NIRFlex N-500
Polarization interferometer
Novel rugged crystal interferometer for superior performance
- Polarization interferometer (click to enlarge)
The BUCHI polarization interferometer
BUCHI has adapted the successful quartz polarization interferometer by using crystals with a very high refractive index. This leads to optimum resolution and a compact design maintaining the advantage of insensitivity to mechanical disturbances.
The reason for the superior performance of this interferometer in rough environments is the difference of spatial movements and the optical path shifts for the two light beams in a birefringent crystal of variable thickness. Whereas in a Michelson type interferometer mechanical distortions directly affect the interference, such effects are reduced by a factor of 10 to 20 in a crystal interferometer.
Optimum resolution
Because NIR absorption bands are relatively broad in the case of liquids as well as for solid samples, resolutions higher than 8 cm-1 no longer improve the performance of NIR applications. On the contrary: Data collection with a higher resolution than the natural line width leads within a given measurement time to a poorer signal-to-noise ratio.
Furthermore, the data sets become unnecessarily large. It is therefore possible to keep the design of a polarization interferometer for optimum resolution extremely compact without losing its intrinsic advantage of ruggedness.
Principle of the polarization interferometer
When light strikes an anisotropic crystal, it is split into two components. These are polarized perpendicularly to each other and traverse the crystal at different phase velocities. The heart of the interferometer is its two prisms, which together form a cuboid. As soon as the previously 45° polarized light enters the crystal, the two partial beams traverse at different velocities.
As a result, they undergo a phase shift relative to each other which is systematically varied by moving one prism relative to the other. This changes the polarization of the combined beam. Therefore, after the second polarizer, a sinusoidal variation of the radiation is observed for monochromatic light. Radiation of several frequencies results in a superposition of various sine waves: an interferogram.
- Principle of the polarization iterferometer (click to enlarge)
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