NEW DEVELOPMENTS IN FT SPECTROMETER DESIGN

Laser fringes have long been used to establish the x-axis in interfero metric spectrometry, but solutions for the intensity axis have been fa r less satisfactory. Now we are seeing the rapid commercial developmen t of low-cost, medium speed sigma-delta analog-to-digital converters, developed for stereo audio applications. A single chip provides two ch annels of 20-bit precision at 50 kHz, a significant improvement over m any current systems of much greater cost and complexity. However, whil e the laser works in the spatial domain, this converter operates stric tly in the time domain; it cannot be triggered. We have developed a br idge between these two domains: the adaptive digital filter (ADF), whi ch 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 comp lexity of an interferometric control and data acquisition system from hardware to software. For example, flexible subdivision (to increase t he free spectral range) is easily obtained via a simple and efficient algorithm, free of laser ghosts. Compensation for drive velocity varia bility is also possible, requiring only a modest amount of computer me mory.