Polarization-dependent loss compensator

1. A polarization-dependent loss (PDL) compensator, comprising:

a substrate having an input surface and an output surface opposite the input surface;

an anti-reflective coating formed on the output surface; and

a partial reflective coating formed on the input surface.

2. The PDL compensator of claim 1 , wherein the PDL compensator includes a PDL that depends on an incident angle of an optical signal with respect to the partial reflective coating.

3. The PDL compensator of claim 1 , wherein the partial reflective coating comprises material layers of alternating index of refraction.

4. The PDL compensator of claim 3 , wherein the material layers of alternating index of refraction comprise at least one layer of tantalum oxide (Ta2O5) and at least one layer of silicon dioxide (SiO2).

5. The PDL compensator of claim 4 , wherein the at least one layer of Ta205 comprises one layer of Ta205 having a quarter wave optical thickness (QWOT) of 0.1523 nanometers and a thickness of 28.24 nanometers, and wherein the at least one layer of SiO2 comprises one layer of SiO2 having a QWOT of 0.6920 nanometers and a thickness of 183.66 nanometers.

6. The PDL compensator of claim 1 , wherein the PDL of the PDL compensator is wavelength-dependent.

7. The PDL compensator of claim 6 , wherein the partial reflective coating comprises:

a first layer of tantalum oxide (TaO5) having a quarter wave optical thickness (QWOT) of 1.3450 nanometers and a thickness of 249.37 nanometers;

a second layer of silicon dioxide (SiO2) having a QWOT of 1.5269 nanometers and a thickness of 405.26 nanometers;

a third layer of TaO5 having a QWOT of 1.4674 nanometers and a thickness of 272.07 nanometers;

a fourth layer of SiO2 having a QWOT of 1.5272 nanometers and a thickness of 405.34 nanometers;

a fifth layer of TaO5 having a QWOT of 1.3236 nanometers and a thickness of 245.40 nanometers; and

a sixth layer of SiO2 having a QWOT of 1.2590 nanometers and a thickness of 334.15 nanometers.

8. The PDL compensator of claim 6 , wherein the partial reflective coating comprises:

a first layer of tantalum oxide (TaO5) having a quarter wave optical thickness (QWOT) of 1.0269 nanometers and a thickness of 190.39 nanometers;

a second layer of silicon dioxide (SiO2) having a QWOT of 0.9348 nanometers and a thickness of 248.11 nanometers;

a third layer of TaO5 having a QWOT of 0.5283 nanometers and a thickness of 97.95 nanometers;

a fourth layer of SiO2 having a QWOT of 0.5837 nanometers and a thickness of 154.92 nanometers;

a fifth layer of TaO5 having a QWOT of 0.9006 nanometers and a thickness of 166.98 nanometers; and

a sixth layer of SiO2 having a QWOT of 0.3284 nanometers and a thickness of 87.16 nanometers.

9. The PDL compensator of claim 1 , wherein the anti-reflective coating comprises:

a first layer of tantalum oxide (TaO5) having a quarter wave optical thickness (QWOT) of 0.3208 nanometers and a thickness of 59.48 nanometers;

a second layer of silicon dioxide (SiO2) having a QWOT of 0.3507 nanometers and a thickness of 93.08 nanometers;

a third layer of TaO5 having a QWOT of 1.1100 nanometers and a thickness of 205.80 nanometers; and

a fourth layer of SiO2 having a QWOT of 1.1049 nanometers and a thickness of 269.37 nanometers.

10. The PDL compensator of claim 1 , wherein the substrate comprises fused silica.

11. A method to compensate polarization-dependent loss (PDL), the method comprising:

receiving, at a PDL compensator, an input signal subject to PDL within a propagation channel in which a first of two polarization states of the input signal is attenuated more than a second of the two polarization states, the PDL compensator including a substrate having an input surface and an output surface opposite the input surface, an anti-reflective coating formed on the output surface, and a partial reflective coating formed on the input surface;

rotating the PDL compensator such that an incident angle of the input signal with respect to the input surface of the PDL compensator is effective to compensate the PDL of the input signal within the propagation channel; and

compensating the PDL of the input signal.

12. The method of claim 11 , wherein compensating the PDL of the input signal comprises attenuating the second of the two polarization states of the input signal more than the first of the two polarization states.

13. The method of claim 12 , wherein attenuating the second of the two polarization states of the input signal more than the first of the two polarization states comprises attenuating, by the PDL compensator, the second of the two polarization states of the input signal by an amount equal to an attenuation of the first of the two polarization states by the propagation channel less an attenuation of the first of the two polarization states by the PDL compensator.

14. The method of claim 11 , wherein rotating the PDL compensator comprises at least one of rotating the PDL compensator about a first axis of rotation that is orthogonal to a propagation direction of the input signal or rotating the PDL compensator about a second axis of rotation that is orthogonal to the propagation direction of the input signal and to the first axis of rotation.

15. The method of claim 11 , wherein compensating the PDL of the input signal comprises transmitting the input signal through a partial reflective coating that includes material layers of alternating index of refraction, a substrate, and an anti-reflective coating that includes material layers of alternating index of refraction.

16. An optical channel monitor (OCM) comprising:

a propagation channel including a plurality of optical elements, wherein the propagation channel includes a polarization-dependent loss (PDL); and

a PDL compensator disposed in the propagation channel and configured to at least partially compensate the PDL of the propagation channel, wherein the PDL compensator comprises:

a substrate having an input surface and an output surface opposite the input surface;

an anti-reflective coating formed on the output surface; and

a partial reflective coating formed on the input surface.

17. The OCM of claim 16 , wherein the plurality of optical elements include:

a mirror positioned to receive and reflect an optical signal from an input of the OCM;

an imaging lens positioned to receive the optical signal after reflection by the mirror;

an expanding prism positioned to receive the optical signal after transmission through the imaging lens;

a downwardly directed mirror positioned to receive the optical signal after transmission through the expanding prism and to reflect the optical signal a first time;

a microelectromechanical systems (MEMS) mirror positioned to receive the optical signal after reflection by the downwardly directed mirror the first time and to reflect the optical signal back to the downwardly directed mirror for reflection of the optical signal a second time; and

a grism positioned to receive the optical signal after reflection by the downwardly directed mirror the second time.

18. The OCM of claim 17 , wherein the PDL compensator is disposed between the input of the OCM and the imaging lens.

19. The OCM of claim 17 , further comprising:

a substrate on which the mirror, the imaging lens, the expanding prism, the downwardly directed mirror, the MEMS mirror, and the grism are mounted; and

a rotation assembly that couples the PDL compensator to the substrate, wherein the rotation assembly is configured to rotate the PDL compensator about one or more axes of rotation.

20. The OCM of claim 16 , wherein:

the anti-reflective coating comprises N1 layers of tantalum oxide (Ta2O5) and N1 layers of silicon dioxide (SiO2) arranged in an alternating manner with the N1 layers of Ta2O5;

N1 is an integer greater than or equal to 1;

the partial reflective coating comprises N2 layers of Ta2O5 and N2 layers of SiO2 arranged in an alternating manner with the N2 layers of Ta2O5; and

N2 is an integer greater than or equal to 1.