Optical signal coupling apparatus

OPTICAL SIGNAL COUPLING APPARATUS

BACKGROUND OF THE INVENTION

As optical signal and optoelectronic communications 5 have come into prominence in recent years, numerous optical couplers have been devised to couple one fiber-optic cable to another. Generally these couplers have been directed to coupling two stationary cables together. However, more recently, the need to couple moving fiber-optic 10 cables together has come into being. In response to this need, devices such as the Fiber Optic Rotating Joint (FORJ) have been developed. In a FORJ coupler, an optical fiber such as that used in communications, is coupled through a rotating joint to another optical fiber. In this manner, one of the 15 optical fibers can move or pivot while the other remains stationary. One illustrative use of such a FORJ coupler is on military tanks wherein a FORJ coupler is mounted on the base of the rotating turret to maintain communications with the body of the tank. An example of one FORJ coupler is the 2o "Off-Axis FORJ" which is available from Litton PolyScientific.

One problem encountered in FORJ coupler design is significant signal loss as the signal is transmitted through the FORJ device. Another problem encountered in FORJ 25 devices is undesired wear and friction within the device as portions of the device rotate. Moreover FORJ couplers such as the one referenced above require digital data and need active electronics at the input and output of the coupler. Insertion loss of this FORJ coupler can be quite high, for 30 example approximately 20 dB. The amplitude of the output signal can also vary significantly when this FORJ coupler is rotated.

Accordingly, one object of the present invention is to provide a fiberoptic rotating joint optical coupler which reduces signal loss as an optical signal is transmitted through the coupler. 40

Another object of the present invention is to provide a fiberoptic rotating joint optical coupler which reduces friction within the optical coupler.

In accordance with one embodiment of the present invention, an optical coupler is provided including a transmitter 45 optical fiber member having a generally curved upper surface and a substantially flat lower surface running lengthwise along the transmitter optical fiber member. The transmitter optical fiber member also includes an input end. A first cladding layer is situated on the upper surface of the 50 transmitter optical fiber member. The optical coupler further includes a receiver optical fiber member having a generally curved lower surface and a substantially flat upper surface running lengthwise along the receiver optical fiber member, the receiver optical fiber member including an output end. A 55 second cladding layer is situated on the lower surface of the receiver optical fiber member, the flat surface of the receiver optical fiber member being oriented to face the flat surface of the transmitter optical fiber member in spaced apart relationship thereto. An index matching member is situated 60 between the flat surface of the receiver optical fiber member and the flat surface of the transmitter optical fiber member. The index matching member couples light from the transmitter optical fiber member to the receiver optical fiber member while permitting the transmitter optical fiber mem- 65 ber to move with respect to the receiver optical fiber member. In this manner, incident light which is supplied to

the input end of the transmitter optical fiber member exits the flat surface of the transmitter optical fiber member, passes through the index matching member, enters the flat surface of the receiver optical fiber member and exits the output end of the receiver optical fiber member.

Another embodiment of the optical coupler includes a first annular member and a second annular member which is rotatably mounted to the first annular member. The coupler includes a transmitter optical fiber member which is coupled to the first annular member. The transmitter optical fiber member includes a first cladding layer for containing an optical signal provided thereto and a substantially flat lower surface through which the optical signal passes. The optical coupler further includes a receiver optical fiber member which is coupled to the second annular member. The receiver optical fiber member includes a second cladding layer for containing an optical signal provided thereto and a substantially flat upper surface through which the optical signal passes. An index matching member is situated between and in slidable contact with the lower surface of the transmitter optical fiber member and the upper surface of the receiver optical fiber member. This slidable index member couples the optical signal between the transmitter optical fiber member and the receiver optical fiber member while matching the indices of refraction of the transmitter and receiver optical fiber members.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following description and accompanying drawings.

FIG. 1A is a simplified perspective representation of one embodiment of the optical coupler of the invention.

FIG. IB is a cross section of an optical fiber cable employed in fabricating the optical coupler of FIG. 1A.

