Optical connector using a polarization-maintaining optical fiber

Abstract: An optical connector comprising polarization-maintaining optical fiber (5) means for maintaining a polarization plane by applying stress in an axial direction perpendicular to the optical axis of said optical fiber means (5), which are inserted into ferrule means (1). The section of the ferrule means (1) perpendicular to said optical axis has an oval hole (2). The major axis (3) of said oval hole (2) coincides with the line connecting the two stress-applying parts (7) the polarization-maintaining optical fiber means (5). The minor axis (4) of said oval hole (2) being approximately equal to the diameter of said polarization-maintaining optical fiber means (5) which is fixed in said hole (2) with an adhesive means (8).

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical connector using a polarization-maintaining optical fiber according to the introduction portion of claim 1 Such an optical connector is generally known for instance by the JP-A-56 161 512 Optical fibers with high internal birefringence have been developed to maintain the polarization plane of the transmitted light over a long distance. In the most common type of polarization-maintaining optical fiber, the birefringence is caused by a stress anisotropy in the core. The stress anisotropy is achieved by doping material to have a larger thermal expansion coefficient than that of the silica substrate. An example of such a polarization=maintaining optical fiber the structure described by Y. Sasaki et al. in "Fabrication of Polarization-Maintaining Optical Fibers with Stress-Induced Birefringence", REVIEW of the Electrical Communication Laboratories, Vol. 32, No. 3, 1984 is well known. The optical fiber described in the paper comprises a core and parts arranged around the core and applying stress to the core in a fixed direction. In the optical fiber, by making one of two orthogonal polarization planes of the light incident on the optical fiber coincident with the direction of the stress on the core (main stress direction), the polarization plane of the light propagating along the core is maintained in the direction of that stress. However, if some external force prevents the maintaining of the polarization plane, the polarization plane will rotate, causing deterioration of the polarization extinction ratio (power ratio of the two orthogonal polarizations). Therefore, utmost attention has to be paid to maintaining the polarization plane. However, as no adequate attention is paid to the preservation of the polarization plane in a conventional optical connector using a polarization-maintaining optical fiber, there is the problem of rotation of the polarization plane, resulting in a substantial deterioration of the polarization extinction ratio. To further elaborate on this point, in a conventional optical connector, when a polarization-maintaining fiber is fixed within a hole of a ferrule made of glass or some other ceramic with adhesive, the fiber is fixed off the center of the hole, resulting in uneven thickness of the adhesive layer. The uneven thickness of the adhesive layer, affecting the fiber as an external stress at the time of hardening of the adhesive, disturbs the direction of the internal stress applied in advance to the fiber. As a consequence, the polarization plane will rotate in response to the external stress, causing a substantial deterioration in the polarization extinction ratio. If a polarization-maintaining fiber having a polarization extinction ratio of over 40 dB, for instance, is used in a conventional optical connector, the polarization extinction ratio will deteriorate to somewhere between 15 and 40 dB, resulting in a fluctuation of the deterioration of the polarization extinction ratio from about 0 to 25 dB. SUMMARY OF THE INVENTION An object of the present invention, therefore, is to provide an optical connector whose polarization plane is free from rotation by the stresses resulting from the hardening of adhesive when a polarization-maintaining optical fiber is fixed with the adhesive to the inside of the ferrule of the optical connector. According with the present invention said object is obtained with the features of the characterizing portion of claim 1. Accordingly the present connector comprises a polarization-maintaining optical fiber for maintaining the polarization plane by applying stress in a prescribed axial direction perpendicular to the optical axis of the optical fiber. The fiber is inserted and fixed with adhesive into a ferrule, whose section perpendicular to the optical axis has an oval hole. The major axis of the oval hole coincides with the direction in which the stress is applied to the fiber, and the minor axis of the same is approximately equal to the diameter of the fiber. Further preferred embodiments of the invention are mentioned in additional subclaims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1A shows a front view of one preferred embodiment of the invention; FIG. 1B is an enlarged section of FIG. 1A along line A-A; and FIG. 2 is a partially cut-out oblique view of the embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1A shows a front view of the embodiment, and FIG. 1B is an enlarged section of FIG. 1A along line A-A. In FIGS. 1A and 1B, an oval hole 2 is opened in the direction of the central axis of a glass-made ferrule 1 for connecting a polarization-maintaining optical fiber 5. The minor axis 4 of the oval hole 2 is set equal to the diameter of the fiber 5 so that adhesive 8 can concentrate in the area along the major axis 3 of the oval hole 2. The fiber 5 comprises a core 6 and a clad having a pair of stress-applying parts 7 arranged in symmetrical positions with respect to the core 6. The stress-applying parts 7 are doped with 15 mol% of B₂O₃ to enlarge the coefficient of thermal expansion, and the core 6 and the clad surrounding it are primarily made of SiO₂, whose coefficient of thermal expansion is smaller than that of the stress-applying parts 7. Therefore, during the production of the optical fiber 5, the thermally expanded stress-applying parts 7 contract when the optical fiber cools off, and pull the surrounding SiO₂. As a result, the core 6 is pulled toward the stress-applying parts 7, and this pulling force turns into the stress applied to the core 6. The fiber 5 is so fixed in the oval hole 2 with the adhesive 8 that the line connecting the two stress-applying parts 7 of the fiber 5 substantially coincides with the major axis 3 of the hole 2. The adhesive 8, of an ultraviolet-hardened type, is hardened by irradiating ultraviolet rays from outside the glass-made ferrule 1. In this embodiment, since the adhesive 8 concentrates in the area along the major axis 3, the direction of stress applied to the core 6 by the hardening and contraction of the adhesive 8 is substantially identical with that of the stress attributable to the stress-applying parts 7. As a result, there is no rotation of the polarization plane by the hardening of the adhesive 8, and a fluctuation of deterioration in the polarization extinction ratio is reduced to no more than 10 dB. Although the Sasaki-proposed type optical fiber is used for maintaining the polarization plane in the aforementioned embodiment, an optical fiber of any other structure may as well be used only if stress is applied to its core in a monoaxial direction perpendicular to the central axis. It also should be noted that adhesive need not be an ultraviolet-hardened type. FIG. 2 is a partially cut-out oblique view of an optical connector having the ferrule illustrated in FIGS. 1A and 1B. To simplify description, the direction of the tip of the ferrule is supposed to be forward, and the reverse direction, backward. In the figure, a fiber cord 11 is inserted in the rear part of a rubber boot 12 and a cylindrical member 13 in the front part thereof. The rear part of the cylindrical member 13 fixes the fiber cord 11, and the front part of same is screwed into a housing 14. The housing 14 is arranged around a coated fiber 11a, resulting from the fiber cord 11 stripped of its external cover, and into its front part are inserted a spring 15 and the rear part of a ferrule 10. The external circumference of the front part of the housing 14 is screwed to the rear part of a frame 16. The ferrule 10 comprises first, second, third and fourth cylindrical parts 10a, 10b, 10c and 10d, each differing in diameter from others. The first cylindrical part 10a has an oval hole, in which is fixed a bare fiber 11b with adhesive. The bare fiber 11b results from the elimination of the silicon coat from the coated fiber 11a, and corresponds to the polarization-maintaining optical fiber of FIG. 1B. Therefore, its section perpendicular to the central axis of the first cylindrical part 10a is the same as what is shown in FIG. 1B. The coated fiber 11a is inserted and fixed within the second through fourth cylindrical parts 10b to 10d. Around the second cylindrical part 10b is arranged a flange 16a of the frame 16, and the flange 16a and the third cylindrical part 10c serve as stoppers to limit the shifting of the ferrule 10 in the direction of its central axis. A location key 18 is fixed partially on the external circumference of the front part of the frame 16. The location key 18 operates to position and prevent the rotation of the frame 16 when the optical connector is connected to an adapter. The key 18 is fixed in alignment with the direction of internal stress to represent the direction of the internal stress attributable to the stress-applying parts of the bare fiber 11b. A coupling nut 17 is arranged on the external circumference of the frame 16, and on its front part is formed a thread for fitting with an optical adaptor. As hitherto described, the present invention makes it possible, when a polarization-maintaining optical fiber is fitted to an optical connector ferrule, to reduce the deterioration of the polarization extinction ratio and further to minimize its fluctuation.

