Two-tone optical heterodyne spectroscopy using buried double heterostructure lead-salt diode lasers

Two-tone optical heterodyne detection using single mode buried double heterostructure lead-salt diode lasers is studied. Residual amplitude modulation with these lasers is found to be substantially smaller than has been noted in previous work with mesa stripe lasers, permitting improved sensitivity for measurement of small optical absorption. Minimum detectable absorption well below 10(-5) is measured at a laser power of 3 microW, in good agreement with signal-to-noise calculations using the experimental values of the FM and AM modulation indices. Optical heterodyne detection with two-tone modulation at 145 and 155 MHz is found to be nearly an order of magnitude more sensitive than conventional second harmonic detection at kilohertz frequencies. In addition to improved detection sensitivity, the double heterostructure lasers offer the important advantages of single mode operation above liquid nitrogen temperature.