Tilt-compensated interferometers

TILT-COMPENSATED INTERFEROMETERS 5.2.1. Cube-Comer Interferometers 

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Perkin-Elmer (now PerkinElmer) (PE) Corporation designed several types of tilt-compensated interferometers for their FT-IR spectrometers. All of these interferometers operate on a similar principle, which was first described by Sternberg and James in 1964 [22]. In these interferometers, two or more mirrors are mounted on a common base plate which is rotated to give rise to the path difference in two arms of the interferometer. 

A successM variant of this principle where the OPD is introduced by rotating a refractive element has been designed by Mold and Tlimer of PerkinElmer Ltd. [24,25]. When tested in comparison with several other tilt-compensated interferometers, this instrument had better resistance to the effects of tilt and shear than that of any other instrument tested. For example, the modulation efficiency of this interferometer in the near infrared did not change by more than 0.1% when the temperature was changed by more than 5°C [26]. This instrument has one fairly important disadvantage, however, in that it is far more expensive instrument to build than competitive designs and has not been introduced commercially. 

One potential disadvantage of refractively scanned interferometers is the effect of dispersion on the wavenumber accuracy. However, the variation of n with wave-number is known and this effect can be compensated accurately by the appropriate software. This disadvantage is more than offset by the fact that the stationary cube corners in each arm of the interferometer give it a remarkable immunity from the effects of tilt and shear. As a result, this interferometer is often used for process monitoring by FT-IR spectrometry. This interferometer has been installed as the key component of process monitors in well over 100 plants. 

In the Bomem F.T. spectrometer the simplicity of the flat mirror interferometer is maintained while at the same time compensating for errors in the orientation of the moving mirror by means of an active servo controlled tilt adjustment system. 

Fig. 5.8.7 Interferometer with double passing and cat s eyes is compensated for small errors in tilt of optical components as well as lateral shift of the carriage motion. The ATMOS interferometer has a similar design.

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