FIG. 1C is a cross section of the optical coupler of FIG. 1A taken along section line 1C—1C.

FIG. ID is a cross section of the optical coupler of FIG. 1C taken along section line ID—ID.

FIG. 2A is a more detailed perspective view of the optical coupler of FIG. 1A.

FIG. 2B is a cross section of the optical coupler of FIG. 2A taken along section line 2B—2B.

FIG. 2C is a close up view of the portion of optical coupler of FIG. 2B in which a transmitter and receiver portion are situated.

FIG. 3A is a simplified cross section of another optical coupler which is similar to the optical coupler of FIG. ID except for a different index matching member.

FIG. 3B is a cross section of an optical fiber cable employed in fabricating the optical coupler of FIG. 3A.

FIG. 3C is a cross section of the optical coupler of FIG. 3A taken along section line 3C—3C.

FIG. 4A is a more detailed perspective view of the optical coupler of FIG. 3A.

FIG. 4B is a cross section of the optical coupler of FIG. 4A taken along section line 4B—4B.

FIG. 4C is a close up view of the portion of optical coupler of FIG. 4B in which a transmitter and receiver portion are situated.

FIG. 4D is a cross section of the portion of optical coupler of FIG,. 4C taken along section line 4D—4D.

20

FIG. 5A is a perspective view of another embodiment of the optical coupler of the present invention.

FIG. 5B is a cross section of the optical coupler of FIG. 5A taken along section line 5B—5B.

FIG. 5C is a close up view of the portion of optical 5 coupler of FIG. 5B in which a transmitter and receiver portion are situated.

FIG. 5D is a cross section of the portion of optical coupler of FIG. 5C taken along section line 5D—5D. 10

FIG. 6A is a close up cross sectional view of the portion of another embodiment of the optical coupler which houses a transmitter and receiver portion.

FIG. 6B is a cross section of the optical coupler of FIG. 6A taken along section line 6B—6B. 15

FIG. 7A is a close up cross sectional view of the portion of another embodiment of the optical coupler which houses a transmitter and receiver portion.

FIG. 7B is a cross section of the optical coupler of FIG. 7A taken along section line 7B—7B.

FIG. 8A is a perspective view of another embodiment of the optical coupler of the invention which is adapted for linear translation motion.

FIG. 8B is a cross section of the optical coupler of FIG. 25 8A taken along section line 8B—8B.

DETAILED DESCRIPTION OF THE
INVENTION

30

FIG. 1A shows a simplified representation of one embodiment of the optical coupler of the present invention as optical coupler 10. Coupler 10 is adapted for rotational motion and includes a transmitter optical fiber member 15 and a receiver optical fiber member 20. Transmitter and 3J receiver optical fiber members 15 and 20 respectively include input 15A and output 20A, respectively. Transmitter optical fiber member 15 is situated in spaced apart relationship with respect to receiver optical fiber member 20, the distance between members 15 and 20 being designated, S. It 4Q is noted that FIG. lAis not drawn to scale. In actual practice, members 15 and 20 are significantly closer together than shown in FIG. 1A. Also, in actual practice, an index matching fluid 22 is situated between, and in slidable contact with, members 15 and 20 within space S as described later. 45

As seen in FIG. 1A, a portion of transmitter optical fiber member 15 is wound to close on itself and forms a circular or ring-like transmitter portion 25. Transmitter portion 25 thus exhibits a generally annular geometry. Transmitter portion 25 closes on itself at end 15B. Similarly, a portion of 50 receiver optical fiber member 20 is wound to close on itself and form a circular or ring-like receiver portion 30. Receiver portion 30 closes on itself at end 20B. Transmitter portion 25 and receiver portion 30 are vertically aligned. Transmitter portion 25 is rotatably mounted with respect to receiver 55 portion 30 so as to permit rotation of transmitter portion 25 while receiver portion 30 remains stationary or vice versa.

