JP2006317624A - Optical circulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a reflective type optical circulator which is designed to improve characteristics by preventing variance of insertion loss for each round trip optical path, as well as preventing generation of PDL. <P>SOLUTION: The optical circulator is structured by arranging all waveguides with equal spaces apart from the center point of each waveguide, equalizing each crystal axis direction of two birefringent elements, and making polarization components reflected in point symmetry in a reflecting body. In addition, the shifted quantity of abnormal light beams in the first birefringent element is set larger than the shifted quantity in the second birefringent element; and the reflection point of each polarization component in the reflecting body and the lens optical axis are linearly arranged while the center point of each waveguide and the lens optical axis are arranged nonlinearly. Then, the first birefringent element and a phase element are positioned, in a manner such that each propagation position is situated nonlinearly in the abnormal light beam emitted in a forward direction from the first birefringent element and in the normal light beam made incident in a direction reverse to that of the first birefringent element. Furthermore, in the optical circulator, the second birefringent element is arranged so that the light beam in the forward direction is made incident to the second birefringent element, prior to reflection at the reflecting body. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical circulator used in fields such as an optical communication system and optical measurement.

  An optical circulator is one non-reciprocal optical device that plays an important role in fields such as optical communication systems and optical measurement. The optical circulator has at least three or more ports. For example, in the case of an optical circulator having three ports represented by numbers 1, 2, 3, the forward direction 1 → 2, 2 → 3, 3 → 1. Then, the propagation light has a low loss, and light is propagated as a high loss output in the reverse direction 3 → 2, 2 → 1, 1 → 3.

  By the way, the optical circulator has a problem that light is propagated from one port opposed to the other along the propagation direction to the other port, and the number of optical elements is large. It was. In addition, if the number of ports is increased, the number of optical elements is further increased, resulting in a further increase in size and difficulty in increasing the number of ports. Therefore, a reflection-type optical circulator that includes a reflector and forms a round-trip optical path has been filed as an optical circulator that is smaller than the conventional structure without increasing the number of optical elements even when the number of ports is increased (for example, (See Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2000-231080 (page 2-5, FIG. 1-6) Japanese translation of PCT publication No. 2002-528765 (pages 38-39, FIG. 12)

  As shown in FIG. 12, the optical circulator 100 of Patent Document 1 uses four array-type optical fibers 101 at the entrance / exit side port (light entrance / exit section), and between the array-type optical fiber 101 and the lens 102. In addition, a birefringent element 103, a first phase element 104 composed of two half-wave elements, a polarization plane rotating element 105, and a composite birefringent element 106 composed of two birefringent elements are arranged, and the lens 102 is further sandwiched between them. The second phase element 107 and the reflector 108 are arranged on the opposite side.

  Further, as shown in FIG. 13, the optical circulator 109 of Patent Document 2 is an optical circulator that uses a non-row type optical fiber bundle 110 at an input / output side port (light input / output portion), and includes two birefringent elements 111a. And 111b, two Faraday rotators 112 and 113 (or a polarization plane rotating element 114), and a reflecting prism 115 as a reflector. The two Faraday rotators 112 and 113 are configured to rotate in opposite directions.

  However, when viewed from the z-axis direction of FIG. 12, the four optical fibers 101 constituting the input / output side port of the optical circulator 100 have an array shape arranged in a line as shown in FIG. Therefore, the centers of all the optical fibers P1 to P4 cannot be arranged at equal distances with respect to the center point C on the diagonal line connecting the centers of all the optical fibers P1 to P4, and the outer optical fibers P3 and P4 cannot be arranged at the same distance. The distance from the center point C increases as it goes to. Therefore, an optical path length difference occurs between the optical path lengths of the inner round-trip optical path (for example, P1 → P2) and the outer round-trip optical path (for example, P3 → P4). It has occurred. Therefore, it is difficult for the conventional optical circulator 100 to stabilize the insertion loss.

