WO2014132046A2 - Photoacoustic chemical detector - Google Patents

Photoacoustic chemical detector Download PDF

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Publication number
WO2014132046A2
WO2014132046A2 PCT/GB2014/050548 GB2014050548W WO2014132046A2 WO 2014132046 A2 WO2014132046 A2 WO 2014132046A2 GB 2014050548 W GB2014050548 W GB 2014050548W WO 2014132046 A2 WO2014132046 A2 WO 2014132046A2
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WO
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Prior art keywords
photoacoustic
light source
sensor
liquid
light
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PCT/GB2014/050548
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French (fr)
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WO2014132046A3 (en
Inventor
Hideaki Page
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Scytronix Ltd
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Priority to GB1303546.4 priority Critical
Priority to GB201303546A priority patent/GB2511327A/en
Application filed by Scytronix Ltd filed Critical Scytronix Ltd
Publication of WO2014132046A2 publication Critical patent/WO2014132046A2/en
Publication of WO2014132046A3 publication Critical patent/WO2014132046A3/en

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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/01Arrangements or apparatus for facilitating the optical investigation
    • G01N21/03Cuvette constructions
    • G01N2021/0378Shapes
    • G01N2021/0382Frustoconical, tapered cell
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G01N2021/1704Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases

Abstract

The invention relates to a photoacoustic chemical detector for detecting one or more target chemicals in a sample gas or liquid, a method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector, and a photoacoustic sensor for use in a photoacoustic chemical detector. The light source (e.g. Fabry-Perot laser) output is encoded by controlling the drive current, drive voltage and/or operating temperature in order to produce a known multimode emission (having known peaks of intensity at known frequencies) that is used to irradiate a sample gas or liquid within a specially designed photoacoustic sensor, which generates an acoustic signal that is detected by a microphone. The encoded drive signal applied to the light source results in a varying trace in the photoacoustic signal that can be used to analyse and identify the of the sample in the photoacoustic cell. The photoacoustic signal is the convolution of the programmed frequency encoding of the light source with the absorption spectrum of the sample.

Description

Photoacoustic Chemical Detector

FIELD OF THE INVENTION The invention relates to a photoacoustic chemical detector for detecting one or more target chemicals in a sample gas or liquid, a method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector, and a photoacoustic sensor for use in a photoacoustic chemical detector. BACKGROUND OF THE INVENTION

Laser based spectroscopy is an established technique for chemical detection. Typically, laser light is tuned to the resonant absorption modes of target molecules and atoms. Light matching the vibrational frequency of these modes is absorbed whereas light of other frequencies is not absorbed. This absorption can be measured with a suitable detector. Semiconductor lasers have been used to realise practical spectroscopic systems; they are convenient as they are small and robust, all solid state and may be driven electrically. Typically, a single frequency laser, such as that based on a distributed feedback laser, a coupled cavity device, or external cavity laser is scanned over the spectroscopic region of interest and the resulting transmission or absorption of the beam is measured to provide a unique fingerprint that is characteristic of the chemical sample that is being analysed. However, single mode lasers with useful tuning properties are difficult to realise practically and are often costly to produce.

Numerous embodiments of laser, sample cell, and detector have been used to realise practical laser based spectroscopy systems. These fall into two broad categories. 1 ) Optical detection, where the light transmitted through a sample is measured on an optical detector such as a photodiode or otherwise. 2) Photoacoustic detection, where is absorbed radiation is transformed into pressure and heat, which are converted into an acoustic wave that can be measured with a microphone. However, many of the known systems are expensive or difficult to implement. We have therefore appreciated the need for an improved photoacoustic chemical detector.

SUMMARY OF THE INVENTION

The present invention therefore provides a photoacoustic chemical detector for detecting one or more target chemicals in a sample gas or liquid, the photoacoustic chemical detector comprising: a light source for emitting light comprising two or more discrete optical modes; a photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a sensor signal in response to acoustic energy created when received light from the light source interacts with a sample gas or liquid contained within the photoacoustic sensor, the sample gas or liquid comprising one or more target chemicals to be detected; a controller electrically coupled to the light source and the photoacoustic sensor, wherein the controller is configured to: supply a drive signal to the light source such that the light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; read the sensor signal output from the photoacoustic sensor; and detect one or more target chemicals in a sample gas or liquid using the sensor signal.

Advantageously, by controlling the light source to emit a plurality of known discrete modes, where each mode has a defined frequency and intensity, this enables the sample gas or liquid to be sampled at multiple different wavelengths simultaneously. In preferred embodiments, the drive signal defines one or more of an operating current, an operating voltage and an operating temperature of the light source.

The controller may also be configured to: supply a second drive signal to the light source such that the light source controllably emits light comprising a second plurality of discrete modes, wherein the second drive signal is different to the first drive signal; read the sensor signal output from the photoacoustic sensor in response to the second drive signal; and detect one or more target chemicals in a sample gas or liquid using the sensor signal output from the photoacoustic sensor in response to the first and second drive signals. Applying a second (and subsequent) drive signals, where the light emitted by the light source in response to the second or subsequent drive signals defines different operating modes of the emitted light, enables the sample gas or liquid to be sampled at a second plurality of wavelengths simultaneously.

The controller may also be configured to detect one or more target chemicals in a sample by comparing the received sensor signal with a known sensor signal produced when a known gas or liquid is exposed to the light source when driven with the drive signal, and detecting and identifying one or more target chemicals when the received sensor signal is substantially similar to the known sensor signal.

Alternatively, the controller may be configured to detect one or more target chemicals by deconvolving the frequency spectrum and intensity levels of the emitted light from the received sensor signal to reveal a sample spectrum comprising a frequency and intensity response of the sample gas or liquid, and detecting one or more target chemicals using the sample spectrum.

