Frequency modulation spectroscopy with lead-salt diode lasers: a comparison of single-tone and two-tone techniques.

We present a theoretical and experimental comparison of single-tone and two-tone frequency modulation (FM) spectroscopy using lead-salt diode lasers. Our analysis reveals those diode laser operating characteristics that are necessary for high sensitivity performance using either technique for IR absorption measurements. High sensitivity performance using these techniques requires laser diodes having low incidental amplitude modulation and a small variation in FM/AM phase shift over a suitable diode tuning range. Neither requirement is met with the present mesa-stripe lead-salt diode laser technology.