POLARIZATION-INSENSITIVE, EVANESCENT-WAVE, FUSED COUPLER WITH MINIMAL ENVIRONMENTAL SENSITIVITY

Abstract: An optical coupler and method of making thereof. The coupling ratio of the coupler is polarization-insensitive. The optical coupled described herein is made from single-mode optical fibers (12, 14). Each optical fiber has a length of nearly exposed core (20, 22) which is fused to the exposed core of the other optical fiber while the fibers are maintained in parallel juxtaposition with one another without twisting (24). By creating a fused core coupler from single-mode optical fibers in which the cores are in parallel juxtaposition with one another, the coupling ratio of the subject invention does not change with changes in polarization of light passing through each single-mode fiber and, thus, is polarization insensitive.

Description: POLARIZA JON-INSE SI IVE, EVANESOSNT-WAVE, FUSED COUPLER WITH MTJffl AL ENVIRONMENTAL SENSITIVITY BACKGROUND OF THE INVENTION A . FIELD OF THE INVENTION The present invention relates generally to op¬ tical waveguide couplers and more specifically to optical couplers using single-mode optical fibers that are re¬ sistant to polarization changes. B. DESCRIPTION OF THE PRIOR ART Fiber optic couplers are commonly used to transfer light from one optical waveguide to another for a variety of functions. For example, fiber optic coup¬ lers are used in the communications field to rapidly con¬ vey large amounts of data. Other applications include measurement systems in which high-precision measurements are made based upon an environmentally dependent shift in either the phase or wavelength of light traveling through an optical waveguide. Early couplers used for measure¬ ment applications had several disadvantages. For in¬ stance, the coupling ratio of early couplers typically varied with the polarization of the light passing through the coupler. Furthermore in some cases, the phase of the light passing through the coupler varied with polariza¬ tion which was unsatisfactory for certain interferometric sensor aplications such as the passive quadrature demodu¬ lator coupler matrix. Much of the technology developed in the past ten years for manufacturing couplers has centered around couplers which use multimode fibers. Multi ode optical fibers are fairly large fibers, having a core diameter on the order of 40 microns to 200 microns. Some multimode fibers may be as large as 4 millimeters. While multimode fibers can be used to produce couplers for transferring polarized light from one fiber to another, they are typi- cally undesirable because polarized light transmitted in a multimode optical fiber has a tendency to become depo¬ larized. Single-mode fibers, on the other hand, are more capable of transmitting polarized light with minimal de¬ polarization of light during transmission over small dis- tances. Due to the method in which single-mode fibers act to transmit only one mode of light, the core diameter of a single-mode fiber is approximately ten times smaller than the core diameter of a multimode fiber. This has led to serious difficulties in manufacturing couplers using single-mode fibers. The invention described here- inbelow represents an advancement in the art in that it teaches methods for coupling single-mode fibers and for producing polarization-insensitive optical couplers. Before describing the applicants* invention however, it is necessary to briefly describe the physical characteristics of a typical optical fiber. In most in¬ stances, a fiber comprises an inner core having a rela¬ tively high index of refraction, a cladding having a relatively low index of refraction, and a substrate hav- ing a relatively high index of refraction, but not neces¬ sarily equal to the index of refraction of the core. This type of fiber is commonly called "W-fiber" by those skilled in the art because the index profile of the fiber appears to resemble a "W". It should be noted that other types of fibers do not have an outer substrate having a relatively high index of refraction. SUMMARY OF THE INVENTION The subject development comprehends a polarization-insensitive optical device for coupling light which includes a first single-mode optical fiber having a first longitudinal segment having an inner core having a relatively high index and a cladding having a relatively low index of refraction. The optical device also includes a second single-mode optical fiber having a second longitudinal segment having an inner core having a relatively high index of refraction and a cladding having a relatively low index of refraction. A coupling region is included in the subject device having the first and second longitudinal portions fused together in parallel juxtaposition to one another. The coupler further in- eludes a housing means for maintaining