Measuring distributed polarization crosstalk in polarization maintaining fiber and optical birefringent material

1. A method for measuring distribution of polarization coupling in an optical birefringent medium, comprising:

coupling a linearly polarized light of a broadband spectrum into an optical birefringent medium in a direction along which the optical birefringent medium supports two orthogonal polarization modes due to optical birefringence to produce an optical output signal out of the optical birefringent medium;

directing the optical output signal out of the optical birefringent medium into an optical delay device which causes an added optical delay greater than an optical delay between the two orthogonal polarization modes caused by the optical birefringent medium in the optical output signal to produce a modified optical output signal;

directing modified optical output signal to transmit through a linear optical polarizer which is polarized in a direction to cause a mixing between the two orthogonal polarization modes in optical transmission light of the linear optical polarizer;

directing the optical transmission light of the linear optical polarizer into an optical interferometer to obtain optical interference of light between the two orthogonal polarization modes in the optical birefringent medium; and

processing the obtained optical interference to identify locations of the polarization coupling between the two orthogonal polarization modes in the optical birefringent medium.

2. The method as in claim 1 , comprising:

using a fiber-based interferometer as the optical interferometer.

3. The method as in claim 2 , comprising:

using a fiber-based optical delay device as the optical delay device.

4. The method as in claim 1 , wherein:

the optical interferometer is a free-space interferometer.

5. The method as in claim 1 , comprising:

processing the obtained optical interference to obtain magnitudes of the polarization coupling at the identified locations.

6. The method as in claim 1 , comprising:

processing the obtained optical interference to obtain an attenuation ratio between the two orthogonal polarizations.

7. The method as in claim 1 , comprising:

the optical interferometer is a Michelson interferometer including a beam splitter that splits the optical transmission light of the linear optical polarizer into a first beam and a second beam, a fixed mirror in an optical path of the first team to reflect the first beam back to the beam splitter and a movable reflector in an optical path of the second beam to reflect the second beam back to the beam splitter so that the reflected first and second beams overlap with each other at the beam splitter to cause the optical interference between the two orthogonal polarization modes in the optical birefringent medium.

8. The method as in claim 1 , comprising:

using a length of polarization maintaining (PM) fiber as the optical birefringent medium to place the PM fiber as a sensing medium for sensing stresses along a structure; and

processing the obtained optical interference to measure a stress distribution of the structure.

9. The method as in claim 8 , wherein:

the structure is a bridge.

10. The method as in claim 8 , wherein:

the structure is a tunnel.

11. The method as in claim 8 , wherein:

the structure is a dam.

12. The method as in claim 8 , wherein:

the structure is a pipeline or pipe for transporting a liquid or gas.

13. The method as in claim 8 , wherein:

the structure is a building.

14. The method as in claim 1 , comprising

using a length of polarization maintaining (PM) fiber as the optical birefringent medium; and

processing the obtained optical interference to measure a stress distribution of the PM fiber to evaluate a quality of the PM fiber.

15. A device for measuring distribution of polarization coupling in an optical birefringent medium, comprising:

a mechanism that couples a linearly polarized light of a broadband spectrum into an optical birefringent medium in a direction along which the optical birefringent medium supports two orthogonal polarization modes due to optical birefringence to produce an optical output signal out of the optical birefringent medium;

an optical delay device located downstream from the optical birefringent medium in an optical path of the optical output signal out of the optical birefringent medium to produce a modified optical output signal, the optical delay device structured to cause an added optical delay greater than an optical delay between the two orthogonal polarization modes caused by the optical birefringent medium in the optical output signal to produce the modified optical output signal;

a linear optical polarizer which is polarized in a direction to cause a mixing between the two orthogonal polarization modes in optical transmission light of the linear optical polarizer, the linear optical polarizer placed in an optical path of the modified optical output signal to produce the optical transmission light;

an optical interferometer located to receive the optical transmission light of the linear optical polarizer and structured to obtain optical interference of light between the two orthogonal polarization modes in the optical birefringent medium; and

a processing device that processes the obtained optical interference to identify locations of the polarization coupling between the two orthogonal polarization modes in the optical birefringent medium.

16. The device as in claim 15 , wherein

the processing device is configured to process the obtained optical interference to obtain magnitudes of the polarization coupling at the identified locations.

