Abstract

The motivation is replacement of numerous thermocouples on the vacuum liner of a magnetic confinement fusion experiment. Fiber optics′ advantages over hardwire include immunity from EMI, no voltage insulation requirements, and the capability of averaging temperature over finite lengths. Costs per channel are minimized by using a common light source, a common phase conjugator, and multimode fiber components. Phase conjugation permits multimode fibers to be used with single-mode fiber sensitivity. Each interferometer gets argonion laser light through a multimode star coupler arm spliced to the interferometer multimode 2×2 input coupler. One arm of each interferometer is piezoelectrically modulated at 20 kHz with amplitude of ~2.4-rad phase shift. Both arms have GRIN lens fiber terminations that project beams with 0.8-mm vertical displacements onto a self-pumped BaTiO₃ phase conjugator. Three interferometer pairs can fit vertically on a 5-mm high crystal. An additional three pairs can be displaced horizontally on the same crystal with fiber lengths differing from the first three pairs by more than the laser coherence length to avoid phase crosstalk. Processing the detected light at two harmonics of the modulation frequency can give calibrated temperature-induced phase shifts unambiguously from fractional to multiple fringe counts.

© 1989 Optical Society of America