the first and sec¬ ond longitudinal portions substantially straight and for maintaining the first and second longitudinal portions in a stable environment. The invention further comprehends a method of making polarization-insensitive optical couplers compris¬ ing the steps of exposing first and second longitudinal cladding segments of first and second single-mode optical fibers; maintaining the optical fibers in linear parallel juxtaposition with one another along a portion of the ex- posed longitudinal segments; fusing the parallel juxtapo¬ sition segments of the fibers together to form a coupling region, and rigidly encapsulating the coupling region of the optical fibers while maintaining the linear parallel juxtaposition of the fibers. in view of the preceding, an object of the in¬ vention described herein is to provide a polarization- insensitive coupler which is simple to fabricate using single-mode optical fibers. It is a further object of the invention to pro¬ vide an environmentally stable single-mode optical fiber. It is a further object of the invention to pro¬ vide a single-mode coupler device having controlled coup¬ ling. These objects are given only by way of example; thus, other objectives and advantages inherently achieved by the disclosed invention may occur to those skilled in the art. The scope of the invention is to be limited on¬ ly by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1. is a frontal view of one embodiment of the coupler described herein indicating the coupling re- gion; Fig. 2. is a frontal view of one embodiment of the coupler described herein illustrating a quartz hous¬ ing in which the housing is broken away to provide a view of the coupling region; Fig. 3 is an illustration partially in phantom of one embodiment of the subject coupler after encapsula- tion ? and Fig. 4 is an isometric illustration of one em¬ bodiment of the equipment used to fabricate the couplers described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As discussed above, single-mode fibers are ex¬ tremely fragile because of their minute size. The sub¬ ject development, therefore, has been directed to provid- ing a single-mode coupler which is relatively rugged, and is relatively insensitive, or stable, with respect to changes in polarization. Referring now to Fig. 1, first and second single-mode optical fibers 12, 14 are provided in the preferred embodiment of the subject development. Each optical fiber 12, 14 has a respective substrate 16, 18 and core and cladding 20, 22. The diameter of the core of each single-mode fiber is on the order of 5 mi¬ crons whereas the diameter of the cladding of each fiber is on the order of 75-125 microns. Thus, it should be recognized that when the core 20, 22 of each fiber is exposed, it is in a relatively delicate, or fragile, state. In the preferred embodiment of the subject devel¬ opment, each fiber 12, 14 has a longitudinal segment having a core and cladding diameter 20, 22 of approxi- mately 15-50 microns. It should be obvious to one skilled in the art that the length of the longitudinal segment can be varied greatly without deviating from the intent of the subject invention. In the preferred embodiment, the exposed core length is approximately one centimeter in order to ensure complete coupling between the first optical fiber 12 and the second optical fiber 14. The longitudinal segments 20, 22 are fused together in coupling region 24. The invention described herein comprehends that throughout the coupling region, the first and second fibers 12 and 14 have cores which are maintained in linear parallel juxtaposition with one another. Referring now to Fig. 2, the fused fibers 12, 14 of Fig. 1 are illustrated in a rigid housing 26. The method of placing the fused fibers within the housing will be discussed in greater detail hereinbelow. it is important to note, at this point however, that the hous¬ ing provides a rigid support to fused fibers 12 and 14 throughout coupling region 24. Referring now briefly to Fig. 3, after fibers 12 and 14 have been positioned in housing 26, the entire assembly is encapsulated in an rubber-like material 28. Encapsulation of the assembly provides a further means for stabilizing the subject device. The encapsulation material may provide strain relief to the ends of the fi¬ bers which extend beyond the housing because the housing may be encapsulated by a dipping technique in which the ends of the fibers are also coated with the encapsulation material. The following description represents the cur¬ rently preferred method of making the subject couplers. In the preferred embodiment, two single-mode fibers are prepared. A variety of single-mode fibers may be used, depending on the wavelength of operation of the coupler. Each coupler would be designed to be laser