  Further, in the optical circulator 100, the birefringent element 103 and the composite birefringent element 106 are configured so that the crystal axis directions thereof are perpendicular to each other, and unless the optical fibers P1 to P4 are arranged in a line, Optical coupling between the round-trip optical path and each end face of the optical fibers P1 to P4 cannot be obtained. From the above, with the configuration of the optical circulator 100, it was impossible to prevent variations in insertion loss while ensuring the operation as an optical circulator.

  On the other hand, the entrance / exit side port composed of the three optical fibers P1 to P3 of the optical circulator 109 is configured in a non-line type as shown in FIG. 15 when viewed from the z-axis direction of FIG. The centers of the optical fibers P1 to P3 are arranged at equal distances with respect to the center point C on the diagonal line connecting the centers of the fibers P1 to P3. Therefore, variation in insertion loss for each round-trip optical path is prevented. However, since the birefringence elements 116 to 119 are configured to shift only one of the two polarization components, polarization dependent loss (PDL) has occurred.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a reflection-type light whose characteristics are improved by preventing both occurrence of PDL and variation in insertion loss for each round-trip optical path. It is to realize a circulator.

According to a first aspect of the present invention, an optical element unit including a first birefringent element, a phase element, a first polarization plane rotating element, a second birefringent element, and a lens are provided. A light incident / exit section in which at least three waveguides are arranged is disposed on one end side of the element section, and a reflector is disposed on the opposite side of the optical element section with the lens interposed therebetween. In the axial direction, the second polarization plane rotation element is disposed either before or after the lens, and the phase element is constituted by two phase optical elements of the first and second phase optical elements, and the first in the forward direction When light separated into two polarization components of the ordinary ray and the extraordinary ray by the birefringence element is incident on the second birefringence element, the polarization planes of the two polarization components are the same in the second direction on the optical surface. Phase element so that it is perpendicular to the crystal axis direction of the birefringent element of And the direction of rotation of the first polarization plane rotation element are set, and the dimension of the second birefringence element is set to one of the forward path and the return path of the light that reciprocates when reflected. In an optical circulator set to transmit only light,
All waveguides are arranged at equal intervals from the center point on the diagonal line connecting the centers of the waveguides,
Furthermore, the first and second birefringent elements are arranged so that the crystal axis directions on the optical surfaces of the first and second birefringent elements are parallel to the same direction.
In the reflector, the two polarized components are reflected point-symmetrically,
The extraordinary ray shift amount in the first birefringent element is set larger than the extraordinary ray shift amount in the second birefringent element;
While the reflection point on the reflector and the optical axis of the lens are arranged on the same straight line in the light propagation direction, the center point on the diagonal line connecting the centers of the waveguides and the optical axis of the lens Arranged on the same straight line,
A polarization component that becomes an extraordinary ray when transmitted through the first birefringent element in the forward direction and the reverse direction is transmitted through the first phase optical element, and becomes an ordinary ray when transmitted through the first birefringent element in the reverse direction and the forward direction. The polarization component is transmitted through the second phase optical element, and further, the propagation position of the extraordinary ray emitted from the first birefringence element in the forward direction and the propagation position of the ordinary ray incident on the first birefringence element in the reverse direction. And the first birefringent element and the phase element are positioned so that they are non-collinear,
The optical circulator is characterized in that the second birefringent element is arranged so that the light in the forward direction is incident on the second birefringent element before being reflected by the reflector.

  The invention according to claim 2 is the optical circulator according to claim 1, wherein the second polarization plane rotating element is replaced with a quarter-wave element.

  According to the optical circulator of the present invention, it becomes possible to configure an optical circulator by arranging all the waveguides at equal distances with respect to the central point on the diagonal line connecting the centers of the respective waveguides. It can also be operated as a circulator. Accordingly, it is possible to stabilize the insertion loss by preventing variation in insertion loss for each round-trip optical path in the optical circulator.

  Furthermore, it is possible to prevent the occurrence of PDL which is caused by shifting only one polarization component when transmitting through the second birefringent element. In addition, since the two polarized components are reflected point-symmetrically by the reflector, the optical path length difference between the two polarized components before and after the reflection can be made zero, thereby preventing the occurrence of PDL. . Further, by arranging the second birefringent element so that the light in the forward direction is incident on the second birefringent element before being reflected by the reflector, the occurrence of PDL is prevented and the size of each optical element is reduced. Can be achieved.

