Continuously tunable high-pressure CO2 laser for spectroscopic studies on trace gases

A high-pressure CO₂ laser with unique characteristics in terms of continuous tunability and emission bandwidth is presented. It is operated at a pressure of 11.5 bar and transversely excited by short, high-voltage pulses generated by a double LC inversion circuit. Auxiliary discharges parallel to the electrodes provide a sufficient free-electron density through UV ionization of the laser gas mixture. The laser resonator consists of a near-grazing-incidence grating setup in which the grating is positioned at a large incidence angle of 77°. A theoretical model for the calculation of the emission bandwidth is presented and its predictions are compared to direct measurements and show excellent agreement. The achieved very narrow bandwidth of 0.018 cm^-1 constitutes the ultimate wavelength resolution of any detection system using this laser as radiation source. It allows the resolution of any fine structure in the spectra of absorbing gases at atmospheric pressure. Continuous tunability has been achieved over 76 cm^-1 between 932 cm^-1 and 1088 cm ^-1 with minimum pulse energies in excess of 10 mJ. The narrow bandwidth precludes the occurrence of mode-pulling effects so that the laser exhibits a linear wavelength tuning behavior throughout the entire emission range. The calibration of the laser wavelength is performed by photoacoustic measurements on low pressure CO₂ gas. An absolute accuracy of ±10^-2 cm^-1 is achieved. A great potential improvement in detection selectivity can thus be expected from a scheme with the high-pressure CO₂ laser as radiation source