CA1260742A - Bidirectional optical fiber coupler - Google Patents

Abstract

ABSTRACT
A bi-directional, fiber optic coupler operative to receive from an optical fiber, modulated light for detection thereof, and for applying to the same fiber, modulated light for transmission to a remote location. The coupler comprises a body which can be conveniently and economically molded as a unitary plastic element and in which V-channel is formed to receive and securely hold the optical fiber. The channel terminates in a slanted reflecting face which is readily coated with a dielectric layer to provide a beam splitting function, or in the case of dual frequency transmissions, a frequency dependent layer to provide multi-plexing or demultiplexing. The slanted reflecting face is located at the bottom of an emitter cavity in which an emitter assembly is readily press-fit to provide predetermined alignment between light eminating from the emitter assembly and the reflecting face from which the light is reflected into the adjacent termination of optical fiber. A detector cavity is formed within the body and position to receive light passing through the reflecting face. the transiting light is refracted downward toward the detector cavity which is positioned off axis. An index of refraction matching material may be applied between the optical fiber and reflecting face in which case the reflecting face passes the incoming light directly along the axis. A portion of the transmitted light is totally internally reflected by the bottom surface of the emitter cavity which is formed to provide a mirror surface than redirects that light toward the detector cavity. a lens may be molded directly into the coupler body to gather light passing through and refracted by the reflecting face. For the portion of light generated by the emitter and passing through the reflective face, a reflective wedge is provided in the bottom of thecoupler body to direct that radiation away from the detector to reduce cross talk.

Description

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~IELD AND BACKGROUND OF THE INVENIION
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¦ . The present invention relates to a coupIer for Iight carried by an optical .fiber.

2 I In particular it is desirAble to be able to provide a single coupler for bi~irectional operation of an optical fiber such that radiation transmitted along the fiber from a 4 remote location can be detected to provîde electricP~ signals corresponding to the 5 modulation on.the light. At the same time light, modulated in accordance with 6 ! desired electrical signals, is to be applied to the opticsl fiber for transmission to 7 the remote location. Such a function is important to bi directional communica-8 tions utilizing optical fibers, 9 In providing such a coupling function, it is desirable that lthe coup~ing to ~nd from the optical fiber be accomplished in a single device. It is also import~nt that 11 the attenuation in the radiation supplied to snd received ~rom the fiber be as 12 minimal as possible. ~inally, it is import~nt that cross talk, or reception by the 13 detector of a portion of the radi~tion intended for tr~nsmissioll along the ~iber ~
14 the remote location, be kept acceptably low so as not to impair the signal qu~lity of the detected incoming light.
16 Such couplers are likely to be ~tili~ed in great numbers making it important 17 for economical, high volume production to be available for su~h a coupler. At the 18 same time such couplers are likely to be installed in ~e field where oDmplex 19 alignment procedures are impracticaL

I BRIEF SUMMARY OF ~HE INYENTION
1 In ~ccordance with the teaching o~ the present inve~tion, a coupler fs provided 21 for receiving and detecting light transmitted slong an optical fi~er from a remote 22 location and for applying to the fiber f~r transmission to the remote location light 23 gènerated in response to electrical si~als. The eouplèr is con~ren~ently molded as !i . I
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a unitary plastic part which can.be efficiently and economically reproduced in 2 large numbers. An inclined face is located at the junction of the incoming and

3 ~ outgoing beams and is dielect~ic co~ted to provide a beam splitting function.

