WO2010081739A1 - Photoacoustic gas analyser with interferometric modulation - Google Patents

Photoacoustic gas analyser with interferometric modulation Download PDF

Info

Publication number
WO2010081739A1
WO2010081739A1 PCT/EP2010/000255 EP2010000255W WO2010081739A1 WO 2010081739 A1 WO2010081739 A1 WO 2010081739A1 EP 2010000255 W EP2010000255 W EP 2010000255W WO 2010081739 A1 WO2010081739 A1 WO 2010081739A1
Authority
WO
WIPO (PCT)
Prior art keywords
photoacoustic
light
analysing system
signal
gas
Prior art date
Application number
PCT/EP2010/000255
Other languages
French (fr)
Inventor
Andras Miklos
Judit Angster
Zlatko Dubovski
Original Assignee
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. filed Critical Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Publication of WO2010081739A1 publication Critical patent/WO2010081739A1/en

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems 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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/24—Probes
    • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/45—Interferometric spectrometry
    • G01J3/453—Interferometric spectrometry by correlation of the amplitudes
    • G01J3/4535—Devices with moving mirror
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems 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
    • G01N2021/1704—Systems 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

Definitions

  • Patent application Photoacoustic Gas Analyser with lnterferometric Modulation
  • the invention concerns a photoacoustic gas analyser containing a light source, an interferometer for modulating the light intensity and a photoacoustic gas cell for detecting the photoacoustic signal generated by light absorption in the gas.
  • Photoacoustic spectroscopy becomes increasingly accepted as a method for detecting the presence and measuring the concentration of substances that absorb light. Due to the absorbed light energy the material that contains the absorbing substance becomes warmer and expands. In the cases of periodic or pulsed illumination a pressure wave, that means a sound wave, is generated in the material, which can be detected by a pressure transducer. The received electric signal, that is the photoacoustic signal, can be regarded as a measure of the concentration of the substance that absorbs the light.
  • a small closed photoacoustic cell In the case a) mostly a small closed photoacoustic cell is used.
  • the solid or liquid sample is poured in a small cup.
  • the sample is illuminated by an intensity modulated infrared light beam through a transparent window.
  • the light energy absorbed in the sample generates a thermal wave in the sample, which causes a periodic temperature oscillation at the surface of the sample.
  • the gas that fills the closed cell is periodically heated by the sample, thus a pressure oscillation is generated in the gas. This pressure oscillation is measured by a small microphone mounted into the PA cell, or connected to the PA cell by a capillary.
  • the above described method has another advantage; the possibility of depth profiling. Since the information about the absorption of the light deep in the sample has to be brought to the sample surface for generating a PA signal in the gas, the penetration depth of the thermal wave plays an important role in PA spectroscopy. The thermal penetration depth, however, depends on the modulation frequency. Thus, a thicker layer of the sample contributes to the PA signal at a low modulation frequency, than at a higher modulation frequency. This effect can be used by depth profiling of the sample using a scan over an appropriate range of modulation frequencies. The frequencies are quite low; the range of ⁇ 5 Hz - 200 Hz is usually applied.
  • the high sensitivity of photoacoustic detection is used for detecting gas concentrations in the ppmV - ppbV (parts per million in volume - parts per billion in volume) range. Since the photoacoustic signal is proportional with the light power, mostly powerful tunable lasers are applied, while the photoacoustic cell is an acoustic resonator with high signal amplification. The light intensity is modulated at the resonance frequency, which is mostly in the kHz range.
  • photoacoustic gas detectors and analysers that utilize broadband light sources instead of tunable lasers. These kinds of devices are much simpler and cheaper than a system with tunable laser(s) or other tunable infrared light source. However, the sensitivity and selectivity of such devices is usually much smaller than that of photoacoustic detectors with laser excitation.
  • a much better method is the direct modulation of the light intensity. This can easily be done at diode lasers or quantum cascade lasers by modulating the laser current. Photoacoustic detectors with broadband light sources apply mostly a low frequency mechanical chopper.