In the deconvolving embodiment, controller may be configured to detect the one or more chemicals in the sample gas or liquid using pattern recognition to identify known spectral identities of one or more gases or liquids in the sample spectrum.

The photoacoustic sensor may comprise: a housing having an optical window at one end, the optical window for receiving light emitted from the light source, and a microphone at an end of the housing opposing the optical window, wherein the housing defines a cavity between the optical window and the microphone, the cavity for containing a sample gas or liquid, and wherein the microphone is configured to generate and outputting the sensor output in response to acoustic energy generated by interaction of received light with a sample gas or liquid held within the cavity. The housing of the photoacoustic sensor may flare outwardly from the optical window to define an acoustic energy generation portion in which acoustic energy is generated by interaction of received light with a sample gas or liquid held within the cavity. Advantageously, by flaring outwardly in such a way, this enables more light from the light source to be captured compared to other photoacoustic cells that do not have a flared portion. The housing of the photoacoustic sensor may also comprise an acoustic energy coupling portion between the acoustic energy generation portion and the microphone, the acoustic energy coupling portion being configured to couple acoustic energy generated in the acoustic energy generation portion to the microphone. This enables efficient coupling of the acoustic signal.

The housing of the photoacoustic sensor may also flare inwardly from the acoustic energy generation portion to define the acoustic energy coupling portion. Such flaring guides the acoustic signal towards the microphone.

The housing may also comprise an acoustic waveguide between the acoustic energy coupling portion and the microphone for coupling acoustic energy to the microphone. This enables efficient guiding of the acoustic signal towards the microphone.

The housing of the photoacoustic sensor between the microphone and acoustic energy generation portion may also comprise an acoustically reflective portion, the reflective portion being configured to transmit a portion of the generated acoustic energy to the microphone and configured to reflect a portion of the generated acoustic energy back to the acoustic generation portion thereby forming an acoustic resonator within the sensor. Such a resonator may increase the efficiency of the detection of the chemicals in the sample gas or liquid.

In embodiments, the microphone may be substantially surrounded by an electrical shield. This may increase the signal to noise ratio of the signal, since less electrical interference will be picked up by the microphone.

The photoacoustic chemical detector may also comprise a reference photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a reference sensor signal in response to acoustic energy created when received light from the light source interacts with a reference gas or liquid contained within the photoacoustic sensor, the reference gas or liquid comprising one or more known chemicals in known concentrations; wherein the reference photoacoustic sensor is electrically coupled to the controller, and wherein the controller is configured to read the reference sensor signal output from the reference photoacoustic sensor; and detect one or more target chemicals in a sample gas or liquid using the sensor signal and the reference sensor signal.

Such a reference photoacoustic sensor provides a system whereby the sample gas may be compared to known concentrations of known chemicals in a reference gas or liquid in order to aid in the identification of the chemicals in the sample gas or liquid.

In embodiments comprising the reference photoacoustic sensor, the light source may be configured to emit light from a first facet and a second facet, and wherein the photoacoustic sensor is optically coupled to the first facet, and the reference photoacoustic sensor is optically coupled to the second facet.

In embodiments of the photoacoustic chemical detector, the light source may be mounted on a first surface of a mount, the mount comprising a base opposing the first surface and walls between the first surface and base, wherein the first surface of the mount has a geometry substantially the same as the light source, and wherein walls flare outwardly from the first surface to the base. Such a geometry enables the light source to be mounted such that front and rear facets may be accessible for capture of the emitted light by the photoacoustic sensor.

The walls of the mount may flare outwardly either continuously or in a stepped geometry.

The base of the mount may comprise one or more thermoelectric elements configured to heat or cool the mount, and wherein the thermoelectric elements are electrically coupled to the controller.

In further embodiments, the photoacoustic chemical detector may comprise: a second light source for emitting light comprising two or more discrete optical modes; a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the second photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected; wherein the second photoacoustic sensor and second light source are electrically coupled to the controller, and wherein the controller is configured to: supply a second drive signal to the second light source such that the second light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; read the second sensor signal output from the second photoacoustic sensor; and detect one or more target chemicals in the second sample gas or liquid using the sensor signal.

Such a photoacoustic chemical detector enables first and second sample gases or liquids to be sampled simultaneously. In this embodiment, the first and second sample gases may be substantially the same gas or liquid or a different gas or liquid.

In an alternative embodiment, a photoacoustic chemical detector (for detecting one or more target chemicals in a sample gas or liquid) comprises: a first and second light source for emitting first and second light, each of the first and second light comprising two or more discrete optical modes; a first photoacoustic sensor optically coupled to the first light source for receiving light emitted from the first light source, and being configured to output a first acoustic sensor signal in response to acoustic energy created when received light from the first light source interacts with a first sample gas or liquid contained within the photoacoustic sensor, the first sample gas or liquid comprising one or more target chemicals to be detected; a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second acoustic sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected; a microphone acoustically coupled to the first and second photoacoustic sensors for receiving the first and second acoustic outputs and generating a sensor signal; a controller electrically coupled to the first and second light sources and the microphone, wherein the controller is configured to: supply a first and second drive signal to the respective first and second light sources such that the each of the first and second light sources controllably emit light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; read the sensor signal output from the microphone; and detect one or more target chemicals in the first and second sample gases or liquids using the sensor signal. Such a photoacoustic chemical detector enables first and second sample gases or liquids to be sampled simultaneously. In this embodiment, the first and second sample gases may be substantially the same gas or liquid. In any of the above described embodiments, the light source may be a Fabry-Perot quantum cascade laser, a Fabry-Perot diode laser, or a multimode fibre laser. Furthermore, the controller may be configured to drive the laser to produce a continuous wave output or a pulsed output. The present invention also provides a method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector, comprising a light source for emitting light comprising two or more discrete optical modes; a photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a sensor signal in response to acoustic energy created when received light from the light source interacts with a sample gas or liquid contained within the photoacoustic sensor, the sample gas or liquid comprising one or more target chemicals to be detected; and a controller electrically coupled to the light source and the photoacoustic sensor, the method comprising: supplying a drive signal to the light source such that the light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; reading the sensor signal output from the photoacoustic sensor; and detecting one or more target chemicals in a sample gas or liquid using the sensor signal. In the method, the drive signal defines one or more of an operating current, an operating voltage and an operating temperature of the light source.

