Spectrophone assembly for identifying and detecting multiple species

A spectrophone assembly comprises a single detector chamber, a plurality of lasers, a gas inlet for supplying a gas sample to the single detector chamber, and at least one microphone. The detector chamber has an internal geometry arranged to be simultaneously acoustically resonant at a plurality of different resonant frequencies. Each laser operates at a different wavelength and is positioned to emit radiation into the single detector chamber, and is operable to emit radiation that is amplitude modulated at a frequency rate corresponding to a particular resonant frequency different from the resonant frequency of each other laser, simultaneously with each other laser. The microphone(s) are positioned in the single detector chamber so that each microphone is located at or near a maximum of a corresponding acoustic resonance defined by the internal geometry of the detector chamber.

FIELD OF THE INVENTION

This invention relates to the identification and determination of the concentrations of multiple species in a gas sample by means of a spectrophone.

BACKGROUND OF INVENTION

In laser based photo acoustic spectroscopy, each molecular specie in a detection chamber is basically detected from the response to illumination by laser radiation of a Specific wavelength. Absorption of such radiation by a specie in the detector chamber at the specific wavelength produces an amplitude modulated pressure which is detected by a microphone in the detector chamber. Generally, if more than one specie is involved, and interference from unwanted species is to be taken into account, then operation at corresponding different wavelengths is required. The procedure involved ultimately sorts out different species and/or interfering components.

Such a procedure normally requires the operation of a spectrophone at different wavelengths in time sequence, that is to say requires that the laser be tuned in time ordered sequence to different wavelengths. When each wavelength arises from a different and separate source, such as a set of semiconductor lasers each operating at a different wavelength, the illumination from each such laser is injected into the spectrophone in timed sequence. Thus, measurement of multiple species cannot be carried out simultaneously and consequently requires more time for the species to be identified and their concentrations determined.

It is therefore an object of this invention to provide a method simultaneously identifying and determining the concentrations of multiple species by means of a spectrophone.

SUMMARY OF INVENTION

In one aspect, the present invention is directed to a spectrophone assembly. The spectrophone assembly comprises a single detector chamber, a plurality of lasers, a gas inlet for supplying a gas sample to the single detector chamber, and at least one microphone. The detector chamber has an internal geometry arranged to be simultaneously acoustically resonant at a plurality of different resonant frequencies. Each laser operates at a different wavelength and is positioned to emit radiation into the single detector chamber, and is operable to emit radiation that is amplitude modulated at a frequency rate corresponding to a particular resonant frequency different from the resonant frequency of each other laser, simultaneously with each other laser. The at least one microphone is positioned in the single detector chamber so that each microphone is located at or near a maximum of a corresponding acoustic resonance defined by the internal geometry of the detector chamber.

In one embodiment, the internal geometry of the detector chamber comprises at least two cylindrical tubes internally connected to one another.

In one embodiment, the spectrophone assembly further comprises at least one power meter, a filter, a plurality of phase-lock amplifiers, and a computer. The at least one power meter is sensitive to the frequency rate and wavelength of each laser and is positioned to receive radiation from the lasers after said radiation has passed through the detector chamber. The filter is coupled to the at least one power meter and operable to separate exit power measured by the at least one power meter into power components for each of the frequency rates. Each phase-lock amplifier is coupled to a corresponding microphone to receive a signal therefrom, referenced to the frequency rate corresponding to the position of its respective microphone, and operable to evaluate the microphone signals and convert them into direct current values. The computer is coupled to the filter to receive the power components therefrom and to the phase-lock amplifiers to receive the direct current values therefrom, and is operable to use the power components and the direct current values to generate specie identifications. The assembly may include a single power meter sensitive to the frequency rate and the wavelength of each laser, or a plurality of power meters, each power meter being sensitive to the frequency rate and the wavelength of a particular laser.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first toFIG. 1of the drawings, a spectrophone assembly includes a cell or chamber3which in use is illuminated by two lasers1,2with different wavelengths λ1, λ2respectively. The chamber3is formed by two cylindrical tubes with internal length and diameter L1, D1and L2, D2respectively, the two tubes being internally connected and vacuum closed to the outside.

The lasers1,2are power modulated at frequencies ω1and ω2by modulators1aand2arespectively and, as references, these frequencies are sent to separate phase-lock amplifiers10,11. The laser beams are guided into the chamber3by fiber optics or wave guides4and the two beams are combined by a dichroic mirror5which transmits the beam from laser1and reflects the beam from laser2.

The chamber3is closed by two radiation transmitting windows6through which the laser beams pass before being received by a radiation power meter7adjacent the exit window6. A single power meter7is used to measure the exit powers for both power beams simultaneously. Thus, the power meter7must have a response time fast enough to detect beams at both modulation rates ω1and ω2. Alternatively, separate power meters each sensitive to a corresponding modulation rate may be used.

The exit power P measured by power meter7is separated by a filter8into power components for ω1and ω2which are then sent to a computer14for signal normalization purposes. A gas sample containing trace amounts of the species of interest is passed into and out of the chamber3through valved ports9. The gas sample is typically air at a pressure of about one atmosphere. The sample may be a static gas fill or may be continuously flowed through the chamber3. The necessary electric power supplies are of course provided as will be readily apparent to a person skilled in the art.

