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High-frequency-modulation spectroscopy with a lead-salt diode laser.

For the first reported time, high-frequency optical heterodyne spectroscopy has been performed with tunable lead-salt diode lasers. N2O transitions at 1259.2 cm−1 were observed by modulating the injection current at 50–200 MHz and detecting the heterodyne beat signal generated by the frequency- and amplitude-modulated light. By proper adjustment of the rf phase in the phase-sensitive detection circuitry, the background contributions due to laser amplitude modulation could be completely suppressed. The sample induced signal is measured against zero background and detected at radio frequencies at which diode lasers have little noise.