  Further, by providing the reflector and configuring the optical circulator with a reciprocating optical path, the overall length of the optical circulator can be shortened, and the optical circulator can be downsized.

Hereinafter, the best embodiment of the optical circulator according to the present invention will be described in detail with reference to FIGS. As shown in FIGS. 1 and 2, the optical circulator 1 of the present invention is an optical device comprising a first birefringent element 2, a phase element 3, a first polarization plane rotating element 4, and a second birefringent element 5. An element portion and a lens 6 are provided. Further, a light incident / exit section 9 is disposed on one end side of these optical element sections, and a reflector 8 and a second polarization plane rotating element 7 are disposed on the opposite side of the optical element section with the lens 6 interposed therebetween. Yes. It is desirable to apply an antireflection coating such as SiO ₂ / TiO _{2 on} each optical surface of each optical element. The description and illustration of the magnets that apply magnetism to the polarization plane rotating elements 4 and 7 are omitted.

  1 and 2, the light propagation direction 9 to the reflector 8 when the light propagation direction is the z-axis, the horizontal direction in the plane orthogonal to the z-axis is the x-axis, and the vertical direction is the y-axis. The arrangement of The x-axis to z-axis correspond similarly in other drawings. The light incident / exit portions 9 are arranged by inserting and fixing an optical fiber as a waveguide in each of three insertion holes formed at equal intervals and parallel (within 3 degrees of parallelism) on a ferrule (not shown). It is. Here, for convenience of explanation, the three optical fibers are called ports P1 to P3, respectively.

  As shown in FIG. 11, when the light incident / exit section 9 is viewed toward the reflector 8 along the z axis, the center of all optical fibers (core axis) from the center point C on the diagonal connecting the fiber centers. The optical fibers P1 to P3 are arranged so that the distance to the However, the distance Lf between adjacent optical fibers is determined in consideration of the relationship between the back focus of the lens 6 and the thickness and separation width of the birefringent element 2.

  The plurality of optical fibers P1 to P3 are configured by a single mode fiber (SMF), and a graded index fiber (GIF) is bonded to the light incident / exit end face. ing. By providing the GIF, the mode field diameter (MFD) at the light incident / exit end of each of the optical fibers P1 to P3 is enlarged, and the spread angle of the emitted light is suppressed to be small. The enlargement of the MFD is not limited to the installation of the GIF, but an optical fiber may be subjected to TEC processing or a micro lens may be installed.

The first birefringent element 2 separates the light emitted from the optical fibers P1 to P3 into an ordinary ray and an extraordinary ray, and combines the ordinary ray and the extraordinary ray that are reflected by the reflector 8 and returned. It is an optical element which performs. The first and second birefringent elements 2 and 5 include birefringence elements such as rutile (TiO ₂ ), calcite (CaCO ₃ ), yttrium osovanadate (YVO ₄ ), and alpha barium vodate (αBaB ₂ O ₄ ). A refractive single crystal is used. Also, the direction of the crystal axis X22 (see FIGS. 3 and 7) with respect to the optical surface of the first birefringent element 2 is about 42 to 50 degrees with respect to the normal of the optical surface so as to obtain the maximum separation width ( Most preferably, it is set to 47.8 degrees.

  The phase element 3 rotates the polarization plane of each polarization component (ordinary ray and extraordinary ray) transmitted through the first birefringent element 2 by 45 degrees, for example, TBIG (Terbium Bismuth Iron Garnet), GBIG A garnet such as (Gadolinium / Bismuth / Iron / Garnet) or a reciprocal polarization plane rotating element such as quartz or a half-wavelength element is used. When the half-wavelength element is used, as shown in FIG. 3, the first phase optical element 3a in which the crystal axis X31 direction is inclined 22.5 degrees with respect to the y axis and the crystal axis X32 direction is inclined 22.5 degrees with respect to the x axis. The second phase optical element 3b is overlapped with the bonding surface 3c in the y-axis direction, and bonded with an optical adhesive such as an ultraviolet curing type. It is assumed that the adhesive does not protrude from the bonding surface 3c to the optical surface. When a reciprocal polarization plane rotating element is used, it is desirable that it is as thin as possible, such as a zero-order single plate or a primary single plate. When a higher-order wave plate is used, wavelength characteristics and temperature characteristics are deteriorated.

