 is such that light from optical fibers 503 and 504 is incident on the surface at an incident angle. In some embodiments, the incident angle is arranged to be about 9°, although any incident angle that reduces reflection of light back towards optical fibers 503 and 504 can be used. In this way, collimator 501 reduces the reflections of light back into optical fibers 503 and 504, or back into the circulator itself, where the reflected light could otherwise interfere with the light beam. Therefore, surfaces 513 and 515 reduce the return loss of the optical circulator. In general, the angle of surfaces 513 and 514 is arranged to maximize the optical throughput of the collimator while maximizing the return loss.

Figure 5B shows an embodiment of collimator 501. In the embodiment of collimator 501 shown in Figure 5B, housing 510 includes an inner housing 516 and an outer housing 517. Outer housing 517 may be a metallic tube (e.g., stainless steel tubing) or plating such as a gold plating, which may ultimately facilitate mounting and packaging of collimator 501 with the remaining components of the optical circulator. Inner housing 516 may be of any material including, for example, a stainless steel tubing and fused silica tubing. Inner housing 516 may be for facilitating the mounting of transmission section 51 1 and fiber receiving section 512 within housing 510. Tighter tolerances between inner housing 516 and transmission section 512 and fiber receiving section 511 are attained if inner housing 516 is of fused silica tubing because of the close match in thermal expansion characteristics. Transmission section 512 and fiber receiving section 511 are rigidly mounted within housing 510.

As an example, one embodiment of collimator 501 is shown in Figures 5 A and 5B. Collimator 501 can operate in a wavelength range from visible to near infrared light. The example shown in Figures 5A and 5B is appropriate for operation at wavelengths of about 1550 nm and around 1310 nm, which are among the wavelengths appropriate for fiber communcation applications. Collimator 501 has an outer housing 517 which is a gold plated stainless steel tubing having inner diameter of about 2.80 mm and outer diameter of about 3.20 mm. Inner housing 516 is a fused silica tubing with inner diameter of about 1.80 mm and outer diameter of about 2.80 mm. Inner housing 516 is inserted and epoxied into outer housing 517. Transmission portion 512 can include any lens, including a GRIN lens or an aspherical lens in order to focus light beams 519 and 520 into optical fibers 503 and 504. Additionally, microlenses 540 may be constructed on the ends of optical fibers 503 and 504 to aid in coupling light beams into the optical fibers.

As an example, fiber receiving portion 511 can be a fused silica rod having an outer diameter of about 1.80 mm and an elongated access for receiving optical fibers. Transmission portion 512 and fiber receiving portion 511 are cut at an angle of about 9° from a direction along the rod and polished to form surfaces 513 and 514, respectively, and inserted into housing 510. Surfaces 513 and 514 may also be coated with an anti-reflection coating to reduce reflection around the operating wavelengths, such as about 1550 nm or 1310 nm, in order to reduce insertion loss. Transmission portion 512 and fiber receiving portion 511 are inserted into inner housing 516 such that the two polished surfaces 513 and 514 are parallel and separated by a small gap determined by the position of the focal plane of transmission portion 512. The length of housing 510 is approximately 10 mm. The length of fiber receiving portion

511 is about 5 mm and the length of transmission portion 512 is about 4.5 mm. The end of transmission portion 512 is polished to form a surface 518. Surface 518 may also be coated to reduce reflection at around the operating wavelength.

Fiber receiving portion 511 includes access 515 arranged longitudinally through its center. Access 515, as is shown in Figure 5C, is an elongated cylindrical hole having a height of one fiber diameter (about 0.125 mm for standard telecommunication single mode fiber) and about two fiber diameters in width. Two optical fibers are inserted into access 515 such that they are securely held in collimator 501. In general, access 515 has a height arranged to accommodate the diameter of one (1) optical fiber and width arranged to accommodate any number of optical fibers. Fused silica tubing and rod similar to that shown in Figures 5B and 5C can be purchased from several companies, including American Quality Quartz, Inc.

In Figure 5A, optical beams 519 and 520 are coupled to optical fibers 504 and 503, respectively, which are the ports of the resulting circulator. Typical beam diameter for a wavelength of about 1550 nm is about 0.5 mm. Optical beams 519 and 520 are angularly separated by an angle θ. For example, in one embodiment for use at a wavelength of about 1550 nm system θ is approximately 3.68°. Prism 502 is arranged such that light beams 519 and 520 are parallel with each other on the side of the prism opposite that of collimator 501 and towards the optical circulator (see Figure 4A).

Prism 502 can be of any optical material that is transparent at the operating wavelength of the optical circulator, including fused silica, glass, plastic, and crystal materials. In one embodiment, the prism is of fused silica. Fused silica has an index of refraction of about 1.444 at a wavelength of 1550 nm. The apex angle α of prism 502 is matched to the angular separation θ of light beams from collimator 501. The apex angle can be determined by the equation

^ sm l \ a ++-— = nsina ,

where θ is the angle between beams 519 and 520 at collimator 501 and n is the index of refraction of prism 502 at the operating wavelength. The apex angle α is arranged so that light beams incident on the angled surfaces of prisms 502 exit prism 502 parallel to the optical axis (the Z direction in Figure 4 A) of the optical circulator.

In some embodiments, prisms 502 may include only one angled surface or may be formed from combinations of several half-prisms. Figure 5D shows a prism 502 formed from two half prisms 521 and 523 situated on either side of a flat portion 522. Collimator 501 in Figure 5D accommodates three optical fibers 503, 504 and 550, each of which operates as a port. Each optical fiber (i.e., input port) accommodated by collimator 501 requires one an appropriate portion of prism 502 in order to couple the light beams from the optical fibers into light beams parallel to the Z axis (Figure 4) of optical circulator 400. Light beams 519 and 520 are coupled to right-angle portions 521 and 523, respectively. Light beam 551, which is already parallel with the Z axis, is coupled to flat portion 522. The three light beams, on exit from prism 502, are parallel with the Z axis.

As shown in Figure 5A, the separation between parallel beams 519 and 520 is determined by the separation / between collimator 501 and prism 502. Therefore, tuning of the beams in order to couple them into an optical circulator (see Figure 1) can be accomplished by adjusting the separation / between collimator 501 and prism 502. Additionally, some fine tuning of the beams at the optical circulator can be accomplished by a slight (i.e., up to a few degrees) angular rotation of prism 502 relative to the optical axis of collimator 501.

Figures 5E, 5F and 5G show embodiments of microlenses 540 on optical fiber 541 which may be fabricated at the tip of optical fibers, such as fibers 503 and 504, that terminate in collimator 501. Techniques for constructing such lenses are well known. Significant increases in coupling efficiencies can be accomplished using such lenses. Figure 5E shows a conventional flat termination 540 of optical fiber 541. Figure 5F shows microlenses 540 as a hemispherical lens, which results in a higher effective numerical aperature of optical fiber 541. Figure 5G shows microlens 540 as a tapered lens with even higher effective numerical aperture. When the microlens fiber shown in Figure 5F or 5G is used in optical fibers 503 and 504 in collimator 501, alignment of an optical circulator utilizing collimator 501 is simplified. In addition, microlenses are easier to form, requires no additional grinding and polishing of the fiber ends, and can be produced in a much shorter time than conventional fiber terminatoions. Figure 5H shows an embodiment of a microlens that functions as a collimator 550. Collimator 550 includes single-mode optical fiber 551 and graded index multimode fiber 552 spliced to single-mode optical fiber 551. The index of refraction of graded index multimode fiber 552 decreases with distance from the optical axis 553 of fiber 552. A properly chosen length L for multimode fiber 552 results in a collimated output beam 554 from optical fiber 551. The length L is the length necessary to result in a collimated beam. Embodiments of optical circulators that include a microlens collimator such as collimator 550 shown in Figure 5H do not include an alignment prism such as prism 502 in Figures 5 A through 5D. Additionally, the beam size is smaller using a microlens collimator, resulting in smaller crystal sizes for birefringent crystals 403 and 406. Therefore, an embodiment of optical circulator 400 where collimators 401 and 408 are bundles of optical fibers terminated with microcollimators 550 can be much smaller physically than if collimators

401 and 408 are collimators 501 as shown in Figure 5 A.

Figure 6A shows the crystallographic directions of a birefringent crystal 600. Birefringent crystal 600 is of any birefringent material that is transparent at the operating wavelength of the optical circulator. Examples of birefringent materials that can be used to form birefringent crystal 600 include, for example, YV0 , LiNb0₃, LiTa0₂, MgF₂, β-BaB0 , calcite, and rutile. As an example of a birefringent crystal for use as birefringent crystals 403 and 406 of optical circulator 400 (Figure 4), consider birefringent crystal 600 to be a uniaxial crystal.

In an uniaxial crystal, the lattice parameters are such that a = b ≠ c , where a refers to the length of the unit cell in the [100] crystallographic direction, b refers to the length of the unit cell in the [010] crystallographic direction and c refers to the length of the unit cell in the [001] crystallographic direction. The crystal directions, referred to vectorially as a , b and c , are shown with reference to birefringent crystal 600. In YV0 , for example, a=b=7.1193 A and c=6.2892 A with a , b and c being orthogonal to one another. LiNb0₃, a trigonal symmetry crystal, has lattice parameters a= 5.148 A and c = 13.863 A.

Birefringent crystal 600 separates an incident light beam into an ordinary beam and an extraordinary beam. For nearly normal incidence, the ordinary beam is substantially undeflected while the extraordinary beam is deflected by birefringent crystal 600. The overall deflection of the extraordinary beam from the ordinary beam is referred to as the "walk-off deflection. Birefringent crystal 600, as shown in Figure 6A, should be cut along crystal directions that maximize the "walk-off deflection of the extraordinary beam and which chooses the "walk-off direction in a convenient fashion for use in an embodiment of the optical circulator.

The maximum walk-off distance in an uniaxial crystal occurs when a light beam is incident on the crystals at an angle of co_M from the c direction in the crystal. The angle CO_M is given by

tanω_M = n„

where r__e is the extraordinary index and ι__o is the ordinary index. For example, in YV0

_i_e=2.1486 and r__o = 1.9447 at a wavelength of 1550 nm, yielding a direction of maximum walk- off of about 47.85° from the c crystallographic direction. In LiNb0₃, alternatively, ι__e=2.13806 and n₀ = 2.21122 at 1550 nm for a maximum walk-off at a direction of about 44.04° from the c crystallographic direction. In one embodiment of the invention, a birefringent crystal 610 is cut and oriented such that the walk-off direction of the extraordinary beam is from a top portion of the crystal towards a bottom portion of the crystal (in about a 45° downward angle from the +Y direction of axis 410). Birefringent crystal 610 may be cut to yield walk-off directions other than that shown. The walk-off direction and the polarizations of the ordinary and extraordinary light rays, in general, are determined by the interactions of the electric field of the light beam with the crystal of birefringent crystal 600. In general, crystals are cut and polished such that, when the crystals are arranged on an optical flat, the desired walk-off directions are obtained.

In one embodiment of the invention, birefringent crystal 600 is YV0 cut such that one surface is the <010> plane and the surface upon which light beams are incident is the <101> plane. In this notation, the <010> plane is the plane of the crystal that is normal to the b crystallographic direction and the <101> plane is the plane that is normal to the vector d + c .

In YV0₄, the extraordinary beam, which is linearly polarized normal to the b crystallographic direction, is deflected in a direction peφendicular to both the b direction and the n direction. Additionally, fortuitously, a crystal cut along the [101] direction (i.e., having a face that is the <101> plane) is arranged such that an incident beam normal to the <101> plane makes an angle of about 48.5° with the c direction, which is very nearly the direction of maximum walk-off of about 47.85°. The [101] direction and the [010] direction are both easily located by X-ray diffraction techniques, so the crystal is easily oriented for cutting. In crystal 600 of Figure 6 A, cut along the <101> and <010> planes, the deflection of the extraordinary beam is substantially in the <101> plane and is parallel with the <010> plane. Crystals are typically cut into rectangular pieces having height H, width W and length

L. The two surfaces along the length of the crystal are polished to an optical quality and coated with an anti-reflective coating for the operating wavelength of the optical circulator. The length of the crystal, L, determines the actual walk-off distance d. For YV0₄, the walk-off distance for a crystal cut in this fashion is about 0.1 L. In LiNb0₃, the maximum walk-off distance d is about 0.03 L. Therefore, optical circulators utilizing YV0₄ crystals can be made roughly 3 times shorter than optical circulators utilizing LiNb0 crystals. In most embodiments, a birefringent crystal required to separate beams of different polarizations exhibits a walk-off distance of about 0.7 mm to about 1 mm. The minimum height H and the width W of the crystal are determined by the walk-off distance d of the crystal and by how many ports that the optical circulator is capable of supporting. The width of the crystal must be sufficient to support one set of light beams, which is slightly larger than the diameter of the light beam from the optical fibers. In many embodiments, the minimum width W of birefringent crystal 600 for a three or four port circulator is about 3 times the diameter of a light beam. In some embodiments of the invention, birefringent crystals 403 and 406 and crystals utilized in non-reciprocal rotators 404 and 405 can include multiple ones of birefringent crystal 600. Optical circulator 400 having crystals 403 and 406 and non-reciprocal rotators 404 and 405 constructed in this manner can have any number of individual ports.

Crystal 600 of Figure 6A is cut so that, as the crystal sits on an external plane that is parallel with the a-c crystallographic plane, the walk-off distance is horizontal. This crystal cut is best suited to provide walk-off deflections in the X-Z plane (Figure 4 A), which is suitable for birefringent crystals in non-reciprocal rotators 404 and 405. Crystal cut 610 in Figure 6A, however, is suitable for birefringent crystals 403 and 406 of optical circulator 400 (Figure 4A). Crystal 610 is cut in the <101> plane for maximum walk-off and oriented such that light is incident from the [101] direction. Crystal 610 is also cut in a plane that is the a-c plane rotated around the [101] direction by about 45° so that the walk-off direction is out of the X-Z plane

(see Figure 4) and towards the -Y direction. In general, as was previously discussed, crystal 610 may be cut and mounted within optical circulator 400 so that the walk-off direction of incident extraordinary light beams is in any direction perpendicular to the Z direction. In other embodiments, crystal 610 can be cut along any plane that is peφendicular to the direction of maximum walk-off.

Figure 6B shows birefringence in crystal 600 of Figure 6A. The crystal is cut such that light is incident on crystal 600 along the direction of substantially maximum walkoff (i.e., the [101] direction in YV0 ) crystallographic direction. Additionally, the b crystallographic direction is out of the page as is indicated by the symbol Θ. In uniaxial crystals, such as YV0 , incident light having a polarization that is along the b-crystallographic direction in the crystal is undeflected while light polarized peφendicularly to the b crystallographic direction is deflected by the birefringent crystal. In general, the crystals can be cut and mounted in an optical circulator in any fashion required to attain a desirable walk-off direction.

Figure 7A shows an example of a non-reciprocal rotator 700 that may be used as non- reciprocal rotator 404 of Figure 4A. Non-reciprocal rotator 700 includes Faraday rotator 701, birefringent crystal 702, Faraday rotator 703 and half- wave plate 704. Faraday rotators 701 and 703 can be any non-reciprocal polarization rotator, such as, for example, a Faraday rotator material.

Faraday rotator materials include Y.I.G. crystals, Bi -added thin film crystals and bismuth-substituted rare earth iron garnet films and are sold commercially by several companies including Lucent Technologies, Inc. The composition of the Bi-added thin film crystals include a combination of, for example, (YbTbBi)₃Fe₅0ι₂ and (AdBi)₃(GeAlGa)₅0ι₂, or of Y.I.G. and Y_3XBi_xFe₅0₁₂. Garnet films include bismuth-substituted rare-earth iron garnet (BiRE)₃(FeGa)₅0i₂ such as Bi_0.75Euι.₅Hθo_.75Fe _.ιGao_.9θι₂ and (BiTb)₃(FeGa)₅0ι₂ that are sold by Lucent Technologies. As a non-reciprocal rotator material, a Faraday rotator is magnetized along a direction, so that the polarization of light propagating along that direction is rotated by the material. Some Faraday rotators, such as (BiRE)₃(FeGa)₅0ι₂, maintain a magnetization without application of external magnets. Others, however, require application of an external magnetic field to the Faraday rotator, which increases the size of an optical circulator utilizing that material.

A Faraday rotator rotates the polarization of the light beam by an amount given by

θ = VBl .

where B is the magnetic field strength, 1 is the length of the Faraday rotator along the direction of beam propagation, and V is a material dependent constant, the Verdet constant. As such, Faraday rotators rotate the polarization of an incident beam clockwise with respect to the +Z axis if the Faraday rotator is magnetized in the +Z direction and rotates the polarization of an incident light beam from counterclockwise with respect to the +Z direction if the Faraday rotator is magnetized in the -Z direction, regardless of the direction of propagation of the light beam.

