1. Field of the Invention
This invention relates to a tunable laser trace gas analyzer and a method of reducing low frequency and high frequency power non-linearity errors in operating a laser in a gas analyzer to determine the light absorption characteristics of a sample gas.
2. Description of the Prior Art
Tunable laser gas analyzers are known as are methods of operation of said analyzers. Thurtell, et al. U.S. Pat. No. 5,331,409 describes a tunable laser trace gas analyzer and method of operation thereof where the sample and reference gas are maintained at the same temperature and pressure. The Thurtell, et al. patent describes low pass filtering to reduce high frequency fringe errors but does not suggest any low frequency fringe error rejection. Also, the analyzer described in Thurtell, et al can suffer from concentration drift due to high frequency fringe problems and there is no laser power non-linearity correction suggested. Further, the analyzer of Thurtell, et al. necessarily has a much greater length than the analyzer of the present invention.
The Whittaker, et al. U.S. Pat. No. 5,267,019 describes a method and apparatus for reducing fringe interference in laser spectroscopy. The apparatus and method use a reference gas and this patent does not suggest using odd cosine transforming or calculating the harmonics on a continuous basis and will suffer from concentration drift due to high frequency fringe problems.
The Tell, et al. U.S. Pat. No. 5,173,749 describes a method and apparatus to spectroscopically measure the concentration of a gas in a sample whereby a light of a laser diode is intensity and frequency modulated by a modulation signal to lock the light to an absorption line of the gas within a reference cell of predetermined pressure and concentration. The structure uses both a sample gas and a reference gas.
Reid J. has co-authored numerous papers on tunable diode lasers. For example, see Applied Optics, January 1978, pp. 300-307.