Optical couplers and methods for making same

1. An optical coupler for use with a polarization-maintaining input optical fiber and polarization-maintaining output optical fiber, the optical coupler comprising:

a tube having a wide end, a narrow end having an end face, and a taper therebetween;

a polarization-maintaining feed-through optical fiber having a first end having an end face and a second end, the polarization-maintaining feed-through optical fiber being disposed within the tube from the wide end to the narrow end, at least the narrow end of the tube being fused around a first length of the polarization-maintaining feed-through optical fiber including the first end of the polarization-maintaining feed-through optical fiber to form a coupler end face comprising the end face of the tube and the end face of the first end of the polarization-maintaining feed-through optical fiber,

wherein the polarization-maintaining feed-through optical fiber has an outer diameter no greater than about 200 μm at the coupler end face and throughout the region in which the tube is fused around it,

wherein the diameter of the core of the polarization-maintaining feed-through optical fiber is not substantially tapered as it traverses the tube from the wide end of the tube to the end face of the optical coupler;

wherein the polarization-maintaining feed-through optical fiber comprises a core, a cladding surrounding the core and forming the outer surface of the polarization-maintaining feed-through optical fiber, and one or more birefringence-inducing elements disposed in the cladding, wherein the one or more birefringence-inducing elements do not extend beyond a diameter of 150 μm, and do not extend to the outer surface of the cladding of the polarization-maintaining feed-through optical fiber in the region in which the tube is fused around it.

2. The optical coupler according to claim 1 , wherein the wide end of the tube has an annular end face.

3. The optical coupler according to claim 1 , wherein the diameter of the polarization-maintaining feed-through optical fiber does not substantially change from the wide end to the narrow end of the tube.

4. The optical coupler according to claim 1 , wherein at a position along its length that is disposed within the wide end of the tube, the polarization-maintaining feed-through optical fiber has an outer diameter that is substantially larger than its outer diameter along its first length around which the tube is fused.

5. The optical coupler according to claim 1 , wherein the polarization-maintaining feed-through optical fiber has a narrower diameter along its first length around which the tube is fused than it does in a region where the tube is not fused around it.

6. An optical fiber device comprising an optical coupler according to claim 1 .

7. The optical fiber device according to claim 6 , further comprising the polarization-maintaining output optical fiber optically coupled to the coupler end face of the optical coupler.

8. The optical fiber device according to claim 7 , wherein the polarization-maintaining output optical fiber optically coupled to the coupler end face has a birefringence that is substantially greater than the birefringence of the polarization-maintaining feed-through optical fiber.

9. The optical fiber device according to claim 8 , wherein the outer diameter of the coupler end face is not substantially greater than the diameter of the polarization-maintaining output optical fiber.

10. The optical fiber device according to claim 8 , further comprising the polarization-maintaining input optical fiber optically coupled to the second end of the polarization-maintaining feed-through optical fiber.

11. The optical fiber device according to claim 7 , wherein the polarization-maintaining input optical fiber comprises a core, a cladding surrounding the core, and one or more birefringence-inducing elements disposed in the cladding, the one or more birefringence-inducing elements extending radially in the cladding to a diameter of greater than about 200 μm.

12. An optical fiber laser or amplifier comprising the optical fiber device according to claim 6 .

13. A method for manufacturing an optical coupler according to claim 1 , the method comprising:

providing a tube having a wide section, a narrow section and a taper therebetween;

providing a polarization-maintaining feed-through optical fiber having a diameter of less than about 200 μm along at least a length thereof, wherein the polarization-maintaining feed-through optical fiber comprises a core, a cladding surrounding the core and forming the outer surface of the polarization-maintaining feed-through optical fiber, and one or more birefringence-inducing elements disposed in the cladding, wherein the one or more birefringence-inducing elements do not extend beyond a diameter of 150 μm, and do not extend to the outer surface of the cladding of the polarization-maintaining feed-through optical fiber along the length thereof;

disposing the polarization-maintaining feed-through optical fiber at least partially within the tube, so that it extends from the wide section into the narrow section, and so that the length having the diameter of less than about 200 μm is disposed within the narrow section of the tube;

collapsing at least the narrow section of the tube onto the polarization-maintaining feed-through optical fiber along at least part of the length having the diameter of less than about 200 μm, forming a fused section; and

cleaving the fused section to form a coupler end face comprising an end face of the tube and an end face of the polarization-maintaining feed-through optical fiber.

14. The method according to claim 13 , wherein collapsing at least the narrow end of the tube onto the polarization-maintaining feed-through optical fiber includes refraining from collapsing the wide section of the tube onto the polarization-maintaining feed-through optical fiber.

15. The method according to claim 13 comprising refraining from tapering the core of the polarization-maintaining feed-through optical fiber.

16. The optical coupler according to claim 1 , wherein the diameter of the polarization-maintaining feed-through fiber in its length around which the tube is collapsed is less than about 75% of its diameter at a position along its length that is disposed within the wide end of the tube.

17. The optical coupler according to claim 1 , wherein the polarization-maintaining feed-through optical fiber has a length of less than about 50 cm.

18. The optical coupler according to claim 1 , wherein the birefringence of the polarization-maintaining feed-through optical fiber in the region in which the tube is collapsed is less than about 50% of the birefringence of the polarization-maintaining input optical fiber.

19. The optical coupler according to claim 1 , wherein the polarization-maintaining feed-through optical fiber has an outer diameter no greater than about 150 μm over a length extending at least 1 cm from the coupler end face.

20. The optical coupler according to claim 1 , wherein the diameter of the polarization-maintaining feed-through fiber at its second end is greater than about 200 pm.

21. An optical coupler for use with a polarization-maintaining input optical fiber and polarization-maintaining output optical fiber, the optical coupler comprising:

a tube having a wide end, a narrow end having an end face, and a taper therebetween;

a polarization-maintaining feed-through optical fiber having a first end having an end face and a second end, the polarization-maintaining feed-through optical fiber being disposed within the tube from the wide end to the narrow end, at least the narrow end of the tube being fused around a first length of the polarization-maintaining feed-through optical fiber including the first end of the polarization-maintaining feed-through optical fiber to form a coupler end face comprising the end face of the tube and the end face of the first end of the polarization-maintaining feed-through optical fiber,

wherein the polarization-maintaining feed-through optical fiber has an outer diameter at the coupler end face no greater than about 75% of the diameter of the polarization-maintaining input optical fiber, and

wherein the diameter of the core of the polarization-maintaining feed-through optical fiber is not substantially tapered as it traverses the tube from the wide end of the tube to the end face of the optical coupler;

wherein the polarization-maintaining feed-through optical fiber comprises a core, a cladding surrounding the core and forming the outer surface of the polarization-maintaining feed-through optical fiber, and one or more birefringence-inducing elements disposed in the cladding, wherein the one or more birefringence-inducing elements do not extend beyond a diameter of 200 μm, and do not extend to the outer surface of the cladding of the polarization-maintaining feed-through optical fiber in the region in which the tube is fused around it.

22. The optical coupler according to claim 21 , wherein the diameter of the polarization-maintaining feed-through fiber in its length around which the tube is collapsed is less than about 50% of its diameter at a position along its length that is disposed within the wide end of the tube.

23. The optical coupler according to claim 21 , wherein the diameter of the polarization-maintaining feed-through fiber in its length around which the tube is collapsed is less than about 75% of its diameter at its second end.