Method and Apparatus for Background Signal Reduction in Opto-Acoustic Absorption Measurement

This invention relates to opto-acoustic absorption detectors commonly referred to as spectrophones, and more particularly to a method and apparatus for measuring the concentrations of absorbing gases in a sample cell using opto-acoustic detection of absorption.

There is an increasing interest in detecting trace amounts (approaching 0.01 parts per billion) of atmospheric pollutant gases in an air sample. However, the residual background signal, such as that produced by the heating of the cell windows, limits the sensitivity of opto-acoustic absorption detectors. Terrence F. Deaton, David A. Depatie and Thomas W. Walker report their efforts to overcome this problem of sensitivity with a differential cell in a paper titled "Absorption Coefficient Measurements of Nitrous Oxide and Methane at DF Laser Wavelengths," Applied Physics Letters, Vol. 25, No. 6, 300-303 (1975). Two identical cells were used in tandem. Both were first filled with non-absorbing gas, and the pressure differential signal between the cells was then minimized while the laser beam was transmitted through both cells to "zero" the instrument. Absorption measurements were then made by filling one cell with a sample. The pressure differential signal between the two cells represents the absorption coefficient of the test gas. Residual pressures which are present as background signals due to absorption by the windows of the cell is effectively balanced out by this double cell arrangement, but it has the problem that extreme care must be exercised in filling both cells to the same pressure during the balancing procedure and during the measuring procedure. Still another approach to the problem referred to, but not discussed in their paper, is the development of a "windowless resonant cell" by David A. Depatie. The problem with that approach is said in that paper to be that "the pickup of ambient acoustic noise increases."

In accordance with the present invention, the residual background signal in an opto-acoustic pressure detector is reduced by alternately diverting a beam of light to a second path through a second pair of windows in a sample cell. The lengths of the two paths are significantly different. The beam intensity in the two paths is then adjusted through suitable means to minimize the detected pressure difference between the two paths while the beam wavelength is tuned away from the absorption lines of the sample in the cell, thereby reducing any residual background signal due to absorption pressure of the window material. When the beam wavelength is thereafter tuned to the absorption line of any trace constituent of a gas sample in the cell, the pressure difference between the two paths through the two cells is due only to absorption by the specific constituent of interest in the cell having a spectral line at the wavelength of the laser.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.

FIG. 1 is a schematic diagram of an exemplary embodiment of the present invention.

FIG. 2 is a diagram of waveforms useful in understanding the present invention.

Referring now to FIG. 1, a sample cell 10 and tunable laser 11 is shown in an arrangement for opto-acoustic measurement of trace amounts (approaching 0.01 parts per billion) of atmospheric pollutant gases in an air sample. The cell is provided with an inlet valve 10a and an outlet valve 10b for sampling the air at some convenient pressure using a suitable pump (not shown) for first purging the cell while both valves are open, charging the cell with only valve 10a open and then closing the valve 10a when the desired static pressure is reached as measured (by means not shown).

The normal operation of an opto-acoustic detector requires tuning the laser wavelength to the absorption line of the molecules expected in the gas sample in the cell. The energy absorbed by the molecules increases the pressure in the cell. The increase in pressure is detected through a transducer 12, such as a capacitor microphone, and displayed on a meter 13 or on a cathode ray tube (CRT) 14 via an amplifier 15 implemented as a lock-in amplifier for sensitive, narrow bandwidth amplification in order to eliminate noise. That pressure is then a measure of the amount of the molecules, i.e., the concentration of the trace constituent of interest. The problem with this normal operation is that windows 16 and 17 for the laser beam path through the cell will absorb energy and cause an increase in pressure within the cell. The resulting background signal due to absorption losses in the cell windows is often large as compared to the absorption by trace constituents of a sample, making it impossible to use opto-acoustic techniques for detecting the presence of trace constituents.

