Quantum noise-limited FM spectroscopy with a lead-salt diode laser

This paper reports the demonstration of quantum-noise-limited sensitivity in frequency modulation spectroscopy (FMS) with a lead-salt diode laser operating in the 4.8-microm region. Experimentally determined sensitivities of 10(-7) in a 1-Hz bandwidth have been demonstrated for detection of CO and CO(2) in a single-pass absorption cell. The high sensitivity obtained with our FMS apparatus can be attributed to the use of improved lead-salt diode lasers, a new optical fringe reduction method, and careful attention to optical processing of the diode laser beam.