High-Sensitivity Detection with Tunable Diode Lasers

Abstract

<p>This thesis describes methods for high-sensitivity detection of trace gases using tunable diode lasers (TDL). TDL absorption spectrometers have been widely used to acquire infrared spectra since 1970. However, high-sensitivity measurements have been limited mainly to the detection of low-pressure gases by harmonic techniques. The detection of atmospheric-pressure gases and the development of a rapid sweep technique for the accumulation of weak-absorption data over ~0.4 cmˉ¹ spectral regions are emphasized in this thesis. The results of this work may be divided into four areas where major accomplishments have been realized.</p> <p>Optical interference fringes generated by scattered light often limit the sensitivity of measurements performed with TDL spectrometers. The effect of fringes can be minimized by applying a jitter modulation. An investigation was undertaken to understand and quantify the effects of the jitter on the fringe and absorption signals. Simple analytical expressions describing the effects of the jitter modulation are derived and compared to experiment. Good agreement between theory and experiment is found.</p> <p>The limiting noise sources of a TDL spectrometer were carefully investigated and identified. Detection methods insensitive to the noise were developed. Application of these methods leads to sensitivity limits equivalent to detecting an absorption of ~10ˉ²% for atmospheric-pressure gases and ~10ˉ³% for low-pressure gases over path lengths ≤ 200 m. This represents a substantial improvement over previous work.</p> <p>A rapid-sweep technique capable of detecting absorptions due to low-pressure gases of ~10ˉ³% over path lengths ≤ 200 m is described. Data accumulation by the rapid-sweep technique allows direct determination of line shapes and linewidths from the measurements.</p> <p>An investigation of the TDL output power and frequency stabilitiy was performed as these laser characteristics directly affect the achievable sensitivity. The development of a new method to determine the TDL linewidth is reported. This technique permits the linewidth of any tunable laser to be quickly and easily measured.</p> <p>Throughout this thesis, the sensitivity of the various detection techniques is demonstrated by the detection of pollutants and trace gases. Whenever applicable, techniques for further increasing the sensitivity are discussed.</p>