Claims: An optical connector with a polarization-maintaining optical fiber (5, 11) comprising: ferrule means (1, 10) having a hole (2) receiving said optical fiber (5, 11), characterized by said hole (2) having an oval cross-section perpendicular to the optical axis of said fiber (5, 11), said fiber (5, 11) having a core (6) and stress-applying parts (7), said stress-applying parts (7) having a higher coefficient of the thermal expansion than the rest of said fiber (5, 11) and being disposed to apply stress to the core in a monoaxial direction perpendicular to the central axis of the fiber and adhesive means (8) fixing said fiber (5, 11) in said oval hole (2) whereby the major axis (3) of said oval hole (2) concides with the direction in which the stress is applied to said fiber (5, 11) and the minor axis of the same is approximately equal to the diameter of said fiber (5, 11). An optical connector as set forth in claim 1, wherein two longitudinal stress-applying parts (7) extend axially within said fiber (5, 11) symmetrically on opposite sides of said core; and wherein the major axis of said oval hole (2) is colinear with a line connecting said internal stress-applying parts (7). An optical connector as set forth in claim 1, wherein said fiber (5,11) and said core (6) are primarily made of SiO₂, whose coefficient of thermal expansion is smaller than that of said stress-applying parts (7) which are doped with B₂O₃ to enlarge the coefficient of thermal expansion. An optical connector as set forth in claim 1, wherein the direction of stress applied to the core (6) by the hardening and contraction of said adhesive (8) is substantially identical with that of the stress attributable to the stress-applying parts (7). An optical connector as set forth in claim 1, wherein said adhesive (8) is concentrated in the area along the major axis (3) of said oval hole (2).

Inventors: HIRATSUKA TOSHIO C O NEC CORPO, HIRATSUKA, TOSHIO C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1987
Main classification: G02B6
IPC codes: G02B6