  • the modulation of the light intensity in the measuring PA cell is provided by two infrared beams that enter into the PA cell alternatively.
  • One beam passes a gas cell filled with the target gas, where the light at the absorption lines of the gas is strongly absorbed, the other beam passes a reference cell filled with a nonabsorbing gas.
  • the intensities of the two beams are adjusted to the same value.
  • intensity modulation in the PA cell occurs only at those wavelengths, where the light is absorbed in the gas cell, i.e. at the absorption lines of the target gas. Consequently, the same gas produces the largest PA signal in the PA cell.
  • the different components of the broadband infrared light are modulated at different frequencies.
  • the light transmitted through a sample is detected by an infrared photodetector, and the spectrum of the detector signal is calculated by Fourier transformation. Since each frequency components of the spectrum correspond to a given wavelength, an optical transmission spectrum can be obtained. From the difference of the transmission spectra with and without sample the optical absorption spectrum of the sample can be determined.
  • FTIR spectrometers can also be used with photoacoustic detection (FT-IR Photoacoustic Spectroscopy).
  • a photoacoustic detector is used instead of the infrared photodetector.
  • the US company MTEC Photoacoustics, Inc. (www.mtecpas.com) produces a photoacoustic detector (Model 300) that can be used by several commercially available FTIR spectrometers. This detector is similar to that described in case a) above. It can be placed into the sample compartment of the FTIR spectrometer. The IR beam is then focused to the sample inside the PA cell. The different frequency components of the microphone signal are proportional with the optical absorption of the sample at the corresponding wavelength. Thus, the absorption spectrum of the sample can be determined.
  • the photoacoustic detector is mostly used with the so-called "step-scan" mode of the FTIR spectrometer.
  • the movable mirror of the interferometer is moved stepwise. Constructive interference enhances the light intensity at a wavelength, where the path difference corresponds to an integer multiple of that wavelength. Then the mirror is brought to oscillation around that position. The amplitude of the oscillation is much smaller than the half of the wavelength. Thus, mostly the phase of the light will be modulated. This phase modulation is used for generating the photoacoustic signal in the PA detector.
  • the FT-IR photoacoustic spectroscopy is mostly used for depth profiling of solid samples (thin films, layered structures, biological samples, etc.). This method is not applied for measuring gas concentrations.
  • an interferometer can modulate the light intensity.
  • the US patent US2006/0262316 A1 describes a "System and method for interferometric photoacoustic spectroscopy".
  • the system is essentially a photoacoustic gas detector with a laser source and a photoacoustic detector cell.
  • the intensity of the laser beam is modulated by means of a Michelson-type interferometer.
  • the modulated beam is then used for generating the photoacoustic signal and also for determining the wavelength of the laser accurately.
  • the main task of the presented invention is to overcome the disadvantages of the State of the art and to provide a low-priced (reasonably priced) method and an associated device for gas analysis.
  • the photoacoustic analysing system has a light source designed to emit a broadband or band limited light beam.
  • the system has further an interferometer, designed to receive the light beam, to modulate the light and to transmit the light to the photoacoustic gas detector.
  • the basic principle of using modulated light is known from the FTIR spectroscopy.
  • a light beam will be divided in a first and second light beam.
  • the light beams have different path lengths and will then be brought together. If the difference is equal ⁇ or an integer multiple of ⁇ the power of the two beams will be added due to constructive interference. If the difference is equal ⁇ /2 or an integer multiple ⁇ /2 due to destructive interference there is no resulting light beam.
  • the path length of the first or the second light beam is dependent on the position of a mirror, moving the mirror changes the power of the light beam produced by interfering the first light beam and the second light beam. This changing the power is important for producing a photoacoustic signal. If the power is constant over the time, the gas will be heated until a stationary state, where the heating by the incident light is equal to the cooling losses to the ambience. If the power of the incident light changes, the gas will be heated temporary. The resulting expansion leads to acoustic pressure, which can be detected.
