Measuring Optical Spectral Property of Light Based on Polarization Analysis

• US 20090207409A1
• Filed: 01/09/2009
• Published: 08/20/2009
• Est. Priority Date: 02/04/2008
• Status: Active Grant

• Current Assignee: Luna Innovations, Inc.
• Original Assignee: General Photonics Corporation (General Cable Corporation)
• Inventors: Yao, Xiaotian Steve
• Granted Patent: US 8,345,238 B2
• Time in Patent Office: Days
• US Class Current: 356/365
• CPC Class Codes: G01J 3/1895 using fiber Bragg gratings or gratings integrated in a waveguide
• CPC Class Codes: G01J 3/447 Polarisation spectrometry

Abstract

Apparatus, systems and techniques for measuring optical spectral property such as the frequency and the optical spectrum of light based on polarization analysis.

Claims

1. A method for measuring spectrum of light, comprising:

directing light under measurement through a differential group delay (DGD) device to produce output light with a DGD value representing a difference in the group delay between two orthogonal optical polarizations of the light under measurement;

measuring optical polarization and a degree of polarization of the output light corresponding to the DGD value; and

processing the measured optical polarization and degree of polarization for the output light to produce an optical spectrum of the light under measurement.

2. The method as in claim 1, comprising:

controlling the DGD device to increase the DGD value to decrease a minimum resolvable frequency spacing of the optical spectrum.

3. The method as in claim 1, comprising:

controlling the DGD device to reduce the DGD value to increase a spectral range of the optical spectrum.

4. The method as in claim 1, comprising:

processing the measured optical polarizations and the respective corresponding DGD values at a fixed optical frequency of the light under measurement to determine a value of the fixed optical frequency.

5. The method as in claim 1, wherein the DGD device comprises:

a polarization beam splitter to split the light under measurement into a first optical beam in a first optical polarization along a first optical path and a second optical beam along a second, different optical path in a second optical polarization that is orthogonal to the first optical polarization;

a delay mechanism that changes a relative optical path difference between the first and second optical paths to produce the DGD values; and

a polarization beam combiner located at an interception of the first and second optical paths to combine light in the first optical path in the first optical polarization and light in the second optical path in the second optical polarization to produce a combined output beam as the output light.

6. The method as in claim 1, wherein the DGD device comprises:

a polarization beam splitter to split the light under measurement into a first optical beam in a first optical polarization along a first optical path and a second optical beam along a second, different optical path in a second optical polarization that is orthogonal to the first optical polarization;

a first Faraday reflector in the first optical path to reflect light in the first optical path back towards the polarization beam splitter;

a second Faraday reflector in the second optical path to reflect light in the second optical path back towards the polarization beam splitter; and

a delay mechanism that changes a relative optical path difference between the first and second optical paths to produce the DGD values;

wherein the polarization beam splitter combines reflected light in the first optical path in the second optical polarization and reflected light in the second optical path in the first optical polarization to produce a combined output beam as the output light.

7. The method as in claim 1, wherein the DGD device comprises:

a plurality of variable DGD units cascaded to form an optical path through which the light under measurement is directed to produce the output light,

wherein each variable DGD unit comprises;

a polarization rotator operable to control a polarization of received light in response to a unit control signal;

a birefringent segment formed of a birefringent material and located to receive output light from the polarization rotator and to transmit received light; and

a unit control element, coupled to the polarization rotator to supply the unit control signal, to control light received by the birefringent segment in a first polarization to cause a first optical delay in light output by the birefringent segment and in a second orthogonal polarization to cause the second, different optical delay in light output by the birefringent segment.

8. The method as in claim 1, wherein the DGD device comprises:

a plurality of variable DGD units cascaded to form an optical path through which the light under measurement is directed to produce the output light,

wherein each variable DGD unit comprises;

a polarization rotator operable to control a polarization of received light in response to a unit control signal; and

a birefringent segment formed of a birefringent material and located to receive output light from the polarization rotator and to transmit received light; and

wherein the method comprising;

operating a unit control element, coupled to the polarization rotators to supply the unit control signal to each of the polarization rotators, to control light received by the birefringent segment in a first polarization to cause a first optical delay in light output by the birefringent segment and in a second orthogonal polarization to cause the second, different optical delay in light output by the birefringent segment.

