New developments in FT spectrometer design

Laser fringes have long been used to establish the x-axis in interferometric spectrometry, but solutions for the intensity axis have been far less satisfactory. Now we are seeing the rapid commercial development of low-cost, medium speed sigma-delta analog-to-digital converters, developed for stereo audio applications. A single chip provides two channels of 20-bit precision at 50 kHz, a significant improvement over many current systems of much greater cost and complexity. However, while the laser works in the spatial domain, this converter operates strictly in the time domain; it cannot be triggered. We have developed a bridge between these two domains: the adaptive digital filter (ADF), which not only permits us to use this converter to obtain measurements at arbitrary times, but as a bonus shows us how to move much of the complexity of an interferometric control and data acquisition system from hardware to software. For example, flexible subdivision (to increase the free spectral range) is easily obtained via a simple and efficient algorithm, free of laser ghosts. Compensation for drive velocity variability is also possible, requiring only a modest amount of computer memory.