Faraday rotators 701 and 703 are arranged such that the polarization of a light beam is rotated clockwise (with respect to the direction of propagation of the light beam) if the light beam is propagated along the direction of the magnetic field and counterclockwise (with respect to the direction of propagation of the light beam) if the light beam is propagated along the direction opposite that of the magnetic field. For example, (BiRE)₃(FeGa)₅0ι₂ of thickness approximately 325 μm rotates the polarization of the incident light beam by about 45° ± 1°, although tighter tolerances may be available. In general, Faraday rotators 701 and 703 are arranged to rotate the polarization of light beams to match the polarization requirements of birefringent crystal 702. In non-reciprocal rotator 700 shown in Figure 7 A, Faraday rotator 701 is magnetized in the -Z direction and Faraday rotator 703 is magnetized in the +Z direction, with respect to axis 710. The propagation of light is along the Z axis. In one embodiment, Faraday rotator 701 and 703 rotate the polarization of a light beam propagating along the Z-axis by about 45°. Therefore, the polarization of a light beam propagating in the +Z direction, as is beam A of Figure 7A, is rotated by about 45° counterclockwise (with respect to the +Z direction). If

Figure 7A depicts non-reciprocal rotator 404 of optical circulator 400 (Figure 4), then light ray A of Figure 7A may be linearly polarized in a direction about 45° from the +Y direction towards -X (See 403b of Figure 4B) as is shown in X-Y cross-section 705. A beam cross- section, such as cross-section 705, is an X-Y projection of the polarization light beam with the +Z direction directed into the page and the +Y direction towards the top of the page. After

Faraday rotator 701, the polarization of beam A is along the Y direction, as is shown on cross- section 706.

Birefringent crystal 702 is cut such that polarizations along the Y direction are substantially undeflected and polarizations along the X direction are deflected. The crystal can be cut to substantially achieve the maximum walk-off distance and therefore the direction of propagation of the light beam, the +Z direction, is along the [101] direction in YV0₄. Birefringent crystal 702 is then arranged such that the b-crystallographic axis of the crystal is directed along the Y-direction of optical circulator 400 (Figure 4A) so that the walk-off direction is in the X-Z plane. Faraday rotator 703 rotates the polarization of beam A clockwise (with respect to the +Z direction) by about 45°. Half-wave plate 704 is an active crystal that rotates the polarization, in this case counter-clockwise by about 90°, in order to output a beam of light at B whose polarization is rotated by about 90° from the beam of light entering the non-reciprocal rotator 700, as is shown in cross-section 707. In Figure 7A, consider a light beam entering non-reciprocal rotator 700 at port B with the polarization indicated by cross section 707. The polarization of the light beam is rotated clockwise by about 90° in half-wave plate 704 and about 45° clockwise by Faraday rotator 703, with respect to the +Z direction, in order to become polarized in the X direction, as is shown in cross section 708. The polarization is then peφendicular to the b-crystallographic direction of birefringent crystal 702 and is therefore deflected by birefringent crystal 702. Birefringent crystal 702 is arranged such that the walk-off direction is in the -X direction of axis 410 for beams traveling in the -Z direction. The polarization of the beam is then rotated clockwise by Faraday rotator 701 in order to exit non-reciprocal rotator 700 at port C with the polarization indicated by cross-section 709.

Figure 7B shows the same non-reciprocal rotator 700 as shown in Figure 7A, but reoriented with respect to axis 710 in order to operate as non-reciprocal rotator 405 of Figure 4A. Non-reciprocal rotator 700 in Figure 7A is rotated by about 180° about the +Y direction in order to be suitably oriented to operate as non-reciprocal rotator 405 of Figure 4A, as is shown in Figure 7B. Consider a light beam entering at port A with the polarization indicated by cross- section 710. The polarization indicated by cross-section 710 is peφendicular to the polarization indicated by cross-section 705 of Figure 7 A. The polarization of light beam A is rotated about 90° clockwise (with respect to the +Z direction) by half- wave plate 704 and about 45° counterclockwise by Faraday rotator 703 to become polarized in the Y direction, as is indicated by cross-section 711. This polarization is aligned with the b crystallographic direction of birefringent crystal 702, and therefore light beam A is undeflected by birefringent crystal 702. Faraday rotator 701 rotates the polarization of light beam A by about 45° clockwise to arrive at the polarization at port B indicated by cross-section 712. The polarization of a light beam incident on non-reciprocal rotator 700 at port B, polarized as shown in cross section 712, and propagating along the -Z direction, is rotated about 45° clockwise (with respect to the +Z direction) by Faraday rotator 701 to become polarized in the X direction, as is shown by cross-section 713. The light beam from port B, therefore, is deflected in the -X direction by birefringent crystal 702. The polarization of the light beam is rotated by Faraday rotator 703 and by half-wave plate 704 to exit non-reciprocal rotator 700 at C with the polarization indicated by cross-section 714.

Figures 8 A and 8B show another example of non-reciprocal rotation. Figure 8 A shows a non-reciprocal rotator 800 that includes Faraday rotators 801 and 803, birefringent crystal 802 and rotator 804. Faraday rotator 801 and 803 are magnetized in the opposite direction of

Faraday rotators 701 and 703 of Figure 7A, respectively. Faraday rotator 801, as oriented in Figure 8A, is magnetized in the +Z direction and Faraday rotator 803 is magnetized in the -Z direction, with respect to axis 810. Figure 8A and 8B show non-reciprocal rotator 800 oriented for substitution as non-reciprocal rotator 404 and 405, respectively. In Figure 8 A, light beam 805 is incident at A, propagating in the +Z direction of axis

810, and polarized as indicated in cross-section 820. The polarization of light beam 805 is rotated by about 45° clockwise (with respect to the +Z direction) by Faraday rotator 801 to become light beam 806 which is polarized as indicated in cross-section 821. Again, birefringent crystal 802 is arranged such that the b crystallographic direction is aligned with the +Y axis and a beam polarized in the X direction will be deflected in the +X direction. A beam polarized along the Y direction will be undeflected by birefringent crystal 802.

Light beam 806, which is polarized along the Y direction, is therefore undeflected by birefringent crystal 802. The polarization of light beam 806 is rotated by about 45° counterclockwise by Faraday rotator 803 and about 90° clockwise by half-wave plate 804 so that light beam 807 is polarized peφendicularly to light beam 805, as is shown in cross-section 822.

If light beam 807 is entering non-reciprocal rotator 800 at port B, with the polarization shown in cross-section 822, then the polarization of light beam 807 is rotated by about 90° counterclockwise (with respect to the +Z direction) in rotator 804 and about 45° counterclockwise by Faraday rotator 803 in order that light beam 808 is polarized along the X direction, as is shown in cross-section 823. A light beam polarized along the X direction and propagating in the -Z direction is deflected in the -X direction by birefringent crystal 802. The polarization of light beam 808 is rotated about 45° clockwise (with respect to the +Z direction) by Faraday rotator 801 to exit at port C as light beam 809. Light beam 809 is polarized peφendicularly to light beam 807, as is indicated by cross-section 824.

Figure 8B shows non-reciprocal rotator 800 oriented for use as non-reciprocal rotator 405 (Figure 4A). In essence, non-reciprocal rotator 800 of Figure 8 A, oriented appropriately for use as non-reciprocal rotator 404 (Figure 4A), is rotated by about 180° around the +Y direction to be oriented appropriately for use as non-reciprocal rotator 405, as is indicated in Figure 4B.

Light beam 810, originating at port A of optical circulator 400 (see Figure 4A), enters non-reciprocal rotator 800 at A and is polarized along a direction about 45° from X towards Y, as is shown in cross-section 825. The polarization of light beam 810 is rotated by about 90° clockwise (with respect to the +Z direction) by half- wave plate 804 and about 45° clockwise by Faraday rotator 803 to become light beam 811, which is polarized along the +Y direction as is shown by cross-section 826. Light beam 811, therefore, is undeflected by birefringent crystal 802. The polarization of light beam 811 is rotated by about 45° counterclockwise (with respect to the +Z direction) by Faraday rotator 801 to exit non-reciprocal rotator 800 at port B as light beam 812. The polarization of light beam 812 is about 45° from +Y towards -X, as is indicated by cross-section 827.

If light beam 812 is entering non-reciprocal rotator 800 at port B, then the polarization of light beam 812 is rotated by about 45° counterclockwise (with respect to the +Z direction) to become light beam 813, which is polarized along the X direction as is indicated by cross- section 828. Light beam 813, therefore, is deflected along the -X direction by birefringent crystal 802. The polarization of light beam 813 is rotated by about 45° clockwise (with respect to the +Z direction) by Faraday rotator 803 and by about 90° counterclockwise by half-wave plate 804 to become light beam 814, which is polarized identically with light beam 810 as is shown in cross-section 829 and 825, respectively.

Yet another embodiment of non-reciprocal rotator elements 404 and 405 is shown in Figures 9A and 9B, respectively. Figure 9A shows a non-reciprocal rotator 900 oriented appropriately for use as non-reciprocal rotator 400 (Figure 4A). Coordinate system 910 indicates the orientation of non-reciprocal rotator 900, which corresponds to the orientation of non-reciprocal rotator 400 in optical circulator 400 (Figure 4A). Non-reciprocal rotator 900 in Figure 9A is oriented such that Faraday rotators 901 and 903 are both magnetized along the +Z directions. As for non-reciprocal rotator 404 (Figure 4A), consider a light beam 904 originating from port A of optical circulator 400 and arriving at port A of non-reciprocal rotator 900. Light beam 904 is polarized along a direction of about 45° from +Y towards +X, as is indicated in cross-section 920. The polarization of light beam 904 is rotated by about 45° clockwise (with respect to the +Z direction) so that light beam 905 is polarized along the Y direction, as is indicated in cross-section 921. Again, birefringent crystal 902 is cut such that the b crystallographic axis is oriented with the Y direction. Therefore, light beam 905 is undeflected by birefringent crystal 902. The polarization of light beam 905 is rotated by about 45° clockwise (with respect to the +Z direction) by Faraday rotator 903 so that the polarization of light beam 906 is along a direction orthogonal to the polarization of light beam 904, as is indicated in cross-section 922.

Additionally, consider light beam 906, having the polarization indicated in cross-section 922, incident on non-reciprocal rotator 900 at input B. The polarization of light beam 906 is rotated by about 45° clockwise (with respect to the +Z direction) by Faraday rotator 903 so that light beam 907 is polarized along the X direction, as is indicated in cross-section 923. Light beam 907, therefore, is deflected by birefringent crystal 902. The polarization of light beam

907 is rotated by about 45° clockwise (with respect to the +Z direction) by Faraday rotator 901 to exit as light beam 908 at input C. Light beam 908 is polarized along the same direction as light beam 904, as is indicated by cross-section 924.

Non-reciprocal rotator 900 in Figure 9 A is reoriented by about a 180° rotation of rotator 900 around the +Y direction in order to obtain the orientation indicated in Figure 9B. The orientation of non-reciprocal rotator 900 shown in Figure 9B is appropriate for use as non- reciprocal rotator 405 of Figure 4A.

Light beam 910, originating from port A of optical circulator 400 is incident at input A of non-reciprocal rotator 900 of Figure 9B and has the polarization indicated by cross-section 925. The polarization of light beam 910 is rotated about 45° counterclockwise (with respect to the +Z direction) in Faraday rotator 903 so that light beam 911 is polarized along the Y direction, as is indicated by cross-section 926. As such, light beam 911 remains undeflected by birefringent crystal 902. The polarization of light beam 911 is rotated by about 45° counterclockwise (with respect to +Z) by Faraday rotator 901 in order to exit at input B as light beam 912, which is polarized in a direction peφendicular to that of light beam 910 as is shown by cross-sections 927 and 925, respectively.

Consider light beam 912, which is polarized in a direction about 45° from +Y towards +X as is shown in cross-section 927, to be propagating in the -Z direction and incident at input B of non-reciprocal rotator 900. The polarization of light beam 912 is rotated about 45° counterclockwise (with respect to the +Z direction) by Faraday rotator 901 in order to become light beam 913, which is polarized along the X direction as is indicated by cross-section 928. Light beam 913 is deflected in the -X direction by birefringent crystal 902. The polarization of light beam 913 is rotated by about 45° counterclockwise (with respect to the +Z direction) by Faraday rotator 903 in order to become light beam 914 polarized in the same direction as light beam 910, as is indicated by cross-section 929.

The embodiments of non-reciprocal rotators 404 and 405 illustrated as non-reciprocal rotators 700, 800 and 900 shown in Figures 7A through 9B are illustrative of non-reciprocal rotators for the example embodiment of non-reciprocal illustrated in Figures 4A through 4E. One of ordinary skill in the art will recognize that other embodiments of optical circulators according to the present invention will require non-reciprocal rotators having different characteristics.

Figure 10A shows a projection of optical circulator 400 in the X-Z plane (as defined by axis 410 of Figure 4A). As before, optical circulator 400 includes collimators 401 and 408, alignment prisms 402 and 407, birefringent crystals 403 and 406, and non-reciprocal rotators 404 and 405. In Figure 10A, non-reciprocal rotator 405 is positioned directly beneath (i.e., towards the -Y direction) non-reciprocal rotator 404. In one embodiment of the invention, non- reciprocal rotator 400 includes only one alignment prism 402. Embodiments having more than two ports have at least one alignment prism and embodiments having more than three ports have two alignment prisms.

Birefringent crystal 403 has length L in the Z direction. Birefringent crystal 406 has a length L' in the Z direction which, if both birefringent crystals are of the same material and cut, is identical with L. Otherwise, the length of birefringent crystal 406 L' is such that light beams that are separated in birefringent crystal 403 are recombined in birefringent crystal 406. Non- reciprocal rotators 404 and 405 each include a birefringent crystal whose length is arranged to deflect in an X direction the distance between light beams from adjoining ports.

Figures 10B through 10E illustrate the cuts and orientation of birefringent crystals 403 and 406 and the birefringent crystals for non-reciprocal rotators 404 and 405 in an embodiment of optical circulator 400 where the birefringent crystals are not identically cut. Figures 10F through 1 OH show an embodiment where birefringent crystals 403 and 405 and the birefringent crystals of non-reciprocal rotators 404 and 405 have identical cuts.

Figure 10B shows an X-Y projection of one embodiment of birefringent crystal 403. Birefringent crystal 403 is cut such that the direction of maximum walk-off (nearly the [101] direction in YV0 ), , is peφendicular to one face. This is indicated in Figure 10B by the symbol Θ, indicating a vector projection out of the page (the -Z direction). The crystal is therefore positioned in optical circulator 400 such that n is aligned with the -Z axis (see axis 410). Birefringent crystal 403 is further arranged so that the b crystallographic direction is rotated an angle θ towards -X from +Y. A light beam incident on birefringent crystal 403, propagating in the +Z direction, and having a polarization directed an angle θ from Y towards - X, as is indicated by polarization state 1010, is undeflected while a light beam having a polarization directed an angle 90-θ from Y towards +X, as is indicated by polarization state 1011 is deflected. The walk-off direction, indicated by walk-off vector 1012, vt , is at an angle of θ from -X towards -Y. Therefore, if θ is 45°, as w has been defined in the examples discussed above, the polarization of the ordinary (or undeflected) light beam is 45° from Y towards -X, the polarization of the extraordinary light beam is 45° from Y towards +X, and the walk-off direction is 45° from -X towards -Y.

In many embodiments of optical circulator 400, birefringent crystal 403 is cut such that the surfaces of birefringent crystal 403 are peφendicular to the axis of coordinate system 410. Cutting the crystals in this fashion allows for simplified alignment of crystals and ease of mounting within a circulator package. The dimension of the crystal in the Y direction are at least large enough to accommodate the Y direction walk-off distance. The dimension of birefringent crystal in the X direction is large enough to accommodate light beams from each port of optical circulator 400.

Figure 10E shows a birefringent crystal cut for birefringent crystal 406. In most embodiments, birefringent crystal 403 and 406 are of identical materials and therefore the two crystal cuts will be the same. Birefringent crystal 406 shown in Figure 10E is identically oriented and identically cut with birefringent crystal 403 shown in Figure 10B. In some embodiments, where birefringent crystal 403 and birefringent crystal 406 are of differing materials are oriented differently, the two crystals are not identically arranged or cut.

In Figure 10E, birefringent crystal 406 is cut such that the direction of maximum walk- off , n , is directed along the -Z direction and oriented such that the b-crystallographic axis is at an angle θ from +Y towards -X. The polarization of the ordinary beam, along the b- crystallographic axis, is shown by polarization state 1019. The polarization of the extraordinary beam is peφendicular to the b-crystallographic axis, as is shown by polarization state 1020. The walk-off direction is the same as in birefringent crystal 403, as is shown by walk-off vector 1021.