To significantly improve the sensitivity of the opto-acoustic detector, a reflecting chopper 20 is employed to direct the laser beam via mirrors 21 and 22 through a second pair of windows 23 and 24. Lenses 25 and 26 are simply for focusing the laser beam through the cell windows. The chopper alternates the laser beam into the two paths, preferably with equal time through each path, although that is not necessary. By adjusting the light beam power through attenuators 27 and 28, it is possible to compensate for any time difference.

The alternate beam path through the second pair of windows is employed to balance out residual pressures due to absorption by the first pair of windows 16 and 17, although all windows are made of the same material and thickness, but that is not a requirement.

It should be noted that the alternate (reflected) beam path has a length l.sub.r through the cell much shorter than the primary (direct) path which has a length l.sub.d. Since the pressure, P, in the cell is proportional to the absorption coefficient, .alpha., and the path length, l, for a fixed cell volume, v, the direct path pressure (P.sub.d .perspectiveto.kl.sub.d, where k=.alpha./v) will be much greater than the reflected path (P.sub.r .perspectiveto.kl.sub.r). Consequently, as the laser beam is switched through the two paths, it modulates the cell pressure as shown in waveform C of FIG. 2. While the laser wavelength is turned off resonance of an absorption line of the sample gas, the attenuators 27 and 28 are adjusted so that the rms value of the first harmonic of the pressure difference (variations in amplitude of the waveform C) is minimum. The waveform displayed on the CRT will then be as shown by the dotted line waveform C in FIG. 2, assuming only the attenuator 28 is adjusted in this example. If both are adjusted, the dotted line waveform will have substantially the same form (approaching a straight line), but at a lower amplitude. Both attenuators may be conventional crystals for transmitting the polarized light of the laser with means for rotating the axis of the crystal relative to the axis of polarization of the light to attenuate the light transmitted.

In operation, the cell is charged with a sample of mixed gases and the laser wavelength is tuned away from the absorption line of the trace constituent suspected to be present in the sample. The relative beam intensity of the two transverse radiation paths is then adjusted to balance out the background signal due to absorption by the window material. That is done by operating on one or the other, or both, of the attenuators 27 and 28 in the beam paths, and observing the difference in pressure in the cell displayed on the CRT, as shown in waveforms C of FIG. 2. The solid line waveform C is indicative of the windows 23 and 24 absorbing more energy than the windows 16 and 17, but any unbalance may be opposite to that. By so adjusting the attenuators as to minimize the amplitude of any modulation present in that waveform C, any residual background in signal due to absorption by the windows 16 and 17 is balanced out.

Once the balancing procedure has been completed, the attenuators are locked in their adjusted position and the laser wavelength is tuned to the absorption lines of the constituent gases of interest. The pressure difference (amplitude of modulation) in the detector output displayed on the CRT as shown in waveform D is a measure of the amount (concentration) of a constituent associated with the wavelength of the beam. For example, if three pollutants are suspected of being present in an air sample, the laser is tuned to the wavelength of the absorption line of each in sequence, noting at each wavelength the amplitude of the modulation on the detector output, i.e., on the output of the pressure transducer. The detected pressure amplitude is the measure of the amount of each pollutant present. Trace gas concentrations as low as 0.01 parts per million can be detected with this differential-path cell because it significantly reduces the background signal. The adjustment of the relative power in the two beam paths need not be readjusted if the power output of the laser varies. This is an important advantage. No special reference gas is required for this balancing procedure. That is another important advantage.

This differential-path cell may be used in situ for air pollution monitoring because it is rugged, simple, and occupies minimal space. It may also be used to equal advantage in a laboratory for measuring the concentration of each gas in a mixture of known gases, or for measuring absorption coefficients of known gas samples.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and equivalents may readily occur to those skilled in the art. For example, both sets of windows could be set at Brewster's angle to minimize reflection of light by the window material, particularly reflection of light from inside surfaces back through the sample in the cell. Use of Brewster's angle will, of course, permit arrangements for the two light paths other than at right angles to each other. Also light sources other than lasers could be used, but for sensitivity on the order of 0.01 parts per billion, a laser is required. Other suitable light sources might be ordinary white light sources directed in a beam through filters that pass a selected narrow band of wavelengths as required. Consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.