The chamber3is provided with microphones M1, M2. The acoustic responses chosen for operation should be sufficiently separated in frequency space such that there will be no overlap in the response from the microphones. This can be effected by proper design and selection of the internal geometry of the chamber3.FIG. 2illustrates a typical frequency separation of the two acoustic resonances in this embodiment. Such frequency separation provides a basic filter between the responses of the microphones and also filters substantially all acoustic noise and/or erroneous signals arising from outside the bandwidths of the subject resonances.

Referring now again toFIG. 1, if the specie to be detected has an absorptivity at λ1, the absorption will produce gas heating which, because of the fixed chamber volume, causes a pressure change modulated at a frequency of ω1which is sensed by internal microphone M1. The resulting electronic signal es1, from microphone M1is fed to and measured by lock-in amplifier10. The modulation rate ω1corresponds to an acoustic resonance frequency at ω1which amplifies the pressure changes at this modulation rate. The frequency bandwidth of the resonance is sufficiently narrow to effectively prevent frequency overlap, within its bandwidth, with other resonances and thereby filters out acoustic signals from any source at frequencies outside the bandwidth of the resonance at ω1. The resonance frequency is determined by the internal geometry of the chamber3. Microphone M1is located at or near a maximum of the pressure standing wave12, the amplitude of which is shown inFIG. 1.

In a similar manner, microphone M2is located near a maximum of pressure standing wave13in the side arm with length and diameter L2, D2. The resonance in this case is at frequency ω2and outside the bandwidth of the resonance at ω1. The signals es1and es2from microphones M1and M2respectively are fed for processing to separate phase-locked (lock-in) amplifiers10,11, each referenced to the corresponding frequencies ω1and ω2respectively. The phase-locked amplifiers evaluate the microphone signals, convert them into direct current values and subsequently feed them into the computer14. The exit powers of the two beams P(ω1) and P(ω2) required for normalization of the microphone signals are also fed into the computer14. The computer analyzes the computer data and produces the specie identifications and their concentrations for display.

The acoustic resonances of the chamber3are defined by its internal geometry in accordance with the following equation:
ωkmn=πc[(k/L)2+(βmn/R)2]1/2(1)
where ωkmnis the acoustic resonance frequency, in radians per second, defined by a cylindrical section of length L between the end boundaries and of internal radius R, c is the velocity of sound for the gas at the pressure and temperature inside the chamber3, k is an integer having values corresponding to longitudinal harmonics, and βmnis the nthroot of the derivative of the Bessel function Jm(πβ), of order m, with respect to β.

It should be noted that the acoustic resonance is a pressure standing wave where the boundaries defining the length L can be any discontinuity in the cross section, such as the window boundary at each end of the chamber3. The windows need not even be present, i.e. the ends of the cell may be open. The basic fact is that these boundaries define the standing wave nodes of zero pressure. Each tubular section of the cell (as shown being utilized by microphones M1and M2) has available to it a number of resonances defined by the values of k, m and n and the sum and difference resonance frequencies by various combinations of resonances arising from the two sections shown inFIG. 1. All of the differing resonances can be used to increase the number of radiation sources of different wavelengths illuminating the chamber3where each source is modulated and its microphone response is processed at its resonance frequency.

There are many internal geometrical configurations of the chamber3which promote acoustic resonances. To illustrate the principles involved,FIGS. 3a,3b,4a,4b,5aand5bshow schematics of some configurations (with the pressure profile indicated by dotted lines) for some resonant pressure standing waves. The maxima are the most desirable locations for a microphone.

FIGS. 3aand3bshow fundamental resonance modes available with the geometry illustrated. The expansion bulbs16,17,18define the lengths to determine the resonance frequency by equation (1) for each case. The expansion bulbs16,17,18provide an abrupt change in the tube cross section which is sufficient to force a pressure node (zero pressure point) within the vicinity of the entrance to the bulb. The chamber15need not be closed at both ends. As shown, one end is open.FIG. 3shows the best locations X for a microphone for each resonance. A single microphone of sufficient frequency bandwidth can be located in an overlapping region of two or more resonances where the pressure value in each case is non-zero.

FIGS. 4aand4bshow a configuration with two resonance side arms19,20to provide difference resonance frequencies, the side arms19,20having non-equal length and/or diameters as illustrated. Various other resonances, all of different frequencies, can of course be obtained from the geometry shown.

FIGS. 5aand5bshow a configuration similar to that ofFIGS. 4aand4bexcept that the side arm tubes19,20are replaced by the geometries of disks21,22of radii R1, R2respectively. The acoustic resonances in these disks will be dominated by the second term (π c βmn/R, i.e. the radial patterns of standing pressure waves, of equation (1). Additional resonance frequencies are also obtainable from harmonics or overtones and combinations in terms of sums and difference frequencies of these fundamental resonances. Further, the same principles apply to rectangular cross sections or any geometry where there will be points between which pressure standing waves can be produced.

Thus, as described above, a photo acoustic cell can be simultaneously illuminated by a number of radiation sources, each of different wavelength, and simultaneously analyzing a gas sample in the cell for its specie identifications and concentrations. The gas pressure in the chamber may be in the range of from about 0.1 TORR to as high as practically possible, and the detectable concentration of a specie may range from a trace to 100%.

The advantages and other embodiments of the invention will now be readily apparent to a person skilled in the art, the scope of the invention being defined in the appended claims.