CA1171704A - Fiber optic switch - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides modules for interfacing optical fibers with very low light loss and with provision for monitoring of the optical signal. The modules according to the present invention are character-ized by the precise tolerances required in high capacity optical communication systems and yet may be mass pro-duced at reasonable costs. A device according to the present invention comprises a transparent imaging element having a curved reflective surface at one end and pre-aligned fiber insertion holes at the other end. The transparent element is characterized by an index of refraction equal to that of the fiber core, and the fibers are glued in their respective holes with index matching cement.

Description

~'71~

FIBER OPTIC SWITCH
Field of the Invention This invention relates generally to optical S fiber communications, and more specifically to modules for intercouplinq of light from or to fibers and perform-ing monitoring, splitting, 6witching, duplexing and multiplexing functions.
Backqround Of The Invention As existing communication systems have become increasingly overloaded, optical transmission through transparent fibers has been found ~o provide a means of achieving a smaller cross-section per message, thus enabling an increased capacity within existing conduit constraints. The basic medium of transmission is an optical fiber. A first type of fiber is a 6tepped index fiber which comprises a transparent core member and a transparent cladding, the core member having a higher index of refraction than the cladding. Light is transmit-ted through the core, and contained within the core byinternal reflection. So long as the light does not deviate from the fiber axis by more than the complement of the critical angle for the core-cladding interface, total internal reflection with substantially no loss resultE. A second type of fiber is a graded index fiber whose refractive index gradually decreases away from the fiber axis. Transmission i6 highly relia~le, and i5 11'~ 17(~4 substantially insensitive to electrical noise, cross coupling between channels, and the like.
A~ with any communication medium, once a suit-able transmission line has been found, the need arises S for modules to couple sources and detectors to the line, couple lines together, perform switching, splitting, duplexing, and multiplexing functions. Ultimately, the total system can be no more reliable than these modules.
When it is considered that the core of a typical optical communication fiber is characterized by a diameter of only 60 microns, it can be immediately appreciated that such modules must be fabricated and installed to highly precise tolerances.
In order to realize the inherent reliability of optical fiber communication systems, the modules them-selves must be highly reliable since they are t~pically installed in relatively inaccessible locations (e.g.
within conduits running under city streets, etc.). Given this requirement, it can be seen that it would be highly desirable to have monitoring signals that would verify the operation of the modules and the integrity of the fibers themselves. A further re~uirement for a satis-factory optical communication system is that the modules introduce a minimum of loss into the system. It has only been with the development of extremely high transparency fibers that optical fiber communication has become prac-tical, and the introduction of lossy modules would con-~iderably undercut the advantages and efficacy of such systems.
Unfortunately, existing devices for interfacing fibers to sources, detectors, and each other, have proved to be lossy, bulky, delicate, and expensive. Thus, while fiber optic communication ~ystems are proving to be highly advantageous they are prevented from realizing their fullest potential.

