<title>Continuously tunable frequency-doubled CO<formula><inf><roman>2</roman></inf></formula>-laser-based photoacoustic spectrometer for trace gas detection</title>

Abstract

We present a pulsed photoacoustic (PA) spectrometer based on a frequency doubled, continuously tunable high pressure CO2 laser. A quasi-phasematched diffusion-bonded GaAs crystal is implemented for second harmonic generation (SHG) and has advantages over well-known non-linear infrared (IR) materials such as AgGaSe2 and ZnGeP2, like high-average power capabilities, and high damage threshold. GaAs has a large non-linear coefficient (d14 = d36 = 150 pm/V), is transparent in the range 1 - 16 m, features low absorption and high thermal conductivity, but it is not birefringent. The periodically poled crystal is anti-reflection-coated (AR) for the 10 m and 5 m range, and consists of 53 GaAs plates of ~106 m thickness for first-order quasi-phase-matching. With 80 mJ pump energy we achieved up to 1 mJ SHG pulse energy. A non-resonant PA gas cell equipped with an 80-microphone-array is employed sealed-off and in flow mode. We use the fundamental as well as the SHG radiation for our spectroscopic measurements. To our best knowledge, we present the first PA spectra measured with such a system in the 5 m range, e. g., on NO buffered in N2 at atmospheric pressure and room temperature. The performance of this novel PA spectrometer is illustrated by various examples of biological interest.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.