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A high-sensitivity spectroscopic system has been constructed with a GaAlAs diode laser by using two-tone frequency-modulation spectroscopy. We have demonstrated an absorption sensitivity of 3 × 10−7 in a 0.84-Hz bandwidth by using the Doppler-broadened water vapor absorption line at 12 238.32 cm−1. The sensitivity is limited by laser excess noise and is approximately five times less than the sensitivity expected on the basis of detector noise.
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Wang et al. (1989) studied this question.
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