Double-pass fiber amplifier

1. Optical apparatus, comprising:

an optical fiber amplifier including an all-fiber polarization splitter-combiner having first second, third and fourth ports, a polarization-rotating arrangement, and a polarization maintaining gain-fiber, the amplifier being arranged such that light plane-polarized in a first polarization-orientation enters the amplifier via the first port of the polarization splitter-combiner, exits the first polarization splitter-combiner via the second port thereof and 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, re-enters the first polarization splitter-combiner via the third port thereof following the polarization-rotation, exits the first polarization splitter-combiner via the second port thereof and makes a second pass through the gain fiber in a second polarization-orientation, is polarization-rotated by the polarization rotating arrangement back into the first polarization orientation, enters the first polarization splitter-combiner by the third port thereof and exits the first polarization splitter-combiner via the fourth port thereof.

2. The apparatus of claim 1 , wherein the gain fiber has a proximal end thereof fiber-coupled to the second port of the polarization splitter-combiner and wherein the polarization rotating arrangement is a 90° cross-splice between a distal end of the gain-fiber and a proximal end of a polarization maintaining coupling fiber, and a distal end of the polarization maintaining coupling fiber is fiber-coupled to the third port of the polarization splitter-combiner.

3. The apparatus of claim 2 , wherein the polarization maintaining coupling fiber is a passive polarization maintaining fiber.

4. The apparatus of claim 1 , wherein the first and second passes through the gain-fiber are in the same direction.

5. Optical apparatus, comprising:

an optical fiber amplifier including an all-fiber polarization splitter-combiner, a polarization-rotating arrangement, and a polarization maintaining 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 in the same direction as the first pass, and exits the amplifier via the polarization splitter-combiner.

6. The apparatus of claim 5 , wherein the polarization splitter-combiner has first second, third and fourth ports, and the first-orientation polarized light enters the polarization splitter-combiner by the first port thereof, exits the polarization splitter-combiner via the second port thereof, makes the first pass through the gain-fiber and re-enters the polarization splitter-combiner via the third port thereof following the polarization-rotation, exits the polarization splitter-combiner via the second port thereof, makes the second pass through the gain-fiber, is polarization-rotated by the polarization rotating arrangement back into the first polarization orientation, enters the polarization splitter-combiner by the third port thereof and exits the polarization splitter-combiner via the fourth port thereof.

7. The apparatus of claim 6 , wherein the gain fiber has a proximal end thereof fiber-coupled to the second port of the polarization splitter-combiner, the polarization rotating arrangement is a 90° cross-splice between a distal end of the gain-fiber and a proximal end of a polarization maintaining coupling fiber, and a distal end of the polarization maintaining coupling fiber is fiber-coupled to the third port of the polarization splitter-combiner.

8. The apparatus of claim 7 , wherein the polarization maintaining coupling fiber is a passive polarization maintaining fiber.

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

a) injecting light plane-polarized in a first polarization-orientation into the first port of an all-fiber polarization splitter-combiner having four ports;

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

c) rotating the polarization of the light;

d) coupling light exiting the gain fiber into the third port of the splitter combiner;

e) coupling the once amplified light exiting the second port of the splitter combiner back into the gain fiber where the light is amplified during the second pass therethrough;

f) rotating the polarization of the light;

g) coupling light exiting the gain fiber into the third port of the splitter combiner; and

h) coupling the twice amplified light out of the fourth port of the splitter-combiner.

10. A method as recited in claim 9 , wherein the first and second passes of the light through the gain fiber are in the same direction.

11. A method as recited in claim 9 , wherein the steps of rotating the polarization of light are performed using a 90 degree cross-splice.