Furthermore, the method may comprise: supplying a second drive signal to the light source such that the light source controllably emits light comprising a second plurality of discrete modes, wherein the second drive signal is different to the first drive signal; reading the sensor signal output from the photoacoustic sensor in response to the second drive signal; and detecting one or more target chemicals in a sample gas or liquid using the sensor signal output from the photoacoustic sensor in response to the first and second drive signals. The step of detecting one or more target chemicals in a sample gas or liquid may comprise: comparing the received sensor signal with a known sensor signal produced when a known gas or liquid is exposed to the light source when driven with the drive signal; and detecting and identifying one or more target chemicals when the received sensor signal is substantially similar to the known sensor signal.

The step of detecting one or more target chemicals in a sample gas or liquid may comprise: deconvolving the frequency spectrum and intensity levels of the emitted light from the received sensor signal to reveal a sample spectrum comprising a frequency and intensity response of the sample gas or liquid, and detecting and identifying one or more target chemicals using the sample spectrum.

The method also may comprise detecting the one or more chemicals in the sample gas or liquid using pattern recognition to identify known spectral identities of one or more gases or liquids in the sample spectrum.

The method may further comprise: reading a reference sensor signal output from a reference photoacoustic sensor; and detecting one or more target chemicals in a sample gas or liquid using the sensor signal and the reference sensor signal, wherein the reference photoacoustic sensor is optically coupled to the light source for receiving light emitted from the light source, and being configured to output the reference sensor signal in response to acoustic energy created when received light from the light source interacts with a reference gas or liquid contained within the photoacoustic sensor, the reference gas or liquid comprising one or more target chemicals in known concentrations.

The method may further comprise: supplying a second drive signal to a second light source such that the second light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; reading a second sensor signal output from a second photoacoustic sensor; and detecting one or more target chemicals in the second sample gas or liquid using the sensor signal, wherein the second light source is configured to emit light comprising two or more discrete optical modes, and wherein the second photoacoustic sensor is optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output the second sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the second photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected. Alternatively, a method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector comprising: a first and second light source for emitting first and second light, each of the first and second light comprising two or more discrete optical modes; a first photoacoustic sensor optically coupled to the first light source for receiving light emitted from the first light source, and being configured to output a first acoustic sensor signal in response to acoustic energy created when received light from the first light source interacts with a first sample gas or liquid contained within the photoacoustic sensor, the first sample gas or liquid comprising one or more target chemicals to be detected; a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second acoustic sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected; a microphone acoustically coupled to the first and second photoacoustic sensors for receiving the first and second acoustic outputs and generating a sensor signal; and a controller electrically coupled to the first and second light sources and the microphone, the method comprising: supplying a first and second drive signal to the respective first and second light sources such that the each of the first and second light sources controllably emit light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity; reading the sensor signal output from the microphone; and detecting one or more target chemicals in the first and second sample gases or liquids using the sensor signal.

In any of the above described methods, the light source may comprise a Fabry-Perot quantum cascade laser, a Fabry-Perot diode laser, or a multimode fibre laser that is driven to produce a continuous wave output or a pulsed output.

The present invention also provides a photoacoustic sensor for use in a photoacoustic chemical detector, the sensor comprising: a housing having an optical window at one end, the optical window for receiving light emitted from a light source, and a microphone at an end of the housing opposing the optical window, wherein the housing defines a cavity between the optical window and the microphone, the cavity for containing a sample gas or liquid, and wherein the microphone is configured to generate and outputting the sensor output in response to acoustic energy generated by interaction of received light with a sample gas or liquid held within the cavity.

The housing may flare outwardly from the optical window to define an acoustic energy generation portion in which acoustic energy is generated by interaction of received light with a sample gas or liquid held within the cavity.

The housing may comprise an acoustic energy coupling portion between the acoustic energy generation portion and the microphone, the acoustic energy coupling portion being configured to couple acoustic energy generated in the acoustic energy generation portion to the microphone.

The housing may also flare or taper inwardly from the acoustic energy generation portion to define the acoustic energy coupling portion. Advantageously, this configuration helps to concentrate the acoustic energy generated in the acoustic energy generation portion towards the microphone from a larger volume where the sound is generated.

The housing may comprise an acoustic waveguide between the acoustic energy coupling portion and the microphone for coupling acoustic energy to the microphone. In such a photoacoustic sensor, the housing between the microphone and acoustic energy generation portion may comprise an acoustically reflective portion, the reflective portion being configured to transmit a portion of the generated acoustic energy to the microphone and configured to reflect a portion of the generated acoustic energy back to the acoustic generation portion thereby forming an acoustic resonator within the sensor.

