Stark effect spectrophone for continuous absorption spectra monitoring

Abstract

A Stark effect spectrophone is provided using a pulsed or continuous wave laser (33) having a beam with one or more absorption lines of a constituent of an unknown gas. The laser beam is directed through windows (31,032) of a closed cell (30) while the unknown gas to be monitored is caused to flow continuously through the cell between electric field plates (34, 35) disposed in the cell on opposite sides of the beam path through the cell. The plates are so disposed as to be divergent, e.g., flat plates at an oblique angle relative to each other, or plates shaped according to a mathematical function so that, with constant voltage applied across the plates, there is a linear variation in electric field strength along the beam path. Discrete pressure transducers (37) are positioned at field strength points of interest. When the beam is pulsed, energy absorbed by the gas then present in the cell will increase at each point along the beam path according to the spectral lines of the constituents of the gas for the particular field strengths at those points. The pressure measurement at each point during each pulse of energy will yield a plot of absorption as a function of electric field for simultaneous detection of the gas constituents of interest. Signal averagers (39) are provided to permit repeatedly pulsing the laser for averaging before recording and processing the data. When the beam is continuous (i.e., not pulsed), the bias voltage may be modulated to dither the electric field at each point. Still other techniques that may be used for modulation of the absorbed energy include polarization rotation, wavelength modulation and conventional 50% duty cycle chopping of the incident beam.