specific, that is, the fiber used for one wavelength of light may be made from a different material, and have a different core diameter and index of refraction than a coupler designed for another wavelength of operation. Important charac¬ teristics of any fiber chosen include: good concentric¬ ity, excellent core uniformity, and an appropriate re- fractive index profile. While various lengths of optical fiber may be used, the present procedure in the labora¬ tory is to use two lengths of optical fiber that are 50 O PI SRNAΥ\0^ centimeters long. Each fiber has a plastic jacket and an RTV (room temperature vulcanizing silicone) coating. The plastic jacket is cut away and any RTV is removed with a hydrofluoric acid rinse. Referring now to Fig. 4, after the fibers have been prepared as discussed above, each fiber 12 and 14 is placed in respective groves 30-33 of clamping devices 36, 38. The fibers are then cleaned with a hydrofluoric acid/water/alcohol rinses according to techniques known to those skilled in the art. After each fiber has been inspected for cleanliness, etching of the fibers is ini¬ tiated. It is the object of the etching portion of the procedure to remove the substrate 16 and 18 of each fiber 12 and 14 (Fig. 1) to expose core and cladding 20 and 22 along a coupling region 24 of each of the fibers so as to provide an appropriate index profile. An appropriate in¬ dex profile is one in which the outer surface, or clad¬ ding, of each fiber has a lower index of refraction than the core of each fiber. Although a variety of etching techniques may be used, in view of the particular fiber used in the preferred embodiment, it is desirable to etch the subject fibers by a heated etching technique. In this technique, the fibers are placed in close proximity to an etching station which is heated by a thermoelectric module. A drop of etchant is placed on top of the etch¬ ing station to etch a longitudinal' portion of the fiber. After the fiber has been etched to the desired diameter, the fibers are then rinsed with water to prevent further etching. The very fragile etched longitudinal segments 20, 22 are then brought in parallel juxtaposition with one another by wrapping opposite ends of each exposed core length to one another with a thread or other wrap- ping mate-rial 42, 44. In the preferred embodiment, it has been found that a thin plastic sheet is desirable to prevent breakage of the fragile cores 20, 22. After the cores have been temporarily wrapped as discussed above to provide the desired parallel juxtaposition special relationship, a small drop of adhesive material is deposited on each end 46, 48 of the exposed core length adjacent to threads 42, 44. The glue is allowed to cure so that the cores 20, 22 remain in parallel juxtaposition with one another throughout coupling region 24 when threads 42 and 44 are removed from the exposed cores 20, 22. The coupling region of the fibers 12, 14 are then heated. In the laboratory technique used in the preferred embodiment, a lighted torch is simply passed along the coupling region 24 while the longitudinal segments are in axial tension until the segments 20, 22 are fused together throughout the length of the coupling region. A quartz tube 26 is then measured and cut to the desired length to surround the exposed cores of the subject coupler. In one embodiment, a slot extending the length of the quartz tube 26 may be provided to allow insertion of the tube about fibers 12, 14. In another embodiment, the tube may be cut in half longitudinally to allow each half to be simply placed about the fibers. The ends of the fibers 12, 14 are then glued to the ends 81, 83 of the quartz tube to suspend the etched and fused segments of the fibers containing the coupling region 24 in the approximate center of the tube 26. The glued assembly is then allowed to cure as necessary. The quartz tube is then dipped in a material to provide an elastic covering over tube 26. OMPI SUBSTITUTE SHEET The invention described herein is insensitive to changes in polarization due to its unique design. In other single-mode, evanescent-wave couplers, birefrin¬ gence resulting from twists, bends, stresses, or strains imposed on the fibers will cause changes in polarity of light transmitted through the fibers. It is common prac¬ tice in coupler manufacture to twist a pair of fibers about one another in order to maintain contact during fusing. By eliminating the need to twist the fibers about one another and by making bends in the fiber as gradual as possible, the polarization dependence of the coupling ratio is eliminated. ' Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration an example only, and is not to be taken by way of limitation; the spirit and scope of this invention being limited only by the terms of the appended claims.