Figures 10C and 10D show one embodiment of crystal cuts and orientations of the birefringent crystals 1004 and 1005, respectively, of non-reciprocal rotators 404 and 405. In Figure 10C, birefringent crystal 1004 is cut and oriented such that the b-crystallographic direction is aligned with Y and the direction of maximum walk-off, n , is directed along -Z, as indicated by the vector b and the Θ symbol, respectively. In most embodiments birefringent crystal 1004 is cut with surfaces peφendicular to the X direction, the Y direction and the Z direction for easier alignment and mounting. With the orientation of birefringent crystal 1004 shown in Figure 10C, a light beam propagating in the +Z direction and polarized along the Y direction, as is indicated by polarization state 1013, is undeflected. A light beam propagating in the +Z direction and polarized along the X direction is deflected in the -X direction, as indicated by polarization state 1014 and walk-off vector 1015.

In Figure 10D, one embodiment of birefringent crystal 1005 is shown. In this embodiment, birefringent crystal 1005 is cut and oriented such that the b-crystallographic direction is aligned with the +Y direction and the direction of maximum walk-off, h , is aligned with the +Z direction, as is shown by the vector b and the ® symbol, respectively. In most embodiments, crystal 1005 is cut along surfaces peφendicular to the X direction, the Y direction and the Z direction. With the orientation of birefringent crystal 1005 shown in Figure 10D, a light beam propagating in the +Z direction and polarized along the Y direction is undeflected, as is shown by polarization state 1016. A light beam propagating in the +Z direction and polarized along the X direction is deflected in the -X direction, as is shown by polarization state 1017 and walk-off vector 1018, respectively. In most embodiments, birefringent crystal 1005 of Figure 10D and birefringent crystal 1004 of Figure 10C are identically cut crystals which differ only in physical orientation.

The physical orientation of birefringent crystal 1005 of Figure 10D is the physical orientation of birefringent crystal 1004 of Figure 10C rotated around Y by about 180°. In Figures 7A and 7B, non-reciprocal rotator 700 of Figure 7B is physically orientated as non- reciprocal rotator 700 of Figure 7A rotated around Y by about 180°. In Figures 8A and 8B, non-reciprocal rotator 800 of Figure 8B is physically orientated as non-reciprocal rotator 800 of Figure 8A rotated around Y by about 180°. In Figures 9A and 9B, non-reciprocal rotator 900 of Figure 9B is orientated as non-reciprocal rotator 900 of Figure 9A rotated around Y by about 180°. In this fashion, non-reciprocal rotators 404 (Figure 4A) and 405, or at least the birefringent crystal portion of non-reciprocal rotators 404 and 405, can be manufactured from one larger optical device. The larger device is then cut in half and the two halves appropriately oriented and positioned in optical circulator 400 to form non-reciprocal rotator 404 and non- reciprocal rotator 405. In this fashion, the need for matching non-reciprocal rotators 404 and 405 to one another is eliminated.

In other embodiments birefringent crystal 1005 of Figure 10D is identically oriented with birefringent crystal 1004 of Figure IOC. In that case, the remaining components of non- reciprocal rotators 404 and 405 (such as Faraday rotators 701 and 703 and rotator 704 of the embodiment shown in Figures 7A and 7B) are appropriately mounted onto birefringent crystals 1005 and 1004, respectively. One skilled in the art will recognize that components such as Faraday rotators can also be mounted to birefringent crystals 1001 and 1002. Figures 10F, 10G and 10H show an embodiment of the crystal cuts where birefringent crystals 403 and 406 and non-reciprocal rotators 404 and 405 all utilize a single cut of the birefringent crystal. Optical circulator 400, therefore, can be formed, using three birefringent crystals having a single cut. Figures 10F and 10H show the orientation and cuts of birefringent crystals 403 and 406, respectively. In Figure 10F, birefringent crystal 403 is oriented in the same way with respect to optical circulator 400 as is birefringent crystal 403 of Figure 10B. The cut of birefringent crystal 403 of Figure 10F, however, differs. In Figure 10F, birefringent crystal 403 is cut such that the direction of maximum walk-off, f , is directed along the -Z direction. Two other faces are cut peφendicular to the b-crystallographic direction. The remaining two faces are cut parallel with the b-crystallographic direction. Birefringent crystal 403 of Figure 1 OF is oriented such that the b-crystallographic direction is at an angle of θ from +Y towards -X. In Figure 10F, physical orientation is accomplished through positioner 1001 however any method of properly positioning crystal 403 with respect to the rest of optical circulator 400 can be used. The embodiments of birefringent crystal 406 shown in Figure 10H is cut and oriented, with positioner 1002, identically with birefringent crystal 403 of Figure 10F.

A light beam having a polarization along the b-crystallographic direction, as shown by polarization state 1022 of Figures 10F and polarization 1028 of Figure 10H, is undeflected by birefringent crystals 403 and 406, respectively. A light beam having a polarization peφendicular to the b-crystallographic axis, as shown by polarization state 1023 of Figure 10F and polarization state 1029 of Figure 10H, is deflected as indicated by walk-off vector 1024 of Figure 10F and walk-off vector 1030 of Figure 10H, respectively, of birefringent crystals 403 and 406, respectively. In Figure 10G, non-reciprocal rotators 404 and 405 share a common birefringent crystal

1006. Birefringent crystal 1006 is identically cut with birefringent crystal 403 of Figure 10F and physically oriented such that the b crystallographic axis aligns with the Y direction and the direction of maximum walkoff , n , is direction along the -Z direction. Therefore, a light beam polarized along the Y axis, as shown by polarization state 1025, is undeflected while a light beam polarized along the X axis, as is shown by polarization state 1026, is deflected along the -

X direction, as is shown by the walk-off vector 1027.

An embodiment of optical circulator 400 constructed using the embodiments of birefringent crystals shown in Figures 10F, 10G and 10H is constructed using only one cut of crystal, which reduces production costs for the crystals. Only one cutting sequence needs to be indicated to a crystal manufacturer, which reduces the time required to process crystals.

Additionally, the time required to "match" crystals and assemble the final to optical circulator is reduced because tolerances for producing one type of crystal are more easily controlled than tolerances for production of several cuts of crystals.

In the embodiments of the optical circulators shown in Figures 4A through 10H, the optical circulators have a definable upper portion and lower portion with respect to the Y axis. For example, in optical circulator 400 of Figure 4 A, non-reciprocal rotator 404 is in an upper portion and non-reciprocal rotator 405 is in a lower portion of optical circulator 400. In these embodiments, collimator 401 is in the upper portion of optical circulator 400 and collimator 408 is in the lower portion of optical circulator 400. Additionally, optical circulator 400 sits flat on an X-Z plane. In general, the X-Z plane has no reference outside of optical circulator 400 itself. In some situations, for example such as when an optical circulator is to be mounted on a silicon substrate, it is desirable that the optical circulator be arranged such that the collimators are coplanar with respect to a particular plane, i.e., collimators 401 and 408 are equidistant above a particular externally defined plane.

Figure 11 A shows an orientation and cut of birefringent crystals such that optical circulator 1100 can be mounted on an external plane, such as a silicon substrate or other mounting surface, and all of the input and output ports of the circulator lie in a plane parallel to the external plane. Optical circulator 1100 includes birefringent crystals 1103 and 1106 and non-reciprocal rotators 1104 and 1105 (which may be combined into a single non-reciprocal rotator 1115). Optical circulator 1100 may also include alignment prisms 1102 and 1107 and collimators 1101 and 1108. In some embodiments, some or all of collimators 1101 and 1108 and some or all of alignment prisms 1102 and 1107 are absent. In some embodiments, collimators 1101 and 1108 are groups of optical fibers terminated with microlens collimators and alignment prisms 1102 and 1107 are absent. Coordinate axis 1110, indicating that Figure 11 A is a Y-Z projection of optical circulator 1100, provide reference coordinates to Figure 11 A.

In Figure 11 A, substrate l l l l is notched to hold collimators 1101 and 1108 (i.e., collimators 1101 and 1108 are mounted in V-grooves 1116 and 1117, respectively, in substrate l l l l). In an embodiment where optical fibers are terminated with microlens collimators, substrate l l l l can be notched to position and affix each individual optical fiber. Birefringent crystals 1103 and 1106, as well as non-reciprocal rotators 1104 and 1105, are mounted on a flat portion of substrate l l l l. In most embodiments, collimators 1101 and 1108 are physically larger than birefringent crystals 1103 and 1106, indicating that the flat portion of substrate l l l l is higher than the notched portion of substrate l l l l. The relationship between the flat portion of substrate l l l l and the notched portion of substrate l l l l is determined by the optical alignment needs of optical circulator 1100.

Figure 1 IB shows a geometric projection into the X-Y plane (indicated by coordinate axis 1110 of Figure 1 IA) with respect to birefringent crystals 1103 and 1106 in order to indicate the geometry required to attain a coplanar series of ports. The projection indicates a three-port circulator where ports A and C are input through collimator 1101 and alignment prism 1102 but port B is coupled directly to a fiber without alignment prism 1107. Birefringent crystal 1100, as indicated, is cut such that the direction of maximum walk-off, n , is peφendicular to a face and aligned in the -Z direction. The position and orientation of alignment prism 1102 is indicated. The apex of alignment prism 1102 makes an angle θ with the b-crystallographic direction of birefringent crystals 1103 and 1006 and is aligned with the b-crystallographic direction of birefringent crystals of non-reciprocal rotators 1104 and 1105. In Figure 4A, for example, the Y-direction coincides with the apex of alignment prism 402.

In Figure 11 A, collimator 1101 is aligned with the apex of alignment prism 1102 and couples light beams from ports A and C to inputs A and C, respectively. As was previously described, (see, e.g., Figures 4A through 4E) the light beams entering at input A are routed to exit at input B of optical circulator 1100. In the three-port circulator embodiment, collimator 1108 is arranged adjacent to input B in order to receive the light beam from input A and couple it to an optical fiber at port B.

Figure 1 IB shows an X-Y projection, looking towards the +Z direction, through optical circulator 1100. In this case, the orientation of the birefringent crystals relative to one another is the same as has been previously discussed. The apex of alignment prisms 1 102 is arranged to make an angle θ with the b crystallographic axis of birefringent crystals 1103 and 1106 and is arranged to align with the b-crystallographic axis of the birefringent crystals of non- reciprocal rotators 1105 and 1104. The apex of alignment prism 1107 is shown for reference only; the three-port circulator shown in Figure 1 IB does not include alignment prism 1107.

Coplaner optical circulator 1100 can be obtained from optical circulator 400 (Figure 4A) by rotating optical circulator 400 about the +Z direction until the ports are coplaner and then cutting and polishing the surfaces of each of the crystals to lay on the flat portion of substrate l l l l. Figure 11 B illustrates the geometry for the case of a three-port circulator that includes alignment prism 1102 but not alignment prism 1107 so that the light beam at input B is coupled directly into collimator 1108. In that situation, collimator 1108 is aligned directly with input B. Collimator 1101, however, is aligned with the apex of alignment prism 1102 such that input A falls to one side of the apex and input C falls at an equal distance to the opposite side of the apex. The separation between input A and input C is designated as S. The walk-off distance, the lateral displacement (i.e., in the -X direction of axis 1110 of Figure 1 IA) between input A and input B, is W. In general, a line connecting collimator 1101 and collimator 1108 in the X-Y projection makes an angle β with the line connected input A and input B. The line connecting point A and point B is peφendicular to the b-crystallographic direction of birefringent crystal 1103. For general θ, S and W, the angle β is given by the equation

where W is the walk-off distance, S is the separation between inputs A and C, and θ is the angle between the apex of alignment crystal 1102 and the b-crystallographic axis of birefringent crystal 1103. Coplanar optical circulator 1100, then, is arranged such that flat surfaces 1112, 1113 and 11 14 are parallel with the line connected collimator 1101 and collimator 1108. Therefore, birefringent crystal 1103 is cut such that the b-crystallographic direction is aligned with an angle β from +Y (axis 1110 of Figure 1 IA) towards +X. The apex of alignment prism 1102 is, then, at an angle of θ + β from +Y towards +X. Embodiments of optical circulator 1100, then, are functionally the same as embodiments of optical circulator 400 that have been previously described. Optical circulator 1100 is optical circulator 400 of Figure 4A rotated clockwise about the +Z axis by an angle of θ + β and with a surface along the new X-Z plane polished flat. For example, if θ is arranged to be about 45°, and W is V2 S (as is the case illustrated in Figures 4B through 4E), then β is about 18.43°. Coplanar optical circulator 1100, therefore, is arranged such that the b-crystallographic axis of birefringent crystal 1103 is directed along a direction about 18.43° from +Y towards +X in axis 1110 of Figure 1 IA. Alignment prism 1102 is arranged such that its apex makes an angle of about 63.43° from +Y towards +X. The birefringent crystals of non-reciprocal rotators 1105 and 1104 are arranged such that the b crystallographic direction is directed at an angle of about 18.43° from +X towards +Y. The other components of non-reciprocal rotators 1105 and 1104 (as described in Figures 7A through 9B) are arranged accordingly. The orientation of optical circulator 1100, then, is the same as that of optical circulator 400 (Figure 4A) rotated by about 63.63° counterclockwise around the +Z axis.

In an embodiment of optical circulator 1100 that includes alignment prism 1107 (i.e., primarily embodiments having four ports or more), collimator 1108 is aligned with the apex of alignment prism 1107. In that circumstance, a line connecting collimator 1101 and collimator 1108 is parallel with the line connecting A and B. The b-crystallographic axis of birefringent crystal 1103 should be aligned with Y in order to arrange coplanar optical circulator 1100.

Therefore, the orientation of optical circulator 1100 is the same as that of optical circulator 400 (Figure 4A) rotated counterclockwise by an angle θ about +Z.

Figures 12A through 12D show yet another embodiment of an optical circulator having a minimum number of optical components according to the present invention. As shown in Figure 12 A, optical circulator 1200 includes birefringent crystals 1203 and 1207, a half- wave plate 1204, and a Faraday rotator 1205. Half- wave plate 1204 is arranged to rotate the polarization of a light beam propagating in the +Z direction (with reference to coordinate axis 1206) by about 45° clockwise. Faraday rotator 1205 is arranged to rotate the polarization of a light beam by about 45° clockwise (with reference to the +Z direction). Optical circulator 1200 also includes mirror assembly 1208, which includes transparent portion 1213 and two 45° mirrors 1209 and 1210. Light beams from ports A and C are coupled into optical circulator 1200 by alignment prism 1202 and collimator 1201. The non-reciprocal rotator 1280 formed by half- wave plate 1204 and Faraday rotator 1205 does not include a birefringent crystal. Figure 12A shows a randomly polarized light beam 1220 entering at port A of optical circulator 1200. Light beam 1220 is directed along the +Z direction by alignment prism 1202 and passes through transparent portion 1213 of mirror assembly 1208. The polarization state of light beam 1220 is random, as shown in cross-section 1250. Birefringent crystal 1203 is arranged such that a light beam polarized along the X direction is undeflected and a light beam polarized along the Y direction is deflected in the -Y direction. Therefore, light beam 1220 is separated into light beam 1221, which is polarized along the X direction as shown in cross-section 1251, and light beam 1222, which is polarized along the Y direction as is shown in cross-section 1255. The polarization of light beam 1221 is rotated by about 90° by rotator 1204 and Faraday rotator 1205 (about 45° clockwise with respect to the +Z direction by rotator 1204 and another about 45° clockwise by Faraday rotator 1205) and becomes polarized along the Y direction, as is shown by cross-section 1252. Similarly, the polarization of light beam 1222 is rotated by about 90° into a polarization along the X direction by the combination of rotator 1204 and Faraday rotator 1205, as is shown by cross-section 1254. Light beam 1221 is deflected by birefringent crystal 1207, which is arranged such that it recombines light beams which have been separated by birefringent crystal 1203. Light beam 1223, the combination of light beams 1221 and 1222, exits optical circulator 1200 at port B. Light beam 1223 is randomly polarized, as is shown by cross-section 1253. Figure 12B shows light beam 1224, which is randomly polarized as is shown in cross- section 1256, entering optical circulator 1200 at port B. Light beam 1224 is separated into light beam 1225, polarized along the Y direction, as is shown in cross-section 1257, and light beam 1226, polarized along the X direction as is shown in cross-section 1259, by birefringent crystal 1207. Light beam 1225 is the extraordinary beam, which is deflected by birefringent crystal 1207, while light beam 1226 is the ordinary beam, which remains undeflected. The polarization of light beam 1225 is not altered by the combination of Faraday rotator

1205 (which rotates the polarization by about 45° clockwise with respect to the +Z direction) and rotator 1204 (which rotates the polarization by about 45° counterclockwise). Similarly, the polarization of light beam 1226 is not altered by the combination of Faraday rotator 1205 and rotator 1204. Therefore, light beam 1225 remains the extraordinary beam which is polarized along the Y axis as is shown in cross-section 1258, in birefringent crystal 1203 and is again deflected in the +Y direction. Light beam 1226 is again the ordinary beam which is polarized along the X direction as shown in cross-section 1260, in birefringent crystal 1203 and remains undeflected. Light beam 1226 is deflected into the +Y direction by mirror 1209 and into the -Z direction by mirror 1210. Mirror assembly 1208 is arranged such that light beam 1225 passes just over the top of mirror assembly. Although not directly combined (light beams 1226 and 1225 remain slightly separated), light beams 1226 and 1225 are coupled into port C by the combination of alignment prism 1202 and collimator 1201. In some embodiments, the coupling of light beams 1226 and 1225 into port C is assisted by terminating the optical fiber of port C with a microlens as shown in Figures 5F and 5G.

