CA1171705A - Fiber optical multiplexer/demultiplexer - 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

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FIBER OPTICAL MULTIPLEXER/DEMULTIPLEXER

Field of the Invention This invention relates generally to optical fiber communications, and more specifically to modules for intercoupling 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 tr.ansmission through transparent fibers has been found to provide a means of achieving a smaller cross-section per message, thus enabling an increased capacity within existing conduit constraints. The basic medium of t~ansmission is an optical fiber. A first type of fiber is a 6tepped index fiber which compri~es a transparent core member and a transparent cladding, the core member having a higher inde~ 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 anqle for the core-cladding interface, total internal reflection with 6ubstantially no loss results. A ~econd type of fiber i5 a graded index fiber whose refractive index gradually decreases away from the fiber axis. Transmi~sion i5 highly reliable, and is 7i~S

~ubstantially insensitive to electrical noise, ~ross coupling between channel~, and the like.
As with any communication medium, once a ~uit-able transmission line has been found, the need arises 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 typically 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 monitorinq signals that would verify the operation of the modules and the integrity of the fibers themselves. A further requirement 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-6iderably undercut the advantages and efficacy of such fiystems .
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 sy~tems are proving to be highly advanta~eous they are prevented from realizing their fullest potential.

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Summarv 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 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 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 matching cement. The holes facili-tate precision alignment and provide mechanical strength.
The curved reflective surface is charactexized 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 3~ short length of fiber.
The use of a transparent imaging element char-acterized ~y an index of refraction equal to that of the fiber core has the important advantage that fresnel reflec~ion at the fiber end, a significant potential ~ource of loss of the signal, i5 eliminated. Also, ~ I, l'7i~
refraction whi~h would spread the li~ht, thus necessitating a larger reflective surface, is avoided. Moreover, 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 provide many years of trouble free operation. A further advantage of the monolithic structure wherein reflective light losses are avoided is that reflected liaht pulses that could affect other communication line within the system are avoided.
According to the present invention there is provided a device for permitting multiple optical signals of differing wavelength to be transmitted simultaneously on a single optical fiber comprising:
imaging reflective means characterized by a focal plane wherein a point in said focal plane is imaged in said focal plane;
means for registering an end of said optical fiber at a first location within said focal plane;
classifying means cooperating with said imaging reflective means for imaging light of a first wavelength emanating from said fiber end at a first image location in said focal plane and imaging light of a second wavelength emanating from said fiber end at a second image location within said focal plane and displaced from said first image position.

1 I', 1'7~5 Preferably, said imaging reflective means and said classifying means together comprise a concave reflection grating.
Preferably also, classifying means comprises a dichroic beam splitter.
Preferably also, second means for registering a source of light of said first wavelength at said first image location;
and means for registering a detector sensitive to light of said second wavelength at said second image location, whereby said device functions as a duplexer.
Preferably also, second and third means for registering respective sources of light of said first and second wavelengths at said first and second image locations whereby said device functions as a multiplexer.
Preferably also, second and third means for registering respective detectors sensitive to light of said first and second wavelengths at said first and second image locations whereby said device functions as a demultiplexer.
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.

J. 1'7 1 7~5 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 SB 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. SB
showing a mechanism for achieving increased precision;
Figs. 7A and 7B are simplified cross-sectional views of alternate embodiments of two colored duplexers;