9. The method as in claim 8, wherein the birefringent material includes a birefringent crystal.

10. The method as in claim 8, wherein the birefringent material includes a polarization maintaining (PM) fiber.

11. The method as in claim 8, wherein different birefringent segments in different variable DGD units have different lengths along the optical path.

12. The method as in claim 11, wherein two adjacent different birefringent segments differ in length by a constant factor.

13. The method as in claim 12, wherein said constant factor is 2.

14. The method as in claim 11, wherein lengths of the different birefringent segments increase successively along the optical path from a first variable DGD unit that receives the light under measurement to a last variable DGD unit that outputs the output light.

15. The method as in claim 11, wherein the variable DGD units includes a first variable optical delay unit whose birefringent segment is formed of a first birefringent material and a second variable optical delay unit whose birefringent segment is formed of a second birefringent material that has birefringence different from said first birefringent material.

16. The method as in claim 11, wherein at least one birefringent segment in one variable optical delay unit is formed of a birefringent material that is responsive to an index-control signal to change a refractive index and thus an associated optical delay therein to fine tune a total variable optical delay.

17. The method as in claim 1, comprising:

placing an optical sensor at a target location that interacts with light at a resonance wavelength of the optical sensor and responds to a parameter to change the resonance wavelength of the optical sensor;

directing probe light to the optical sensor to interact with the optical sensor to produce returned probe light at the resonance wavelength of the optical sensor associated with the parameter;

directing the returned probe light as the light under measurement into the DGD device; and

processing the optical spectrum of the returned probe light to extract information on the parameter.

18. The method as in claim 1, comprising:

placing optical sensors at different target locations, the optical sensors interacting with light at different resonance wavelengths, respectively, each responding to a parameter to change a respective resonance wavelength of the optical sensor;

directing probe light to the optical sensors to interact with the optical sensors to produce returned probe light comprising light from the optical sensors caused by interaction with the probe light at the different resonance wavelengths of the optical sensors associated with the parameter;

directing the returned probe light as the light under measurement into the DGD device; and

processing the optical spectrum of the returned probe light to extract information on the parameter at each of the optical sensors.

19. The method as in claim 1, comprising:

measuring a change in the optical polarization of the output light; and

processing the measured change in the optical polarization to determine a direction of change in the frequency of light.

20. The method as in claim 19, wherein the change in the optical polarization of the output light is a change in an angle of a respective state of polarization vector on the Poincare Sphere.

21. A device for measuring spectrum of light, comprising:

a differential group delay (DGD) device positioned to receive light under measurement and to produce output light with a DGD value representing a difference in the group delay between two orthogonal optical polarizations of the light under measurement;

an optical detector positioned to receive the output light from the DGD device to measure a state of polarization and a degree of polarization of the output light; and

a processing device that receives and processes measurements of the state of polarization and the degree of polarization from the optical detector to produce a spectrum of the light under measurement.

22. The device as in claim 21, wherein:

the DGD device comprises;

a polarization beam splitter to split the light under measurement into a first optical beam in a first optical polarization along a first optical path and a second optical beam along a second, different optical path in a second optical polarization that is orthogonal to the first optical polarization;

a delay mechanism that changes a relative optical path difference between the first and second optical paths to produce the DGD values; and

a polarization beam combiner located at an interception of the first and second optical paths to combine light in the first optical path in the first optical polarization and light in the second optical path in the second optical polarization to produce a combined output beam as the output light.

23. The device as in claim 21, wherein:

the DGD device comprises;

a polarization beam splitter to split the light under measurement into a first optical beam in a first optical polarization along a first optical path and a second optical beam along a second, different optical path in a second optical polarization that is orthogonal to the first optical polarization;

a first Faraday reflector in the first optical path to reflect light in the first optical path back towards the polarization beam splitter;

a second Faraday reflector in the second optical path to reflect light in the second optical path back towards the polarization beam splitter; and

a delay mechanism that changes a relative optical path difference between the first and second optical paths to produce the DGD values;

wherein the polarization beam splitter combines reflected light in the first optical path in the second optical polarization and reflected light in the second optical path in the first optical polarization to produce a combined output beam as the output light.

24. The device as in claim 21, wherein the DGD device comprises:

a plurality of variable DGD units cascaded to form an optical path through which the light under measurement is directed to produce the output light,

wherein each variable DGD unit comprises;

a polarization rotator operable to control a polarization of received light in response to a unit control signal;

a birefringent segment formed of a birefringent material and located to receive output light from the polarization rotator and to transmit received light; and

a unit control element, coupled to the polarization rotator to supply the unit control signal, to control light received by the birefringent segment in a first polarization to cause a first optical delay in light output by the birefringent segment and in a second orthogonal polarization to cause the second, different optical delay in light output by the birefringent segment.