4 i Incoming light is transmitted along an opticaI fiber placed in a channel. The light j is divided by the beam splitting face with a portion directed tow~rd a detector 6 1¦ locsted within a molded-in coupler cavity. A separate emitter cavity is provided within the coupler bcdy to pre lign an emitter assembly with respect to the 8 reflecting face to insure that light from the emitter is directed toward the 9 reflecting face for reflection into the channel located optical fiber. Emitter radiation passing through the reflecting face is directed away from the detector11 cavity and may be absorbed by an absorbing eoating on the coupler body to prevent 12 cross taL'c with the incoming lighto .
13 Typically the interface between the optical fiber termination and the 14 reflecting face is filled with an index of refraction matching material such that the light passes on a2~s directly throu~h the reflecting face, and ic partially reflected 16 by the bottom surface of the emitter cavity as it passes toward the detector 17 cavity. Without the inde~ matching material, the radiQtion directed toward the 18 detector cavity is refracted at the beam splitting interface with the coupler body 19 flnd the detector may be displaced sligSltly off axis from the optical fiber to best receive this light. Typically, a relatively large active area is available on the 21 detector so that good coupling can be achieved in spite of poor alignment or a large 22 intervening distance from the fiber to detector. The emitter cavity may be 23 located more remotely from the renecting face and imaging optics utili~ed to 24 concentrate the emitter light directly onto the renecting face. This allows the detector cavity to be located more proximate to the refle~ting face and a~roid 26 ~ intervening coupler body reflecting surfaces.

Il i il , o 1 Where desired, a focusing ~ens for the incoming radiation received after 2 passing through the reflecting f~ce can be molded directly into the coupler bodg to 3 interface with small area detectors.
4 The emitter cavity is typically configured to pro~ide a pressfit with the

5 ¦ emitter assembly that provides predetermined aJignment of tha emitter output6 light with respect to the reflecting f~ce and of the position of ~e reflect d beam 7 with respect to the optical ~iber termination to insure optîm~ coupling of the 8 emitter radiation into the optical fiber.
9 The coupler can be provided with a dielec~ic coating of a frequency sensitive nature on the r~flective face to provide 2 multiplexing or demultiplexing 11 flmction utilizing two emitters or two detectors and frequenc~ separated radiation la in the optical fiber.

BRIEF DESCRIPTIO~ OF l'HE I)RAWING
13 These and other features of the present inY~ntion are more fully set forth 14 below in the solely e~emplary detailed description and acco~T~eanying drawing of which:
16 Figs. lA and lB ~re respectively partially cut-away and sectional siews OI
17 alternative coupler bodies according to the present invention;
18 Figs. 2A and 2B are di~grammatic views of conventional emitters for use in 19 - the present invenffon;
Figs. 3A and 3B are section 1 views of ~ portion of the couple~ body of 21 Figs. lA and lB, showing incoming radiation paths with and without inde2~ of 22 ~ refraction matching;
23 ~ Fig. 4 is a sectional view of a modification to the coupler body of ~igs. lA
24 and lB; ~nd 11 ~

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11 Fig. S is a sectional view of a further modification to the coupler body of 2 ¦I the present ;nvention.
I
¦ DETAILED DESCRIPTlON OF THE INVENTION
3 I The present imention contemplates a bi directional coupler for light trans-4 ¦ mitted along an optical fiber. The coupler is conveniently molded as a unitary 5 ¦ plastic structure and provides high efficiency with low cross talk in coupling

6 radiaffon between the optical fiber and an emitter or ~ detector. Multiple~ng and

7 ! demultiplexing functions are also provided by the coupler using two ernitters or

8 detectors and radiation at different frequencies.