1 1'f ~7 ~)4 SummarY of the Invention The present invention provides modules for interfacing optical fibers with very low light 106s and with provision for monitoring of the optical 6ignal. The modules according to the present invention are character-ized by the precise tolerances required in high capacity optical communication 6ystems and yet may be mass pro-duced at reasonable costs.
A device according to the present invention comprises a transparent imaging element having a curved reflective surface at one end and prealigned fiber inser-tion holes at the other end. The transparent element is characterized by an index of refraction equal to that of the fiber core, and the fibers are glued in their respec-tive holes with index matc~ing cement. The holes facili-tate precision alignment and provide mechanical strength.
The curved reflective surface is characterized by a focal plane having the property that a point source of light at a first location in the focal plane is imaged at a second complementary location in the focal plane, and the fiber insertion holes maintain the ends of the fibers at suit-able complementary locations within the focal plane. In this context, the term "fiber insertion hole" should also be taken to include a hole sized to maintain a light 60urce or detector at a given location within the focal plane. In some applications, the source or detector would be directly mounted to the transparent imaging element, while in other applications the source or detec-tor would communicate with the imaging element via a short length of fiber.
The use of a transparent imaging element char-acterized by an index of refraction equal to that of the fiber coxe has the important advan~age that fresnel reflection at the fiber end, a 6ignificant potential 60urce of 106s of the signal, i~ eliminated. Al60, t 1', ~7~4 refraction which would spread the liyht, thus l~ecessitating a larger reflective surface, is avoided. Moreo~er, the use of prealigned fiber insertion holes wherein the fiber ends are cemented into automatic registered position with index matching cement results in a monolithic structure that is dimensionally stable and sufficiently rugged to yrovide many years of trouble free operation. A further advantage of the monolithic structure wherein reflective light losses are avoided is that reflected light pulses that could affect other communication line within the system are avoided.
According to the present invention there is provided a switch for selectively communicating optical information between an input optical fiber and one of a plurality of output optical fibers comprising:
means dPfining an imaging reflective surface characterized by a focal plane such that an object point in said focal plane is focused in said focal plane, said reflective surface being further characterized by an optical axis;
fiber positioning means for registering respective ends o said input fiber and said output fibers to respective locations within said focal plane;
pivot means for allowing sufficient rotation of said reflective surface about a point between said focal plane and said reflective surface to provide a plurality of orientations wherein that the end of said input fiber is imaged at the ends of said output fibers; and t l ',' 17~4 indexing means for maintaining said reflecting surface in a selected one of said plurality of positions.
Preferably, the switches include a body of transparent material having a first end contoured to define said imaging reflective surface and a second end provided with cylindrical bores terminating at said focal plane to at least partially define said fiber positioning means.
Preferably also, the indexing means comprises:
an open frame rigidly coupled to said fiber positioning means and having portions defining a corresponding plurality of inwardly opening tapered depressions a member rigidly coupled to said reflective surface and having a corresponding plurality of outwardly protruding tapered wedge elements adapted to mate with said depressions on said frame such that when one of said wedges is seated in its corresponding depression, said reflective surface is in said corresponding position.
For a further understanding of the nature and advantages of the present invention, reference should be had to the remaining portions of this specification and to the attached drawings.

t ~'~'1 7~k4 Brief Description Of The Drawings Fig. 1 is an isometric cut-away view of a fiber/fiber coupler according to the present invention;
Fig. 2 is a simplified cross-sectional view of the coupler of Fig. l;
Fig. 3 is a simplified cross-sectional view of a source/fiber coupler;
Figs. 4A and 4B are simplified cross-sectional views of different embodiments of a splitter according to the present invention;
Figs. 5A and 5B are simplified cross-sectional views of alternate embodiments of a switch according to the present invention;
Fig. 6 is an exploded view of the switch of Fig. 5B
showing a mechanism for achieving increased precision;
Figs. 7A and 7B are simplified cross-sectional views of alternate embodiments of two colored duplexers;

~ 7 ~

Figs. 8A, 8B and 8C are 6implified cross-sec-tional views of multiplexer and demultiplexer embodi-ments;
Figs. 9A and 9B show a directional monitor.
5DescriDtion Of The Preferred Embodiments The present invention relates to modules for interfacing optical fibers with each other, with light sources, and with detectors. This is generally accom-plished by positioning detectors, ~ources, or respective ~0 ends of 6uch fibers in a focal plane as will be described below. It will be immediately apparent to one of ordi-nary skill in the art that an input fiber and a light source may be ~ubstituted for one another, that an output fiber and a detector may be substituted for one another, lS and that the system may be "time reversed" by inter-changing inputs and outputs. Therefore, while the de-scription that follows is in specific terms, such equiva-lent systems will be made readily apparent.
Fig. 1 is an isometric cut-away view of a fiber/fiber coupler 10 according to the present inven-tion. Coupler 10 couples input and output fiber optic cables 12 and 13 having respective fibers 14 and 15 therein 60 that optical information traveling within the core of input fiber 14 i~ transmitted to the core of output fiber 15 with low loss. An electrical output signal proportional to the optical signal power in fiber 14 is provided by monitor unit 16 at an electrical output terminal 17 (preferably a "BNC" output connector).
Fiber~ 14 and 15 optically communicate with a tran parent imaging element 20 within a housing 21 as will be de-scribed below, the optical communication requiring pre-cise registration of the ends of the fibers. Gross ~echanical positioning of the fiber optic cables is accompli~hed by a clamping mechanism 22 comprising grooved mating body portions 25 for positioning and ~ 7~