  The first polarization plane rotation element 4 is a non-reciprocal polarization plane rotation element that rotates the polarization plane of the polarization component that has passed through the phase element 3, and is as thin as possible with a rotation angle of about 45 degrees in the operating wavelength band. use. For example, garnet, TBIG, GBIG, etc. are optimal. In the present embodiment, a clockwise garnet single crystal is used when viewed in the z-axis direction. However, when a counterclockwise garnet is used, the crystal axes of the first and second phase optical elements 3a and 3b are used. X31 and X32 may be inclined at -22.5 degrees with respect to the y-axis and -22.5 degrees with respect to the x-axis, respectively.

  The first polarization plane rotation element 4 may be replaced with the position of the phase element 3. Since the spread angle of the transmitted light is constant, the polarization plane rotating element 4 can be used in a place where the beam diameter of the light is smaller when it is replaced. For this reason, it is possible to reduce the possibility of propagating the propagation light due to the assembly error of the optical circulator 1 or the processing error of the constituent optical elements.

  As shown in FIG. 1, the second birefringent element 5 has a size that allows only one light of the forward path or the return path of the optical path (shown by a solid line and a broken line in FIG. 1) to reciprocate when reflected. It is set to such a size. Further, the crystal axis X52 direction of the second birefringent element 5 with respect to the normal of the optical surface is set to θ1 = about −42 to 50 degrees (most preferably −47.8 degrees) with respect to the z axis. The direction of the crystal axis X51 on the optical surface is parallel to the y-axis direction. Furthermore, the direction of the crystal axis X21 on the optical surface of the first birefringent element 2 is also set parallel to the y-axis direction. Therefore, the two birefringent elements 2 and 5 are arranged so that the crystal axes X21 and X51 of the first and second birefringent elements 2 and 5 are parallel to the same direction.

  The lens 6 collimates or converges incident light, and an aspherical lens, a ball lens, a plano-convex lens, a distributed refractive lens, or the like can be used. However, as the lens 6, a lens having a back focus in which the constituent optical elements 2 to 5 can be arranged between the light incident / exit section 9 is used. In this embodiment, an aspheric lens is used.

  As the second polarization plane rotation element 7, a nonreciprocal polarization plane rotation element rotated by 45 degrees, for example, garnet, TBIG, GBIG or the like is used. In FIGS. 1, 2, 4, and 8, the second polarization plane rotation element 7 is disposed behind the lens 6 in the direction of the optical axis X6 of the lens 6. However, it may of course be disposed in front of the lens.

The reflector 8 is a reflection mirror that reflects the light transmitted through the second polarization plane rotating element 7. In this embodiment, as an example, a total reflection mirror in which the surface of the substrate is coated with SiO ₂ / TiO ₂ is used. It was.

  Next, the operation of the optical circulator 1 will be described with reference to FIGS. (A) to (L) of FIGS. 5 and 6 are diagrams showing the polarization state of light in the forward direction in the optical circulator 1, and in each optical path section indicated by reference numerals (A) to (L) in FIG. Corresponds to the polarization state. 5 and 6, the horizontal direction is the x-axis, the vertical direction is the y-axis, and the direction toward the paper surface is the z-axis. For convenience of explanation, the vertical and horizontal directions are divided into eight, and the horizontal direction is 1 to 8. In the vertical direction, a to h indicate the propagation position of the polarization component in each optical path cross section. In FIG. 1, a reciprocating optical path from the optical fiber P1 (hidden under P3) to the optical fiber P2 through the reflector 8 is defined as “forward direction”, and light is transmitted from the optical fiber P2 through the reflector 8. The round-trip optical path toward the fiber P3 is defined as “reverse direction”.