9 The coupler comprises a body 12, illustrated in Figs. lA and 1B, and is conveniently molded ss 8 unitary acrylic or polycarbonate plastic piece. For this 11 purpose, Lexan, ~ ~ademark for a moldable polycarbonate plastic of the General 12 ~lectric Company, is usable. The body 12 has on one f~e 14 a detector cavity 16 13 and on a second face 18 an emitter cavity 20. A trough 22 is cut in the face 1~
14 providing a first wall 24 and facing second wall (not shown) snd terminating in a V-shaped channel 26. The V-shaped chsnnel 26 extends from a surface,28 of the body16 1a, opposite the surface 14, into the body 12, terminating at a 45 angled reflecting 17 face 30 located ~t the bott~m of the emitter cavity 20. The face 30 is typi~y 18 coated with a dielectric layer to function as a beam splitter for radistion received 19 from an optical fiber 32, or from an emitter assembl~,T inserted within the caYity 20.
21 ~ The reflecting face 30 ~s precisely located with respect to the cavity 20 to 22 ,¦ receive light from an emitter assembly within the emitter cavity 20 and having 23 precisely concentric or centered beam of radiation eminating therefrom. The 24 reflecting face 30 provides precise alignment between the radiation received from 25 ¦ the emitter assembly and the term~tion of the opticsl fiber 32 for optimal 26 ! coupling of the emitter radiation into the fiber 32.

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~IVari us concentric~y radiating emitter ~ssembdes are avail~ble =e shown 2 1 by Figs. 2A and 2B. In ~ig. 2A, the emitter includes an active, light emitting 3 1 semiconductor element 9, the radiation from which is focused by a spheric~l 4 1¦ lens 11 to a point preferably outside a window 13 in a metal contsiner 15. The 5 ~I focal point is positioned to ~oincide with the location of the fiber termination after 6 ¦ reflection by face 30. Emitters are available with focal points located beyond the 7 ! window, and highly concentric to the container I5. The coupler body 12 is 8 I dimensioned to pro-ride an interference fit of the emitter of Fig. 2A into the 9 cavity 20 to permit consistent alignment without field adjustment~ Another form

10 of emitter is shown in ~ig. 28 where a pigtail 17 of optical fiber transmits the light 11emitted by the semiconductor element to the front surface 19 as an intense spot 12 Examples of available emitters are the Motorola M~OE108F, Laser Diode Labs 13 IRE161 or Spectronics "sweetspot" LED.
14The ~roove of the channel 26 provides precise placement o~ the optical 15fiber 32 and permits its termination to be located and butted as close to the 16 surf~ce 30 as possible for consistent alignment. The fiber 32 is typically cemented 17 within the channel 26 and such cement may include an epoxy filling the region18 between the termination of the ~lber 32 and reflecting face 30 with an index or 19 refraction matching that of the body 12 to prevent refraction of the radiation 2D passing through the beam splitting reflecting surface 30.
21ln coating the reflecting surface 30 with a dielec~ic layer in order to 22 provide, typically, a 50/50 beam splitting function, it is noted that the coating need 23 I not be confined to the surface 30 alone but m~y be more generally applied as is 24 ! more convenient in eoating technologies without impairing the ~ction of the25 ¦ coupler 26 ¦ The emitter cavity 20 terminates in a bottom surface 34 which has a pair OI
27 ~ raised ledges 36 that locate the inserted emitter just above the bo~tom surface 34 !i . , C~ ~ lZ607'~2 to pre~ent contact ther ewith and to facilitate, where desired, totQl internal I reflection of radiation p~ssing from ~lber 32 through face 30 into the body 123 1I towards detector cavity 16, as will be explained more fu~ly below. A large srea 4 detector is preferably inst~lled in the cavity 16 to respond to the radiation passing through the face 30 and passing into cavity 16 directly or by total internal 6 reflection frorn the bottom surface 34.
7 ! ~ig lB illustrates in cross secdon further features of an slternative coupler 8 according to She invenffon. In Fig. lB, a block 40 of molded plastic comprises the 9 coupler body. Body 40 has an aperture 42, typically for a small srea detector, to receive light from an optical fiber located within a Y~roove 44 after passing

11 through a reflectir~ face 46 at the ~ner termination of the V groove 44. An .

