A FIXED-DELAY, FREQUENCY-SHIFTED MICHELSON INTERFEROMETER FOR REMOTE AIR TEMPERATURE MEASUREMENT

Abstract

The spectral width of single-frequency radiation scattered in the atmosphere may be used to determine air temperature. In general, the measurement is complicated by pressure dependent changes in the spectral profile of the scattered radiation and by the inherently low received signal levels. A fixed-delay Michelson interferometer minimizes both of these problems by: (1) exhibiting a low sensitivity to pressure induced changes in the scattered spectrum and (2) optimally utilizing the available signal. By frequency shifting the signal in one arm of the interferometer relative to the other it is possible to efficiently modulate the output of the interferometer and make it insensitive to small changes in the center frequency of the scattered spectrum. Laboratory results obtained using a fixed-delay, frequency-shifted Michelson interferometer demonstrate the ability of this instrument to remotely measure air temperatures in the range 290 K to 310 K with an uncertainty of ±2 K with averaging times on the order of seconds at a received signal level of 6 x 10⁻¹⁰ watts.