FIG. IB is a cross section of a fiberoptic cable which may be used to fabricate transmitter optical fiber member 15 and receiver optical fiber member 20. The fiberoptic cable 60 includes a cladding layer 40 which coaxially surrounds an inner fiberoptic core 45. The fiberoptic cable of FIG. IB exhibits a radius of curvature, R, which is within the range of approximately 0.05 mm to approximately 0.5 mm in one embodiment. What is important in selecting the particular 65 radius of curvature of the fiberoptic cable is that the particular radius selected be sufficiently large to permit accurate

surface machining as discussed below. For example, a radius of approximately 0.5 mm or larger is found to be satisfactory. One type of fiber optical cable which may be employed as transmitter optical fiber member 15 and a receiver optical fiber member 20 is 1000 um plastic core fiber optic cable available from Hewlett Packard.

To form the transmitter portion 25 of transmitter optical fiber member 15, the cable of FIG. IB is cut along the dashed line 50. The lowermost portion of the cut cable thus formed is designated as discard portion 32 which is discarded.

FIG. 1C shows a cross section of the periphery of optical coupler 10 of FIG. 1A, namely a slice at a plane through the circumference of transmitter portion 25, receiver portion 30 and matching fluid 22 therebetween. As seen in the assembled optical coupler 10 of FIG. 1C, cut line 50 forms a cut surface 25A which is machined to be substantially smooth.

The receiver portion 30 of receiver optical fiber member 20 is fabricated in substantially the same manner as transmitter portion 25, except that a member 20 with a receiver portion 30 having a cut, machined smooth surface 30A results as shown in FIG. 1C. Machined smooth surfaces 25A and 30A are substantially flat and smooth in this embodiment.

An index matching member or fluid 22 is situated between transmitter portion 25 and receiver portion 30. The function of index matching member 22 will be discussed in more detail later. Many index matching fluids can be used as index matching member 22 according to the particular application providing the index of refraction of the fluid is acceptable. For example, one index matching fluid which may be employed as fluid 22 is Catalog #18094 oil available from Cargille Laboratories Inc., Cedar Grove N.J. 07009. This particular index matching fluid exhibits an index of refraction of 1.6.

FIG. ID is a cross section of optical coupler 10 of FIG. 1C taken along section line ID—ID. Incident light is provided at arrow 55 to fiberoptic core 45 of transmitter Fortion 25. The remaining cladding layer 40 atop transmitter portion 25 assures that light does not vertically escape upward out of transmitter portion 25. However, since the cladding has been removed from the bottom of transmitter portion 25 at surface 25A, light controllably escapes downward through index matching fluid 22 and enters receiver portion 30. The reflection losses which are observed in this optical signal transfer from the transmitter portion to the receiver portion are desirably very low due to index matching member or fluid 22 which couples light from the transmitter portion and the receiver portion. Index matching member 22 is alternatively a transparent solid material or a liquid.

In more detail, when the optical signal enters receiver portion 30, it reflects off the lower cladding layer 40 of receiver portion 30 and propagates back toward the transmitter portion 25. The optical signal bounces between transmitter portion 25 and receiver portion 30 at an angle as shown in FIG. ID. However, the average light propagation is along the length of the fiber core 45 of the transmitter portion and the receiver portion. The signal is thus contained by the fiber cores 45 of transmitter portion 25 and receiver portion 30. A coupling loss of approximately 50% (-3 dB) can be expected from this coupling mechanism. As shown in FIG. ID, the optical signal is equally contained between the two fiber cores 45 of the transmitter and receiver portions as indicated by the light propagation arrows 60 and 62. However, the light output is derived only from the receiver °<0--*-)0~£-)<'

Relationship 2

(kL<R

From FIG. 1C it is seen that in this particular embodiment: Relationship 3

2d+S=L

Inserting this relationship 3 into the stability criterion

20

portion 30, as indicated by output arrow 62. The signal propagating out the end of transmitter portion 25 is lost as indicated by arrow 60.