holding the cables. Elastomeric compression seals 27 provide strain relief when mating portions 25 are tightly fastened to one another, as for example by screwing.
Fig. 2 i~ a cross-sectional view of transparent imaging element 20 with fibers 14 and 15 registered thereto. Imaging element 20 comprises a body 30 of transparent material, body 30 having a curved surface 32 at a first end and paired cylindrical fiber insertion holes 35 and 37 at a ~econd end. Surface 32 is a pol-ished surface and coated with a reflective coating suchas a multilayer dielectric coating that reflects most of the light incident on it from within transparent body 30, but transmits a small fraction. Surface 32 is character-ized by a focal plane 40 having the property that a point source in focal plane 40 is imaged in focal plane 40.
Surface 32 is preferably spherical, in which case focal plane 40 is perpendicular to a radial axis and passes through the center of curvature. Fiber insertion holes 35 and 37 are of a diameter to accomodate fibers 14 and 15 and to maintain the fiber ends at precisely registered locations in focal plane 40 such that the cone of light emanating from the end of fiber 14 is imaged on the end of fiber 15. Body 30 is preferably ~ormed from a trans-parent plastic by an injection molding process. The transparent material i6 chosen to have an index of re-fraction equal to that of the fiber core, and the fiber ends are glued into their re6pective fiber insertion holes with an index matching cement. The fiber insertion holes themselves do not provide the precision alignment, but rather facilitate such alignment which may be carried out in a suitable jig or the like. Once the fibers have been cemented into the holes, mechanical strength is achieved.
Monitor unit ~6 comprises a photodetector 45 and an associa~ed protective window 47. Monitor unit 16 1 ~ ~ 17(~4 is located outside transparent body 30 in a position to intercept the light that i~ transmitted by the reflective coating on surface 32. Monitor unit 16 i6 a self con-tained unit which may be inserted into housing 21 if the monitoring function is required. If no monitoring is required, an opaque plug may close off the end of housing 21.
The ends of fibers 14 and 15 are cleaved per-pendicular to the respective axes and located symet-rically about the center of curvature within focal plane40. In order to preserve modes, fiber insertion holes 35 and 37 are inclined with respect to one another ~o that the axes of the respective fibers are directed to a common intersection point 42 on the axis of surface 32.
As discussed above, a light source may be substituted for input fiber 14 without any change in the functioning of the device. Fig. 3 shows a source/fiber coupler 50 ~hat differs from fiber/fiber coupler 10 only in that a light source 52 is substituted for input fiber 12. The purpose of coupler 50 is to transmit the light from source 52 into a fiber S3. Source 52 may be a metal/ceramic "pillbox" light emitting diode or a laser having an optical coupling plastic window 55 and an oil interface 57 to provide optical continuity and index matching. Since light source 52 has a larger diameter than that of a fiber, the complementary optical points within ~he focal plane are moved farther away from the center of curvature to accomodate the larger diameter element. I~order to maintain mode preservation and minimize aberrations, fiber 53' is inclined at a cor-responding larger angle with respect to the optic axis.
Where a monitoring function is carried out, the current from photodetector 45 may be used to provide feedback to the power ~ource driving light source 52 to improve the ~5 linearity of ~he dependence of light output on drive current.