In embodiments of the photoacoustic sensor, the microphone may be substantially surrounded by an electrical shield. LIST OF FIGURES

The invention will now be described by way of example only and with reference to the accompanying figures, in which:

Figure 1 a shows an example absorption spectrum for a sample target chemical;

Figures 1 b to 1d show example frequency spectrum outputs for the light source; Figure 2 shows a photoacoustic chemical detector;

Figure 3 shows a photoacoustic chemical detector with a reference photoacoustic sensor; Figure 4 shows a laser mount;

Figure 5 shows a photoacoustic chemical detector for sampling multiple sample gases or liquids; and Figure 6 shows an alternative photoacoustic chemical detector for sampling multiple gas or liquid samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In brief, the invention utilises cost-effective multimode light sources, such as Fabry- Perot cavity or quantum cascade lasers, which emit a comb of near equally spaced frequencies. The light source output is encoded by controlling the drive current, drive voltage and/or operating temperature in order to produce a known multimode emission (having known peaks of intensity at known frequencies) that is used to irradiate a sample gas or liquid within a specially designed photoacoustic sensor, which generates an acoustic signal that is detected by a microphone. The encoded drive signal applied to the light source results in a varying trace in the photoacoustic signal that can be used to analyse and identify the of the sample in the photoacoustic cell. The photoacoustic signal is the convolution of the programmed frequency encoding of the light source with the absorption spectrum of the sample. Laser frequency control and detection

The invention is based on exploiting a laser that emits multiple discrete modes simultaneously with a specially-designed photoacoustic sensor system. Several types of laser emit multiple discrete modes in this way and all these types of laser be exploited in this invention. An example of such a laser is a semiconductor laser diode based on a Fabry-Perot cavity or a Fabry-Perot quantum cascade laser that emit a comb of near equally spaced frequencies that are defined by the dimensions of the laser cavity and the optical constants of the enclosed gain medium.

The multiple frequencies may be shifted in a controlled way by programming the drive conditions of the laser such as voltage, current, and temperature. These parameters may be controlled separately or in combination in a predetermined sequence.

This controlled programming of the mode structure differs from previous approaches. For example, US 5,917,193, random, uncontrolled variations in the intensity of fixed modes in the emission spectrum of a C02 laser were induced by varying the drive current. However, the laser modes are not frequency tuneable as they are fixed by the C02 molecules in the laser gain medium. Other published techniques include sweeping the mode spectrum in a simple linear fashion by ramping the drive current of a laser, and operating the laser in continuous wave mode i.e. drive by a direct current not pulses. In the present invention, chemical detection and identification may be achieved in a very different manner to conventional laser based spectroscopy. In brief, the method encodes the emission frequencies of the multimode laser with a controlled temporal pattern of variation of frequency and amplitude. The encoding may be achieved by varying the drive current, voltage, and operating temperature with a predetermined sequence of control signals. This produces a frequency and amplitude encoding of the emission spectrum; a complex series of shifts in frequency and amplitude that follow the programmed signal. The response of the laser emission spectra to this encoding is specific to the type of laser used. This effect is illustrated in Figure 1 . At each point in the programmed series Figures 1 b, 1c and 1 d, the distribution of optical power amongst the emitted modes from the laser and the optical frequencies of the comb is different but controlled. In this way, the absorption spectrum, Figure 1 a, of the chemicals in the photoacoustic cell (i.e. the target chemicals to be detected) are simultaneously sampled at many discrete points corresponding to the positions of the teeth in the mode comb, illustrated by the dashed lines "A". The next point in the programmed series will results in a different set of sampled points. This shift in the teeth of the mode comb is shown by the dashed lines "B". The total absorption of light is the sum of the absorption at each sample each tooth in the mode comb; the total absorption gives rise to a photoacoustic signal. The encoded sequence applied to the laser results in a varying trace in the photoacoustic signal that can be used to analyse and identify target chemicals in the sample gas or liquid in the photoacoustic cell. The photoacoustic signal is the convolution of the programmed frequency encoding of the laser with the spectrum of the sample.

Advantageously, an exact knowledge of the emission frequencies of the Fabry-Perot laser is not required. The only requirement is that the centre of the emission comb of the laser preferably coincides with the spectroscopic region of interest. The photoacoustic signal will be a unique pattern that can be used to identify the gas or liquid.

The measured photoacoustic signal contains many variations and features that may be exploited through pattern recognition algorithms to analyse the chemical constituents of the cell. For example, the signal will change from high to low as each tooth in the mode comb pass a string absorption feature in the spectrum. Alternatively, de-convolution techniques may then be used to extract the spectrum of the sample. Furthermore, such distributed, multiplexed, and even pseudo-random sampling is regularly exploited in compressive sampling techniques. Likewise, our encoded emission spectrum forms a basis set of measurements that may be analysed through compressive sensing and result in the recovery of the emission spectrum in the sample in the photoacoustic cell.

This mode of detection has several advantages:

1 ) useful spectra can be recovered from a few simple measurements below the Nyquist criteria using compressive sampling techniques. Conventional scanning tuneable diode laser (TDL) spectroscopy systems have to sample at high resolution to extract the data. Although, the result is the same, the multimode technique results in an important simplification in the hardware.

2) Multiplex spectral measurements results in higher signal to noise ratios due to the Fellgett advantage. As we sample many points simultaneously across the spectrum, the signal to noise decreases as the square root of the number of samples.

3) The noise immunity of the system is improved as pseudo random sampling across the absorption spectrum is an incoherent process and has a high probability to be out of phase with extraneous pick-up in the signal chain.

4) Weighted sampling may be employed to enhance the signal of interest and diminish the signal from uninteresting or unwanted parts of the spectra. For example, the programmed mode sequence can be varied to dwell of a critical region of the spectra where there is strong variation is photoacoustic signal. Conversely, the programmed sequence may equally be chosen to minimise the sampling of less important regions of the spectrum where the signal is weak, or confusing peak may exist. In this way the signal to noise can be enhanced for the detection of specific chemicals in a mixed sample.

The method is well suited to the exploitation of lasers that can only operating in pulsed mode. These pulsed lasers are often simpler to fabrication and hence cheaper. Pulse mode operation is usually undesirable as the dynamic variations in output frequency and amplitude during the drive pulse are considered as problematic instabilities. Here we exploit these dynamic effects to our own advantage.

Photoacoustic cell

An example photoacoustic chemical detector using a photoacoustic cell or sensor is shown in Figure 2.