It is noted that the above described coupling mechanism can be implemented in a linear coupling device wherein the 5 transmitter portion and receiver portion are linearly vertically aligned as well as the rotational coupling approach described above. In either case, the optical signal is contained by the transmitter portion on top and the receiver portion on the bottom. Minimum signal loss through the 10 coupler is achieved by using the index matching member 22, for example an index matching fluid or transparent solid, between transmitter portion 25 and receiver portion 30.

In a coupler wherein the optical fiber core 45 of transmitter portion 25 exhibits an index of refraction of approxi- 15 mately 1.6 and optical fiber core 45 of receiver portion 30 exhibits an index of refraction of approximately 1.6, to match transmitter portion 25 to receiver portion 30, matching member 22 is selected to have the same index of refraction of approximately 1.6. The indices of refraction of optical fiber core 45 of transmitter portion 25, optical fibre core 45 of receiver portion 30 and index matching member 22 are selected to be approximately equal. Index matching member 22 effectively eliminates the discontinuity between the fiber core 45 and the atmosphere (air) such that reflection losses are reduced to a minimum. In other words, the index 25 matching fluid or oil eliminates the plastic-air interfaces associated with the fiber of the transmitter portion and the fiber of the receiver portion so as to allow light to travel freely between the two fibers with low loss.

It is noted that the coupling mechanism of optical coupler 30 10 uses the fiberoptic core-cladding interface of transmitter portion 25 and the fiberoptic core-cladding interface of receiver portion 30 as mirrors for containing the optical signal transmitted therethrough. Fiberoptic cores 45 can be either stepped index or graded index and still exhibit the desired mirror effect. These curved mirrors are effectively located at the outer surfaces of the fiberoptic cores 45, namely the surfaces thereof which face cladding layers 40.

As seen in FIG. 1C, the distance between the outer surface of the fiberoptic core 45 of transmitter portion 25 and the outer surface of fiberoptic core 45 of receiver portion 30 is designated, L, namely the distance between the "mirrors". It is noted that L is measured through the lateral centers of the respective cores 45. The stability criterion for this curved 4J mirror structure is given by the relationship:

Relationship 1

expressed in relationship 1 yields:

35

40

50

wherein R=Rj=R2= the radius of curvature of the optical fiber. Rx and R2 are the radii of curvature of fiber cores 45 of transmitter portion 25 and receiver portion 30.

It is noted that maximum containment of the optical signal 55 between the effective mirrors occurs when the following condition is true:

60

65

Relationship 4

0<2d+S<R

Thus, R, the radius of curvature of fiberoptic cores 45 of transmitter portion 25 and receiver portion 30 is equal to 500 um. for a 1 mm diameter fiber. When these values are substituted in relationship 3, it is found that L<0.5 mm for this particular embodiment.

FIG. 2A depicts a more detailed perspective representation of optical coupler 10. The structure of coupler 10 is more readily appreciated from the cross section shown in FIG. 2B. FIG. 2B is a cross section of optical coupler 10 of FIG. 2A taken along section line 2B—2B. Coupler 10 includes an inner cylindrical member 65 and an outer cylindrical member 70 as shown. Inner cylindrical member 65 is substantially L-shaped in cross section. More particularly, inner cylindrical member 65 appears as an inverted L in the cross section in FIG. 2B. An upper annular ball bearing assembly 75 and a lower annular ball bearing assembly 80 are situated between inner cylindrical member 65 and outer cylindrical member 70. Ball bearing assemblies 75 and 80 permit inner cylindrical member 65 and outer cylindrical member 70 to rotate substantially freely with respect to each other.

FIG. 2C is a close-up view of the portion of coupler 10 of FIG. 2B which houses transmitter portion 25 and receiver portion 30. Most of transmitter portion 25 is situated in an annular groove 82 which is semi-circularly recessed in lower wall 84 of L leg 86 of inner cylindrical member 65. As seen in FIG. 2C, a portion of transmitter portion 25 extends downwardly into the horizontal portion of the channel 88 formed between lower wall 84 of inner cylindrical member 65 and upper wall 90 of outer cylindrical member 70. Receiver portion 30 is situated in an annular groove 92 which is semi-circularly recessed in upper wall 90 of outer cylindrical member 70.