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Fiqs. 8A, 8B and 8C are 6implified cross-sec-tional views of multiplexer and demultiplexer embodi-ments;
Figs. 9A and 9B 6how a directional monitor.
5Description Of The Preferred Embodiments The present invention relates to modules for interfacing optical fibers with each other, with light ~ources, and with detectors. This is generally accom-plished by positioning detectors, ~ources, or respective 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 6ubstituted for one another, that an output fiber and a detector may be substituted for one another, lS and that the 6ystem may be "time reversed" by inter-changing input6 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 is transmitted to the core of output fiber 15 with low loss. An electrical output signal proportional to the optical ~ignal power in fiber 14 i5 provided by monitor unit 16 at an electrical output terminal 17 (preferably a "BNC" output connector).
Fiber6 14 and 15 optically communicate with a transparent imaging element 20 within a housing 21 as will be de-scribed below, the optical communication reguiring pre-cise registration of the ends of the fiber~. Gross mechanical positioning of the fiber optic cable~ is accomplished by a clamping mechani6m 22 comprising grooved mating body portions 25 for positioning and holding the cables. Elastomeric compression 6eals 27 provide ~train relief when mating portions 25 are tightly fastened to one another, as for example by screwing.
Fig. 2 is a cross-sectional view of tran6parent 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 second 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 formed from a trans-parent plastic ~y 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 respective fiber insertion holes with an index matching cement. ~he fiber insertion holes themselves do not provide the precision alignment, but rather facilitate such alig~ment which may be carried out in a suitable jig or the li~e. Once the fibers have been cemented into the holes, mechanical strength is achieved.
Monitor unit 16 compri~es a ~hotodetector 45 and an associated protective window 47. Monitor unit 16 ~'~

~ 1 ~ 17~5 is located outside transparent ~ody 30 in a position to intercept the light that is transmitted by the reflective coating on 6urface 32. Monitor unit 16 is ~ 6elf 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. -~ he ends of fibers 14 and 15 are cleaved per-pendicular to the re6pective 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 that differs from fiber/fiber coupler 10 only in that a light ~ource 52 is substituted for input fiber 12. The purpose of coupler S0 is to transmit the light from source 52 into a fiber 53. 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 the focal plane are moved farther away from the center of curvature to accomodate the larger diameter element. I ~ rder to maintain mode preservation and minimize aberrations, fiber 53' is inclined at a cor-responding larger angle with respect to the optic a~is.
Where a monitoring function i6 carried out, the current from photodetector 45 may be used to provide feedback to the power ~ource driving light source 52 to improve the linearity of the dependence of light output on drive current.

_ g _ 117~7~ 5 Fig. 4A 6hows a first embodiment of a two-way 6plitter 60 for dividing the light carried by an input fiber 61 between fir~t and 6econd output fiber6 62 and 65. As in the coupler, the basic element of 6plitter 60 i~ a transparent body 68 having a reflective surface at one end and fiber insertion holes at the other end.
~owever, the reflective 6urface is continuous but not mathematically ~mooth, compri6ing abutting 6pherical 6urface segments 70 and 72. Spherical 6urface 6egments 70 and 72 are characterized by the 6ame radius but have respective center~ of curvature 75 and 77 that are dis-placed from the axi6 of input fiber 61. In particular, center of curvature 75 i~ midway between the end of fiber 61 and the end of fiber 62; center of curvature 77 i~
midway between the end of fiber 61 and fiber 65. Gen-erally, for an N way 6plitter, N pie-shaped surface 6egments having wedge angles 360 and respective 6phere 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. ~hi6 embodiment differ6 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 6egment. In particular, a transparent bo~y ~2 is configured with a wedge-shaped depression 92 which defines respective plane interfaces 95 and 97 tha~ come together at an apex 100 on the axi6 of input fiber 82. ~he half cone that reflects from plane 6urface 95 impinges on a fir6t curved reflec-tive 6egment 102 and i6 focused on the end of output fiber 85. Similarly the other half cone i6 incident on a ~econd curved reflective 6egment lOS and focused on output fi~er 87. Thi6 embodiment i~ typically ea6ier to t l'; 1'7~5 fabricate than ~e embodiment of Fig. 4A 6ince all the curved 6egments, if 6pherical, may be located with a common center of curvature. The differing point6 of focus are achieved by providing a wedge angle of slightly more than 90. Generally, for an N-way 6plitter with N >

