Double-pass fiber amplifier

1. An optical apparatus, comprising:

an optical fiber amplifier including an all-fiber polarization splitter-combiner, a polarization-rotating arrangement, and a gain-fiber, the amplifier being arranged such that light plane-polarized in a first polarization-orientation enters the amplifier via the polarization splitter-combiner, makes a first pass through the gain fiber in the first polarization-orientation, is polarization rotated by the polarization-rotating arrangement into a second polarization-orientation perpendicular to the first polarization-orientation, makes a second pass through the gain fiber in a second polarization-orientation, and exits the amplifier via the polarization splitter-combiner in the second polarization-orientation and wherein the polarization-rotating arrangement is provided by a 45 degree fiber-based rotator arranged so that the light will traverse the 45 degree fiber rotator twice prior to reentering the gain fiber.

2. An apparatus as recited in claim 1 , wherein said polarization rotating arrangement includes a 45 degree cross splice located in a passive polarization maintaining fiber.

3. An apparatus as recited in claim 2 , further wherein said polarization-rotating arrangement further includes a reflector for causing the light to traverse the 45 degree cross splice twice in opposite directions prior to reentering the gain fiber.

4. An apparatus as recited in claim 3 , wherein said reflector is a fiber Bragg grating.

5. An apparatus as recited in claim 3 , wherein the first and second passes through the gain fiber are in opposite directions.

6. An apparatus as recited in claim 1 , wherein said polarization-rotating arrangement includes a stressed fiber.

7. An apparatus as recited in claim 6 , further wherein said polarization-rotating arrangement further includes a reflector for causing the light to traverse the stressed fiber twice in opposite directions prior to reentering the gain fiber.

8. An apparatus as recited in claim 7 , wherein said reflector is a fiber Bragg grating.

9. An apparatus as recited in claim 7 , wherein the first and second passes through the gain fiber are in opposite directions.

10. An apparatus as recited in claim 1 , wherein the first and second passes through the gain fiber are in opposite directions.

11. A method of amplifying light comprising the steps of:

a) injecting light plane-polarized in a first polarization-orientation into an all-fiber polarization splitter-combiner;

b) coupling light exiting the splitter-combiner into a gain fiber where the light is amplified during a first pass therethrough;

c) rotating the polarization of the light by passing the light through a 45 degree fiber-based rotator arranged so that the light will traverse the 45 degree fiber rotator twice in opposite directions;

d) coupling the light back into the gain fiber where the light is amplified during a second pass therethrough;

e) coupling the twice amplified light into the splitter-combiner; and

f) coupling the twice amplified light out of the splitter-combiner.

12. A method as recited in claim 11 , wherein the first and second passes of the light through the gain fiber are in opposite directions.

13. Method as recited in claim 12 , wherein step (d) is performed by coupling the light back into the end from which the light exited the gain fiber on the first pass therethrough.

14. A method as recited in claim 11 , wherein the 45 degree fiber based rotator is a 45 degree cross splice located in a passive polarization maintaining fiber.

15. A method as recited in claim 11 , wherein the 45 degree fiber based rotator is a stressed fiber.