  When viewed in a matrix, the position of the optical fiber P1 is between 5 and 6 in the horizontal direction and between g and h in the vertical direction, as shown in FIG. In this embodiment, such a position is represented as (5-6, g-h). Reference symbol C is the center point, and reference symbol R is a reflection point of the polarization component in the reflector 8.

  The light incident on the optical fiber P1 is emitted to the first birefringent element 2 and, as shown in FIG. 5B, an ordinary ray perpendicular to the crystal axis X21 and a parallel extraordinary ray in the birefringent element 2. Are separated into two polarization components. In the forward direction, the propagation position of the extraordinary ray emitted from the first birefringent element 2 is (5-6, cd) from FIG.

  The separated polarization components are incident on the first phase optical element 3a and the second phase optical element 3b for each polarization component (see FIG. 3). In FIG. 5B, the solid line between d and e represents the joint surface 3c. In the forward direction, the polarized component that was an extraordinary ray when transmitted through the first birefringent element 2 is transmitted to the first phase element 3a, and the polarized component that was an ordinary ray is transmitted to the second phase optical element 3b. As described above, the phase element 3 tilts the crystal axis X31 at 22.5 degrees with respect to the y-axis and X32 at 22.5 degrees with respect to the x-axis. When transmitted, as shown in FIG. 5C, the polarization directions rotate 45 degrees in opposite directions, and the polarization directions become the same direction.

  Then, these lights pass through the first polarization plane rotating element 4 and are rotated clockwise (clockwise) as shown in FIG. 5D, and the crystal of the second birefringent element 5 is rotated. The direction of polarization is perpendicular to the direction of the axis X51. Next, these lights are incident on the second birefringent element 5. The second birefringent element 5 is arranged only on the optical path on one side as shown in FIGS. 1, 3 and 5D so that light propagating in the forward direction is incident before being reflected by the reflector. Be placed.

  As described above, since the directions of the crystal axes X21 and X51 are parallel, when the polarization planes of the two polarization components enter the second birefringent element 5, they are orthogonal to the direction of the crystal axis X51. Therefore, as shown in FIG. 5E, in the forward direction, the two polarization components are emitted without being shifted by the second birefringent element 5 while maintaining the polarization direction at the time of incidence.

  From the above, when the second birefringent element 5 is incident in the forward direction, the crystal of the phase element 3 between the two birefringent elements so that the polarization planes of the two polarization components are aligned perpendicular to the crystal axis X51 direction. The directions of the axes X31 and X32 and the rotation direction of the first polarization plane rotating element 4 are set.

  Next, the light transmitted through the second birefringent element 5 is refracted by a predetermined angle by the lens 6, but the polarization state does not change. The refraction angle at this time is determined by the center position of the light from the optical axis X6 of the lens 6 and the focal length of the lens 6. Next, the light transmitted through the lens 6 is incident on the second polarization plane rotating element 7, and the plane of polarization is rotated by 45 degrees as shown in FIG.

  Then, the two polarized light components emitted from the second polarization plane rotating element 7 are reflected by the reflector 8 on the side opposite to the incident angle so as to be point symmetric at one point R (FIGS. 4 and 5). (F), see FIG. 6 (G)). As can be seen from FIGS. 1 and 2, in the optical circulator 1 of the present invention, the reflection point R on the reflector 8 and the optical axis X6 of the lens 6 in the vicinity thereof are identical in the light propagation direction (z-axis direction). The reflector 8 and the lens 6 are positioned and arranged so as to be on the line. On the other hand, the center point C on the diagonal line connecting the centers of the optical fibers and the optical axis X6 of the lens are non-collinear as is apparent from FIGS. The lens 6 is positioned and arranged. In this way, the light is reflected by the reflector 8 to form a reciprocating optical path, whereby the entire length of the optical circulator 1 can be shortened.

  Next, the reflected light (see FIG. 6G) is transmitted again through the second polarization plane rotating element 7, whereby the polarization plane is further rotated by 45 degrees.

  Next, the two polarization components are transmitted through the lens 6 again, and are emitted at positions symmetrical to the case of FIG. 5E with respect to the optical axis X6 of the lens 6, as shown in FIG. 6H. . At this time, the polarization state does not change before and after the lens 6.