In order to reduce the timing difference (PMD) of propagation between light beams of differing polarizations to a desired level, it is necessary that the effective optical paths of light beams 1225 and 1226 be substantially identical. The effective optical path determines the time of travel of the polarization separated light beams through the optical circulator. If the time of travel is not the same, an optical pulse signal input to the optical circulator at port B will be broadened (i.e., the polarization mode dispersion, PMD). If the time of travel is substantially different, and the optical pulse signal input is of short duration, the optical signal input could be separated into two signals of opposite polarizations. Therefore, it is desirable to have the optical path lengths between the two polarizations be substantially identical so that the dispersion is as low as possible.

The ordinary beam, light beam 1226, passes entirely through both of the crystals along their lengths. The extraordinary beam, light beam 1225, is deflected by both crystals, thereby traveling a longer path. Additionally, the indices of refraction in birefringent crystals 1203 and 1207 for the ordinary and extraordinary beams are different. Light beam 1225 experiences an index of refraction of rie and light beam 1226 experiences an index of refraction of n_o. The effective path of light beam 1226 through each of birefringent crystal 1203 or 1207, then, is r__oL, where L is the length of one of birefringent crystals 1203 and 1207 and assuming that birefringent crystal 1203 is equivalent to birefringent crystal 1207. The total effective path of light beam 1225 through one of birefringent crystals 1203 or 1207 is n((o)VZ.² + W² , where L is the length of the crystal, n(ω) is the index seen by the extraordinary beam when the crystal is cut at an angle ω from the c crystallographic axis (see Figure 6A), and W is the walk-off distance through the crystal. Therefore, the effective path difference Δ between light beams 1225 and 1226 is

= 2n₀L + ,h - 2n(ω) ύ + W² + (« - X)d;

where n_g is the index of refraction of transparent portion 1213 of mirror assembly 1208, h is the path length followed by light beam 1226 in mirror assembly 1208, and d is the thickness of mirror assembly 1208. In optical circulator 1200, h is equal to 2W-ε, where ε is the closest separation between beams 1225 and 1226 at mirror assembly 1208. Additionally, assuming that W = αL and h 2αL-ε, where α is the walk-off constant, then Δ is given by Δ = 2Ln₀ - 2Ln(ω)Jx + a² + (2aL - ε)n + (n - X)d

As can be seen from this expression, positive crystals (where r__e>n(ω )>_i_o) yield less of a path difference, and therefore less polarization mode dispersion. For embodiments where birefringent crystals 1203 and 1207 are of YV0₄ and transparent portion 1213 is of fused silica, n₀= 1.9447, n_e = 2.1486, n_g = 1.445, and ω=48.5°. The effective path difference where ε=0.5 mm, L=7 mm and d=l .5 mm, then, is Δ = 0.3 mm, yielding a timing difference (or PMD) between beams 1225 and 1226 of approximately lps.

In many applications, timing differences of this magnitude are tolerable and an optical circulator such as optical circulator 1200 can be serviceable. The slight separation between light beams 1225 and 1226 does not prevent both beams from being coupled into the same optical fiber in collimator 1201, especially if the optical fiber is terminated with a microlens 540 as shown in Figures 5F and 5G.

Some applications, however, require a timing difference of less than .1 ps. In one embodiment of optical circulator 1200, crystals 1203 and 1207 are cut along a direction other than the direction of maximum walk-off in order to better match path differences. In the above equation, Δ is set to 0 and the resulting equation can be solved for n(ω). For YV0₄ and the physical parameters (L, ε, d, h) listed above, ω = 54° minimizes Δ. Minimizing Δ results in a minimum timing difference, and therefore a minimum PMD.

Another consideration is the polarization dependent loss (PDL) between light beams 1225 and 1226 of Figure 12B. The PDL should be kept below about 0.1 db. In Figure 12B, light beam 1225 remains a longer time in birefringent crystals 1203 and 1207 and therefore is subjected to a little more absoφtion and scattering type losses in those crystals. Light beam 1226, however, suffers additional losses at both mirrors 1209 and 1210, as well as the losses due to propagation in the material of mirror assembly 1208. Total internal reflection losses at mirrors 1209 and 1210 can be kept small, less than about .01%, so that the PDL is low. In actuality, the difference in losses between light beams 1225 and 1226 can be kept below 0.1 dB.

One advantage of optical circulator 1200 is that it has relatively few components. Because of the relatively few components, optical circulator 1200 can be easily and inexpensively produced. Additionally, optical circulator 1200 is coplaner by nature (i.e., collimator 1201 and port B both lie in a plane parallel with the Y-Z plane) and can be mounted on a silicon substrate as described above.

Figure 12C shows optical circulator 1200 having four ports and illustrates propogation of light from port C to port D. A portion of a light beam from port C, randomly polarized light beam 1227, is coupled into optical circulator 1200 through collimator 1201 and alignment prism 1202 such that it is incident on mirror 1210. The polarization of light beam 1227 is shown in cross-section 1261. Mirror 1210 redirects light beam 1227 into the -Y direction and mirror 1209 reflects light beam 1227 into the +Z direction. Birefringent crystal 1203 separates light beam 1227 into light beams 1228 and 1229. Light beam 1228, polarized along the X direction, is undeflected by birefringent crystal 1203. The polarization of light beam 1228 is rotated about 90° by the combination of half- wave plate 1204 and Faraday rotator 1205. Light beam 1228, now polarized along the Y direction, as is shown in cross-section 1268, is deflected by birefringent crystal 1207. Light beam 1229, polarized in the Y direction, as is shown in cross-section 1267 is deflected by birefringent crystal 1203. The polarization of light beam 1229 is rotated about 90° by the combination of rotator 1204 and Faraday rotator 1205. Light beam 1229, now polarized along the X direction, as is shown in cross-section 1269, is undeflected in birefringent crystal 1207 to recombine with light beam 1228. Resultant randomly polarized light beam 1230, as is shown in cross-section 1270, is coupled into port D by alignment prism 1211 and collimator 1212.

In many embodiments, another portion of the light beam from port C, randomly polarized light beam 1235, will pass over the top of mirror 1210 and be separated into light beams 1236 and 1237 in birefringent crystal 1203. Light beams 1236, having polarization along the X direction as shown in cross-section 1262, and 1237 having polarization along the Y direction as shown in cross-section 1263, propagate through birefringent crystal 1203, rotator 1204, Faraday rotator 1205, and birefringent crystal 1207 in the same fashion as light beams 1228 and 1229, respectively, to recombine into light beam 1238. A second mirror assembly 1231, having mirrors 1232 and 1234 and transparent portion 1233, and being substantially similar to mirror assembly 1208, is positioned between birefringent crystal 1207 and alignment prism 1211. Light beam 1238, as it exits birefringent crystal 1207, is deflected in the -Y direction by mirror 1232 and into the +Z direction by mirror 1234. Light beam 1230 is unaffected by mirror assembly 1231. Beam 1238 is coupled, along with light beam 1230, into port D of optical circulator 1200 by alignment prism 1211 and collimator 1212.

Figure 12D shows the cut of birefringent crystal 1203. Birefringent crystal 1207 is cut substantially identically if birefringent crystal 1207 is of the same birefringent material as birefringent crystal 1203. If the materials of birefringent crystals 1203 and 1207 differ, then their cuts may not be the same. The extraordinary index of refraction depends on the orientation of the crystal (i.e., the relationship between the light beam propagation and the optical axis). Birefringent crystal 1203 is cut to minimize the path difference between light beams 1226 and 1225 of Figure 12B. Therefore, the crystal is rotated slightly from the direction of maximum walk-off, ή_m , in order to adjust the indices of refraction so that the optical paths between light beams are substantially identical. In YV0 , for example, the optimum angle between the crystallographic c-direction and , ή_m , the optimal direction, is about 54° for the structural parameters mentioned above (see Figure 12B), instead of about 48° for optimal walkoff. Although other walk-off directions can be obtained, in order that the walk-off direction in birefringent crystal 1203 be in the -Y direction, the b-crystallographic direction of the crystal is directed along the -X direction.

Figure 13 shows an embodiment of a packaged optical circulator 1300. In general, an optical circulator can have any number of ports. As an example, Figure 13 shows packaged optical circulator 1300 having three ports. Packaged optical circulator 1300 includes a collimator prism assembly 1330, a circulator component assembly 1333 and a collimator assembly 1332. Collimator prism assembly 1330, circulator component assembly 1333, and collimator assembly 1332 are positioned and affixed to housing 1350 to form component assembly 1331. Component assembly 1331 is sealed within outer housing 1334. Outer housing 1334 is sealed with retainer caps 1321 and 1322 as well as boots 1324 and 1323.

As an example, Figure 13 shows a component assembly 1331 having optical circulator components similar to optical circulator 400 shown in Figure 4A. Package 1300 can include a circulator having any number of ports, but a three-port circulator is shown in Figure 13. Package 1300 includes collimator 1301, alignment prism 1302, birefringent crystal 1303, non- reciprocal rotators 1304 and 1305, birefringent crystal 1306, and collimator 1308. If package 1300 included a four port circulator, then collimator 1308 accommodates two optical fibers and a second alignment prism be located between collimator 1308 and birefringent crystal 1306, i.e. a collimator prism assembly similar to that shown as collimator prism assembly 1330 is substituted for collimator assembly 1332 in Figure 13. Non-reciprocal rotator 1305 is, in the X-Z projection shown in Figure 13, hidden beneath non-reciprocal crystal 1304.

Other embodiments of optical circulator 1300 can include any of the embodiments of optical circulators that have already been discussed. Packaged optical circulator 1300 can be of any length and diameter. One embodiment of optical circulator 1300 has an overall length (excluding caps 1321 and 1322 and boots 1324 and 1323) of about 56 mm and an outside diameter of about 8.5 mm.

Production and cutting of birefringent crystals (such as birefringent crystals 1303 and 1306 or the birefringent crystals of non-reciprocal rotators 1304 and 1305) have been previously discussed. The optical surfaces (where light beams are incident) are polished to optically quality and coated with a anti-reflecting film. Crystals cut and polished to the specifications of this invention can be ordered from various crystal manufacturers. The crystals also are ground flat on one surface. In Figure 13, light propagates along the Z axis and at least the Y-Z plane of each crystal is ground flat by the manufacturer. In one embodiment, birefringent crystals 1306 and 1303 are produced from YV0₄ and are cut to a length of about 10 mm, a width of about 2.5 to 3.5 mm and a height of about 3 mm, with the length being the dimension along the Z axis, the width being along the X axis, and the height being along the Y axis. Non-reciprocal rotators 1304 and 1305 also include birefringent crystals, which are cut to about 7 mm in length, about 2.5 mm in width and about 1 to 1.5 mm in height. The surfaces oriented perpendicularly to the Z axis, i.e. the direction of light propagation, are polished.

Figure 14A shows a coated optical component 1400. The optical surfaces of each of the components of optical circulator 1300 are coated with anti-reflecting films in order to reduce reflections at the operating wavelengths. A coated optical surface is shown in Figure 14A. In one embodiment, the operating wavelength is 1550 nm + about 20 nm, although optical circulators may also operate in other wavelength ranges as well. Although any coating that minimizes the reflectance of light at those wavelengths may be used. It is desirable that the reflectance at the surface be reduced to less than 0.4% over the entire range of operating wavelengths. With proper coating controls, reflectance can be reduced to as low as 0.1 %, as is shown in Figure 14B. The quality of reflection coating can be monitored by measuring the reflectance of surfaces in a spectrophotometer having a reference arm, such as, for example, a Shimadzu UV-3100 recording spectrophotometer operating in reflectance mode, and utilizing a surface of known reflectance as a reference sample. Fused silica glass, for example, provides a reflective surface having a reflectance of about 3.3%.

Figure 14A shows an embodiment of the surface coating. Coating 1401 is deposited on component 1403 and coating 1402 is deposited on surface 1401. Other embodiments can include any number of coating layers. Additionally, any coating technology can be used, including electron beam evaporation, sputtering techniques such as ion beam sputtering, and ion beam assisted evaporation. In one embodiment, coating 1402 is Si0 deposited by electron beam deposition and coating 1401 is Zr0₂ deposited by electron beam deposition.

Figure 15A illustrates construction of non-reciprocal rotators such as non-reciprocal rotators 1305 and 1304 of Figure 13 or non-reciprocal rotators 700, 800 or 900 as shown in Figures 7A, 8 A, and 9 A, respectively. A non-reciprocal rotator jig includes optical flat 1501 and right-angle locators 1502 and 1503. In general, a non-reciprocal rotator includes a birefringent crystal, two Faraday rotators, and possibly other rotators as well. Non-reciprocal rotator 1510 includes Faraday rotators 1511 and 1514 and half- wave plate 1513.

Production of birefringent crystal 1512 has been discussed above. Faraday rotator material is purchased, for example, from Lucent Technologies and cut appropriately for use as Faraday rotators 1511 and 1514 in non-reciprocal rotator 1510. Figure 15B shows Faraday rotator material 1520 and the cut of Faraday rotator material 1520 into individual Faraday rotators 1521. Cutting the Faraday rotator material is accomplished using a precision diamond blade cutting machine. Faraday rotators 1511 and 1514 are large enough to cover at least the portions of birefringent crystal 1510 through which light beams will pass. In one embodiment, Faraday rotators have the dimension of about 1 mm by 2 mm. Faraday rotators 1511 and 1514 can be coated by the manufacturer to reduce reflections.

Wave plates can also be purchased from a number of suppliers, for example Melles Griot or Newport, and cut to a size that covers the portion of birefringent crystal 1510 through which light beams will pass to form rotator 1513. Figure 15C shows wave plate material 1530 being cut to individual rotators 1531. Wave plate material is also coated by the manufacturer in order to reduce reflections.

In Figure 15 A, each of the components of non-reciprocal rotator 1510 is arranged on optical flat 1501 and between right-angle locators 1502 and 1503. Right angle positioners 1502 and 1503, which are any devices with one surface that can be held at a right angle to optical flat 1501, are then pushed such that the components are close together in order to compact non- reciprocal rotator 1510. Actual separation between components of non-reciprocal rotator 1510 is not important to the operation of non-reciprocal rotator 1510. A slight air gap is left between individual components, the components are not pressed together. In Figure 15 A, air gaps 1516 and 1517 is left between birefringent crystal 1512 and rotator 1513 and rotator 1513 and Faraday rotator 1514, respectively. In most embodiments, air gaps 1516 and 1517 of less than about .05 mm is desirable, however air gaps of any size are allowable. If the components become optically connected, then the coatings on all components need to be adjusted because of the absence of index of refraction 1 material (i.e., air) adjacent the coating material. In some embodiments, components are positioned against a back-guide 1515, which assists in aligning components with appropriately cut surfaces by preventing rotations of one component relative to another. In some embodiments, a spacer can be inserted, for example, between Faraday rotator 1511 and birefringent crystal 1512 in order to maintain the air gap. To permanently affix the components together, a small bead of epoxy 1504 is applied to the exposed non-optical surfaces of non-reciprocal rotator 1510 where adjoining components of rotator 1510 meet. In one embodiment, a quick-fix UV cured epoxy, such as that sold by Epoxy Technology, Inc., part no. UV 4481, is applied in spots and exposed to UV to form UV epoxy tacks 1504, which hold the components in place. Any epoxy that can be quickly cured can be used to form UV epoxy tacks 1504. Figure 15D shows assembled non-reciprocal rotator

1510 with cured UV epoxy tacks 1504 holding the components in place.

The viscosity of the UV epoxy is low, however, and will flow onto the optical surfaces between components of rotator 1510. To prevent the epoxy from entering areas where light beams will pass, the viscosity of the epoxy should be increased. The viscosity can be increased in several ways, including exposing the epoxy to air in order to evaporate away the included solvent and mixing the epoxy with another material, for example silica powder. The viscosity is increased before being applied to rotator 1510 to form epoxy tacks 1504.

