High frequency modulation spectroscopy with tunable GaAs and lead salt diode lasers

Abstract

Tunable diode lasers covering the 0.8-30-μm spectral region are attractive light sources for the detection of gaseous species in applications like stack gas monitoring, pollution control, and industrial process monitoring. High frequency modulation spectroscopy with semiconductor lasers combines the simple and convenient tuning and modulation methods available for diode lasers with the high sensitivity and fast time response of optical heterodyne spectroscopy. We have tested the sensitivity limits of this technique, using NO2 and N2O absorption lines, and for the first time have implemented it with a lead salt diode laser. Semiconductor lasers can readily be frequency modulated at rf frequencies by directly modulating the injection current. However, the desired frequency modulation is accompanied by additional amplitude modulation. The zero background property of pure frequency modulation spectroscopy can be restored by careful adjustment of the local oscillator phase in the phase sensitive detection circuitry. The sample-induced signal arises from zero background and is detected at rf frequencies where diode lasers have little noise. The minimum detectable NO2 absorption (signal-to-noise ratio ≅ 1) was 1 × 10−7 for 1-Hz bandwidth and 5 × 10−4 for 20-MHz bandwidth. The sensitivity limit was essentially given by shot noise. The inherently fast response time of high frequency modulation spectroscopy is of particular interest for the investigation of gas kinetics and combustion processes.