25. The device as in claim 21, wherein the DGD device comprises:

a plurality of variable DGD units cascaded to form an optical path through which the light under measurement is directed to produce the output light,

wherein each variable DGD unit comprises;

a polarization rotator operable to control a polarization of received light in response to a unit control signal; and

a birefringent segment formed of a birefringent material and located to receive output light from the polarization rotator and to transmit received light; and

wherein the device comprises;

a control unit, coupled to the polarization rotators to supply the unit control signal to each of the polarization rotators, to control light received by the birefringent segment in a first polarization to cause a first optical delay in light output by the birefringent segment and in a second orthogonal polarization to cause the second, different optical delay in light output by the birefringent segment.

26. The device as in claim 24, wherein the birefringent material includes a birefringent crystal.

27. The device as in claim 24, wherein the birefringent material includes a polarization maintaining (PM) fiber.

28. The device as in claim 24, wherein different birefringent segments in different variable DGD units have different lengths along the optical path.

29. The device as in claim 28, wherein two adjacent different birefringent segments differ in length by a constant factor.

30. The device as in claim 29, wherein said constant factor is 2.

31. The device as in claim 28, wherein lengths of the different birefringent segments increase successively along the optical path from a first variable DGD unit that receives the light under measurement to a last variable DGD unit that outputs the output light.

32. The device as in claim 28, wherein lengths of the different birefringent segments decrease successively along the optical path from a first variable DGD unit that receives the light under measurement to a last variable DGD unit that outputs the output light.

33. The device as in claim 28, wherein the variable DGD units includes a first variable optical delay unit whose birefringent segment is formed of a first birefringent material and a second variable optical delay unit whose birefringent segment is formed of a second birefringent material that has birefringence different from said first birefringent material.

34. The device as in claim 28, wherein at least one birefringent segment in one variable optical delay unit is formed of a birefringent material that is responsive to an index-control signal to change a refractive index and thus an associated optical delay therein to fine tune a total variable optical delay.

35. The device as in claim 21, wherein the DGD device comprises:

an optical polarization beam splitter (PBS) having a first port, a second port and a third port, wherein light at a first optical polarization transmits through the PBS between the first port and the second port, and light at a second optical polarization orthogonal to the first optical polarization that enters at either of the second port and the third port is reflected to the other of the second port and the third port;

a first optical path having a first end optically coupled to the second port of the PBS to receive light from the second port and to direct light to the second port;

a polarization reflector coupled to a second end of the first optical path to reflect light received from the first optical path back to the first optical path with a reflected optical polarization that is orthogonal to a polarization of the light that initially enters the first optical path upon exiting the second port of the PBS;

a second optical path having a first end optically coupled to the third port of the PBS to receive light from the second port and to direct light to the third port; and

a reflector coupled to a second end of the second optical path to reflect light received from the second optical path back to the second optical path with a reflected optical polarization that is the same as an optical polarization of the light at the third port when entering the second optical path.

36. The device as in claim 35, wherein, in DGD device,

the polarization reflector is a Faraday reflector coupled to a second end of the first optical path to reflect light received from the first optical path back to the first optical path with a reflected optical polarization that is, at each location along the first optical path, orthogonal to an optical polarization of the light when traveling in the first optical path from the PBS towards the Faraday reflector prior to the reflection.

37. The device as in claim 35, wherein, in the DGD device,

the polarization reflector comprises a reflector and a quarter wave plate whose one of two orthogonal principal polarization axes is at 45 degrees with a polarization axis of light exiting the second port of the PBS, and

wherein the quarter wave plate is located between the PBS and the reflector.

38. The device as in claim 21, comprising:

a probe light source that produces probe light;

an optical sensor at a target location that receives the probe light and interacts with the probe light at a resonance wavelength of the optical sensor, the optical sensor responding to a parameter to change the resonance wavelength of the optical sensor and to produce returned probe light at the resonance wavelength of the optical sensor associated with the parameter, wherein the returned probe light is directed into the DGD device as the light under measurement; and

a processing unit that processes the optical spectrum of the returned probe light to extract information on the parameter.

