A single-frequency Ho:Tm:YLF laser, operating at an eye-safe wavelength of 2 mum, has been developed with tuning characteristics optimized for spectroscopy of absorption features. The laser frequency was stabilized to three different absorption lines of carbon dioxide by a wavelength modulation technique. Long-term frequency drift has been eliminated from the laser, and shorter-term jitter has been reduced to within 13.5 MHz of the absorption line center. This stabilized laser is an ideal injection seed source for a differential absorption lidar system for measurement of atmospheric gases.
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A theoretical treatment of modulation absorption spectroscopy, applicable when amplitude and frequency modulation are present simultaneously, is given. The effects of any relative phase difference between these two types of modulation are incorporated and the theory is applicable to an experiment involving detection at any harmonic order of the modulation frequency. The results obtained are given in terms of the derivatives of the absorption lineshape function and bring out the connection to Nth-order derivative spectroscopy naturally. General expressions for any gaseous molecular absorption line are given. It is shown that certain advantages are obtained when one performs higher harmonic detection, including an increase in the wavelength resolution. Experimental results are obtained and compared with the predictions of the theory developed.
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