12 emitter cavity 48 terminates just above the reflecting face 46 with a set of wedges

13 50 used to position the emitter assembly above a bottom face 52 of the emitter

14 cavity 48. In the case where an index matching ~luid is applied within the V~roove 44, light emitting from the optical fiber in the groove 44 will transit the reflecting 16 face 46 in the a~al direction, without refraction. A ~ocusing sur~ce 54, 17 interfacing with cavity 42 in the light path, directs the transmit~ed light toward a 18 detector 56. The deteetor 56 includes a lens 58 which images the light transmitted 19 through the face 46 onto a light sensitive element.
The body 40 of Fig ~ includes a deflecting wedge 55 whi~h is positioned in 21 the path of radiation from ~n emitter 49 located in cavity 48 that transits the 22 reflecting f~ce 46; the other portion being reflected along the axis of the ch~NIel 23 ¦l 44 into the fiber. The ~nterface between wedge SS and the surrounding aJr will 24 1~ transmit most of the emitter light striking it from the ~ace 46. A portion is I reflected and the shape of wedge 55 is provided to d~ect away from the ~etector 26 cavity 42 the frenel réflected portion of the radiation-so as to avoid cross talk. ln 27 ~ ~ ~ddition, e bcdy 4D may be corted with rn abrorbing layer in rr-~ected locationr in ii ' ~~ '.
!l , 1~ 126~)7~ j order to îurther Attenuate the radi~tion reflected by the wedge 55 and thus further 2 ¦ eliminate cross t,~. The wedge 55 may be formed of a suc~ession o~ different3 j angles grAdually steepening and thus approximating a curve. This is an e~ficient 4 ¦ molding technique. The sh~pe is adapted to the dimensions of the particular body 5 1 ~0.
6 Pigs. 3A and 3B illustrate the effeet on light of the presence and absence of 7 index of refraction matching m~terial, such as an epoxy. In Fig. 3A, an optical 8 fiber 62 is provided in a channel 64 in which the region between a reflecting f,~ce 9 66 and a termination 68 of the fiber 62 is not filled with an index of refraction matching materi,~l such as an epoxy. In that case, light passing through the surfa,ce 11 66 will be refracted downwardly off the axis of the fiber 62 toward a ~etector, not 12 shown. As illustrated in Fig. 3A, the emitter is shown to include an optic~ fiber 13 pigtail 78 surrounded by a ferule 80 providing an interference fit within a cavity 14 76. The pigtail 78 trar~smits the radiation from the emitting semicondu~or material down to a locati~n proximate to the reflecting surface 66. A de~lecting16 wedge is provided directly below the surface 66 to reflect away from the detector 17 radiation from the emitter as noted ~bove. Ln Fig. 3B, the presence of Rn index 18 matching material produces a direct transmission of the light through face 66 19 without refraction. An upper diverging side to the beam will strike the bottom 80 of emitter cavity ~6 and be totally internally reflected back to the detector, 21It may be desirable to position the emitter away ~rom its location proxim~te 22to the reflecffng ~ace at the termin~tion of the optical fiber hannel in order to 23il permit closer locaUon of tl~e detestor assembly and detector cavity snd to aYoid 24¦~ the presence of any interfaces or borders of the ~oupler body in the path of 25j I radiation coupled from the optical ~lber through the reflecting face in~ the 261 detector. Bmbodiments which accompLish this objective are illustrated in Figs. 4 271 ~nd S. In Fig. 4, a coupler body 100 h~s an emitter cavity 102 terminat ng at a il i 1 !
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distance above a reflechng face 104 located at .the terminaffon of a ~' groove 2 channel 106 adapted to hold and align an optical fiber 108 in botll lateral position 3 ¦ and axially as close to the s~face 104 as possible~ The emitter cavity 102 is 4 ! illustrated to have an emitter 110 posit;oned therein with a terminal cap 112 used to define the depth of penetration of the assembly of the emitter 110. The cap 112 6 has a window 114 through which radiation is imaged by a sphericsl lens 116 from n 7 ligh~ emitting semiconductor element 118. Light is imaged by the lens 116 through 8 1 the window 114 for reflecffon, or be~m splitting, by the re~lecting face 104 and is 9 ~applied to the fiber 108. Typically the radiation imaged by the lens 116 will have its waist located at or near the location of the termination of the ~iber 108.
11 The lens 116 is employed to gather as much radiation as possible from the .
12 emitting element 118 for transmission over the distance between it and the 13 reflecting suface 104 and into the termination of the ~lber 108. By loc~ting the 14 emitter assembly 110 we~l above the reflecting face 104, a dete~tor cavity 120 cQn be placed to receive radiation tra~ismitted through the reflecting face 104 without ~6 interference from adjacent faces of the coupler body 100 such as the emitter 17 cavity. A detector assembly 122 is typically placed ~ithin the cavity 120, and 18 positioned by a hermetic sealing cap 124. In the case o$ a small area detector, as 19 illustrated, a spherical lens 126 is preferably utilized for ~o¢using the sligh~
2D diverging beam transmitted by the beam splitting face 104 onto a detec'cing : 21 - element 128. If further lensing is r~ired, the plasff~ body may have a lens 22 molded as shown in Pig. lB. This lens need not be pre~se as even smsll detector 2~ areas are typically larger than emitter ~reas.
24 1The embodiment of Fig. 4 is illustrated with a further modification ~n Fig. 5 in which the right angle between the emitted beam and the optical fiber is redueed 28 to an acute angle by positioning, within a coupler body 130, a cavity 132 to the 27 right of the cavity i~lustrated in Fig. S. ln this case, radiation proYided from an _ g _ ~ lZtiV'~