  • the photoacoustic gas detector includes in a preferred embodiment of the invention at least one acoustic resonator for enhancing the photoacoustic signal.
  • the light source can be a broadband source, like a blackbody resonator, or a broadband IR-emitter, which emits broadband infrared light in the wavelength range of 2 ⁇ m - 20 ⁇ m. Also possible are incandescent lamps. Alternative, maybe also in addition, a source with limited bandwidth can be used. This can be for example light emitting diodes (LEDs) and/or superluminescent LEDs and/or an IR-emitter with limited bandwidth.
  • LEDs light emitting diodes
  • superluminescent LEDs and/or an IR-emitter with limited bandwidth.
  • the interferometer includes a reflective beam divider. Light coming from the light source can be reflected in the beam divider for providing two coherent light beams. Further there are means for guiding the light beams to a double mirror. The means for guiding the light can be one mirror for each light beam. The guided light beams reaching the double mirror will be reflected, each light beam at the related side of the double mirror. The first and the second light beam, reflected on the double mirror, will be guided to a common beam. Dependent on the difference of the path length of the first beam and of the second beam there is destructive or constructive difference as described above with respect to the Michelson interferometer.
  • the above described interferometer is especially useful for photoacoustic detection, but not limited to photoacoustic detection. It is also useful for other applications, where wavelength-selective modulation of light is required.
  • the rotating disk can have a profile that allows a linear increase of the radius with the angle from RO to RO+L over a range of 3 ⁇ /2 and a smooth return to RO in the remaining range of ⁇ /2.
  • the mirror can be mounted on a holder that can move along a straight line determined by two linear bearing. It is pressed by a spring to the perimeter of the rotating disk, thus the mirror holder is forced to scan back and forth along a path of L.
  • the rotation speed can be changed easily by the supply voltage. Since the required rotation speed is small compared to the usual ones of DC micromotors a gearbox is inserted between the motor and rotating disk to reduce the speed of rotation into the required range.
  • an appropriate device for example a magnetic or optical encoder, can be attached to the DC motor.
  • This device provides an output signal, whose frequency is proportional to the rotation speed of the motor.
  • the periodic output signal of the encoder can also be used at the later explained AD conversion of the photoacoustic signal.
  • the photoacoustic detector has at least one acoustic pressure sensor, preferably a microphone, designed to transform the sound pressure signal to an electrical signal.
  • acoustic pressure sensor preferably a microphone
  • Microphones are convenient pressure sensors, widely known and commercial available.
  • the photoacoustic gas detector can also include two identical acoustic resonators, especially in a geometrically symmetric arrangement, so that a difference between the sound pressure signal produced in one resonator and the sound pressure signal produced in the other resonator can be determined. So acoustic noise and vibrations from the environment can be suppressed very effectively. Also absorption for example caused by the entrance windows and by gas constituents not to be investigated can be suppressed, since this absorption is equal in both resonators.
  • the acoustic resonator such a way, that incident light is reflected several times in the acoustic resonator. So the incident light beam will be reflected back and forth several times in the resonator and more light will be absorbed. This enhances the acoustic signal.
  • the wavelength of the light which is modulated at a given frequency can be changed. So in an easy way measurements with different wavelengths are possible at a desired modulation frequency, normally at the resonance frequency of the acoustic resonator.
  • analogue acoustic signal for example an alternating current voltmeter.
  • This voltmeter can be connected to the at least one acoustic pressure sensor, normally the microphone. So a signal, which can be processed more easily, is available. So means for preamplifying and filtering the analogue acoustic signal can be used. Furthermore means for calculating the rms (root mean square) value of the preamplified and filtered acoustic signal should be provided to determine the photoacoustic signal.
  • the wavelength resolution of the system is determined by the halfwidth of the acoustic resonance. This method can be applied, when no other components absorb in the wavelength range selected by the acoustic resonance, or the absorption of the substance under investigation is much stronger than the absorption of other substances present in the gaseous sample under investigation.