A separate epoxy, such as Epoxy Technology 353 ND, is applied to non-reciprocal rotator 1510 and cured to permanently form non-reciprocal rotator 1510. In most embodiments, a heat-cured epoxy is used. The heat-cured epoxy is applied to the external surfaces of non-reciprocal rotator 1510 in such a way that it does not interfere with the optical surfaces and so that, when cured, it will secure the components of non-reciprocal rotator 1510. Curing the epoxy often involves heating non-reciprocal rotator 1510 to a temperature of about 65 to 85°C for a period of about 1 hour. The temperature of non-reciprocal rotator 1510 is raised and lowered carefully through the curing process in order to prevent damage to the components. Whatever epoxy is used, care should be taken so that components of non- reciprocal rotator 1510 are not damaged in the curing process. Figure 15E shows non- reciprocal rotator 1510 after application and curing of the permanent epoxy. Epoxy is arranged to form epoxy portions 1505, which permanently hold non-reciprocal rotator 1510 together.

Figures 16A, 16B and 16C show one embodiment of circulator components assembly 1333. Figure 16A shows an X-Z plane projection of assembly 1333. Assembly 1333 includes birefringent crystals 1303 and 1306 along with non-reciprocal rotators 1304 and 1305 mounted within holder 1601 and covered with cover 1602. Non-reciprocal rotators 1304 and 1305 can be assembled according to the procedure outlined in Figures 15 A through 15E. Birefringent crystals 1306 and 1303 are oriented, cut and polished according to processes that have also previously been outlined.

Figure 16B shows an X-Y projection of assembly 1333. Holder 1601 can be of any material, such as stainless steel rod, with a groove 1603. In a stainless steel rod, groove 1603 can be machined along the long axis of the rod. In one embodiment, a stainless steel rod of diameter about 5.20 mm is utilized. Groove 1603 is about 3.6 mm wide (± about .1 mm) and about 2.3 mm deep. In one embodiment, holder 1601 is 29.0 mm long. Cover 1602 is a rectangular plate that sits within groove 1603. Alternatively, cover 1602 can have a curved surface such that, when assembled, the combination of cover 1602 and holder 1601 forms a cylinder.

In assembly, epoxy is applied to side walls 1604 and bottom 1605 of groove 1603 (Figure 16B). Birefringent crystals 1303 and 1306 along with non-reciprocal rotators 1305 and

1304 are positioned along one of side walls 1604 and on bottom 1605. Epoxy is not applied to areas where optical beams will travel. The epoxy may be a heat cured epoxy, a UV cured epoxy or a combination. Finally, epoxy is applied to one side of cover 1602 and the epoxied side is inserted into groove 1603 in order to fully enclose assembly 1331. Only the surfaces peφendicular to the Z axis are left exposed. Because of the cuts of birefringent crystals 1303 and 1306 and the construction of non-reciprocal rotators 1305 and 1304, as has been previously discussed, the optical components of circulator components assembly 1333 can be self-aligning

(i.e., birefringent crystals 1303 and 1306 will be optically aligned with non-reciprocal rotators

1305 and 1304). Positioning of components along the Z axis is not critical and rotations of components relative to one another is minimized by positioning the components flush with one of side walls 1604. Figure 16D illustrates an embodiment of circulator components assembly 1331 that allows adjustments in the relative positions and orientations of birefringent crystals 1303 and

1306 and non-reciprocal rotators 1304 and 1305 for embodiments where the optical components are not self-aligning (see, e.g., Figures 10A and 10F through 10H). As shown in Figure 16D, holder 1601 of Figures 16A through 16C is separated into three holders, holders 1601-1 through 1601-3. Birefringent crystal 1303 is positioned and epoxied into holder 1601-1 and cover 1602-1 is epoxied as is shown in Figure 16B. Non-reciprocal rotators 1304 and 1305 are positioned and epoxied into holder 1601-2 and covered with cover 1602-2. Additionally, birefringent crystal 1306 is positioned and epoxied into holder 1601-3 and covered with cover 1602-3. Holders 1601-1 through 1603 are then positioned into housing 1610. Holder 1601-2 is positioned and affixed within housing 1610 by either epoxy or low temperature solder. Holders 1601-1 and 1601-3 are then positioned on either side of holder 1601-2. At this point, birefringent crystals 1303 and 1306 can be aligned with non-reciprocal rotators 1304 and 1305 by rotating holders 1601-1 and 1601-3 within housing 1610. Alignment can be accomplished, for example, by optimizing the throughput of a laser beam from an external laser 1611 as detected by a detector 1612. When an optimum alignment is obtained, holders 1601-1 and 1601-3 can be affixed within housing 1610 by epoxy or low temperature solder in order to form assembly 1331. Figures 17A through 17D show the construction of another embodiment of optical components assembly 1331. Figure 17D shows this embodiment of completed assembly 1331. Figure 17A shows assembly 1700 which includes birefringent crystals 1306 and 1303 and non- reciprocal rotators 1304 and 1305. Non-reciprocal rotators 1304 and 1305 can be assembled according to the procedure outlined in Figures 15A through 15E. Birefringent crystals 1304 and 1303 are oriented, cut and polished according to processes that have also previously been outlined.

Birefringent crystals 1306 and 1303 along with non-reciprocal rotators 1304 and 1305 are appropriately positioned on an optical flat 1703 and sandwiched between plates 1701 and 1702, which are of any rigid material including glass or fused silica. Positioning of individual components along the Z axis (as shown by axis 1310) is not important, but care should be taken that individual components are not significantly rotated (i.e., enough to substantially affect operation of the resultant optical circulator) with respect to one another.

Plates 1701 and 1702 are prepared by cutting the rigid material to the proper dimensions and forming access holes by, for example, drilling holes in the plates. Plates 1701 and 1702 have accesses 1704 through 1706 and 1707 through 1709, respectively. Accesses

1704 through 1709 are holes, or series of holes, formed in the glass plates (i.e., by any method including drilling) at positions associated with each component. For example, access 1704 is positioned in plate 1701 at a position where birefringent crystal 1306 will be placed; access

1705 is positioned adjacent to the placement of non-reciprocal rotator 1304; access 1706 is located adjacent the placement of birefringent crystal 1303; access 1707 is positioned in plate 1702 adjacent to birefringent crystal 1306; access 1708 is positioned in plate 1702 adjacent to non-reciprocal rotator 1305; and access 1709 is positioned in plate 1702 adjacent to birefringent crystal 1303. When birefringent crystals 1306 and 1303, non-reciprocal rotators 1304 and 1305, and plates 1701 and 1702 are properly positioned, epoxy, for example a UV cured epoxy, is added through accesses 1704 through 1709. The UV epoxy is cured by irradiating the entire assembly with UV light. Permanent epoxy, i.e. thermally cured epoxy, can then be applied along the sides of assembly 1700. The thermal epoxy is then cured, for example by heating to a temperature of 65° to 85° for a period of about one hour, under similar conditions as described above. In some embodiments, curing the epoxy for forming non-reciprocal rotator 1510 and assembly 1703 is accomplished together in one heating of assembly 1700. The optical components of assembly 1700, birefringent crystals 1306 and 1303 and non-reciprocal rotators 1304 and 1305, are optically aligned by the cutting and placing of individual components. Embodiments of the invention may include other numbers of plates. For example, some embodiments may include plates on all sides of assembly 1700. Other embodiments may be arranged such that only one plate suffices, such as, for example, where non-reciprocal rotators 1304 and 1305 are included in a single component (see, e.g., non-reciprocal rotator 1006 of Figure 10G).

Figure 17B shows assembly 1700 of Figure 17A inserted into heat-shrinkable tubing 1710. Figure 17C shows an X-Y cross-section of assembly 1700 inserted into heat-shrinkable tubing 1710 after shrinking the tubing in order to seal assembly 1700 and protect the side- surfaces of birefringent crystals 1306 and 1303 and non-reciprocal rotators 1304 and 1305. The optical surfaces along the Z axis, the optical axis, remain uncovered.

Assembly 1700, covered and protected by shrunk heat-shrinkable tubing 1710, is inserted into tubing 1711, as shown in Figure 17D. Tubing 1711 is any tubing, including stainless steel or glass tubing. In one embodiment, tubing 1711 has an inner diameter of about 3.4 mm and an outer diameter of about 3.8 mm. The surfaces of heat-shrinkable tubing 1710 are coated with epoxy before assembly 1700 is inserted into tubing 1711 in order that assembly 1700 is rigidly held inside tubing 1711. Tubing 1711 may be a standard sized stainless steel tubing that has been bored to the desired inner-diameter.

Tubing 1711, now enclosing assembly 1700, is inserted into tubing 1712. Tubing 1712 is also bored to a diameter that accommodates tubing 1711. Tubing 1712 has a wall thickness greater than that of tubing 1711. In one embodiment, tubing 1711 has a wall thickness of about

.2 mm and tubing 1712 has a wall thickness of about .5 mm. Tubing 1712 is longer than tubing 1711 so that collimators 1301 and 1308 (Figure 13) can eventually be mounted at the openings of tubing 1712.

Tubing 1711 is positioned substantially in the center of tubing 1712. Tubing 1712 includes access holes 1714 and 1713 around the center diameter so that, when tubing 1711 is positioned, there is access to tubing 1711 through tubing 1712. In some embodiments, these access holes are about 3 mm in diameter. Also, in some embodiments the external surface of tubing 1711 and the internal surface of tubing 1712 are plated with a highly conducting material, such as for example gold. Tubing 1711 can then be soldered with a low temperature solder through access holes 1713 and 1714 to tubing 1712. The gold plating and low temperature solder assists in attaining a good solder joint between tubing 1711 and 1712. The resultant optical assembly 1720, then, includes assembly 1700 mounted in tubing 1712 by solder beads 1715 and 1716.

Figures 18A through 18D shows embodiments of collimator assembly 1332 and collimator prism assembly 1330 and their assembly. Figure 1810 shows an embodiment of collimator 1802. Collimator 1810 is formed by positioning and affixing fiber receiving portion 511 within collimator housing 1801, positioning transmission portion 512 relative to fiber receiving portion 511, and preparing optical fibers 503 and 504 and inserting and affixing optical fibers 503 and 504 within fiber receiving portion 511. Preparing optical fibers 503 and 504 can include construction of a microlens at the terminal end of optical fibers 503 and 504 (See microlenses 540 shown in Figures 5E through 5F).

In one embodiment, collimator housing 1801 is of fused silica glass tubing having an inner diameter of about 1.80 mm, an outer diameter of about 2.80 mm, and a length of about 10.0 mm. Fiber receiving portion 511 and transmission portion 512 are affixed within housing 1801 by coating them with epoxy and inserting them within housing 1801. In most embodiments, transmission portion 512 is allowed to extend from housing 1801 by about 1.0 mm. Additionally, fiber receiving portion 511 is inset within housing 1801. Optical fibers 503 and 504 are coated with epoxy and positioned within fiber receiving portion 511.

Figure 18B shows an embodiment of a prism assembly 1811. Prism assembly 1811 includes prism 502 affixed to holder 1802. Holder 1802 can be constructed from any material and has a prism portion 1803 and a insert portion 1804. Although holder 1820 can be of any size and shape, in one embodiment, holder 1820 is constructed from stainless steel tubing having an inner diameter of about 1.80 mm and an outer diameter of about 3.60 mm. The external wall of insert portion 1804 is machined such that its outer diameter is about 3.20 mm. The length of insert portion 1804 is about 3.00 mm. The inside diameter of prism portion 1803 is machined to an inside diameter of about 3.60 mm for a depth of about 0.7 mm. Holder 1802 can then be gold plated to facilitate its mounting. Prism 502 is then inserted into prism portion 1803 and affixed in place by, for example, epoxy.

Figure 18C shows construction of a collimator assembly 1332. Collimator assembly 1332 is formed by affixing, for example by epoxying, collimator 1810 into housing 1820. In one embodiment, housing 1820 is a stainless steel tubing having inner diameter of about 3.20 mm. Housing 1820 can be gold plated in order to facilitate further mounting. In one embodiment, housing 1820 is 10 mm in length and acts as a sleeve for collimator 1810 In general, however, housing 1820 can be of any length.

Figure 18D shows construction of a collimator prism assembly 1330. Assembly 1330 includes a housing 1821. Housing 1821 is a tubing with inner diameter arranged to receive collimator 1810 and insert portion 1804 of prism assembly 1811. In one embodiment, housing 1821 is a gold plated stainless steel having inner diameter of 3.20 mm, outer diameter of 3.60 mm and length 3.60 mm. Collimator 1810 is inserted into one side of housing 1821 and affixed in place by, for example, epoxy. Prism assembly is inserted into the opposite side of housing 1821. Prism 502 can be aligned with respect to collimator 1810 by directing light into optical fibers 503 and 504, monitoring the separation and intensity of the corresponding light beams emerging from prism 502, and adjusting the separation and rotation of prism assembly 1811 with respect to housing 1821. Once the alignment is accomplished, prism assembly 1811 can be affixed by, for example, low temperature solder between insert portion 1804 and housing 1821. Gold plating on the surfaces between insert portion 1804 and housing 1821 assists in soldering them together. In one embodiment, insert portion 1804 is arranged so that when prism assembly 1811 is fully inserted into housing 1821, i.e. by abutting against either housing 1801 or housing 1821, the distance between collimator 1810 and prism 502 is at a preset value. Figures 19A illustrates an embodiment of the final construction and alignment of component assembly 1331. In Figure 19A, optical component assembly 1333 is affixed within housing 1350. In one embodiment, housing 1350 is gold plated stainless steel tubing with inner diameter of about 5 mm and outer diameter of about 6 mm. Assembly 1333 is positioned approximately at the center of housing 1350. Solder access holes 1901 and 1902 are positioned such that assembly 1333 can be soldered into place within housing 1350. Solder access holes 1903, 1904, 1905 and 1906 are positioned for latter attachment of collimator assemblies or collimator prism assemblies.

Figure 19B illustrates alignment of the optical circulator and final attachment of collimator assemblies and collimator prism assemblies to housing 1350. Housing 1350, with circulator component assembly 1333, is inserted into positioner 1912. Positioner 1912 is attached to table 1910 and has positioning controls 1915 (shown as controls 1915a and 1915b). In one embodiment, positioner 1912 is cable of rotating housing 1350 around an X axis and around a Y axis (see axis 1310). Collimator prism assembly 1330 is inserted into positioner 1911. Positioner 1911 has controls 1914 (shown as controls 1914a through 1914b) and is capable of translation along the X, Y and Z axis as well as rotations around the X axis and the

Z axis. An optical source 1917, such as a laser, can be coupled to collimator prism assembly 1330 through an optical fiber. Collimator prism assembly 1330 can then be aligned with circulator component assembly 1333 by adjusting positioners 1912 and 1911 to optimize light throughput through circulator component assembly 1333. Collimator prism assembly 1330 can then be affixed to housing 1350 by soldering it in place through solder accesses 1903 and 1904. Positioner 1911 can then be detached and removed.

Collimator assembly 1332 can then be inserted into positioner 1913. Alternatively, a second collimator prism assembly can be installed. Positioner 1913 has controls 1916 (shown as controls 1916a through 1916e) that allow translations along the X, Y, and Z axis and rotations around the Z axis and the X axis. Again, collimator assembly 1332 can be aligned with circulator component assembly 1333 and collimator prism assembly 1330 by attaching a light source 1917 to optical fibers and optimizing the light throughput through all ports of the circulator. This procedure may require several iterations to arrive at an optimal performance for the circulator. Once the alignment is accomplished, collimator assembly 1332 is soldered in place through solder accesses 1905 and 1906.

Alternatively, collimator prism assembly 1330, circulator component assembly 1333 and collimator assembly 1332, can be optically aligned at once. Light source 1917 is coupled to one of ports A, B or C and detector 1918 is coupled to the next port so that, in a properly aligned circulator, a light beam entering for light source 1917 exits at detector 1918. A rough alignment can be accomplished with a He-Ne laser, for example. More exacting alignments are accomplished with light source 1917 operating at the operating frequency of the circulator, for example about 1 50 nm. Detector 1918 can be one of any type of detector, including photodetectors, IR cameras, or simple screens for visible optical observations of light from source 1917.

Once the optical system is aligned between two adjacent ports, e.g., by adjusting the positions and orientations of collimator assembly 1330, collimator assembly 1332 and circulator component assembly 1333 with positioners 1911, 1913 and 1912, respectively, the system is then aligned between each of the other pairs of ports. For example, once the system is optically aligned between ports A and B, then the system is aligned between ports B and C. The alignment between ports A and B will then need to be readjusted.