_ g _ ~ ~ f .1 7~4 Fig. 4A shows a fir~t embodiment of a two-way splitter 60 for dividing the light carried by an input fiber 61 between fir6t and 6econd output fibers 62 and 6S. As in the coupler, the basic element of splitter 60 is a transparent body 68 havinq a reflective surface at one end and fiber insertion holes at the other end.
~owever, the reflective ~urface is continuous but not mathematically smooth, comprising abutting spherical surface segments 70 and 72. Spherical 6urface 6egments 70 and 72 are characterized by the same radius but have respective centers of curvature 75 and 77 that are dis-placed from the a~is of input fiber 61. In particular, center of curvature 75 is midway between the end of fiber 61 and the end of fiber 62; center of curvature 77 is midway between the end of fiber 61 and fiber 65. Gen-erally, for an N way splitter, N pie-shaped surface segments having wedge angles 360 and respective fiphere centers in a circular array 6urrounding the end of the input fiber would be required.
Fig. 4B is a cross-6ectional view of an alter-nate embodiment of a two-way 6plitter 80 for dividing the light from an input fiber 82 evenly between output fibers 85 and 87. This embodiment differs from the embodiment of Fig. 4A in that each fraction of the input light cone is intercepted by a plane reflecting surface before encountering the corresponding focusing segment. In particular, a transparent body ~2 is configured with a wedge-shaped depression 92 which defines respective plane interfaces 95 and 97 that come together at an apex 100 on the axis of input fiber 82. The half cone that reflects from plane surface 95 impinges on a first curved reflec-tive segment 102 and is focused on the end of output fiber 85. Similarly the other half cone is incident on a second curved reflective segment 105 and focused on output fiber 87. ~his embodiment is typically easier to ~ 70'~

fabricate than the embodiment of Fig. 4A since all the curved segments, if spherical, may be located with a common center of curvature. The differing points of focus are achieved by providing a wedge angle of slightly more than 90~. Generally, for an N-way splitter with N >

2, an N-sided pyramid rather than a wedge is used.
Fig. 5A is a cross-sectional view of a two-way (single-pole/double-throw) switch 110 for selectively directing light traveling along an input fiber 112 to either of paired output fibers 115 and 117. Switch 110 comprises a transparent body 120 having respective fiber insertion holes 122, 125, and 127 at one end, and a continuous, mathematically smooth focusing surface 130 at the other end. Selective switching is accomplished by providing pivoting means to permit reflective surface 130 to rotate relative to the fiber insertion holes about a point 132 intermediate the fiber ends and the reflective surface and located along the axis of input fiber 112.
This is accomplished by fabricating body 120 out of a flexible transparent material and providing the body with a necked portion 135 proximate pivot point 132 of rela-tively small diameter to permit flexing without deforma-tion of the remaining portions of body 120. In particu-lar, when body 120 is flexed about pivot point 132, a body portion 137 moves relative to a body portion 13~ to permit the center of curvature of spherical surface of segment 130 to be selectively directed to a point midway between the ends of fibers 112 and 115 or between the ends of fibers 112 and 117.
The rotation is effected by electromagnetic deflection. A soft steel sleeve 140 surrounds body por-tion 137 having reflective surface 130 thereon and car-ries tapered wedge sections 142 and 143. For an N-way switch, there are N such wedge sections. Corresponding electromagnets 145 and 146 are mounted to the fixed 1 1'71 ~ ~ 4 housing corresponding to each switch position. Each electromagnet includes a yoke 147 and a coil 148. The yoke has portions defining a tapered depression with 6urfaces adapted to mate with the outer surfaces of its respective wedge 6ection on 61eeve 140 in order to index movable body portion 137 to the desired position. Mag-netic latch elements 150 may be provided to maintain a given 6witch position after the respective electromagnet current has been turned off.
Fig. 5B is a 6implified cross-sectional view showing an alternate embodiment of a two-way switch.
This embodiment differs from that of Fig. 5B in that the body comprises two relatively movable portions 155 and 157 having a spherical interface 160 therebetween to define an optical ball bearing. The variable region between body portions 155 and 157 is filled with a sili-cone oil reservoir 162 being bounded by a suitable bel-lows 165. The two mating parts are maintained in tension against one another by a magnet or spring (not shown).
While Figs. 5A and 5B illustrate two-way switches, it will be immediately appreciated that an N-way switch is achieved by the provision of additional input fiber insertion holes, additional indexing electromagnets, and corresponding tapered wedge ~ections on the sleeve.
Fig. 6 illu~trates an additional embodiment of an indexing system suitable for either of the two 6witch embodiments described above, but illustrated for the embodiment of Fig. 5B for definiteness. It will be immediately apparent that the angular positioning of movable body portion 157 with respect to fixed body portion 155 having fiber insertion holes therein is extremely critical to proper operation of the 6witch. In particular, this translates into precise tole,ances on the fabrication of the 61eeve 6urrounding the movable body portion and the location of the electromagnets. It 1 ~ '7 ~7~4 has been found that increased precision of angular orien-tation can be achieved by 6eparatinq the wedges and electromagnet~ from the movable body portion along the axial direction. In particular, an axial lever arm 170 rigidly couples a sleeve 172 surrounding movable body 157 with a soft 6teel ring 175 having tapered wedged portion 177 mounted thereon in the ~ame fashion that tapered wedged portion 142 and 143 were mounted to sleeve 140 in Fig. 5A. Sleeve 172, lever arm 170 and ring 175 are coaxially aligned. Electromagnets, not ~hown, cooperate with wedges 177 and precisely the ~ame manner that elec-tromagnets 145 and 146 cooperated with wedges 142 and 143 in Fig. SA.
Fig. 7A is a simplified cross-sectional view of a duplexer 180 according to the present invention. The purpose of duplexer 180 is to permit optical information to be transmitted simultaneously in both directions on a single fiber 182. This is accomplished by using optical signals of differing wavelengths for the different direc-tional transmission, and incorporating classificationmeans to separate the optical signals. In particular, duplexer 180 couples a source 18S of light of a first wavelength and a detector 187 sensitive to light of a second different wavelength to fiber 182. While source 185 and detector 187 are shown communicating to duplexer 180 by short fibers 190 and 192, such 60urces and/or detectors could be directly mounted to the duplexer.
~uplexer 180 itself comprises a transparent body 195 having a curved surface at one end and fiber insertion holes at the other end. However, in contrast with the devices described above, the curved surface carries a concave reflection grating 197. Grating 197 has the property that light emanating from a point in a curved focal surface i6 imaged at different locations in the focal surface depending on the wavelength of the light.

