INFRARED MODULATOR FOR OPTICAL SENSORS BASED ON A VIBRATING FABRY-PEROT-INTERFEROMETER

The development, construction and space qualification of an IR modulator for f/1 radiation in the 14-16 mu m band is described. The modulation is based on the principle of a vibrating Fabry-Perot (FP) interferometer with gap width variations of plus or minus lambda m/8 approximately equals 2 mu m( lambda //m-center wavelength) which is considered micromechanical. By connecting two such Fabry-Perot interferometers in series (with two outer CdTe plates in fixed positions and one common vibrating inner CdTe plate for harmonic gap width variation) a modulation efficiency of about 75% is attained. CdTe is superior to other materials as far as IR absorption (0. 2%/cm) within the minus 20 degree C to plus 60 degree C temperature range and the 2 to 25 mu m wavelength range is concerned. Its modulation efficiency in a single FP interferometer is however too low because of its low refractive index (n equals 2. 5). The double FP interferometer is designed for small weight, large receiving aperture, small power consumption and high reliability in long-time space operation. The modulator is operated at a selected resonance frequency between 100 Hz and 1 kHz. It can be tuned to other wavelengths in the 2 to 25 mu m range.