Once the optical alignment is optimized between each of the pairs of ports, collimator prism assembly 1330 can be soldered to housing 1350 through solder accesses 1903 and 1904. Again, the external surface of collimator prism assembly 1330 and collimator assembly 1332 may have been gold plated in order to facilitate the welding process. Additionally, welding may be accomplished with conventional electric irons, by laser welding or in any other available fashion.

After collimator prism assembly 1330 is soldered to housing 1350, collimator assembly 1332 may require re-adjustment in order to re-optimize the optical alignment. During this process, collimator prism assembly 1330 should be dismounted from positioner 1911. Once the system is re-aligned, collimator assembly 1332 is soldered to housing 1350 through solder accesses 1905 and 1906. Seals 1920 and 1921 can then formed, for example by soldering, housing 1350 and collimator prism assembly 1330 and collimator assembly 1332 in order to better hold collimator prism assembly 1330 and collimator assembly 1332 in place and to seal the optical components within component assembly 1331. Only optical fibers associated with individual ports, then, are exposed outside of component assembly 1331. Once assembled, component assembly 1331 is removed from positioners 1911, 1912 and 1913 and inserted into outer housing 1334. Outer housing 1334 is tubing or other encasement convenient for holding component assembly 1331. In one embodiment, outer housing 1334 is stainless-steel tubing, outer housing 1334, with component assembly 1331 inserted, is then filled with packing material 1325, such as silicon caulking compound or silicon grease, for example, in order to rigidly hold component assembly 1331 in place within outer housing 1334. The exterior tubing walls at the ends of outer housing 1334 may be reduced in order that cap 1322 and boot 1323 can be inserted over case outer housing 1334 and soldered, screwed, or otherwise affixed in place. Similarly, cap 1321 and boot 1324 are positioned over the opposite end of outer housing 1334 and affixed place. Optical fibers corresponding to the ports of the circulator extend through boots 1323 and 1324. Boots 1323 and 1324 may be rubber material with sufficiently small openings to substantially seal against the optical fibers and prevent atmospheric contamination of optical circulator package 1300. Figure 20C shows an embodiment of a four-port coplanar circulator package 2000. Figure 20A shows four-port circulator 2009. Collimators 2001 and 2007, alignment prisms

2002 and 2008, birefringent crystals 2003 and 2006 and non-reciprocal rotators 2004 and 2005 are positioned, aligned and attached on substrate 2010. In Figure 20B, four-port circulator 2009 is covered by another substrate 2211. Figure 20C shows circulator package 2000 that fully encloses circulator 2009. Other embodiments of coplanar circulator package 2000 can include any number of ports. One skilled in the art will recognize several variations, including a three port circulator having no alignment prism 2008 and where collimator 2007 is coupled to only one optical fiber.

Figures 21 A through 21F illustrates construction of a coplanar circulator such as circulator 2000 (Figure 20 A) or circulator 1100 (Figure 11 A). Figure 21 A shows collimators 2101 and 2107, birefringent crystals 2120 and 2122, and non-reciprocal rotator 2121 mounted on substrate 2010. Collimators 2101 and 2107 are mounted on V-grooves 2011 and 2111, respectively. Birefringent crystals 2120 and 2122 and non-reciprocal rotator 2121 are mounted on flat region 2100 of substrate 2010.

Figure 2 IB shows a silicon substrate 2010 having grooves 2011 and 2111 etched into substrate 2010. Grooves can be formed with well known techniques such as photolithography followed by chemical etching. Grooves 2011 and 2111 are separated by a flat region 2100. Additionally, grooves 2011 and 2111 are offset from one another by a distance corresponding to the separation between collimators that is required by the circulator.

As shown in Figure 21C, a metal layer such as 2 or 3 μm of gold is deposited on substrate 2010 and the substrate is annealed. The metal layer can be deposited by any technique, such as electroplating or evaporation. Figure 21D shows optical component 2013 which has been electroplated with a metal such as gold on surface 2014 which will be in contact with metalized substrate 2010. Figure 2 IE illustrates placement of optical components, such as optical component 2013, onto substrate 2010. Collimators 2101 and 2107 are placed within grooves 2011 and 2111, respectively, while birefringent crystals 2120 and 2122 and non-reciprocal rotator 2121 are placed on flat portion 2100. The components are then aligned. After aligning these components, assembly 2130 is annealed so that the metal layers on the optical components solders to the metallization layer 2012 on substrate 2010.

One method of aligning components includes reflecting a laser beam from the surface of each component and adjusting the position and orientation of that component accordingly. In this fashion, components can be placed onto substrate 2010 one at a time and individually positioned. Additionally, collimators 2101 and 2107, birefringent crystals 2120 and 2122 and non-reciprocal rotator 2121 can be optically aligned similarly to that described with Figure 19B.

Finally, Figure 21 F illustrates final construction of circulator 2000. A second cover substrate 2211, which is prepared in the same fashion as substrate 2010 but without V-grooves 2011 and 2111, is placed over assembly 2130 and in place. Additionally, sidewalls 2212 and 2213 are positioned ane epoxied in place to form package 2000..

The above examples are demonstrative only and should not be inteφreted as limiting. One of ordinary skill in the art will recognize variations that are considered to be included. For example, one embodiment of optical circulator similar to optical circulator 400 of Figure 4A may orient birefringent crystals 403 and 406 such that walk-off directions are in the X-Z plane while non-reciprocal rotators 404 and 405 are oriented to deflect beams out of the X-Z plane (where Y is aligned along the apex of alignment prisms 402 and 407. As such, the invention is limited only by the following claims.

Claims

CLAIMSI claim:

1. An optical circulator having a sequence of at least two ports, comprising: a first alignment prism optically coupled to a first set of ports, the first set of ports being of the at least two ports; a first birefringent crystal optically coupled to the first alignment prism; a non-reciprocal rotator section optically coupled to the first birefringent crystal; and a second birefringent crystal optically coupled between the non^reciprocal rotator section and a second set of ports, the second set of ports being of the at least two ports.

2. The circulator of Claim 1, wherein an optical path difference between a first light beam having a first polarization and a second light beam having a second polarization exiting the circulator at one of the at least two ports, the first light beam and the second light beam being separated from a light beam entering the circulator at the corresponding one of the at least two ports in the sequence, is substantially zero.

3. The circulator of Claim 2, wherein a geometric path difference between the first light beam and the second light beam is substantially zero.

4. The circulator of Claim 1, further comprising: a first collimator coupled between the first alignment prism and the first set of ports; and a second collimator coupled between the second birefringent crystal and the second set of ports.

5. The circulator of Claim 4, wherein each port of the first set of ports and each port of the second set of ports corresponds to an optical fiber; and wherein the first collimator and the second collimator each comprises: a fiber receiving portion for receiving at least one of the optical fibers; and a transmission portion optically coupled to the optical fibers.

6. The circulator of Claim 5, wherein the fiber receiving portion of the first collimator receives at least two optical fibers and the transmission portion of the first collimator optically couples the at least two optical fibers into the first alignment prism; and wherein the fiber receiving portion of the second collimator receives at least one optical fiber and the transmission portion of the second collimator optically couples the at least two optical fibers into the second birefringent crystal.

7. The circulator of Claim 6, wherein each of the at least one optical fibers includes a termination portion at the end of the optical fiber that is inserted into the corresponding one of the first collimator or the second collimator and the termination portion of at least one of the at least one optical fibers includes a flat termination.

8. The circulator of Claim 6, wherein each of the at least one optical fibers includes a termination portion at the end of the optical fiber that is inserted into the corresponding one of the first collimator or the second collimator and the termination portion of at least one of the at least one optical fibers includes a hemispherical microlens.

9. The circulator of Claim 6, wherein each of the at least one optical fibers includes a termination portion at the end of the optical fiber that is inserted into the corresponding one of the first collimator or the second collimator and the termination portion of at least one of the at least one optical fibers includes a tapered microlens.

10. The circulator of Claim 4, wherein the first set of ports comprises at least one optical fiber that include a termination portion that includes a microlens collimator, the first collimator being formed by the first set of ports; and wherein the second set of ports comprises at least one optical fiber that include a termination portion having a microlens collimator, the second collimator being formed by the second set of ports.

11. The circulator of Claim 6, wherein each of the at least one optical fibers includes a termination portion at the end of the optical fiber that is inserted into the corresponding one of the first collimator or the second collimator and the termination portion of at least one of the at least one optical fibers includes a combination of a spherical microlens and a tapered microlens.

12. The circulator of Claim 6 having three ports wherein the fiber receiving portion of the first collimator receives two optical fibers and the fiber receiving portion of the second collimator receives one optical fiber.

13. The circulator of Claim 6, further comprising a second alignment prism coupled between the second birefringent crystal and the second collimator.

14. The circulator of Claim 13 having four ports wherein the fiber receiving portion of the first collimator receives two optical fibers and the fiber receiving portion of the second collimator receives two optical fibers.

15. The circulator of Claim 6, wherein the fiber receiving portion of the first collimator receives at least one spacer fiber.

16. The circulator of Claim 15, wherein at least one of the at least one spacer fiber is positioned to separate the at least two optical fibers.

17. The circulator of Claim 6, wherein the fiber receiving portion of the second collimator receives at least one spacer fiber.

18. The circulator of Claim 17, where at least one of the at least one spacer fiber positioned to separate the at least one optical fibers.

19. The circulator of Claim 6 wherein the first alignment prism includes a first section and a second section separated by a flat section.

20. The circulator of claim 19 having five ports wherein the fiber receiving portion of the first collimator receives three fibers, each of the fibers being optically coupled into a corresponding one of the first section, the flat section or the second section of the first alignment prism, and the fiber receiving portion of the second collimator receives three fibers.

21. The circulator of Claim 1 having more than five ports.

22. The circulator of Claim 19 wherein the second alignment prism includes a first section and a second section separated by a flat section.

23. The circulator of Claim 13 wherein the first alignment prism and the second alignment prism each include at least two half-prism sections.

24. The circulator of Claim 1, wherein the first birefringent crystal and the second birefringent crystal are each of a birefringent material chosen from the group of materials consisting of

YV0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile.

25. The circulator of Claim 24, wherein the birefringent material of the first birefringent crystal is the same as the birefringent material of the second birefringent material.

26. The circulator of Claim 1, wherein the circulator has an optical axis and the first birefringent crystal and the second birefringent crystals are each cut having opposing surface planes peφendicular to the optical axis.

27. The circulator of Claim 1 , wherein the first birefringent crystal and the second birefringent crystal are arranged so as to substantially maximize a walk-off distance between an ordinary and an extraordinary polarization of a randomly polarized light beam propagating along the optical axis in each of the first birefringent crystal and the second birefringent crystal.

28. The circulator of Claim 27, wherein the birefringent material of the first birefringent crystal is of YV0₄ and the optical axis of the circulator is about 48° from a c-crystallographic direction in an a-c crystallographic plane.

29. The circulator of Claim 28, wherein the optical axis of the circulator is about a [101] crystallographic direction.

30. The circulator of Claim 27, wherein the birefringent material of the second birefringent crystal is of YV0₄ and the optical axis of the circulator is about 48° from a c-crystallographic direction in an a-c crystallographic plane.

31. The circulator of Claim 30, wherein optical axis of the circulator is about a [101] crystallographic direction.

32. The circulator of Claim 27, wherein the birefringent material of the first birefringent crystal is of LiNb0₃, and the optical axis of the circulator is about 44° from a c-crystallographic direction in an a-c crystallographic plane.

33. The circulator of Claim 27, wherein the birefringent material of the second birefringent crystal is of LiNb0₃, and the optical axis of the circulator is about 44° from a c-crystallographic direction in an a-c crystallographic plane.

34. The circulator of Claim 26, wherein the non-reciprocal rotator section comprises: a top non-reciprocal rotator coupled between a top portion of the first birefringent crystal and a top portion of the second birefringent crystal; and a bottom non-reciprocal rotator coupled between a bottom portion of the first birefringent crystal and a bottom portion of the second birefringent crystal.

35. The circulator of Claim 34, wherein the top non-reciprocal rotator and the bottom non- reciprocal rotator each comprise a rotator structure having a directional axis such that the polarization of a first polarized light beam propagating in a positive direction along the directional axis is rotated while the polarization of a second polarized light beam propagating in a negative direction along the directional axis opposite the positive direction is rotated and the second polarized light beam is deflected; and wherein the rotator structure of the top non-reciprocal rotator is oriented such that the positive direction of the directional axis is aligned parallel with the optical axis of the circulator and directed from the first birefringent crystal towards the second birefringent crystal; and the rotator structure of the bottom non-reciprocal rotator is oriented such that the negative direction of the directional axis is aligned parallel with the optical axis of the circulator and directed from the first birefringent crystal towards the second birefringent crystal.

36. The circulator of Claim 35, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the negative direction of the directional axis; a second Faraday rotator having a magnetization directed along the positive direction of the directional axis; a birefringent crystal positioned between the first Faraday rotator and the second Faraday rotator such that the opposing polished faces are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk- off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed away from the birefringent crystal; and a half- wave plate having an axis directed along the positive direction of the directional axis, the half-wave plate being optically coupled to the second Faraday rotator opposite the birefringent crystal.

37. The circulator of Claim 35, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the positive direction of the directional axis; a second Faraday rotator having a magnetization directed along the negative direction of the directional axis; a birefringent crystal positioned between the first Faraday rotator and the second

Faraday rotator such that the opposing polished faces are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk- off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed towards the birefringent crystal; and a half- wave plate having an axis directed along the positive direction of the directional axis, the half-wave plate being optically coupled to the second Faraday rotator opposite the birefringent crystal.

38. The circulator of Claim 35, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the positive direction of the directional axis; a second Faraday rotator having a magnetization directed along the positive direction of the directional axis; and a birefringent crystal positioned between the first Faraday rotator and the second Faraday rotator such that the opposing polished faces are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk- off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed towards the birefringent crystal.

39. The circulator of Claim 35, wherein the rotator structure comprises a birefringent crystal having a birefringent material chosen from the group consisting of YV0₄, LiNb0₃, LiTa0 , MgF , β-BaB0₄, calcite, and rutile.

40. The circulator of Claim 39, wherein the opposing faces of the first birefringent crystal, the second birefringent crystal and the birefringent crystal of the non-reciprocal rotator are cut, polished and coated with an anti-reflecting film for the operating wavelength of the circulator.

41. The circulator of Claim 40 wherein the operating wavelength of the circulator is about 1550 nm.

42. The circulator of Claim 40 wherein the operating wavelength of the circulator is about 1310 nm.

43. The circulator of Claim 40, wherein the operating wavelength of the circulator is in the range from the far infrared into the deep UV.

44. An optical circulator, comprising: a first collimator coupled to a first set of optical fibers, the first set of optical fibers including at least one optical fiber, the first collimator receiving light from and directing light into the at least one optical fiber of the first set of optical fibers; a first birefringent crystal optically coupled to the first collimator; a non-reciprocal rotator section optically coupled to the first birefringent crystal; a second birefringent crystal optically coupled to the first birefringent crystal; and a second collimator coupled to a second set of optical fibers, the second set of optical fibers including at least one optical fiber, the second collimator receiving light from and directing light into the at least one optical fiber of the second set of optical fibers.

45. The circulator of Claim 44, wherein the at least one optical fiber of the first set of optical fibers is terminated with a microlens collimator and the first collimator is formed from the at least one optical fiber of the first set of optical fibers.

46. The circulator of Claim 44, wherein the at least one optical fiber of the second set of optical fibers is terminated with a microlens collimator and the second collimator is formed from the at least one optical fiber of the second set of optical fibers.

47. The circulator of Claim 44, further comprising a first alignment prism coupled to the first collimator, the first alignment prism receiving light from the first collimator and outputting substantially parallel input light beams, the first alignment prism receiving substantially parallel output light beams and coupling them into the first collimator;

48. The circulator of Claim 47, further comprising a second alignment prism coupled between the second collimator and the second birefringent crystal.

49. The circulator of Claim 44, wherein the first birefringent crystal and the second birefringent crystals are each of a birefringent material cut with opposing faces peφendicular to an optical axis of the circulator and wherein the optical axis corresponds substantially with a direction of maximum walk-off in the birefringent material.

50. The circulator of Claim 49, wherein the birefringent material of the first birefringent crystal is chosen from the group consisting of YV0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile.

51. The circulator of Claim 49, wherein the birefringent material of the second birefringent crystal is chosen from the group consisting of YY0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile.

52. The circulator of Claim 49 operating, wherein the birefringent material of the first birefringent crystal is YY0₄ and the optical axis of the circulator is directed along a direction that is about 48° from a c crystallographic axis of the birefringent material in an a-c crystallographic plane of the birefringent material.

