Time domain polarization scrambler
The present invention relates to a temporal depolarization assembly. The depolarization assembly operates to produce light that, over some period of time, is depolarized. The depolarization assembly includes first and second variable retarders. The variable retarders are operated to produce a retardance that varies in time. In addition, the modulator axis of a first one of the variable retarders is held at an angle to the modulator axis of the second one of the variable retarders. The frequency at which the retardance of the variable retarders varies is selected to be some fraction of the integration time of a sensor associated with the polarization assembly.
1. A depolarization system, comprising:
a first variable transmission element located along a first optical path;
a first transmission element driver, wherein the first transmission element driver is operated to vary an index of refraction of the first variable transmission element in a first axis of transmission at a first frequency, wherein the first axis of transmission is a birefringence axis of the first variable transmission element;
a second variable transmission element located along the first optical path;
a second transmission element driver, wherein the second transmission element driver is operated to vary an index of refraction of the second variable transmission element in at least one of the first axis of transmission or a second axis of transmission at a second frequency, wherein the at least one of the first and second axes of transmission is a birefringence axis of the second variable transmission element, and wherein the first frequency is different than the second frequency.
2. A depolarization system, comprising:
a first variable transmission element located along a first optical path;
a first transmission element driver, wherein the first transmission element driver is operated to vary an index of refraction of the first variable transmission element in a first axis of transmission at a first frequency;
a second variable transmission element located along the first optical path;
a second transmission element driver, wherein the second transmission element driver is operated to vary an index of refraction of the second variable transmission element in at least one of the first axis of transmission or a second axis of transmission at a second frequency, wherein the first transmission element driver varies the index of refraction of the first variable transmission element in the first axis of transmission so as to produce an optical retardation from minus a first amount, ±0.05 radians, to plus the first amount, ±0.05 radians, and wherein the second transmission element driver varies the index of refraction of the second variable transmission element in the at least one of the first axis of transmission or the second axis of transmission so as to produce an optical retardation from minus the first amount, ±0.05 radians, to plus the first amount, ±0.05 radians.
3. The system of claim 2 , wherein the first amount is 2.4 radians.
4. The system of claim 2 , wherein the first axis of transmission of the first variable transmission element is a modulator optical axis, wherein the second transmission element driver is operated to vary an index of refraction of the second variable transmission element in the first axis of transmission of the second variable transmission element, wherein the first axis of transmission of the second variable transmission element is a modulator optical axis, and wherein the modulator optical axis of the second transmission element is at an angle of 45 degrees with respect to the modulator optical axis of the first transmission element.
5. The system of claim 4 , wherein the first transmission element driver excites the first variable transmission element, and wherein the second transmission element driver excites the second variable transmission element.
6. The system of claim 5 , wherein the first transmission element driver excites the first variable transmission element at the first frequency, and wherein the second transmission element driver excites the second variable transmission element at the second frequency.
7. A depolarization system, comprising:
a first variable transmission element located along a first optical path;
a first transmission element driver, wherein the first transmission element driver is operated to vary an index of refraction of the first variable transmission element in a first axis of transmission at a first frequency;
a second variable transmission element located along the first optical path;
a second transmission element driver, wherein the second transmission element driver is operated to vary an index of refraction of the second variable transmission element in at least one of the first axis of transmission or a second axis of transmission at a second frequency, wherein the first frequency is different than the second frequency, wherein the first variable transmission element and the first transmission element driver together comprise a first photo-elastic modulator, and wherein the second variable transmission element and second transmission element driver together comprise a second photo-elastic modulator.
8. The system of claim 7 , wherein the transmission medium of the first and second transmission elements is fused silica.
9. A depolarization system, comprising:
a first variable transmission element located along a first optical path;
a first transmission element driver, wherein the first transmission element driver is operated to vary an index of refraction of the first variable transmission element in a first axis of transmission at a first frequency;
a second variable transmission element located along the first optical path;
a second transmission element driver, wherein the second transmission element driver is operated to vary an index of refraction of the second variable transmission element in at least one of the first axis of transmission or a second axis of transmission at a second frequency, wherein the first frequency is at least about 50 kHz, wherein the second frequency is equal to the first frequency multiplied by a constant, and wherein the constant is not equal to one.