The space, S, between transmitter portion 25 and receiver portion 30 is filled with index matching fluid 22 as described earlier. To hold index matching fluid 22 between transmitter portion 25 and receiver portion 30, an inner O-ring 93 and an outer O-ring 94 bound fluid 22 as shown. Fluid 22 is thus prevented from escaping from the space, S, between transmitter portion 25 and receiver portion 30. O-ring 93 and O-ring 94 are situated in respective annular grooves 95 and 96 in outer cylindrical member 70.

FIG. 3A shows another embodiment of the invention as optical coupler 100. Optical coupler 100 includes many elements which are similar to those of optical coupler 10 of FIG. 1A-1B. One distinction of optical coupler 100 of FIG. 3A is that optical coupler 100 includes a pliable or deformable index matching member 105 situated between transmitter optical fiber member 115 and a receiver optical fiber member 120. Index matching member 105 is fabricated from optical material, namely transparent material. Transmitter optical fiber member 115 and receiver optical fiber member 120 are similar to transmitter optical fiber member 15 and receiver optical fiber member 20 of FIG.'s 1A-1D except that fiber members 115 and 120 are formed as shown in FIG. 3B as discussed subsequently.

More particularly, fiber members 115 and 120 include transmitter and receiver portions 125 and 130, respectively, which are formed as indicated in FIG. 3B. To form transmitter portion 125 of transmitter optical fiber member 115, the portion of optical fiber member 115 which is to be used 8

to form transmitter portion 125 is cut along center cut line 150 as shown in FIG. 3B. The lower portion 132 of the optical fiber of FIG. 3B is discarded thus leaving transmitter portion 125 remaining. Transmitter portion 125 includes cladding layer 140 and inner fiberoptic core 145. The cut 5 surface 125A thus formed at cut line 150 is machined and polished until surface 125A is substantially smooth.

FIG. 3C is a cross section of the assembled optical coupler 100 of FIG. 3A taken along a section line 3C—3C which passes through pliable matching member 105. Pliable 10 matching member 105 performs the same index of refraction matching function as index matching fluid 22 of optical coupler 10 of FIG. 1A. However, in optical coupler 100 all of the index matching material is confined to pliable or deformable matching member 105. In this particular 15 embodiment, index matching member 105 spans less than the entire circumference of transmitter portion 125 and receiver portion 130.

In optical coupler 100, the receiver portion 130 of receiver optical fiber member 120 is formed in substantially the same 20 manner as transmitter portion 125 of transmitter optical member 115 except the cut surface of the resultant receiver portion 130 is designated as surface 130A. Cut surface 130A is machined to be substantially smooth.

Pliable index matching member 105 is held between 25 transmitter portion 125 and receiver portion 130. Matching member 105 is permitted to travel freely as transmitter portion 125 rotates while receiver portion 130 remains stationary, and vice versa. Pliable index matching member 105 is made of a transparent material such as RTV silicon 30 rubber available from Dow Corning. Index matching member 105 is in physical and optical contact with transmitter portion 125 and receiver portion 130, specifically surfaces 125A and 130A thereof, respectively. Pliable index matching member 105 mates with, and slides with respect to, smooth 35 surfaces 125A and 130A.

Returning now to FIG. 3A, it is seen that optical coupler 100 operates by incident light or the input optical signal being provided to transmitter portion 125 in a direction generally indicated by arrow 155. The input optical signal 40 travels along transmitter portion 125 while being contained therein by total internal reflection due to the cladding layer 140 on the upper surface of transmitter portion 125 and the air interface formed at smooth surface 125A which is the lower surface of transmitter portion 125. This containment is 45 disrupted by deformable index matching member 105. When the input optical signal encounters matching member 105, the optical signal is free to propagate into the receiver portion 130 therebelow. After passing through matching member 105, the optical signal enters receiver portion 130 50 and is contained therein by the same mechanism by which the optical signal was contained in transmitter portion 125. Deformable index matching member 105 is free to travel annularly between transmitter portion 125 and receiver portion 130 as transmitter portion 125 and receiver portion 55 130 rotate with respect to each other. However, propagation of the optical signal between transmitter portion 125 and receiver portion 130 is confined to the location where pliable index matching member 105 is presently located.

