Two-tone optical heterodyne spectroscopy with a tunable lead-salt diode laser

We report the demonstration of two-tone optical heterodyne spectroscopy with a tunable lead-salt diode laser. Using high-frequency modulation of the diode-laser injection current, we demonstrated a detection-sensitivity limit of 1 x 10(-5) to differential absorption from Doppler-broadened NH(3) lines in the 930-cm(-1) region. In our experiment, this limit was set by preamplifier noise and the low quantum efficiency of the photodetector. A substantial increase in sensitivity should be possible by using an improved detector, an improved preamplifier, and higher optical power.