10. A method for depolarizing light, comprising:
collecting light;
operating a first variable retarder at a first frequency, wherein a retardance of the first variable retarder varies in a first axis of transmission at the first frequency, wherein the first variable retarder includes a first transmission element, and wherein the first frequency is equal to an excitation frequency of the first transmission element;
operating a second variable retarder at a second frequency, wherein a retardance of the second variable retarder varies in one of the first axis of transmission and a second axis of transmission at the second frequency, wherein the second variable retarder includes a second transmission element, wherein the second frequency is equal to an excitation frequency of the second transmission element, and wherein the first frequency is not equal to the second frequency;
while operating the first variable retarder at the first frequency, passing collected light through the first variable retarder;
while operating the second variable retarder at the second frequency, passing light passed through the first variable retarder through the second variable retarder, wherein light passed through the second variable retarder is temporally depolarized light.
11. The method of claim 10 , wherein the retardance of the first and second variable retarders varies from a minimum of −2.4 radians, ±0.05 radians, to a maximum of ±2.4 radians, 10.05 radians.
12. The method of claim 10 , wherein the first and second frequencies are of the order of magnitude of 50 kHz.
13. The method of claim 12 , wherein the first frequency is approximately the same as the second frequency.
14. The method of claim 10 , further comprising:
passing the temporally depolarized light to a sensor assembly.
15. A method for depolarizing light, comprising:
collecting light;
operating a first variable retarder at a first frequency, wherein a retardance of the first variable retarder varies at the first frequency;
operating a second variable retarder at a second frequency, wherein a retardance of the second variable retarder varies at the second frequency;
while operating the first variable retarder at the first frequency, passing collected light through the first variable retarder;
while operating the second variable retarder at the second frequency, passing light passed through the first variable retarder through the second variable retarder, wherein light passed through the second variable retarder is temporally depolarized light;
passing the temporally depolarized light to a sensor assembly;
integrating the temporally depolarized light by a detector for an interval equal to at least 2 milliseconds.
16. A depolarization assembly, comprising:
a first variable retarder, including:
a transmission element, wherein the transmission element intersects a first optical path, and wherein the transmission element has a modulation axis;
a transmission element driver, wherein the transmission element driver operates to vary a retardance of the transmission element about the modulation axis at a first predetermined frequency;
a second variable retarder, including:
a transmission element, wherein the transmission element intersects the first optical path, wherein the transmission element has a modulation axis, and wherein the modulation axis of the transmission element of the first variable retarder is rotated with respect to the modulation axis of the transmission element of the second variable retarder;
a transmission element driver, wherein the transmission element driver operates to vary a retardance of the transmission element about the modulation axis at a second predetermined frequency.
17. A depolarization assembly, comprising:
a first variable retarder, including:
a transmission element, wherein the transmission element intersects a first optical path, and wherein the transmission element has a modulation axis;
a transmission element driver, wherein the transmission element driver operates to vary a retardance of the transmission element about the modulation axis at a first frequency;
a second variable retarder, including:
a transmission element, wherein the transmission element intersects the first optical path, wherein the transmission element has a modulation axis, and wherein the modulation axis of the transmission element of the first variable retarder is rotated with respect to the modulation axis of the transmission element of the second variable retarder;
a transmission element driver, wherein the transmission element driver operates to vary a retardance of the transmission element about the modulation axis at a second frequency, wherein the first and second variable retarders are photo-elastic modulators, wherein the transmission elements are formed from fused silica, and wherein the transmission element drivers are piezo electric drivers that operate to introduce an acoustic wave in the transmission elements.
18. The depolarization assembly of claim 17 , wherein the first frequency is equal to a resonant frequency of the transmission element of the first variable retarder.
19. The depolarization assembly of claim 17 , further comprising:
a frame, wherein the first variable retarder is mounted to the frame such that the modulation axis of the transmission element of the first variable retarder is held at a first angle, wherein the second variable retarder is mounted to the frame such that the modulation axis of the transmission element of the second variable retarder is held at a second angle, and wherein the first angle is rotated by about 45 degrees with respect to the second angle.