It is noted that in one embodiment of the invention 60 wherein the index of refraction of the core of transmitter optical fiber member 115 is approximately 1.6 and the index of refraction of the core of receiver optical fiber member 120 is approximately 1.6, that the index of refraction of pliable index matching member 105 is also approximately 1.6. For 65 best results, the indices of refraction of the core of transmitter optical fiber member 115, the core of receiver optical

fiber member 120 and pliable index matching member 105 should match each other as closely as possible.

FIG. 4A depicts a more detailed perspective representation of optical coupler 10. The structure of coupler 100 is more readily appreciated from the cross section shown in FIG. 4B. FIG. 4B is a cross section of optical coupler 100 of FIG. 4 A taken along section line 4B—4B. Coupler 100 includes an inner cylindrical member 165 and an outer cylindrical member 170 which are similar to inner cylindrical member 65 and outer cylindrical member 70 of FIG. 2B except for the subsequently discussed differences.

Inner cylindrical member 165 is substantially L-shaped in cross section. More particularly, inner cylindrical member 165 appears as an inverted L in the cross section in FIG. 4B. An upper annular ball bearing assembly 175 and a lower annular ball bearing assembly 180 are situated between inner cylindrical member 165 and an outer cylindrical member 170. Ball bearing assemblies 175 and 180 permit inner cylindrical member 165 and outer cylindrical member 170 to rotate substantially freely with respect to each other.

FIG. 4C is a close-up view of the portion of coupler 100 of FIG. 4B which houses transmitter portion 125 and receiver portion 130. Transmitter portion 125 and receiver portion 130 are positioned to face each other in spaced apart relationship in a channel 182 between L leg 184 and outer cylindrical member 170. More particularly, transmitter portion 125 rests in an annular groove 186 in lower wall 188 of L leg 184. Similarly, receiver portion 130 rests in channel 182 and in an annular groove 190 in the uppermost portion of outer cylindrical member 170.

Index matching member 105 is situated between transmitter portion 125 and receiver portion 130. Index matching member 105 exhibits a cylindrical shape such that it forms a roller which can rotate as transmitter portion 125 rotates by it on the top and as receiver portion 130 rotates by it on the bottom. More particularly, index matching member 105 is mounted in a bearing assembly 192 as shown in FIG. 4D to permit index matching member 105 to rotate in the above described roller-like fashion. Index matching member 105 includes opposed ends 105A and 105B which are situated in respective bearings 194A and 194B. Bearings 194A and 194B are situated in races 196A and 196B, respectively. It is noted that in one embodiment, deformable or pliable index matching member 105 exhibits a lengthwise dimension as seen in FIG. 4C which is substantially less than the circumference of inner cylindrical member 165 and outer cylindrical member 170.

FIG. 5A shows another embodiment of the invention as optical coupler 200. Optical coupler 200 includes many elements which are similar to those of optical coupler 100 of FIG 4A-4B. Like numbers indicate like elements. FIG. 5B is a cross section of coupler 200 of FIG. 5A taken along section line 5B—5B to show ball bearings 175 and 180 which permit inner cylindrical member 165 to rotate with respect to outer cylindrical member 170.

The transmitter portion of optical coupler 200 is designated as transmitter portion 225 and the receiver portion of optical coupler 200 is designated as receiver portion 230 as shown in FIG.5C. FIG. 5C is a close-up view of the portion of coupler 200 which houses transmitter portion 225 and receiver portion 230. The optical input to transmitter portion 225 is represented as input 255 and the optical output of receiver portion 230 is represented as output 262. The channel formed between inner cylindrical member 165 and outer cylindrical member 170 is again designated as channel 182.

FIG. 5D is a cross section of FIG. 5C taken along section line 5D—5D to more clearly show index matching member