39. The device as in claim 21, comprising:

a probe light source to produce probe light;

optical sensors at different target locations, the optical sensors interacting with light at different resonance wavelengths, respectively, each responding to a parameter to change a respective resonance wavelength of the optical sensor, the optical sensors receiving and interacting with the probe light to produce returned probe light comprising light from the optical sensors caused by interaction with the probe light at the different resonance wavelengths of the optical sensors associated with the parameter, wherein the returned probe light is directed into the DGD device as the light under measurement; and

a processing unit processing the optical spectrum of the returned probe light to extract information on the parameter at each of the optical sensors.

40. The device as in claim 39, wherein each optical sensor includes a fiber Bragg grating sensor.

41. The device as in claim 21, comprising:

means for measuring a change in the optical polarization of the output light; and

means for processing the measured change in the optical polarization to determine a direction of change in the frequency of light.

42. The device as in claim 21, wherein the processing device comprises a mechanism to measure an increase or decrease in frequency of light based on a direction of a change in the optical polarization of the output light.

43. The device as in claim 42, wherein the direction of a change in the optical polarization of the output light is a change in an angle of a respective state of polarization vector on the Poincare Sphere.

44. A method for measuring optical spectral property of light, comprising:

directing light under measurement to pass through a differential group delay (DGD) device to cause a delay between two orthogonal states of polarization of the light under measurement to produce output light;

measuring a state of polarization of the output light; and

processing the measured state of polarization to determine an increase or decrease in frequency of the light under measurement based on a change in the measured state of polarization.

45. The method as in claim 44, comprising:

measuring a change in an angle of a respective state of polarization vector of the light on the Poincare Sphere; and

determining the increase or decrease in frequency of the light based on the measured change in the angle of the respective state of polarization vector of the light on the Poincare Sphere.

46. The method as in claim 44, comprising:

measuring a degree of polarization of the output light; and

processing the measured degree of polarization of the output light to determine an optical spectrum of the light under measurement.

47. The method as in claim 44, comprising:

measuring a number of complete revolutions of the state of polarization for the output light on the Poincare Sphere; and

obtaining an extended spectral range for measuring the light under measurement based on a spectral range determined by a single revolution of the state of polarization and the measured complete revolutions of the state of polarization for the output light on the Poincare Sphere.

48. The method as in claim 44, comprising:

using a wavelength-swept light source to generate the light under measurement;

controlling the DGD device to produce different DGD values on the output light;

at each DGD value, scanning a wavelength of the wavelength-swept light source in a spectral range within a scanning time period to measure values of a state of polarization (SOP) and a degree of polarization (DOP) of the output light at different time instants in the scanning time period;

processing measured SOP and DOP values at each time instant for different DGD values to obtain a center optical frequency or wavelength of the wavelength-swept light source at each time instant;

obtaining a spectral power, which is optical power as a function of the optical wavelength, of the light source at each time instant by Fourier transforming measured DOP values vs. DGD values; and

obtaining a 3-D plot of the optical power as a function of the time instant and as a function of the optical wavelength.

49. The method as in claim 48, comprising:

converting the 3-D plot into a 3-D plot of the optical power as a function of the center wavelength based on the processing of the measured SOP and DOP values at each time instant for different DGD values ting and as a function of the optical wavelength.

50. A device for measuring optical spectral property of light, comprising:

an input port to receive light under measurement;

a differential group delay (DGD) device to receive the light from the input port to cause a delay between two orthogonal principal polarization axes of the DGD device to produce output light;

an optical polarization detection device positioned to receive the output light from the DGD device to measure at least one of a state of polarization and a degree of polarization of the output light; and

a processing device that receives and processes measurements from the optical polarization detection device to extract spectral information of the light under measurement.

51. The device as in claim 50, comprising:

a linear polarizer located between the input port and the DGD device and oriented at 45 degrees in polarization with respect to one principal polarization axis of the DGD device.

52. The device as in claim 50, wherein:

the processing device comprises means for processing measurements of the state of polarization to determine an increase or decrease in frequency of the light under measurement based on a change in the state of polarization.

53. The device as in claim 50, wherein:

the processing device comprises means for processing measurements of the degree of polarization of the output light to determine an optical spectrum of the light under measurement.

54. The device as in claim 50, wherein:

the processing device comprises means for obtaining an extended spectral range for measuring the light under measurement based on a spectral range determined by a single revolution of the state of polarization and a number of complete revolutions of the state of polarization for the output light on the Poincare Sphere.