I j emitter assembly 134 is reflected at an acute angle by a beam splitting face 136 at 2 ¦¦ the termination of a ~-shaped channel 138 which ho~ds an optical fiber 140. To 3 1i accommodate this acute angle of re~lection, the beam splitting face 136 is angled 4 !1 at approximately 60-70with respect to the axis of the optical ~iber 140.!¦ Typically an epoxy fillet 142 is provided around the termination of the fiber 6 ¦i 140 and beam splitting face 136 to match the index of refracffon of the body 130 7 ¦1. for light applied to a detector assembly 144 within ~ detector cavity 146, and to 8 ,. provide a surface 148, substantially perpendicular to the axis of a beam 150 from g I the emitter assembly 134. The 60-70 angle of the beam 5plitting face 136 is OI
lQ ¦~ advanta~e in avoiding polarization selectivity in the por~ions of radiation reflected 11 ¦ and transmitted by the face 136. This is of importance when the dielectric coating .
12 ¦ is to be a dichroic filter for use in multiple~ung. Polsrization effects may preven~
13 I effective wavelength separation in a 45 cemented ~ilter. The geometry of Fig. S
14 ~ also removes the emitter assembly region from the regi~ of the detef~tor assembly

15 I to provide flexibility in the location of the deteetor assembly to accommodate

16 ~ large or small area detectors as desired. It is to be noted that an epoxy fillet may i7 be utilized also at the termination of the opti~al fiber 108 illustrated in :Fig. 4.
18 To function as a frequency multiplea~er or demultiplexer, the surfa~es 30, 19 46, 66, 104, and 136 ~re coated ~ith layers ~at produce reflecffon in one ~equency band and ~ansmission in anothe~, thereby eermitting two emitters or 21 . two detectors to couple to the fiber at distinct frequen~ies.
22 ¦ The above description encompasses a bi~irectional fiber optic coupler for 23 i use in two directional communication over an oEstical ~lbe~r. The exemplaI y 24 i embodiments described above are pro~ided to illustrate the invention, the scope of 25 ~ which is 'd rited solely ir accordance ~ith the fcllowing ~ im Ii ~.
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Claims (18)

