Abstract

High frequency modulation spectroscopy (FM) is a promising tool for high speed and ultrasensitive atmospheric trace gas monitoring. From wideband noise characteristics of a lead salt diode laser a potential sensitivity improvement of 2 orders of magnitude in comparison to conventional derivative (2f) spectroscopy can be derived. This enhancement can be achieved by moving in modulation frequency space from the 1/f-noise dominated region (10 kHz) into a shot noise limited frequency domain at 100 MHz. Ultrasensitive absorption spectroscopy of NO₂ was performed with a tunable lead salt diode laser (TDL). With a detection bandwidth of 200 kHz, an optical density of 2.7 × 10⁻⁵ was detectable at a signal-to-noise-ratio of 1. The detectable optical density could be further improved by reducing the detection bandwidth in agreement with the √Δf-relationship, reaching 2.5 × 10⁻⁶ at a detection bandwidth of 1.56 kHz. This detection limit agrees well with the calculated quantum-limited performance based on the measured laser power, modulation index, and other parameters of the apparatus. These measurements and calculations show that by implementation of the FM technique, the sensitivity of the present TDL absorption spectrometers (TDLAS) can be improved by at least a factor of 10 and possibly even of 100. Such a sensitivity improvement would greatly extend the use of TDLAS for trace gas analysis, especially in atmospheric monitoring.

© 1989 Optical Society of America