  Next, the light transmitted through the lens 6 passes through the space outside the second birefringent element 5 and enters the first polarization plane rotating element 4 as shown in FIGS. 6 (I)), the polarization plane is rotated 45 degrees clockwise as shown in FIG. 6 (J). This state is the same as the polarization state after passing through the phase element 3 from the first birefringence element 2 side as shown in FIG. 5C, although the propagation position in the x-axis direction is different. .

  Then, as shown in FIG. 3 and FIG. 6 (J), these polarization components are transmitted through the first phase optical element 3a and the second phase optical element 3b, respectively, as shown in FIG. 6 (K). In addition, the polarization directions are respectively rotated by 45 degrees in the opposite directions, and the respective polarization directions are orthogonal to each other. In FIG. 6J, the solid line between d and e represents the joint surface 3c.

  After that, the two polarization components are combined by passing through the first birefringent element 2, and as shown in FIG. 6 (L), the optical fiber P2 corresponding to (3-4, gh). Is incident on.

  Next, the operation of the polarization component in the reverse optical path from the optical fibers P2 to P3 will be described with reference to FIGS. In addition, about the thing which overlaps with the matter demonstrated at the time of forward operation | movement, description is abbreviate | omitted or simplified and described. FIGS. 9A and 9B are diagrams showing polarization states of light in the reverse direction in the optical circulator 1, and are cross sections of the respective optical paths indicated by reference numerals A to L in FIG. It corresponds to the polarization state. 9 and 10, the horizontal direction is the x-axis, the vertical direction is the y-axis, and the direction toward the paper surface is the z-axis. For convenience of explanation, the propagation position of the polarization component is shown by dividing into eight parts in the vertical and horizontal directions. Yes.

  As described above, the position of the optical fiber P2 is (3-4, gh), and the light incident on the optical fiber P2 is emitted to the first birefringent element 2 and separated into an ordinary ray and an extraordinary ray. . The separated polarization components are respectively incident on the first phase optical element 3a and the second phase optical element 3b. In FIG. 9K, the solid line between d and e is the joint surface 3c. When the ordinary ray and the extraordinary ray are transmitted through the phase element 3, the polarization directions are rotated by 45 degrees in opposite directions, and the polarization directions are the same (see FIG. 9J).

  Next, each polarization component is rotated clockwise by the first polarization plane rotation element 4 and aligned in the polarization direction orthogonal to the directions of the crystal axes X21 and X51. Next, the light transmitted through the lens 6 is incident on the second polarization plane rotation element 7, and the polarization plane is rotated by 45 degrees (see FIG. 9G).

  Then, the light is reflected point-symmetrically by the reflector 8 and passes through the second polarization plane rotating element 7 again, whereby the polarization plane of each polarization component is further rotated 45 degrees (see FIG. 10E). By this rotation, the polarization directions of the two polarization components are aligned in the same direction as the crystal axis X51 direction of the second birefringent element 5.

  Accordingly, each polarization component incident on the second birefringent element 5 is shifted by the same amount in the y-axis direction, as shown in FIGS. 7 and 10D. Next, these polarization components are rotated by 45 degrees in the same direction by the first polarization plane rotation element 4, and then, as shown in FIGS. 7 and 10C, the first phase optical element 3a and the second phase optical element 3a, respectively. The light is transmitted through the phase optical element 3b. As a result, as shown in FIG. 10B, the polarization directions are rotated 45 degrees in the opposite directions, and the respective polarization directions are orthogonal to each other. In FIG. 10C, the solid line between d and e represents the joint surface 3c.

  After that, the two polarization components are combined by transmitting through the first birefringent element 2, and as shown in FIG. 10 (A), an optical fiber P3 corresponding to (5-6, ef). Incident. From the above, the polarized component that becomes an extraordinary ray when transmitted through the first birefringent element 2 is transmitted to the first phase optical element 3a, and the polarized component that becomes an ordinary ray when transmitted through the first birefringent element 2 is It can be seen that the light is transmitted through the two-phase optical element 3b. Furthermore, the propagation position of the ordinary ray incident on the first birefringent element 2 is (5-6, e−f) from FIG.