In its broadest form, the detector 100 comprises a photoacoustic sensor or cell 102, a light source 108, a microphone 1 16, and a controller and/or processor 106. The photoacoustic sensor 102 comprising a housing, which defines a cavity 104, into which a sample gas or liquid comprising target chemicals to be detected, is contained. In photoacoustic spectroscopy light from a source excites the vibrational modes of the molecules in the gas or liquid in the cell 102. This absorbed energy is converted into a temperature and pressure change. Modulating the light source at a given acoustic frequency results in an acoustic wave in the gas or liquid that can be detected on a microphone 1 16. The design of the photoacoustic cell 102 can be optimised to improve the signal to noise. The photoacoustic sensor 102 can be described in terms of several interconnecting sections. The first sections, an optical window 1 10 and laser beam absorption (or acoustic energy generation) section 1 12 are optimised for light collection from the laser 108. In a typical photoacoustic cell, focusing optics are employed to couple light from the light source into a beam to excite the acoustic modes in the cell. The present invention does not use focusing optics. The light sources 108 have a diverging beam that is directed through a transparent optical window 110 of the photoacoustic cell 102. The window 1 10 is placed in close proximity to the light source 102 so as to capture as much light from the beam as possible.

The entrance portion of the photoacoustic cell is a laser beam absorption (or acoustic generation) section 1 12. At this section, the housing is flared in such a way to capture as much of the beam as possible. The flared section is where the photoacoustic sound generation takes place.

The next section is an acoustic transfer (or acoustic energy coupling) section 1 14 consists of an acoustic coupling section that channels the sound wave from the generation section 1 12 an acoustic waveguide 122. The form of this acoustic horn-like geometry is engineered to maximise the coupling of the acoustic wave to the waveguide section 122. The waveguide section 122 directs the acoustic wave to the microphone 1 16. Section 1 18 consists of a variation in the waveguide geometry, such as an abrupt increase in diameter, which is localised in front of the microphone 1 16. This region serves two purposes. The abrupt change in waveguide dimensions serves to reflect the acoustic wave back into the horn section, thereby forming and acoustic resonator. Furthermore, the acoustic reflectance of this section may be controlled to transmit a fraction of the incident acoustic energy. The length and form of this section is engineered to optimise the coupling of the transmitted acoustic energy to the microphone 1 16. Advantageously, the geometry of the photoacoustic sensor 102 enables acoustic-acoustic coupling from section 1 14 to section 122 and 1 18 and is used to deliver efficiently the signal from the absorption section 1 12 to the microphone 116, whilst allowing enhanced signal to noise from acoustic resonance.

This design differs considerably from other photoacoustic cells in that the coupling of acoustic energy from source, section 1 12 to microphone 1 16 can only take place by acoustic-acoustic transfer. Other designs of photoacoustic cell excite the acoustic mode directly by focusing the beam to through the cell in a controlled manner. In this photacoustic cell, the light excites an acoustic wave in one region, and the flared geometry of the acoustic cell channels the wave by acoustic coupling to a microphone. This feature allows efficient transfer of energy from a diverging light wave to an acoustic wave that in turn couples efficiently to the microphone. Furthermore, in the present invention the acoustic duct or waveguide 122 may be used to direct acoustic energy away from sources of electrical noise into a well-defined electrical shield 120 to improve signal to noise. In this way the microphone 1 16, preamplifier, and detection electronics can be electrically shielded from the exterior, thereby improving the signal-to-noise ratio of the system. The acoustic waveguide 122 is not necessarily straight but may be bent or curved to direct the sound signal to a convenient location in the system.

A controller or processor 106 is electrically coupled to the light source 108 and the microphone 1 16. As such, the controller 106 generates the required drive signal (as described above) in order to produce the required encoded waveform for inputting into the photoacoustic sensor 102 for interaction with the sample gas or liquid contained in the cavity 104.

The controller furthermore receives the sensor signal output from the microphone 1 16 and processes the sensor signal to detect and identify the target chemicals present in the sample gas or liquid. Example techniques for processing the signal are described above with reference to figure 1 . The resulting output from the controller 106 is either an identification of the detected chemicals, a raw data output for further processing by other systems (not shown) or a combination of both. Figure 3 shows a second embodiment of the photoacoustic chemical detector. In this figure, the left-hand portion of the system (150) is the same as the system shown in figure 2 (although the controller 106 is not shown for the sake of clarity). The difference between the photoacoustic chemical detector shown in figures 2 and 3 is that the detector of figure 3 also comprises a reference photoacoustic chemical sensor section 250.

In this system the radiation emitted from the back facet of the laser 108 is exploited to excite a reference photoacoustic cell 202. It is known in photoacoustic systems to use a reference cell to compare the against the signal cell. In the known systems, the laser beam is usually split into a reference and sample beam. This has the disadvantage of reducing the power of the beam. The light emerging from the rear facet of the laser 108 is typically unexploited. In the present invention, the light emerging from the rear of the laser 108 is passed through a second photoacouctic cell 202 of similar design to the photoacoustic sensor 102 containing the sample gas or liquid. As such, the reference photoacoustic sensor 202 comprises an optical window, an acoustic generation section, an acoustic coupling section, a waveguide and a microphone 216. However, the reference photoacoustic sensor 202 contains a known reference gas or liquid of known concentration. The controller (not shown) compares the signal from the reference cell and the sample cell thereby improving system stability.

As with the above systems, since the absorption spectrum in the reference cell is known, de-convolution algorithms may be employed to extract the encoded frequency pattern from the laser. This allows the system to compensate for drift in the frequency response of the laser that may occur overtime.