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

1. A bi-directional coupler for an optical fiber termination, comprising:
a monolithic coupler body of a light transmissive substance of substantially homogeneous index of refraction having an outer surface;
said monolithic coupler having a first integral cavity in communication with said outer surface for receiving a first semiconductor optical transducer along a first axis;
said monolithic coupler body having a second integral cavity in communication with said outer surface for receiving a second semiconductor optical transducer along a second axis;
said monolithic coupler body having an integral internal and elongated optical fiber receiving channel along a third axis which is at a substantially inclined angle to said second axis;
said optical fiber receiving channel terminating on its internal end in an optical surface integrally formed within said monolithic coupler body operative to provide a first light path wholly within said coupler body defined between the first axis of said first integral cavity and the third axis of said optical fiber receiving channel and operative to provide a second light path defined between the second axis of said second integral cavity and the third axis of said optical fiber receiving channel by reflection from said optical surface; and said second integral cavity terminating at said optical fiber receiving channel adjacent to said optical surface.

2. A bi-directional coupler for an optical fiber termination, comprising:
a monolithic coupler body of a light transmissive substance of substantially homogeneous index of refraction having an outer surface;
said monolithic coupler body having a first integral cavity in communication with said outer surface for receiving a light detector along a first axis;
said monolithic coupler body having a second integral cavity in communication with said outer surface for receiving a light emitter along a second axis;
said monolithic coupler body having an integral internal and elongated optical fiber receiving channel along a third axis which is at a substantially inclined angle to said second axis;
said optical fiber receiving channel terminating on its internal end in an optical surface integrally formed within said monolithic coupler body operative to provide a first light path wholly within said coupler body defined between the first axis of said first integral cavity and the third axis of said optical fiber receiving channel and operative to provide a second light path defined between the second axis of said second emitter cavity and the third axis of said optical fiber receiving channel by reflection from said optical surface; and said integral cavity terminating at said optical fiber receiving channel adjacent to said optical surface.

3. The coupler of claim 2, wherein said optical surface is coated to function as a beam splitter for radiation directed toward said optical surface from said second cavity to reflect a portion thereof along said channel and for radiation from said channel to reflect a portion thereof towards said emitter cavity.

4. The coupler of claim 2, further including an emitter assembly for applying light to said optical surface to be reflected towards said channel.

5. The coupler of claims 4 or 3, wherein said channel is V-shaped.

6. The coupler of claim 5, wherein said optical surface forms an angle of 60-70° to said channel and the light path from said emitter cavity to said channel includes an acute angle.

7. The coupler of claims 2 or 4 further including an optical fiber within said channel having a light receiving and emitting termination proximate to said optical surface.

8. The coupler of claim 7 further including index matching epoxy for cementing said optical fiber within said channel and filling the space between the fiber termination and said optical surface.

9. The coupler of claim 2 further including:
a ferruled emitter assembly having a light beam and centered within said emitter cavity;
said optical surface is located to receive said light beam.

10. The coupler of claim 4 wherein said body further includes means for directing away from said detector cavity any light from said emitter passing through said optical surface.

11. The coupler of claim 2, including a lens integral with said body and responsive to radiation passing through said optical surface for focusing the radiation toward said detector cavity.

12. The coupler of claim 2 further including:
an optical fiber within said channel;
index of refraction matching material within said channel between a termination of said fiber and said optical surface.

13. The coupler of claim 2 further including:
an optical fiber within said channel having a termination proximate to said optical surface;
an anti-reflective coating on said optical fiber termination.

14. The coupler of claim 2, further including index of refraction matching material within said detector cavity.

15. The coupler of claim 2, wherein said optical surface includes a frequency dependent dielectric coating adapting said coupler for multiple frequency operation as a multiplexer or demultiplexer.

16. The coupler of claim 2 wherein said body comprises a transparent molded plastic.

17. The coupler of claim 16 wherein said plastic includes an acrylic or polycarbonate.

18. The coupler of claim 2 further including a ledge within said emitter cavity adapted to prevent an emitter assembly positioned within said cavity from contacting said optical surface.