17. The device as in claim 15 , wherein

the processing device is configured to process the obtained optical interference to obtain an attenuation ratio between the two orthogonal polarizations.

18. The device as in claim 15 , wherein

the optical interferometer is a Michelson interferometer including a beam splitter that splits the optical transmission light of the linear optical polarizer into a first beam and a second beam, a fixed mirror in an optical path of the first team to reflect the first beam back to the beam splitter and a movable reflector in an optical path of the second beam to reflect the second beam back to the beam splitter so that the reflected first and second beams overlap with each other at the beam splitter to cause the optical interference between the two orthogonal polarization modes in the optical birefringent medium.

19. The device as in claim 15 , wherein

the optical birefringent medium includes a length of polarization maintaining (PM) fiber as a sensing medium for sensing stresses along a structure, and

the processing device is configured to process the obtained optical interference to measure a stress distribution of the structure.

20. The device as in claim 15 , wherein

the optical delay device is a fiber-based optical delay device.

21. The device as in claim 15 , wherein

the optical interferometer is a fiber-based optical interferometer.

22. The device as in claim 15 , wherein

each of the optical delay device and the optical interferometer is a fiber-based device.

23. A device for measuring distribution of polarization coupling in an optical birefringent medium, comprising:

a light source that produces a light beam of a broad spectral band;

a mechanism that couples light of the light beam in a linear polarization into an optical birefringent medium in a direction along which the optical birefringent medium supports two orthogonal polarization modes due to optical birefringence to produce an optical output signal out of the optical birefringent medium;

a linear optical polarizer which is polarized in a direction to cause a mixing between the two orthogonal polarization modes in optical transmission light of the linear optical polarizer, the linear optical polarizer placed in an optical path of the optical output signal of the optical birefringent medium to produce the optical transmission light; an optical interferometer made of fiber components and located to receive the optical transmission light of the linear optical polarizer and structured to obtain optical interference of light between the two orthogonal polarization modes in the optical birefringent medium;

an optical delay device located between the optical birefringent medium and the optical linear polarizer in an optical path of the optical output signal out of the optical birefringent medium to produce a modified optical output signal that is received by the optical linear polarizer, the optical delay device structured to cause an added optical delay greater than an optical delay between the two orthogonal polarization modes caused by the optical birefringent medium in the optical output signal to produce the modified optical output signal;

an optical detector that receives optical output of the optical interferometer and produces a detector output having information of the obtained optical interference; and

a processing device that processes the obtained optical interference to identify locations of the polarization coupling between the two orthogonal polarization modes in the optical birefringent medium.

24. The device as in claim 23 , wherein:

the optical interferometer includes:

an optical circulator to receive the optical transmission light output by the linear optical polarizer at a first optical circulator port to direct the received light at the first optical circulator port to a second optical circulator port, and to direct light received at the second optical circulator port to a third optical circulator port;

a fiber coupler having a first fiber coupler port coupled to the second optical circulator port, a second fiber coupler port, a third fiber coupler port, a fourth fiber coupler port;

a first fiber coupled to the second fiber coupler port to receive a first portion of the light received by the first fiber coupler port from the second optical circulator port;

a first Faraday mirror coupled to the first fiber to reflect light in the first fiber back to the second fiber coupler port along the first fiber;

a second fiber coupled to the third fiber coupler port to receive a second portion of the light received by the first fiber coupler port from the second optical circulator port;

a second Faraday mirror coupled to the second fiber to reflect light in the second fiber back to the third fiber coupler port along the second fiber, wherein the fiber coupler mixes and overlaps reflected light received at the second fiber coupler from the first fiber and reflected light received at the third fiber coupler form the second fiber to produce a combined light that contains the optical interference and is output at the first fiber coupler port and the fourth fiber coupler port; and

an optical delay device coupled in one of the first fiber and the second fiber to cause an optical delay between the light in the first fiber and the second fiber;

wherein the optical detector receives light output from the third optical circulator port and light output from the fourth fiber coupler port to produce the detector output.

25. The device as in claim 24 , wherein:

the optical detector is a balanced optical detector that includes two optical detectors that receive the light output from the third optical circulator port and light output from the fourth fiber coupler port, respectively.

26. The device as in claim 23 , wherein the optical birefringent medium includes a length of polarization maintaining (PM) fiber as a sensing medium for sensing stresses along a structure, and the processing device is configured to process the obtained optical interference to measure a stress distribution of the structure.