Laser mount

In order to realise a practical working arrangement for this reference photoacoustic sensor, a suitable laser mount is required. Figure 4 shows such a laser mount adapted for the present system, which may be used in conjunction with any of the described embodiments. The laser mount 230 is modified to allow optical access to both facets. One end of the mount (the first surface) is thinned to match the length of the laser 108; the laser chip is then soldered to the thinned end. This allows the windows of the photoacoustic sensor 102 and reference photoacoustic sensor 202 to be brought in close proximity with each facet of the laser 108 and avoids interference with the diverging beams 140, 240 emerging from each facet.

The mount 230 underneath the laser 108 is flared either continually or in a stepped geometry to enhance heat spreading, generated by the laser, into the wider volume of the laser mount. Thermoelectric elements, 302, 304, 306 are used to control the temperature of the laser through the laser mount. These may be mounted singly or in combination around the flared end of the mount to enhance the thermal pumping capacity. The electrical power supplied to these elements may also be encoded as part of the programmed frequency sequence. The thermoelectric elements 302, 304, 306 are electrically coupled to the controller (not shown) for control of their heating or cooling.

Multiplexing

The design of the photoacoustic cell is amenable to multiplexing so that several different chemicals can be detected in separate photoacoustic cells. Such a system is shown for example in figure 5. Photoacoustic sensors 102a, 102b ... 102n are filled with the same sample gas or liquid containing a mixture of chemicals to be detected. Each of the photoacoustic sensors 102a, 102b ... 102n are optically coupled to a respective laser 108a, 108b ... 108n and have respective microphones 1 16a, 1 16b ... 1 16n for outputting a respective sensor signal. Each of the lasers and each of the microphones are electrically coupled to the controller/processor 106.

Each laser may be dedicated to emitting a different spectrum for detecting different absorption spectrums of the sample gas or liquid. The resulting acoustic outputs are input into the controller for processing to detect the target chemicals as described above. The system may be modified to accommodate the reference photoacoustic sensor shown in figure 3. In this case, each of the lasers 108a, 108b ... 108n would be optically coupled to a respective reference photoacoustic sensor (not shown).

Advantageously, the use of several microphones has the virtue of simplicity and direct scalability.

Figure 6 shows an alternative multiplexing scheme in which each of the photoacoustic sensors 1 102a, 1 102b ... 1 102n are optically coupled to a respective laser 1 108a, 1 108b ... 1 108n. In this embodiment, the acoustic energy generated in each of the photoacoustic sensors is channelled via acoustic guides 1 160 and 1 170 to a single common microphone 11 16. The output of the microphone 1 1 16 is electrically coupled to the controller/processor 106. Again, each of the photoacoustic sensors may contain the same sample gas or liquid for chemicals to be detected.

Each of the lasers may be dedicated to detecting a different absorption spectra.

The system may be modified to accommodate the reference photoacoustic sensor shown in figure 3. In this case, each of the lasers 1 108a, 1 108b ... 1 108n would be optically coupled to a respective reference photoacoustic sensor (not shown).

The use of a single microphone has some advantages. The acoustic waveguide network may be used to channel all of the signal into an environment where the pick-up and noise from the external environment may be controlled and minimised (for example using further shielding).

Many attempts at multiplexing laser systems use complex optics to combine the beams from several different lasers into a single detection cell. There is often a considerable cost associated with these optical components and the optical configuration is fragile and susceptible to misalignment from shock, vibration, and thermal gradients.

Conversely, the multiplexed system proposed here in figures 5 and 6 use fewer optical components that do not need tight optical alignment due to the use of the multimode Fabry-Perot lasers and the photoacoustic sensors proposed here. The systems are therefore more cost effective and intrinsically more robust.

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments and modifications will be apparent to a skilled person in the art which lie within the scope of the claims. Any of the embodiments described hereinabove can be used in any combination with one or more of the other embodiments.