  Therefore, in the optical circulator 1 of the present invention, the propagation position of the extraordinary ray emitted from the first birefringent element 2 in the forward direction: (5-6, cd) and the first birefringent element 2 incident in the reverse direction. Position the first birefringent element 2 and the phase element 3 so that the propagation position of the ordinary ray: (5-6, ef) is non-collinear in the light propagation direction (z-axis direction). Arrange.

  5 (A) and (B), FIG. 6 (K) and (L), FIG. 9 (K) and (L), and FIG. 10 (A) and (B). As is apparent from a comparison between the amount of extraordinary ray shift in the birefringent element 2 and the amount of extraordinary ray shift in the second birefringent element 5 in FIGS. 10D and 10E, the optical circulator of the present invention. 1, the shift amount of the first birefringent element 2 is set larger than the shift amount of the second birefringent element 5. For this reason, it is necessary to set the thickness of the first birefringent element 2 to be thicker than that of the second birefringent element 5.

  As described above, the optical circulator 1 according to the present invention sets the shift amounts of extraordinary rays in the first birefringent element 2 and the second birefringent element 5 to non-identical amounts, and the light of the lens 6. From the first birefringent element 2 in the forward direction, the reflection point R of the polarization component on the axis X6 and the reflector 8 is set to be non-collinear with the center point C of each of the optical fibers P1 to P3. It is possible to set the propagation position of the outgoing extraordinary ray and the propagation position of the ordinary ray incident on the first birefringent element 2 in the opposite direction on a non-collinear line. With such a configuration, it becomes possible to configure an optical circulator by arranging all the optical fibers P1 to P3 at equal distances from the center point C, and it is also possible to operate as an optical circulator. . Therefore, it is possible to stabilize the insertion loss by preventing variation in insertion loss for each round-trip optical path in the optical circulator 1.

  Further, by setting the directions of the crystal axes X21 and X51 on the optical surfaces of the first and second birefringent elements 2 and 5 to be parallel to the same direction, the second birefringent element 5 It is possible to shift both polarization components as extraordinary rays. Therefore, it is possible to prevent the occurrence of PDL which is caused by shifting only one polarization component when transmitted through the second birefringent element 5. In addition, since the two polarized components are reflected point-symmetrically by the reflector 8, the optical path length difference between the two polarized components before and after the reflection can be made zero, thereby preventing the occurrence of PDL. Become.

  Further, by disposing the second birefringent element 5 so that the forward light is incident on the second birefringent element 5 before being reflected by the reflector 8, it is possible to prevent the occurrence of PDL and each optical element. It is possible to achieve downsizing. If the second birefringent element 5 is arranged so that the light in the forward direction is incident after being reflected by the reflector 8, the two birefringent elements 5 are shifted together by the second birefringent element 5 in the optical path in the reverse direction. After that, the two polarization components are reflected by the reflector 8. Then, each distance between the reflection point R and the two polarization components is not the same for each polarization component, and PDL occurs. Therefore, the arrangement position of the second birefringent element 5 as described in the present embodiment is preferable.

  If the characteristics required for the optical circulator 1 are not so high, it is possible to change all the optical fibers P1 to P3 to be slightly equidistant from the center point C within a range including tolerance. It is.

  Further, even if a quarter wavelength element is provided instead of the second polarization plane rotating element 5, it is possible to cause the optical circulator 1 to perform the same operation as described above. Further, instead of GIF, a lens may be newly arranged in the vicinity of the light incident / exit end faces of the optical fibers P1 to P3, or a glass plate may be provided in the x-axis direction of the second birefringent element 5.

  The optical circulator of the present invention can be used as a nonreciprocal optical device in an optical communication system, an optical measurement field, or the like.