2, an N-sided pyramid rather than a wedge is u6ed.
Fig. 5A i5 a cross-sectional view of a two-way (single-pole/double-throw) ~witch 110 for 6electively directing light traveling along an input fiber 112 to either of paired output fiber6 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 6mooth focusing surface 130 at the other end. Selective 6witching is accomplished by providing pivoting means to permit reflective 6urface 130 to rotate relative to the fiber insertion holes about a point 132 intennediate the fiber ends and the reflective surface and located along the axis of input fiber 112.
~hi6 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 ~mall 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 138 to permit the center of curvature of spherical 6urface of segment 130 to be ~electively directed to a point midway between the ends of fiber~ 112 and 115 or between the ends of fibers 112 and 117.
The rotation is effected by electromagnetic deflection. A soft ~;teel 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 ~witch, there are N ~uch wedge 6ections. Corresponding electromagnets 145 and 146 are mounted to the fixed housing corresponding to each ~witch position. Each electromagnet includes a yoke 147 and a coil 148. The yoke has portions defining a tapered depression ~ith 6urfaces adapted to mate with the outer ~urfaces of its respective wedge 6ection on 61eeve 140 in order to index movable body portion 137 to the desired position. Ma~-netic latch elements 150 may be provided to maintain a given 6witch position after the respective electromagnet current has been turned off.
Fig. SB is a 6implified cross-sectional view showing an alternate embodiment of a two-way 6witch.
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 therebet~een 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 6hown).
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 6ections on the 61eeve.
Fig. 6 illu6trates an additional embodiment of an indexing system suitable for either of the two 6witch embodiments described above, but illustrated for the embodiment of Fig. SB for definiteness. It will be i~mediately apparent that the angular positioning of movable body portion 157 with respect to fixed body portion 155 having fiber insertion holes therein i6 extremely critical to proper operation of the 6witch. In particular, thi6 translates into precise tolerance6 on the fabrication of the 61eeve 6urrounding the movable body portion and the location of the electromagnet6. It has been found that increased precision of angular orien-tation can be achieved by 6eparating the wedges and electromagnet~ from the movable body portion along the axial direction. In particular, an axial lever arm 170 rigidly couples a 61eeve 172 ~urrounding movable body 157 with a soft ~teel ring 175 having tapered wedged portion 177 mounted thereon in the 6ame fashion that tapered wedged portion 142 apd 143 were mounted to sleeve 140 in Fig. 5A. Sleeve 172, lever arm 170 and ring 175 are coaxially aligned. Electromagnets, not 6hown, cooperate with wedges 177 and precisely the 6ame manner that elec-tromagnets 145 and 146 cooperated with wedges 142 and 143 in Fig. 5A.
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 ~imultaneously in both directions on a single fiber 182. This is accomplished by using optical ~ignals of differing wavelengths for the different direc-tional transmission, and incorporating classification~eans to 6eparate the optical signals. In particular, duplexer 180 couple~ a source 185 of light of a first wavelength and a detector 187 sensitive to light of a 6econd different wavelength to fiber 182. While source 185 and detector 187 are shown communicating to duplexer 180 by short fibers 190 and lg2, 6uch sources and/or detectors could be directly mounted to the duplexer.
Duplexer 180 itself comprises a transparent bod~ 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 6urface i6 imaged at different location6 in the focal 6urface depending on the wavelength of the light.