Claims

CLAIMS:
1. A photoacoustic sensor for use in a photoacoustic chemical detector, the sensor comprising:
a housing having:
an optical window at one end, the optical window for receiving light emitted from a light source,
a microphone at an end of the housing opposing the optical window, an acoustic energy generation portion between the optical window and the microphone in which acoustic energy is generated by interaction of received light with a sample gas or liquid held within the cavity, and
an acoustic energy coupling portion between the acoustic energy generation portion and the microphone, the acoustic energy coupling portion being configured to couple acoustic energy generated in the acoustic energy generation portion to the microphone,
wherein the housing defines a cavity between the optical window and the microphone, the cavity for containing a sample gas or liquid,
wherein the housing flares outwardly from the optical window end to define the acoustic energy generation portion, and the housing flares or tapers inwardly from the acoustic energy generation portion to define the acoustic energy coupling portion, and wherein the microphone is configured to generate and outputting the sensor output in response to acoustic energy generated by interaction of received light with a sample gas or liquid held within the cavity.
2. A photoacoustic sensor according to claim 1 , wherein the housing comprises an acoustic waveguide between the acoustic energy coupling portion and the microphone for coupling acoustic energy to the microphone from the acoustic energy coupling portion.
3. A photoacoustic sensor according to claim 1 or 2, wherein the housing between the microphone and acoustic energy generation portion comprises an acoustically reflective portion, the reflective portion being configured to transmit a portion of the generated acoustic energy to the microphone and configured to reflect a portion of the generated acoustic energy back to the acoustic generation portion thereby forming an acoustic resonator within the sensor.
4. A photoacoustic sensor according to any one of claims 1 to 3, wherein the microphone is substantially surrounded by an electrical shield.
5. A photoacoustic chemical detector for detecting one or more target chemicals in a sample gas or liquid, the photoacoustic chemical detector comprising:
a light source for emitting light comprising two or more discrete optical modes; a photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a sensor signal in response to acoustic energy created when received light from the light source interacts with a sample gas or liquid contained within the photoacoustic sensor, the sample gas or liquid comprising one or more target chemicals to be detected;
a controller electrically coupled to the light source and the photoacoustic sensor,
wherein the controller is configured to:
supply a drive signal to the light source such that the light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
read the sensor signal output from the photoacoustic sensor; and
detect one or more target chemicals in a sample gas or liquid using the sensor signal.
6. A photoacoustic chemical detector according to claim 5, wherein the drive signal defines one or more of an operating current, an operating voltage and an operating temperature of the light source.
7. A photoacoustic chemical detector according to claim 5 or 6, wherein the controller is configured to:
supply a second drive signal to the light source such that the light source controllably emits light comprising a second plurality of discrete modes, wherein the second drive signal is different to the first drive signal;
read the sensor signal output from the photoacoustic sensor in response to the second drive signal; and detect one or more target chemicals in a sample gas or liquid using the sensor signal output from the photoacoustic sensor in response to the first and second drive signals.
8. A photoacoustic chemical detector according to claim 5, 6 or 7, wherein the controller is configured to detect one or more target chemicals in a sample by comparing the received sensor signal with a known sensor signal produced when a known gas or liquid is exposed to the light source when driven with the drive signal, and detecting and identifying one or more target chemicals when the received sensor signal is substantially similar to the known sensor signal.
9. A photoacoustic chemical detector according to claim 5, 6 or 7, wherein the controller is configured to detect one or more target chemicals by deconvolving the frequency spectrum and intensity levels of the emitted light from the received sensor signal to reveal a sample spectrum comprising a frequency and intensity response of the sample gas or liquid, and detecting one or more target chemicals using the sample spectrum.
10. A photoacoustic chemical detector according to claim 9, wherein controller is configured to detect the one or more chemicals in the sample gas or liquid using pattern recognition to identify known spectral identities of one or more gases or liquids in the sample spectrum.
1 1 . A photoacoustic chemical detector according to any one of claims 5 to 10, wherein the photoacoustic sensor comprises:
a housing having an optical window at one end, the optical window for receiving light emitted from the light source, and a microphone at an end of the housing opposing the optical window,
wherein the housing defines a cavity between the optical window and the microphone, the cavity for containing a sample gas or liquid, and
wherein the microphone is configured to generate and outputting the sensor output in response to acoustic energy generated by interaction of received light with a sample gas or liquid held within the cavity.
12. A photoacoustic chemical detector according to claim 1 1 , wherein the housing of the photoacoustic sensor flares outwardly from the optical window to define an acoustic energy generation portion in which acoustic energy is generated by interaction of received light with a sample gas or liquid held within the cavity.
13. A photoacoustic chemical detector according to claim 12, wherein the housing of the photoacoustic sensor comprises an acoustic energy coupling portion between the acoustic energy generation portion and the microphone, the acoustic energy coupling portion being configured to couple acoustic energy generated in the acoustic energy generation portion to the microphone.
14. A photoacoustic chemical detector according to claim 13, wherein the housing of the photoacoustic sensor flares inwardly from the acoustic energy generation portion to define the acoustic energy coupling portion.
15. A photoacoustic chemical detector according to claim 13 or 14, wherein the housing comprises an acoustic waveguide between the acoustic energy coupling portion and the microphone for coupling acoustic energy to the microphone.
16. A photoacoustic chemical detector according to any one of claims 12 to 15, wherein the housing of the photoacoustic sensor between the microphone and acoustic energy generation portion comprises an acoustically reflective portion, the reflective portion being configured to transmit a portion of the generated acoustic energy to the microphone and configured to reflect a portion of the generated acoustic energy back to the acoustic generation portion thereby forming an acoustic resonator within the sensor.
17. A photoacoustic chemical detector according to any one of claims 1 1 to 16, wherein the microphone is substantially surrounded by an electrical shield.
18. A photoacoustic chemical detector according to any one of claims 5 to 17, comprising a reference photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a reference sensor signal in response to acoustic energy created when received light from the light source interacts with a reference gas or liquid contained within the photoacoustic sensor, the reference gas or liquid comprising one or more known chemicals in known concentrations;
wherein the reference photoacoustic sensor is electrically coupled to the controller, and wherein the controller is configured to read the reference sensor signal output from the reference photoacoustic sensor; and
detect one or more target chemicals in a sample gas or liquid using the sensor signal and the reference sensor signal.
19. A photoacoustic chemical detector according to claim 18, wherein the light source is configured to emit light from a first facet and a second facet, and wherein the photoacoustic sensor is optically coupled to the first facet, and the reference photoacoustic sensor is optically coupled to the second facet.
20. A photoacoustic chemical detector according to any one of claims 5 to 19, wherein the light source is mounted on a first surface of a mount, the mount comprising a base opposing the first surface and walls between the first surface and base, wherein the first surface of the mount has a geometry substantially the same as the light source, and wherein walls flare outwardly from the first surface to the base.
21. A photoacoustic chemical detector according to claim 20, wherein the walls flare outwardly either continuously or in a stepped geometry.
22. A photoacoustic chemical detector according to claim 20 or 21 , wherein the base comprises one or more thermoelectric elements configured to heat or cool the mount, and wherein the thermoelectric elements are electrically coupled to the controller.
