Athermal channeled spectropolarimeter
A temperature insensitive (athermal) channeled spectropolarimeter (CSP) is described. The athermal CSP includes a crystal retarder formed of a biaxial crystal. The crystal retarder has three crystal axes, wherein each axis has its own distinct index of refraction. The axes are oriented in a particular manner, causing an amplitude modulating carrier frequency induced by the crystal retarder to be thermally invariant. Accordingly, a calibration beam technique can be used over a relatively wide range of ambient temperatures, with a common calibration data set.
1. An athermal channeled spectropolarimeter (CSP) comprising:
a biaxial crystal retarder;
a second biaxial crystal retarder, wherein the biaxial crystal retarder has a first thickness, the second biaxial crystal retarder has a second thickness, and wherein the first thickness is different from the second thickness;
a polarizer; and
a spectrometer that is in optical communication with the polarizer, wherein the biaxial crystal retarder and the polarizer are optically positioned in series such that a transmission axis of an electromagnetic (EM) signal passes through the biaxial crystal retarder and the polarizer, and wherein the second biaxial crystal retarder is optically in series with the biaxial crystal retarder and the polarizer such that the transmission axis of the EM signal passes through the biaxial crystal retarder, the second biaxial crystal retarder, and the polarizer.
2. The athermal CSP of claim 1 , wherein the biaxial crystal retarder is composed of KTiOPO 4 .
3. The athermal CSP of claim 1 , the EM signal comprising frequencies in the near infrared spectrum.
4. The athermal CSP of claim 1 , wherein the biaxial crystal retarder and the second biaxial crystal retarder are composed of KTiOPO 4 .
5. The athermal CSP of claim 1 , wherein the first thickness is greater than the second thickness.
6. The athermal CSP of claim 5 , the second biaxial crystal retarder positioned between the biaxial crystal retarder and the analyzer.
7. The athermal CSP of claim 1 , the spectrometer being a dispersive spectrometer.
8. The athermal CSP of claim 1 , the spectrometer being a Fourier spectrometer.
9. An athermal channeled spectropolarimeter (CSP) comprising: a first biaxial crystal retarder having a first proximal surface and a first distal surface, the first biaxial crystal retarder comprising: a first crystal axis; a second crystal axis; and a third crystal axis, wherein the first crystal axis, the second crystal axis, and the third crystal axis are orthogonal to one another, the second crystal axis being parallel with the first proximal surface and the first distal surface, the first crystal axis forming an angle of 33.7 degrees relative to the first proximal surface and the first distal surface; a second biaxial crystal retarder positioned optically in series with the first biaxial crystal retarder, the second biaxial crystal retarder having a second proximal surface and a second distal surface, the second biaxial crystal retarder comprising: a fourth crystal axis; a fifth crystal axis; and a sixth crystal axis, wherein the fourth crystal axis, the fifth crystal axis, and the sixth crystal axis are orthogonal to one another, the fifth crystal axis being parallel with the second proximal surface and the second distal surface and angularly offset from the second crystal axis by 45 degrees, the fourth crystal axis forming an angle of 33°7 degrees relative to the second proximal surface and the second distal surface; a polarizer positioned optically in series with the second biaxial crystal retarder, the polarizer configured to allow electromagnetic waves having a polarization direction that is parallel to the second crystal axis to pass therethrough; and a spectrometer in optical communication with tile polarizer, the spectrometer receiving the electromagnetic waves that pass through the polarizer.
10. The athermal CSP of claim 9 , wherein the first biaxial crystal retarder has a first thickness between the first proximal surface and the first distal surface, wherein the second biaxial crystal retarder has a second thickness between the second proximal surface and the second distal surface, and wherein the first thickness is different from the second thickness.
11. The athermal CSP of claim 9 , wherein the biaxial crystal retarder is composed of KTiOPO 4 .
12. The athermal CSP of claim 9 , wherein the first crystal axis has a first index of refraction, the second crystal axis has a second index of refraction, and the third crystal axis has a third index of refraction, wherein the first index of refraction is less than the second index of refraction, and the second index of refraction is less than the third index of refraction.
13. The athermal CSP of claim 12 , wherein the fourth crystal axis has the first index of refraction, the fifth crystal axis has the second index of refraction, and the sixth crystal axis has the third index of refraction.
14. The athermal CSP of claim 9 , wherein the second biaxial crystal retarder has a thickness between the second proximal surface and the second distal surface of between 300 mm and 800 mm.
15. The athermal CSP of claim 9 , the spectrometer being a dispersive spectrometer.
16. The athermal CSP of claim 9 , the spectrometer being a Fourier spectrometer.
17. A method for forming an athermal channeled spectropolarimeter, the method comprising:
positioning a first biaxial crystal retarder optically in series with a second biaxial crystal retarder such that a transmission axis of an electromagnetic (EM) signal subject to measurement passes through the first biaxial crystal retarder and the second biaxial crystal retarder, the first biaxial crystal retarder having a first axis, the second biaxial crystal retarder having a second axis, the first axis offset from the second axis by 45 degrees;
positioning a polarizer optically in series with the first biaxial crystal retarder and the second biaxial crystal retarder such that the transmission axis passes through the polarizer, the second biaxial crystal retarder positioned between the first biaxial crystal retarder and the polarizer, wherein the polarizer is configured to allow EM waves to pass therethrough that have a polarization direction parallel to the first axis of the first biaxial crystal retarder; and
positioning a spectrometer optically in series with the polarizer.
18. The method of claim 17 , wherein the biaxial crystal retarder is KTiOPO 4 .