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Different image points are determined by the ~pacing of the grating lines ~nd the particular wavelength~ in-volved. Thu6, fiber 190 has its end at the complementary position with respect to the end of fiber lB2 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 ~econd wavelength. ~hus, light from source 185 is imaged onto the end of fiber 182 and transmitted away ~rom duplexer 180 while light of the second 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 imaging means are separated. In particular, a dichroic beam splitter interface 202 is reflective ~ith respect to light of the first wavelength and transmissive with respect to light of the econd 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 6urface 202 in order to couple light of the first wavelength between source 185 and fiber 182 and light of the second wavelength between fiber 182 and detector 187. In a duplex ~ystem, a similar duplexer would be employed at at remote end of fiber 182, except that 60urce 185 and detector 187 would be replaced by a detector 6ensitive to light of the first wavelength and a source of light of the second wavelength, respectively.
Fig. 8A ~hows a first embodiment of a three-color multiplexer for 6imultaneously trasmitting optical information from three sources 212, 215, and 217 along a 6ingle fiber 220. Multiplexer 210 comprise~ a transpar-ent body 222 having a concave reflection grating 225 as l ~ JS
de~cribed in connection with duplexer 180. In fact, duplexer 180 could be converted to a two color multi-plexer by 6ub~tituting a 60urce of light of the 6econd wavelength or detector 187.
Fig. 8B 6hows a three color demultiple~er 230 for receiving 6imultaneous transmission of light at three wavelengths along a fiber 232 and sending the light to three detectors 235, 237 and 240. Since the light from the different wavelengths is spatially 6eparated, de-tectors 235, 237 and 240 could be detectors ~hat are 6ensitive to all three wavelengths, although selective wavelength detector6 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 ~hows an alternate embodiment of a three color multiplexer 250 for transmitting light at three wavelengths from respective sources 252, 255 and 257 along a ~ingle fiber 260. This is accomplished by two dichroic beam 6plitter surfaces 262 and 265 and 6epa-rate reflective imaging elements 270, 272, and 275. This embodiment functions substantially the same as duplexer 200 shown in Fig. 7B.
The 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 i~ reguired in the monitoring or splitting operation. Figs. 9A and 9B illustrate a cou-pler 280 having a directional monitoring feature. In particular, a directional coupler 2B0 comprises a body of graded index (~elf focusing) material 2B2 for coupling fir~t and 6econd fibers 2B4 and 285. Graded index mater-ial has the property that a point source at a fir6t axial l ~ ~ 17~s location i~ imaged at a 6econd axial location. Thus, inorder to couple fibers 284 and B25, respective fiber ends are located at complementary axial positions 287 ~nd 288.
A beam ~plitter 6urface 290 i6 interpo~ed at ~n obligue S angle in the path of the light and reflects a 6mall fraction to a suitable detector 292. Due to the oblique inclination, detector 292 only receives light ~hen the light is traveling from fiber 284 to fiber 285.
In 6ummary it can be 6een that the present invention provide6 a 6urprisingly effective ~eries of modules for interfacing optical fibers with a very low liqht 106s and with provisions for monitoring the optical 6ignal. While the above provides a full and complete disclosure of the preferred embodiment of the present invention, variou~ modifications, alternate construc-tions, and equivalents may be employed without departing from the true spirit and 6cope of the invention. For example, the 6plitters and switches described were geo-metrically 6ymmetric devices. ~owever, there is no need for 6uch geometrical symmetry, nor is there any absolute requirement that the fractions of light transmitted be egual or that the 6witching be total. Rather, a 6witch could employ feature6 of a 6plitter as well in order to provide partial switching and partial splitting. More-over, while a common focal plane is 6hown, this is not anabsolute prereguisite. 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 device for permitting multiple optical signals of differing wavelength to be transmitted simultaneously on a single optical fiber comprising:
imaging reflective means characterized by a focal plane wherein a point in said focal plane is imaged in said focal plane;
means for registering an end of said optical fiber at a first location within said focal plane;
classifying means cooperating with said imaging reflective means for imaging light of a first wavelength emanating from said fiber end at a first image location in said focal plane and imaging light of a second wavelength emanating from said fiber end at a second image location within said focal plane and displaced from said first image position.

2. A device as claimed in claim 1 wherein said imaging reflective means and said classifying means together comprise a concave reflection grating.

3. A device as claimed in claim 1 wherein said classifying means comprises a dichroic beam splitter.

4. A device as claimed in claim 1 also comprising:
second means for registering a source of light of said first wavelength at said first image location; and means for registering a detector sensitive to light of said second wavelength at said second image location, whereby said device functions as a duplexer.

5. A device as claimed in claim 1 also comprising second and third means for registering respective sources of light of said first and second wavelengths at said first and second image locations whereby said device functions as a multiplexer.

6. A device as claimed in claim 1 comprising second and third means for registering respective detectors sensitive to light of said first and second wavelengths at said first and second image locations whereby said device functions as a demultiplexer.