23. A photoacoustic chemical detector according to any one of claims 5 to 22, comprising:
a second light source for emitting light comprising two or more discrete optical modes;
a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the second photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected;
wherein the second photoacoustic sensor and second light source are electrically coupled to the controller, and
wherein the controller is configured to:
supply a second drive signal to the second light source such that the second light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
read the second sensor signal output from the second photoacoustic sensor; and
detect one or more target chemicals in the second sample gas or liquid using the sensor signal.
24. A photoacoustic chemical detector for detecting one or more target chemicals in a sample gas or liquid, the photoacoustic chemical detector comprising:
a first and second light source for emitting first and second light, each of the first and second light comprising two or more discrete optical modes;
a first photoacoustic sensor optically coupled to the first light source for receiving light emitted from the first light source, and being configured to output a first acoustic sensor signal in response to acoustic energy created when received light from the first light source interacts with a first sample gas or liquid contained within the photoacoustic sensor, the first sample gas or liquid comprising one or more target chemicals to be detected;
a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second acoustic sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected;
a microphone acoustically coupled to the first and second photoacoustic sensors for receiving the first and second acoustic outputs and generating a sensor signal;
a controller electrically coupled to the first and second light sources and the microphone,
wherein the controller is configured to: supply a first and second drive signal to the respective first and second light sources such that the each of the first and second light sources controllably emit light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
read the sensor signal output from the microphone; and
detect one or more target chemicals in the first and second sample gases or liquids using the sensor signal.
25. A photoacoustic chemical detector according to claim 23 or 24, wherein the first and second sample gases or liquids are substantially the same gas or liquid.
26. A photoacoustic chemical detector according to claim 23, wherein the first and second sample gases or liquids are different gases or liquids.
27. A photoacoustic chemical detector according to any one of claims 5 to 26, wherein the light source is a Fabry-Perot quantum cascade laser, a Fabry-Perot diode laser, or a multimode fibre laser.
28. A photoacoustic chemical detector according to claim 27, wherein the controller is configured to drive the laser to produce a continuous wave output or a pulsed output.
29. A method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector, comprising a light source for emitting light comprising two or more discrete optical modes; a photoacoustic sensor optically coupled to the light source for receiving light emitted from the light source, and being configured to output a sensor signal in response to acoustic energy created when received light from the light source interacts with a sample gas or liquid contained within the photoacoustic sensor, the sample gas or liquid comprising one or more target chemicals to be detected; and a controller electrically coupled to the light source and the photoacoustic sensor, the method comprising:
supplying a drive signal to the light source such that the light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
reading the sensor signal output from the photoacoustic sensor; and detecting one or more target chemicals in a sample gas or liquid using the sensor signal.
30. A method according to claim 29, wherein the drive signal defines one or more of an operating current, an operating voltage and an operating temperature of the light source.
31. A method according to claim 29 or 30, wherein the method comprises:
supplying a second drive signal to the light source such that the light source controllably emits light comprising a second plurality of discrete modes, wherein the second drive signal is different to the first drive signal;
reading the sensor signal output from the photoacoustic sensor in response to the second drive signal; and
detecting one or more target chemicals in a sample gas or liquid using the sensor signal output from the photoacoustic sensor in response to the first and second drive signals.
32. A method according to claim 29, 20 or 31 , wherein detecting one or more target chemicals in a sample gas or liquid comprises:
comparing the received sensor signal with a known sensor signal produced when a known gas or liquid is exposed to the light source when driven with the drive signal; and
detecting and identifying one or more target chemicals when the received sensor signal is substantially similar to the known sensor signal.
33. A method according to claim 29, 30 or 31 , wherein detecting one or more target chemicals in a sample gas or liquid comprises:
deconvolving the frequency spectrum and intensity levels of the emitted light from the received sensor signal to reveal a sample spectrum comprising a frequency and intensity response of the sample gas or liquid, and detecting and identifying one or more target chemicals using the sample spectrum.
34. A method according to claim 31 , comprising detecting the one or more chemicals in the sample gas or liquid using pattern recognition to identify known spectral identities of one or more gases or liquids in the sample spectrum.
35. A method according to any of claims 29 to 34, comprising:
reading a reference sensor signal output from a reference photoacoustic sensor; and
detecting one or more target chemicals in a sample gas or liquid using the sensor signal and the reference sensor signal,
wherein the reference photoacoustic sensor is optically coupled to the light source for receiving light emitted from the light source, and being configured to output the reference sensor signal in response to acoustic energy created when received light from the light source interacts with a reference gas or liquid contained within the photoacoustic sensor, the reference gas or liquid comprising one or more target chemicals in known concentrations.
36. A method according to any one of claims 29 to 35, comprising:
supplying a second drive signal to a second light source such that the second light source controllably emits light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
reading a second sensor signal output from a second photoacoustic sensor; and
detecting one or more target chemicals in the second sample gas or liquid using the sensor signal,
wherein the second light source is configured to emit light comprising two or more discrete optical modes, and
wherein the second photoacoustic sensor is optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output the second sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the second photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected.
37. A method of detecting one or more target chemicals in a sample gas or liquid using a photoacoustic chemical detector comprising: a first and second light source for emitting first and second light, each of the first and second light comprising two or more discrete optical modes; a first photoacoustic sensor optically coupled to the first light source for receiving light emitted from the first light source, and being configured to output a first acoustic sensor signal in response to acoustic energy created when received light from the first light source interacts with a first sample gas or liquid contained within the photoacoustic sensor, the first sample gas or liquid comprising one or more target chemicals to be detected; a second photoacoustic sensor optically coupled to the second light source for receiving light emitted from the second light source, and being configured to output a second acoustic sensor signal in response to acoustic energy created when received light from the second light source interacts with a second sample gas or liquid contained within the photoacoustic sensor, the second sample gas or liquid comprising one or more target chemicals to be detected; a microphone acoustically coupled to the first and second photoacoustic sensors for receiving the first and second acoustic outputs and generating a sensor signal; and a controller electrically coupled to the first and second light sources and the microphone, the method comprising:
supplying a first and second drive signal to the respective first and second light sources such that the each of the first and second light sources controllably emit light comprising a plurality of discrete modes, where each mode has a defined frequency and intensity;
reading the sensor signal output from the microphone; and
detecting one or more target chemicals in the first and second sample gases or liquids using the sensor signal.
38. A method according to any one of claims 29 to 37, wherein the light source is a Fabry-Perot quantum cascade laser, a Fabry-Perot diode laser, or a multimode fibre laser that is driven to produce a continuous wave output or a pulsed output.
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