53. The circulator of Claim 52, wherein the direction is the [101] crystallographic direction.

54. The circulator of Claim 49, wherein the birefringent material of the first birefringent crystal is LiNb0₃ and the optical axis of the circulator is directed along a direction that is about 44° from a c crystallographic axis of the birefringent material in an a-c crystallographic plane of the birefringent material .

55. The circulator of Claim 49, wherein the first birefringent crystal and the second birefringent crystal are identically oriented such that a walk-off direction in the first birefringent crystal is parallel with a walk-off direction in the second birefringent crystal.

56. The circulator of Claim 55, wherein the non-reciprocal rotator section comprises: a top non-reciprocal rotator coupled between a top portion of the first birefringent crystal and a top portion of the second birefringent crystal; and a bottom non-reciprocal rotator coupled between a bottom portion of the first birefringent crystal and a bottom portion of the second birefringent crystal.

57. The circulator of Claim 56, wherein the top non-reciprocal rotator and the bottom non- reciprocal rotator each comprise a rotator structure having a directional axis such that the polarization of a first polarized light beam propagating in a positive direction along the directional axis is rotated while the polarization of a second polarized light beam propagating in a negative direction along the directional axis opposite the positive direction is rotated 5 and the second polarized light beam is deflected; and wherein the rotator structure of the top non-reciprocal rotator is oriented such that the positive direction of the directional axis is aligned parallel with the optical axis of the circulator and directed from the first birefringent crystal towards the o second birefringent crystal; and the rotator structure of the bottom non-reciprocal rotator is oriented such that the negative direction of the directional axis is aligned parallel with the optical axis of the circulator and directed from the first birefringent crystal towards the second birefringent crystal. 5

58. The circulator of Claim 57, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the negative direction of the directional axis; a second Faraday rotator having a magnetization directed along the positive 0 direction of the directional axis; a birefringent crystal positioned between the first Faraday rotator and the second Faraday rotator such that opposing polished faces of the birefringent crystal are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk-off of the birefringent crystal and such that the magnetization of the 5 first Faraday rotator is directed away from the birefringent crystal; and a half- wave plate having an axis directed along the positive direction of the directional axis, the half-wave plate being optically coupled to the second Faraday rotator opposite the birefringent crystal.

0 59. The circulator of Claim 57, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the positive direction of the directional axis; a second Faraday rotator having a magnetization directed along the negative direction of the directional axis; a birefringent crystal positioned between the first Faraday rotator and the second

Faraday rotator such that opposing polished faces are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk-off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed towards the birefringent crystal; and a half- wave plate having an axis directed along the positive direction of the directional axis, the half-wave plate being optically coupled to the second Faraday rotator opposite the birefringent crystal.

60. The circulator of Claim 57, wherein the rotator structure comprises: a first Faraday rotator having a magnetization directed along the positive direction of the directional axis; a second Faraday rotator having a magnetization directed along the positive direction of the directional axis; and a birefringent crystal positioned between the first Faraday rotator and the second Faraday rotator such that the opposing polished faces are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk- off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed towards the birefringent crystal.

61. The circulator of Claim 57, wherein the rotator structure comprises a birefringent crystal having a birefringent material chosen from the group consisting of YV0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0 , calcite, and rutile.

62. The circulator of Claim 61, wherein the opposing faces of the first birefringent crystal, the second birefringent crystal and the birefringent crystal of the non-reciprocal rotator are cut, polished and coated with an anti-reflecting film for an operating wavelength of the circulator.

63. The circulator of Claim 62, wherein the operating wavelength of the circulator is about 1550 nm.

64. The circulator of Claim 62, wherein the operating wavelength of the circulator is about 1310 nm.

65. The circulator of Claim 62, wherein the operating wavelength of the circulator is in the range from the far infrared into the deep UV.

66. The circulator of Claim 55, wherein the non-reciprocal rotator section comprises: a birefringent crystal having opposing faces oriented peφendicular to the optical axis of the circulator such that the optical axis is substantially along a direction of maximum walk-off of the birefringent crystal and oriented such that a walk-off direction in the birefringent crystal of the non-reciprocal rotator section is at an angle with respect to the walk-off direction of the first birefringent crystal.

67. The circulator of Claim 66, wherein the birefringent crystal of the non-reciprocal rotator section, the first birefringent crystal, and the second birefringent crystal are substantially identical crystals and are fully interchangeable by adjusting their relative orientation.

68. The circulator of Claim 66, wherein the angle between the walk-off direction in the birefringent crystal of the non-reciprocal rotator section and the walk-off direction of the first birefringent crystal is about 45°.

69. The circulator of Claim 66, wherein the non-reciprocal rotator section further includes:

a first Faraday rotator having a magnetization directed along a negative direction, the negative direction being parallel to the optical axis of the circulator and directed from the second birefringent crystal towards the first birefringent crystal; a second Faraday rotator having a magnetization directed along the positive direction, the positive direction being parallel to the optical axis and directed opposite the negative direction; a birefringent crystal positioned between the first Faraday rotator and the second

Faraday rotator such that opposing polished faces of the birefringent crystal are peφendicular to the directional axis and the directional axis is substantially a direction of maximum walk-off of the birefringent crystal and such that the magnetization of the first Faraday rotator is directed away from the birefringent crystal; and a half- wave plate having an axis directed along the positive direction of the directional axis, the half-wave plate being optically coupled to the second Faraday rotator opposite the birefringent crystal.

70. The circulator of Claim 66, wherein the non-reciprocal rotator section further includes:

a first Faraday rotator having an upper half, a lower half, and a magnetization directed along a negative direction, the negative direction being parallel to the optical axis of the circulator and directed from the second birefringent crystal towards the first birefringent crystal; a second Faraday rotator having an upper half, a lower half, and a magnetization directed along a positive direction, the positive direction being parallel to the optical axis and directed opposite the negative direction, the first Faraday rotator and the second Faraday rotator being separated by the birefringent crystal of the non-reciprocal rotator such that the magnetization of the first Faraday rotator is directed towards the birefringent crystal and such that the upper half of the first Faraday rotator is opposite the upper half of the second Faraday rotator; a lower half-wave plate having a polarization rotation axis directed along the optical axis, the lower half- wave plate being optically coupled to the lower half of the first Faraday rotator opposite the birefringent crystal; and an upper half- ave plate having a polarization rotation axis directed along the optical axis, the upper half- wave plate being optically coupled to the upper half of the second Faraday rotator opposite the birefringent crystal.

71. The circulator of Claim 66, wherein the non-reciprocal rotator section further includes:

a first Faraday rotator having an upper half, a lower half, and a magnetization directed along a positive direction, the positive direction being parallel to the optical axis of the circulator and directed from the first birefringent crystal towards the second birefringent crystal; a second Faraday rotator having an upper half, a lower half, and a magnetization directed along a negative direction, the negative direction being parallel to the optical axis and directed opposite the positive direction, the first Faraday rotator and the second

Faraday rotator being separated by the birefringent crystal of the non-reciprocal rotator such that the magnetization of the first Faraday rotator is directed away from the birefringent crystal and such that the upper half of the first Faraday rotator is opposite the upper half of the second Faraday rotator; a lower half- wave plate having a polarization rotation axis directed along the optical axis, the lower half- wave plate being optically coupled to the lower half of the first Faraday rotator opposite the birefringent crystal; and an upper half-wave plate having a polarization rotation axis directed along the optical axis, the upper half- wave plate being optically coupled to the upper half of the second Faraday rotator opposite the birefringent crystal.

72. The circulator of Claim 66, wherein the birefringent crystal of the non-reciprocal rotator section includes a top portion and a bottom portion and the non-reciprocal rotator section further includes: a top first Faraday rotator having a magnetization directed along a positive direction, the positive direction being parallel to the optical axis of the circulator and directed from the first birefringent crystal towards the second birefringent crystal; a top second Faraday rotator having a magnetization directed along a positive direction, the top first Faraday rotator and the top second Faraday rotator being separated by the top portion of the birefringent crystal of the non-reciprocal rotator such that the magnetization of the top first Faraday rotator is directed towards the birefringent crystal; a bottom first Faraday rotator having a magnetization directed along a negative direction, the negative direction being parallel to the optical axis and directed opposite the positive direction,; a bottom second Faraday rotator having a magnetization directed along the negative direction, the bottom first Faraday rotator and the bottom second Faraday rotator being separated by the bottom portion of the birefringent crystal of the non- reciprocal rotator such that the magnetization of the bottom first Faraday rotator is directed away from the birefringent crystal.

73. The circulator of Claim 44, wherein the first collimator, the first alignment prism, the first birefringent crystal, the non-reciprocal rotator section, the second birefringent crystal and the second collimator are mounted on a planar substrate such that the first collimator and the second collimator are mounted in a plane parallel with the planar substrate.

74. A coplanar optical circulator, comprising: a first birefringent crystal coupled to a first set of optical fibers; a non-reciprocal rotator coupled to the first birefringent crystal; a second birefringent crystal coupled to the non-reciprocal rotator and to a second set of optical fibers; and wherein the first birefringent crystal, the non-reciprocal rotator, and the second birefringent crystal are mounted on a planar substrate such that the first set of optical fibers and the second set of optical fibers are coplanar with respect to the planar substrate.

75. The circulator of Claim 74 further cormpising: a first collimator coupled between the first set of optical fibers and the first birefringent crystal; and a second collimator coupled between the second set of optical fibers and the second birefringent crystal; and wherein the first colimator and the second collimator are coplanar with respect to the planar substrate.

76. The circulator of Claim 75, wherein each optical fiber of the first set of optical fibers is terminated with a microlens collimator, the first set of optical fibers forming the first collimator, and wherein each optical fiber of the second set of optical fibers is terminated with a microlens collimator, the second set of optical fibers forming the second collimator.

77. The circulator of Claim 75, further including a first alignment prism coupled between the first collimator and the first birefringent crystal.

78. The circulator of Claim 77, further including a second alignment prism coupled between the second birefringent crystal and the second collimator.

79. The circulator of Claim 74, wherein the first birefringent crystal and the second birefringent crystal are each of a birefringent material chosen from the group consisting of YV0 , LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile and each of the first birefringent crystal and the second birefringent crystal having polished opposing faces peφendicular to an optical axis of the circulator.

80. The circulator of Claim 79, wherein the optical axis substantially corresponds with a direction of maximum walk-off in the birefringent material of the first birefringent crystal and the birefringent material of the second birefringent material.

81. The circulator of Claim 80, wherein the birefringent material of the first birefringent crystal and the birefringent material of the second birefringent material is YV0₄ and the optical axis corresponds with a direction of about 48° from an a crystallographic direction in an a-c crystallographic plane of YV0 .

82. The circulator of Claim 80, wherein the birefringent material of the first birefringent crystal and the birefringent material of the second birefringent material is YV0₄ and the optical axis substantially corresponds with a [101] crystallographic direction of YV0₄.

83. The circulator of Claim 80, wherein the first birefringent crystal and the second birefringent crystal are each cut on a mounting plane peφendicular to the polished opposing faces and mounted on a flat portion of the planar substrate.

84. The circulator of Claim 83, wherein the non-reciprocal rotator section is cut on a mounting plane and mounted separating the first birefringent crystal and the second birefringent crystal on the flat portion of the planar substrate.

85. The circulator of Claim 75 wherein the first collimator and the second collimator are each mounted in V-grooves formed in the planar substrate, the V-grooves running parallel with an optical axis of the circulator.

86. The circulator of Claim 76, wherein each optical fiber of the first set of optical fibers and each optical fiber of the second set of optical fibers are mounted in V-grooves fromed in the planar substrate, the V-grooves running parallel with an optical axis of the circulator.

87. The circulator of Claim 75, wherein the first birefringent crystal and the second birefringent crystal are each of YV0₄ cut on a mounting plane peφendicular to two polished opposing faces and mounted on a flat portion of the planar substrate, the circulator having three ports wherein the first collimator is coupled to two (2) optical fibers, the second collimator is coupled to one (1) optical fiber, and the mounting plane of the first birefringent crystal and the mounting plane of the second birefringent crystal makes an angle of about 18° with the b- crystallographic axis of YV0₄.

88. The circulator of Claim 74 wherein the first set of optical fibers includes two (2) optical fibers and the second set of optical fibers includes one (1) optical fiber.

89. The circulator of Claim 74 wherein the first set of optical fibers includes more than two optical fibers and the second set of optical fibers includes more than one optical fiber.

90. An optical circulator having a minimal number of optical surfaces, comprising: a first collimator coupled to at least one optical fiber; a first alignment prism coupled to the first collimator; a first birefringent crystal coupled to the first alignment prism; a first mirror assembly coupled between the first alignment prism and the first birefringent crystal; a second birefringent crystal; a non-reciprocal rotator coupled between the first birefringent crystal and the second birefringent crystal.

91. The circulator of Claim 90, wherein the non-reciprocal rotator includes a Faraday rotator and a quarter-wave plate.

92. The circulator of Claim 91, wherein the first mirror assembly includes a transparent spacer and separated a pair of angled mirrors.

93. The circulator of Claim 92 having a first port, a second port and a third port, the first port and the third port being coupled to the first collimator and wherein a first randomly polarized light beam entering the first port is directed parallel with an optical axis of the circulator by the first alignment prism, the first randomly polarized light beam is separated into a first beam having a first linear polarization and a second beam having a second linear polarization by the first birefringent crystal where the first beam propagates through the first birefringent crystal undeflected while the second beam propagates through the first birefringent crystal deflected in a walk-off direction by a walk-off distance, the first polarization of the first beam is rotated into the second polarization and the second polarization of the second beam is rotated into the first polarization by the non-reciprocal rotator, and the first beam is deflected in the walk-off direction by the walk-off distance and the second beam is undeflected by the second birefringent crystal so that the first beam and the second beam are combined and coupled to the second port; and wherein a second randomly polarized light beam entering the second port is separated into a third beam having the first polarization and a fourth beam having the second polarization by the second birefringent crystal, the third beam being substantially undeflected and the fourth beam being deflected by the walk- off distance in an opposite direction to the walk-off direction, the first polarization of the third beam and the second polarization of the fourth beam are substantially unaltered by the non-reciprocal rotator, the third beam is substantially undeflected and the fourth beam is deflected by the walk-off distance in the opposite direction of the walk-off direction by the first birefringent crystal, the third beam is deflected in the opposite direction of the walk-off direction by one of the pair of angled mirrors in the first mirror assembly, propagates through a length of the transparent portion of the first mirror assembly, and is deflected in a path parallel to the optical axis of the circulator by the second of the pair of angled mirrors in the first mirror assembly, the third beam and the fourth beam is coupled into the first collimator by the first alignment prism so that the third beam and the fourth beam are coupled into the third port.

94. The circulator of Claim 93, wherein the first birefringent crystal and the second birefringent crystal are oriented such that an optical path difference between the third beam and the fourth beam is substantially zero.

95. The circulator of Claim 93, further including a second mirror assembly coupled to the second birefringent crystal, a second alignment prism coupled to the second birefringent crystal and the second mirror assembly, and a second collimator coupled to the second port and a fourth port wherein a third randomly oriented light beam entering and the third port is coupled to the fourth port by the circulator.

96. An optical circulator package comprising: a collimator prism assembly; a output coupling assembly; and a second collimator assembly aligned with the collimator prism assembly and the output coupling assembly, wherein the collimator prism assembly, the output coupling assembly and the optical circulator assembly are inserted into a circulator housing.

97. The circulator package of Claim 96, further including an outer housing wherein the circulator housing is enclosed in the outer housing and padded with a padding material.

98. The circulator package of Claim 97, further including a boot on either end of the outer housing and caps attached to either end of the outer housing that retain the boots in place seal the interior of the outer housing.

99. The circulator package of Claim 96, wherein the second collimator assembly is a collimator prism assembly.

100. The circulator package of Claim 96, wherein the second collimator assembly is a collimator assembly.

101. A method of producing an optical circulator package, comprising: assembling a circulator component assembly; assembling a collimator prism assembly; assembling a second collimator assembly; aligning and affixing the collimator prism assembly, the circulator component assembly, and the second collimator assembly within a circulator housing.

102. The method of Claim 101, further comprising: inserting the circulator housing with the collimator prism assembly, the circulator component assembly and the second collimator assembly into an outer housing; feeding optical fibers from the collimator prism assembly and the second collimator assembly through boots; filling the outer housing with a packing material to hold the circulator housing in place; and sealing the outer housing with the boots held in place by covers.

103. The method of Claim 101 , wherein the second collimator assembly is a collimator prism assembly.

104. The method of Claim 101, wherein the second collimator assembly is a collimator assembly.

105. The method of Claim 101, wherein assembling a collimator prism assembly comprises: assembling a collimator; preparing at least one optical fiber; inserting the at least one optical fiber into the collimator; preparing a prism assembly; aligning and affixing the collimator and prism assembly into a housing.

