High Frequency Optical Heterodyne Spectroscopy with Lead-Salt Semiconductor Diode Lasers

Absorption spectroscopy with lasers has traditionally been limited in sensitivity by noise intrinsic to the laser source. Numerous methods of surmounting this limitation using various forms of sample and source modulation have been proposed and demonstrated. One technique in particular, familiar to most users of lead-salt diode lasers, incorporates low-frequency wavelength modulation of the laser source and detection at either the first or second harmonics of the modulating waveform. Although this technique has been used successfully to measure weak absorptions, it remains fundamentally limited by diode laser amplitude fluctuations because the signal is detected in a frequency regime where the lasers exhibit considerable excess noise. Although the high frequency noise characteristics of lead-salt diode lasers are complex,1,2 one expects their excess noise power to drop considerably at frequencies larger than the intrinsic laser linewidth. Hence, diode laser frequency modulation at radio-frequencies should offer greater immunity to laser noise and consequently a higher sensitivity to absorption than can be obtained with conventional low frequency techniques.