A high resolution submillimeter interferometer system for measurement of electron densities in the 1014-cm-3 ≤ ne ≤ 2 × 1015-cm-3 range has been developed for use in high density tokamaks. Phase modulation at ~1 MHz is accomplished by difference frequency mixing of two cavity tuned laser oscillators. The optically pumped CH3OH lasers, which operate on the 118.8-µtm line, feature a novel output coupling design that permits good mode quality and low beam divergence. The beat signals are detected using a newly developed Ge:Li photoconductor, and a direct measurement of the phase shift is obtained from the time lag between probe and reference signals. The sensitivity of the resulting phase measurement is independent of the instantaneous phase and unaffected by fluctuations in the amplitude or in the frequency of the modulation.
S. M. Wolfe, K. J. Button, J. Waldman, and D. R. Cohn, "Modulated submillimeter laser interferometer system for plasma density measurements," Appl. Opt. 15, 2645-2648 (1976)