In this paper the recent developments in the field of diode-laser photoacoustics will be discussed. Photoacoustic detector designs with high sensitivity and signal-to-noise ratio will be presented. The advantages and disadvantages of different modulation and measurement techniques will be discussed and compared. Simple expressions will be given for an estimation of the sensitivity achievable with an optimized photoacoustic detector. Recent results will be presented for a state-of-the-art dual-resonator differential cell suitable for sensitive trace gas analysis with low electronic and acoustic noise. With this resonant photoacoustic cell and a near-infrared DFB diode laser with 42 mW power at 1.53 mm polar ammonia molecules could be detected with a sensitivity of 200 parts-per-billion volume (ppbv) under flow conditions used to reduce the adsorption problem. By excitation of fundamental vibrations in methane using a pulsed optical parametric oscillator (OPO) with 60 mW output power a sensitivity of 1.2 ppbv has been achieved using this resonant cell. This setup allows sub-ppbv detection of the greenhouse gas methane with a concentration of about 1.7 ppmv in ambient air.
We have combined a LiNbO3 optical parametric oscillator (OPO) and a photoacoustic (PA) open cell to perform air pollution measurements. THe OPO is pumped by a Nd:YAG laser working at 10 Hz. It is tunable between 1.55 micrometers and 4.5 micrometers . The PA cell is designed to be isolated from outside acoustic noise. The OPO output wavelength is tuned between 3.39 micrometers and 3.44 micrometers to monitor absorption lines of methane. The characteristics of the pulses are E equals 2 mJ, (tau) equals 15 ns, (Delta) (nu) equals 2 cm-1. The minimum detected concentration is 1 ppm. Decreasing the background noise, detection of concentrations as low as 20 ppb is expected.