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In some fiber optic sensors, effective noise reduction is achievable by modulating signal radiation at the fiber input end. A small durable modulator is required if a fiber probe is to be inserted in a narrow hazardous environment. The use of plasma absorption in semiconductors is helpful in the development of an all-optical compact modulator that is compatible with various hazardous environments. Since the carrier recombination time in Si (∼0.5 ms) is longer than that in other semiconductors, Si is suitable for the 10–1000 Hz modulation required for sensitive infrared detection. Infrared light modulation at 1 kHz was demonstrated in the 2–12 μm wavelength range by Nd:YAG laser irradiation. This modulation technique was used in infrared fiber optic thermometry, and it successfully eliminated the effects of fiber temperature variation, formerly a serious problem in conventional thermometry. © 2003 American Institute of Physics.