7~4 Different image points are determined by the ~pacing of the grating lines and the particular wavelengths in-volved. Thu6, fiber 190 has its end at the complementary position with respect to the end of fiber 182 for the first wavelength and fiber 192 has it~ end at a comple-mentary position with respect to the end of fiber 182 for the second wavelength. Thus, light from source 185 is imaged onto the end of fiber 182 and transmitted away from duplexer 180 while light of the ~econd wavelength traveling along fiber 182 in a direction toward duplexer 180 is imaged onto the end of fiber 192 and thus trans-mitted to detector 187.
Fig. 7B illustrates an alternate embodiment of a duplexer 200 wherein the classification means and the lS imaging means are separated. In particular, a dichroic beam splitter interface 202 is reflective with respect to light of the first wavelength and transmissive with respect to light of the second wavelength. Beam splitter interface 202 is disposed at approximately 45 from the axis of fiber 182 so that light of the first wavelength is significantly deviated from its original path. Sep-arate reflective imaging elements 205 and 207 cooperate with beam splitter ~urface 202 in order to couple light of the first wavelength between source 185 and fiber 182 and light o~ the second wavelength between fiber 182 and detector 187. In a duplex system, a similar duplexer would be employed at at remote end of fiber 182, except that source 185 and detector 187 would be replaced by a detector sensitive to light of the first wavelength and a source of light of the secon~ wavelength, respectively.
Fig. 8A shows a first embodiment of a ~hree-color multiplexer for ~imultaneously trasmitting optical information from three 60urces 212, 215, and 217 along a single fiber 220. Multiplexer 210 compri~e~ a transpar-ent body 222 having a concave reflection grating 225 as 1 l '; l 7~

