CONTROL AND DATA ACQUISITION FOR A HIGH RESOLUTION DYNAMIC FOURIER TRANSFORM SPECTROMETER

Abstract

A Fourier Transform Spectrometer (FTS) is an interferometric instrument configured so that the optical spectrum of the input to the instrument can be determined from the Fourier transform of the output. Spectral resolution is proportional to the maximum path differences in the interferometer. The Kitt Peak 1-Meter FTS is a high resolution instrument featuring a one-meter maximum path difference. The instrument is of the continuous motion type and utilizes equal time sampling of the interferogram. Realization of the potential of a one-meter path difference instrument requires very precise path difference positioning. Static and dynamic position accuracies of approximately seven Angstrom units are achieved in this instrument. Path difference command and control is accomplished through use of phase-lock techniques. A key element in the implementation is a Zeeman frequency stabilized Zeeman laser. Stabilization is accomplished by phase-locking the Zeeman frequency to a quartz crystal controlled system reference frequency. Path position information is derived by a control position interferometer which utilizes the Zeeman laser so that one cycle in phase at the Zeeman frequency corresponds to one wavelength in path difference at the laser optical frequency. Position control is achieved by phase-locking the output of the position control interferometer to a command signal. The command signal is synthesized from the laser Zeeman frequency by an additional phase-lock loop. Two approaches to minimizing spectral ghosts resulting from velocity errors are developed. One technique is applicable to band-pass designs. The second optimizes response of low-pass designs that include a single high-pass pole to reject the average value of the interferogram. Both techniques result in ghost responses improved by factors of at least ten to one-hundred compared with those of conventional data filters. The instrument includes an autoranging analog-to-digital converter with fifteen bits of resolution and total range of twenty-two bits developed to cope with the large dynamic range of the interferogram. These efforts together with careful design and construction have resulted in a powerful instrument which is in active use.