Claims: We claim: 1. A polarization-insensitive optical device for coupling light comprising: a first single-mode optical fiber having a first longitudinal segment having an inner core having a relatively high index of refraction and a cladding having a relatively low index of refraction; a second single-mode optical fiber having a second longitudinal segment having an inner core having a relatively high index of refraction and a cladding having a relatively low index of refraction; and a coupling region formed by straight portions of said first and second longitudinal segments fused together in linear parallel juxtaposition with one another. 2. A polarization-insensitive optical device as recited in Claim 1, further comprising: encapsulation means for rigidly maintaining said first and second longitudinal segments in linear parallel juxtaposition with one another. 3. A polarization-insensitive optical device for coupling light as recited in Claim 1, wherein said first and second single-mode optical fibers have core diameters of approximately 5 micrometers. 4. A polarization-insensitive optical device for coupling light as recited in Claim 1, wherein said coupling region has said exposed cores fused together along first and second lengths of approximately one centimeter. SUBS 5. A polarization-insensitive optical device for coupling light as recited in Claim 1, wherein said encapsulated means includes a thermally stable tube. 6. A polarization-insensitive optical device for coupling light as recited in Claim 5, wherein said thermally stable tube is quartz and has an inner-diameter in the range of at least one millimeter and an outer di- ameter of at least three millimeters. 7. A polarization-insensitive optical device for coupling light as recited in Claim 5, wherein said tube is coated with rubber about the outer diameter of said tube along the length of said tube and encases each end of said tube to suspend said first and second fibers within said tube. 8. A method of making a polarization-insensi¬ tive optical coupler comprising the steps of: etching first and second longitudinal segments of first and second single-mode optical fibers so that each fiber has an outer cladding having a low index of refraction relative to the index or refraction of an in¬ ner core of each of said fibers; maintaining said optical fibers in parallel juxtaposition with one another along a portion of said longitudinal segments; fusing said parallel juxtapositioned portions of said segments together to form a coupling region; and rigidly encapsulating said exposed cores of said optical fibers while maintaining said parallel jux- taposition of said segments. -12- 9. A method of making polarization-insensi¬ tive optical coupler as recited in Claim 8, wherein said first and second lengths of longitudinal segment of said first and second fibers is in the range of 0.5 to 2cm. 10. A method of making a polarization- insensitive optical coupler as recited in Claim 8, wherein said step of exposing said core lengths includes removing an outer jacket of said first and second fibers along the length of said exposed cores. 11. A method of making polarization-insensi¬ tive .optical couplers as recited in Claim 8, wherein said step of exposing said cores includes rinsing said lengths of said fiber cores to be exposed, in hydro- flouric acid to remove any room temperature vulcanizing rubber which may be surrounding the cladding of said first and second fibers. 12. A method of making a polarization- insensitive optical coupler as recited in Claim 8, wherein said step of maintaining said optical fibers in parallel juxtaposition with one another includes aligning said fibers in a holder having alignment groo es. 13. A method of making a polarization- insensitive optical coupler as recited in Claim 8, wherein said step of maintaining said optical fibers in parallel juxtaposition with one another includes wrapping portions of said longitudinal segments with a material which will not substantially deform said cores of said fibers; gluing said cores of said fibers together while said cores are wrapped; OMPI curing said glue; and unwrapping said cores. 14. A method of making' a polarization-insensi¬ tive optical coupler as recited in Claim 8, wherein said step of rigidly encapsulating said longitudinal segments includes the steps of placing a tube having a length sub- stantially the same as the said first and second lengths of said segments over said coupling region; gluing said ends of said tube to said first and second fibers to sus¬ pend said fibers within said tube and curing said glue. 15. Any and all features of novelty described, referred to, exemplified or shown. .. OMPI

Inventors: KOPERA PAUL M, STOWE DAVID W, KOPERA, PAUL, M., STOWE, DAVID, W.
Applicants: GOULD INC, GOULD INC.
Application year: 1984
Main classification: G02B5
IPC codes: G02B6