The top view which shows the structure of the optical circulator of this invention. The side view which shows the structure of the optical circulator of this invention. The perspective view which shows the arrangement | positioning from the light incident / exit part to the 2nd birefringent element of the optical circulator of FIG. 1, and the polarization state of the propagation light in a forward direction. The perspective view which shows arrangement | positioning from a lens to a reflector of the optical circulator of FIG. 1, and the polarization state of the propagation light in a forward direction. FIG. 2 is an explanatory diagram showing the polarization state of propagating light from the optical fiber P1 until it is reflected by a reflector in the optical circulator of FIG. FIG. 2 is an explanatory diagram showing a polarization state of propagating light from the optical circulator of FIG. 1 until it is reflected by a reflector and enters the optical fiber P2. The perspective view which shows the arrangement | positioning from the light incident / exit part to the 2nd birefringent element, and the polarization state of the propagation light in a reverse direction of the optical circulator of FIG. The perspective view which shows the arrangement | positioning from a lens to a reflector of the optical circulator of FIG. 1, and the polarization state of the propagation light in a reverse direction. FIG. 2 is an explanatory diagram showing a polarization state of propagating light from the optical fiber P2 until it is reflected by a reflector in the optical circulator of FIG. FIG. 2 is an explanatory diagram showing a polarization state of propagating light reflected by a reflector and incident on an optical fiber P3 in the optical circulator of FIG. The block diagram which shows a light incident / exit part when the optical circulator of FIG. 1 is seen toward a reflector from a z-axis direction. The top view which shows an example of the conventional optical circulator. The top view which shows the other example of the conventional optical circulator. The block diagram which shows the light incident / exit part when the optical circulator of FIG. 12 is seen toward a reflector from a z-axis direction. The block diagram which shows the light incident / exit part when the optical circulator of FIG. 13 is seen toward a reflector from a z-axis direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical circulator 2 1st birefringence element 3 Phase element 4 1st polarization plane rotation element 5 2nd birefringence element 6 Lens 7 2nd polarization plane rotation element 8 Reflector 9 Light incident / exit part

Claims (2)

A first birefringent element, a phase element, a first polarization plane rotating element, an optical element unit comprising a second birefringent element, and a lens; and at least three or more guides on one end side of the optical element unit. A light incident / exit section in which waveguides are arranged is disposed, a reflector is disposed on the opposite side of the optical element section across the lens, and a second polarization plane rotating element is disposed in the lens in the optical axis direction of the lens. The phase element is composed of two phase optical elements, the first and second phase optical elements, and the first birefringent element in the forward direction is divided into two rays, an ordinary ray and an extraordinary ray. When the light separated into the polarization components is incident on the second birefringent element, the polarization planes of the two polarization components are in the same direction and orthogonal to the crystal axis direction of the second birefringence element on the optical surface. The crystal axis direction of the phase element and the first polarization plane rotation An optical circulator in which the rotation direction of the optical element is set and the dimension of the second birefringent element is set so as to transmit only one of the forward and backward light paths of the light that reciprocates when reflected. In
All waveguides are arranged at equal intervals from the center point on the diagonal line connecting the centers of the waveguides,
Furthermore, the first and second birefringent elements are arranged so that the crystal axis directions on the optical surfaces of the first and second birefringent elements are parallel to the same direction.
In the reflector, the two polarized components are reflected point-symmetrically,
The extraordinary ray shift amount in the first birefringent element is set larger than the extraordinary ray shift amount in the second birefringent element;
While the reflection point on the reflector and the optical axis of the lens are arranged on the same straight line in the light propagation direction, the center point on the diagonal line connecting the centers of the waveguides and the optical axis of the lens Arranged on the same straight line,
A polarization component that becomes an extraordinary ray when transmitted through the first birefringent element in the forward direction and the reverse direction is transmitted through the first phase optical element, and becomes an ordinary ray when transmitted through the first birefringent element in the reverse direction and the forward direction. The polarization component is transmitted through the second phase optical element, and further, the propagation position of the extraordinary ray emitted from the first birefringence element in the forward direction and the propagation position of the ordinary ray incident on the first birefringence element in the reverse direction. And the first birefringent element and the phase element are positioned so that they are non-collinear,
An optical circulator characterized in that a second birefringent element is arranged so that light in a forward direction is incident on the second birefringent element before being reflected by a reflector. The optical circulator according to claim 1, wherein the second polarization plane rotating element is replaced with a quarter-wave element.

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