106. The method of Claim 105, wherein assembling a collimator comprises: positioning and affixing a fiber receiving portion and a transparent portion within a collimator housing.

107. The method of Claim 106 wherein affixing the fiber receiving portion and the transparent portion within a collimator housing includes epoxying the fiber receiving portion and the transparent portion within the collimator housing.

108. The method of Claim 105, wherein preparing at least one optical fiber includes preparing a microlens at an end of the optical fiber.

109. The method of Claim 108, wherein the microlens includes a spherical lens.

110. The method of Claim 108, wherein the microlens includes a tapered lens.

111. The method of Claim 105, wherein inserting the at least one optical fiber into the collimator comprises: positioning the at least one optical fiber in the fiber receiving portion; and epoxying the at least one optical fiber in place.

112. The method of Claim 111, wherein positioning the at least one optical fiber includes inserting at least one spacer fiber into the fiber receiving portion.

113. The method of Claim 105, wherein preparing a prism assembly comprises: forming a prism holder having a insert portion for insertion into the housing and a prism portion for holding an alignment prism.

114. The method of Claim 113, wherein aligning and affixing the collimator and prism assembly into a housing comprises: epoxying the collimator into the housing; inserting the insert portion of the prism holder into the housing opposite the collimator; adjusting the position of the prism holder with respect to the collimator in order to optimize coupling between the optical fibers and the prism output.

115. The method of Claim 104, wherein assembling a collimator assembly comprises: assembling a collimator; preparing at least one optical fiber; inserting the at least one optical fiber into the collimator; affixing the collimator into a housing.

116. The method of Claim 115, wherein assembling a collimator comprises: positioning and affixing a fiber receiving portion and a transparent portion within a collimator housing.

117. The method of Claim 116 wherein affixing the fiber receiving portion and the transparent portion within a collimator housing includes epoxying the fiber receiving portion and the transparent portion within the collimator housing.

118. The method of Claim 115, wherein preparing at least one optical fiber includes preparing a microlens at an end of the optical fiber.

119. The method of Claim 118, wherein the microlens includes a spherical lens.

120. The method of Claim 118, wherein the microlens includes a tapered lens.

121. The method of Claim 115, wherein inserting the at least one optical fiber into the collimator comprises: positioning the at least one optical fiber in the fiber receiving portion; and epoxying the at least one optical fiber in place.

122. The method of Claim 121, wherein positioning the at least one optical fiber includes inserting at least one spacer fiber into the fiber receiving portion.

123. The method of Claim 101, wherein assembling a circulator component assembly comprises: preparing a first birefringent crystal and a second birefringent crystal; preparing a non-reciprocal rotator section; positioning the first birefringent crystal, the non-reciprocal rotator section, and the second birefringent crystal into a component holder; affixing the first birefringent crystal, the non-reciprocal rotator section, and the second birefringent crystal into the component holder; and sealing the first birefringent crystal, the second birefringent crystal and the non- reciprocal rotator into the holder so that only an optical surface of the first birefringent crystal and an optical surface of the second birefringent crystal are exposed.

124. The method of Claim 123, wherein preparing the first birefringent crystal and the second birefringent crystal each comprises: cutting a birefringent material so that there are two opposing faces peφendicular to an optical axis; further cutting the birefringent material so that there are two side faces peφendicular to the two opposing faces and peφendicular to each other; and coating the two opposing faces with an antireflecting film for an operating wavelength.

125. The method of Claim 124, wherein the optical axis is along a crystallographic direction substantially corresponding to a direction of maximum walk-off.

126. The method of Claim 124, wherein the two side faces are chosen to optimize a walk-off direction.

127. The method of Claim 124, wherein the birefringent material is chosen from the group consisting of YV0₄, LiNb0₃, LiTa0 , MgF , β-BaB0₄, calcite, and rutile.

128. The method of Claim 125, wherein the birefringent material is YV0₄ and the direction of maximum walk-off is about 48° from a c crystallographic axis in an a-c crystallographic plane.

129. The method of Claim 125, wherein the opposing faces are cut peφendicular to a [101] crystallographic direction.

130. The method of Claim 125, further including the step of aligning the birefringent crystal along the [101] crystallographic direction with an x-ray crystallography technique.

131. The method of Claim 123, wherein preparing a non-reciprocal rotator section comprises: forming two non-reciprocal rotators by, for each non-reciprocal rotator, preparing a rotator birefringent crystal having opposing polished faces peφendicular to an optical axis, preparing a first Faraday rotator, preparing a second Faraday rotator, positioning the rotator birefringent crystal between the first Faraday rotator and the second Faraday rotator on a positioning jig such that the opposing polished faces are adjacent the first Farady rotator and the second Faraday rotator, tacking the first Faraday rotator, the birefringent crystal, and the second Faraday rotator with a first epoxy, and affixing the first Faraday rotator, the birefringent crystal, and the second Faraday rotator with a second epoxy; and positioning the two non-reciprocal rotators together such that the first Faraday rotator of one of the two non-reciprocal rotators is adjacent to the half- wave plate of the other non-reciprocal rotator.

132. The method of Claim 131, wherein a magnetization of the first Faraday rotator is along the optical axis and opposite a magnetization of the second Faraday rotator and further including preparing a half-wave plate, tacking the half-wave plate to the second Faraday rotator and affixing the half-wave plate to the second Faraday rotator opposite the rotator birefringent crystal.

133. The circulator of claim 131 wherein a magnetization of the first Faraday rotator is along the optical axis and along a direction equal to a magnetization of the second Faraday rotator.

134. The circulator of claim 131, wherein the first epoxy is a UV cured epoxy, the second epoxy is a heat treatment cured epoxy, tacking includes UV treating the first epoxy, and affixing includes heat treating the second epoxy.

135. The method of Claim 131, wherein preparing the rotator birefringent crystal includes cutting a birefringent material such that it the opposing polished faces are peφendicular to the optical axis wherein the optical axis corresponds substantially to a direction of maximum walk-off of the birefringent material; and further cutting the birefringent material along at least one positioning plane peφendicular to the opposing polished faces such that a walk-off direction is peφendicular to at least one of the at least one positioning plane.

136. The method of Claim 135, wherein the birefringent material is chosen from the group consisting of YV0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile.

137. The method of Claim 135, wherein the birefringent material is YV0₄ and further including aligning a [101] crystallographic direction with the optical axis using an x-ray crystallography technique.

138. The method of Claim 123, wherein preparing a non-reciprocal rotator section comprises: preparing a rotator birefringent crystal having opposing polished faces peφendicular to an optical axis and a top portion and a bottom portion, the top portion and the bottom portion separated by a plane peφendicular to the opposing polished faces; preparing a top first Faraday rotator; preparing a top second Faraday rotator; preparing a bottom first Faraday rotator; preparing a bottom second Faraday rotator; positioning the rotator birefringent crystal on a jig; positioning the top first Faraday rotator and the top second Faraday rotator on the jig such the top first Faraday rotator and the top second Faraday rotator are separated by the top portion of the rotator birefringent crystal; positioning the bottom first Faraday rotator and the bottom second Faraday rotator such that the bottom first Faraday rotator and the bottom second Faraday rotator are separated by the bottom portion of the rotator birefringent crystal and the bottom first Faraday rotator and the top first Faraday rotator are adjacent; tacking the top first Faraday rotator, the top second Faraday rotator, the bottom first Faraday rotator and the bottom second Faraday rotator to the rotator birefringent crystal with a first epoxy; attaching the top first Faraday rotator, the top second Faraday rotator, the bottom first Faraday rotator and the bottom second Faraday rotator tot he rotator birefringent crystal with a second epoxy.

139. The method of Claim 138, wherein a magnetization of the top first Faraday rotator is parallel to the optical axis, opposite a magnetization of the top second Faraday rotator and opposite a magnetization of the bottom first Faraday rotator and parallel with a magnetization of the bottom second Faraday rotator and further including preparing a top half- wave plate, tacking the top half- wave plate to the top second Faraday rotator and affixing the top half- wave plate to the top second Faraday rotator opposite the rotator birefringent crystal, and preparing a bottom half-wave plate, tacking the bottom half- wave plate to the bottom first Faraday rotator and affixing the bottom half wave plate to the bottom first Faraday rotator opposite the rotator birefringent crystal.

140. The circulator of claim 138 wherein a magnetization of the top first Faraday rotator is parallel with the optical axis and along a direction equal to a magnetization of the top second

Faraday rotator, a magnetization of the bottom first Faraday rotator is along a direction equal to a magnetization of the bottom second Faraday rotator, and the magnetization of the top first Faraday rotator is opposite the magnetization of the bottom first Faraday rotator.

141. The circulator of claim 138, wherein the first epoxy is a UV cured epoxy, the second epoxy is a heat treatment cured epoxy, tacking includes UV treating the first epoxy, and affixing includes heat treating the second epoxy.

142. The method of Claim 138, wherein preparing the rotator birefringent crystal includes cutting a birefringent material such that the optical axis corresponds substantially to a direction of maximum walk-off of the birefringent material; and further cutting the birefringent material along at least one positioning plane peφendicular to the opposing polished surfaces such that a walk-off direction is peφendicular to at least one of the at least one positioning plane.

143. The method of Claim 142, wherein the birefringent material is chosen from the group consisting of YV0₄, LiNb0₃, LiTa0₂, MgF₂, β-BaB0₄, calcite, and rutile.

144. The method of Claim 142, wherein the birefringent material is YV0₄ and further including aligning a [101] crystallographic direction with the optical axis using an x-ray crystallography technique.

145. The method of Claim 123, wherein the component holder is a rod with a groove having a floor and a wall, the groove being large enough to admit the first birefringent crystal, the second birefringent crystal and the non-reciprocal rotator cut along its long axis, wherein positioning the first birefringent crystal, the non-reciprocal rotator section and the second birefringent crystal into the component holder includes placing the first birefringent crystal, the second birefringent crystal and the non-reciprocal rotator into the groove and against the floor and the wall.

146. The method of Claim 145 wherein affixing the first birefringent crystal, the non- reciprocal rotator section, and the second birefringent crystal into the component holder includes applying epoxy to the wall and the floor before positioning the first birefringent crystal, the non-reciprocal rotator section, and the second birefringent crystal into the component holder.

147. The method of Claim 146, wherein sealing the holder includes epoxying a cover into the groove of the component holder.

148. The method of Claim 146 wherein the component holder includes a first section for holding the first birefringent crystal, a second section for holding the non-reciprocal rotator and a third section for holding the second birefringent crystal, the first section, the second section and the third section being inserted into a housing such that the first and third section are rotatable with respect to the second section, and wherein positioning the first birefringent crystal, the non-reciprocal rotator, and the second birefringent crystal includes rotating the first section and the third section relative to the second section to optimize optical performance of the first birefringent crystal, the second birefringent crystal and the non-reciprocal rotator, and affixing the first birefringent crystal, the second birefringent crystal and the non- reciprocal rotator includes affixing the first section, the second section and the third section within the housing.

149. The method of Claim 123 wherein the component holder includes two parallel plates having epoxy access holes at locations appropriate for the first birefringent crystal, the non- reciprocal rotator section, and the second birefringent crystal and wherein positioning the first birefringent crystal, the non-reciprocal rotator and the second birefringent crystal includes arranging the first birefringent crystal, the non-reciprocal rotator, and the second birefringent crystal on a flat between the two parallel plates, affixing the first birefringent crystal, the non-reciprocal rotator and the second birefringent crystal includes epoxying the first birefringent crystal, the non-reciprocal rotator and the second birefringent crystal to the two parallel plates through the epoxy access holes, and sealing the first birefringent crystal, the non-reciprocal rotator and the second birefringent crystal includes surrounding the parallel plates with a covering.

150. The method of Claim 149, wherein the covering is heat shrinkable tubing.

151. The method of Claim 150, further including epoxying a second set of parallel plates to the two parallel plates.

152. The method of Claim 101, wherein aligning and affixing the collimator prism assembly, the circulator component assembly, and the second collimator assembly within the circulator housing comprises: affixing the circulator component assembly in the center of the circulator housing; inserting the circulator component assembly into a first positioning jig; inserting the collimator prism assembly into a second positioning jig such that the collimator prism assembly extends into the circulator housing; inserting the second collimator assembly into a third positioning jig such that the second collimator assembly extends into the circulator housing opposite the collimator prism assembly; optically aligning the collimator prism assembly, the circulator component assembly and the second collimator assembly; and affixing the collimator prism assembly and the second collimator assembly to the circulator housing.

153. The method of Claim 152, wherein affixing the circulator component assembly includes placing the circulator component assembly in the center of the circulator housing and soldering it in place.

154. The method of Claim 152, wherein affixing the circulator component assembly includes placing the circulator component assembly in the center of the circulator housing and epoxying it in place.

155. The method of Claim 152, wherein the first positioning jig, the second positioning jig, and the third positioning jig each has positioning controls that control the relative position and aligning of the circulator component assembly, the collimator prism assembly, and the second collimator assembly, and optically aligning comprises: coupling a light source to a first port of the optical circulator package and a light detector a second port of the optical circulator, the second port being where a light beam entering the first port is expected to exit, adjusting the positioning controls of the first jig, the second jig and the third jig until the light beam propagation between the first port and the second port is optimized; and repeating with subsequent ports until the alignment of the optical circulator is optimized.

156. The circulator of Claim 155, wherein affixing the collimator prism assembly and the second collimator assembly to the circulator housing includes soldering the collimator prism assembly and the second collimator assembly to the circulator housing and sealing the collimator prism assembly to the circulator housing and the second collimator assembly to the circulator housing with solder.

157. The circulator of Claim 155, wherein affixing the collimator prism assembly and the second collimator assembly to the circulator housing includes epoxying the collimator prism assembly and the second collimator assembly to the circulator housing and sealing the collimator prism assembly to the circulator housing and the second collimator assembly to the circulator housing with epoxy.

158. The method of Claim 156, further including a second alignment step using the first positioning jig and the third positioning jig after the collimator prism assembly is affixed to the circulator housing and removed from the second positioning jig.

159. A method of producing a coplanar circulator, comprising: preparing a substrate; positioning a first collimator, first birefringent prism, a non-reciprocal rotator section, and a second birefringent prism and a second collimator on the substrate; aligning the first collimator, the first birefringent prism, the non-reciprocal rotator section, the second birefringent prism and the second collimator on the substrate; and affixing the first collimator, the first birefringent prism, the non-reciprocal rotator section, the second birefringent prism and the second collimator to the substrate.

160. The method of Claim 159, further comprising: covering the substrate, the first collimator, the first birefringent prism, the non- reciprocal rotator section and the second birefringent prism with a second substrate; and placing the substrate and the second substrate in a housing, sealing the coplanar circulator.

161. The method of Claim 159, wherein preparing a substrate includes: forming V-grooves in the substrate at locations appropriate for positioning the first collimator and the second collimator; forming a flat portion in the substrate at a location appropriate for positioning the first birefringent crystal, the non-reciprocal rotator, and the second birefringent crystal; and depositing a metal layer on the substrate.

162. The method of Claim 161 wherein photo-lithography is used to form the V-grooves and the flat portion.

163. The method of Claim 161, wherein aligning the first collimator, the first birefringent crystal, the non-reciprocal rotator, the second birefringent crystal and the second collimator comprises: attaching a positioner to the first birefringent crystal, the non-reciprocal rotator, and the second birefringent crystal; coupling a light source to a first port and a detector to a second port of the circulator, the circulator having a plurality of ports, and optimizing the throughput of light between the first port and the second port by adjusting controls on the positioner; rotate the light source and the detector to the next pair of ports in the plurality of ports and continue until the light throughput is optimum for all of the ports.

164. The method of Claim 163, wherein the first collimator, the first birefringent crystal, the non-reciprocal rotator, the second birefringent crystal and the second collimator are metallic plated and affixing comprising heating the substrate and the first collimator, the first birefringent crystal, the non-reciprocal rotator, the second birefringent crystal and the second collimator to solder them together.

165. The method of Claim 164, wherein the metallic plating is gold.

166. An optical circulator, comprising: means for receiving a light beam from a first port of a first set of ports; means for causing the light beams to propagate parallel to an optical axis; means for separating by a walk-off distance the light beam into a first beam having a first polarization and a second beam having a second polarization; means for rotating the first polarization into the second polarization and the second polarization in the first polarization; means for recombining the first beam and the second beam into the light beam; means for coupling the light beam into second port of a second set of ports.

167. The circulator of Claim 166, wherein the means for rotating further includes a means for shifting the first beam and the second beam for light traveling in one direction along the optical axis while not shifting the first beam and the second beam for light traveling in a direction opposite the one direction along the optical axis.

168. The circulator of Claim 167, further including: means for containing the circulator; and means for sealing the circulator from an external environment.