described in connection wi~h duplexer 180. In fact, duplexer 180 could be converted to a two color multi-plexer by sub6tituting a source of light of the second wavelength for detector 187.
Fig. 8B shows a three color demultiplexer 230 for receiving simultaneous transmission of light at three wavelengths along a fiber 232 and ~ending the light to three detectors 235, 237 and 240. Since the light from the different wavelengths is spatially separated, de-tectors 235, 237 and 240 could be detectors that are sensitive to all three wavelengths, although selective wavelength detectors may be preferable. Demultiplexer 230 is substantially identical to multiplexer 210 and comprises a transparent body 242 having a concave reflec-tion grating 245 at one end and fiber insertion holes at the other.
Fig. 8C shows an alternate embodiment of a three color multiplexer 250 for transmitting light at three wavelengths from respective sources 252, 255 and 257 along a single fiber 260. This is accomplished by two dichroic beam splitter surfaces 262 and 265 and sepa-rate reflective imaging elements 270, 272, and 275. This embodiment functions substantially the same as duplexer 200 shown in Fig. 7B.
~he couplers described above have the property that they are bidirectional, that is, that the direction of light travel can be reversed and the device will still function in the same way. ~owever, it sometimes happens that directionality is re~uired in the monitoring or splittin~ operation. Figs. 9A and 9B illustrate a cou-pler 280 having a directional monitorin~ feature. In particular, a directional coupler 280 comprises a body of graded index (self focu~ing) material 282 for coupling first and second fibers 284 and 285. Graded index mater-ial has the property that a point source at a first axial t ~',t7 ~ 4 location i~ imaged at a second ~xial location. Thus, inorder to couple fiber6 284 and ~25, respective fiber ends are located at complementary axial position6 287 and 288.
A beam fiplitter surface 2~0 is interposed at an oblique angle in the path of the light and reflects a ~mall fraction to a suitable detector 292. Due to the oblique inclination, detector 292 only receives light when the light is traveling from fiber 284 to fiber 2~5.
In 6ummary it can be seen that the present invention provides a surprisingly effective series of modules for interfacing optical fibers with a very low light 106s and with provisions for monitoring the optical signal. While the above provides a full and complete disclosure of the preferred embodiment of the present lS invention, various modifications, alternate construc-tions, and equivalents may be employed without departing from the true spirit and Ecope of the invention. For example, the splitters and switches described were geo-metrically symmetric devices. However, there is no need for such geometrical ~ymmetry, nor is there any absolute requirement that the fractions of li~ht transmitted be equal or that the 6witching be total. Rather, a 6witch could employ features of a splitter as well in order to provide partial switching and partial splitting. More-over, while a common focal plane is shown, this is not anabsolute prerequisite. Therefore, the above descriptions and illustrations should not be construed as limiting the scope of the invention which is defined by the appended claims.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A switch for selectively communicating optical information between an input optical fiber and one of a plurality of output optical fibers comprising:
means defining an imaging reflective surface characterized by a focal plane such that an object point in said focal plane is focused in said focal plane, said reflective surface being further characterized by an optical axis;
fiber positioning means for registering respective ends of said input fiber and said output fibers to respective locations within said focal plane;
pivot means for allowing sufficient rotation of said reflective surface about a point between said focal plane and said reflective surface to provide a plurality of orientations wherein that the end of said input fiber is imaged at the ends of said output fibers; and indexing means for maintaining said reflecting surface in a selected one of said plurality of positions.

2. A switch as claimed in claim 1 including a body of transparent material having a first end contoured to define said imaging reflective surface and a second end provided with cylindrical bores terminating at said focal plane to at least partially define said fiber positioning means.

3. A switch as claimed in claim 2 wherein said body is formed of a flexible material and includes a necked portion between said first end and said focal plane such that such pivoting occurs about said narrow portion to define said pivoting means.

4. A switch as claimed in claim 2 wherein said body comprises first and second portions having a spherical interface at a position intermediate said focal plane and said reflective surface, said interface defining said pivot means.

5. A switch as claimed in claim 1 wherein said indexing means comprises:
an open frame rigidly coupled to said fiber positioning means and having portions defining a corresponding plurality of inwardly opening tapered depressions a member rigidly coupled to said reflective surface and having a corresponding plurality of outwardly protruding tapered wedge elements adapted to mate with said depressions on said frame such that when one of said wedges is seated in its corresponding depression, said reflective surface is in said corresponding position.

6. A switch as claimed in claim 5 wherein said member is coupled to said reflective surface by a lever arm to increase the translational motion of said wedge elements for a corresponding angular motion of said reflective surface, whereupon said positions of said reflective surface may be achieved with relatively loose tolerances on said indexing means.