Multichord, near infrared interferometers for the CTX and ZT-40M experiments

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Abstract
A versatile, multichord CO2 interferometer design with a wide dynamic range is adapted to several different experiments. The systems are double pass, Mach–Zehnder, heterodyne, interferometers [that employ a single germanium Bragg cell and 2n+1−1 (n is an integer) 50% beamsplitters to produce 2n interferometer chords], and a single reference beam with matched optical path lengths. Heterodyne signals detected with room-temperature HgCdTe photoresistors are demodulated with digital phase detectors that can track several fringes per microsecond. The real-time electron density readouts provide a considerable savings in ADC channels and computation time when compared with quadrature phase detection schemes. High reliability is achieved with a temperature-controlled wave guide CO2 laser operating on a single mode at intermittent duty. The CTX instrument has been operating with eight chords since March 1984. The ZT-40M instrument, which is under construction, employs a second wavelength to correct for room vibrations in the 0.01 fringe density range.

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