  • ADC analog-to digital converter
  • a digital memory has to be provided.
  • means for evaluating the digital data can be a personal computer. In this case all the presently mentioned evaluation methods can be applied by using appropriate evaluation software. So besides the above mentioned determining the rms-value also the following methods can be used:
  • a lock-in detection method It is possible to pick up a single wavelength within the range given by the resonance curve by applying a lock-in detection method.
  • a reference signal is required, whose frequency corresponds to the wavelength of the absorption line to be measured.
  • This frequency should be provided by an independent source (for example the internal reference of the lock-in amplifier).
  • the selectivity of the method can be significantly (about one order of magnitude) increased, but the sensitivity will be reduced.
  • Another method is the Fourier-analysis of the microphone signal. This analysis results the absorption spectrum weighted by the frequency response of the acoustic resonator. Thus only one wavelength range is amplified from the spectrum; the range that corresponds to the frequency range of the acoustic resonance.
  • the sampling rate of converting to digital data is synchronised to the rotation speed of the rotating disc. So it can be adjusted, that the number of the digital samples during a scan is a power of two and is independent on the rotation speed.
  • the encoder produces M pulses for one rotation of the motor and the gearbox reduces the rotation by a factor of C.
  • fE can be used for adjusting (tuning) the system to the strongest absorption wavelength of the substance to be measured.
  • a sampling rate derived from the encoder frequency may provide significant advantages.
  • an electronic processor is provided to analyse the photoacoustic signal for determining the characteristic properties of the investigated gas and/or to control the analysing system.
  • FIG. 1 the main parts of the photoacoustic gas analyzer are shown;
  • the light comes from the light source 1 to the interferometer 2 for modulating the intensity of the light coming from the light source 1.
  • the modulated light is guided to the photoacoustic detector 3.
  • the electronic unit 4 controls the system.
  • the light source 1 is a broadband blackbody IR-emitter equipped with a parabolic mirror.
  • the output IR power is confined to cylindrical domain of about 8 mm diameter.
  • the construction of the interferometer 2 is shown in Figure 2.
  • the incident light beam is divided to two beams by a reflective beam divider 5.
  • This device is much cheaper than the single crystal beam splitters used in infrared FTIR spectroscopy.
  • Both beams are reflected by mirrors 6 and directed to the moving double mirror 7, whose both surfaces are reflecting.
  • the beams are reflected back by the moving mirror 7 through mirrors 6 to the reflective beam divider 5 where they will be united into a common beam.
  • the interference of both partial beams in the united beam leads to the modulation of the light intensity.
  • Input and output beams of the interferometer are separated by applying a smail angle deviation of the incident IR beam from 90° incidence in the plane that is perpendicular to the plane of Figure 2.
  • the mirror is scanned by a rotating disk 8 having a profile for constant velocity scan. Such a profile is shown in Figure 3.
  • the disk 8 is mounted to the output axis of a planetary gearbox 9 attached directly to the DC micromotor 10.
  • a magnetic encoder 1 1 is attached to the back side of the motor. The output of the encoder provides the TTL signals for controlling the rotation speed of the motor, thus selecting the wavelength of the measurement.
  • the photoacoustic detector 3 as shown in Figure 4 consists of at least one acoustic resonator 12, at least one microphone 13 to measure the sound pressure and arrangements 14 for driving the gas to be measured through the acoustic resonator.
  • the main parts of the electronic unit 4 are shown in Figure 5.
  • the power supply 15 provides the electric supply for the light source, for the DC motor and for the electronics.
  • the motor controller 16 measures and regulates the rotation speed of the DC motor, and provides the sampling signal for AD conversion.
  • the microphone electronics 17 filters and amplifies the microphone signal(s) and converts it (them) to digital data.
  • the main controller 18 collects and evaluates the measurement data, controls the entire measurement procedure, displays the results, performs self tests of the different measurement functions, allows the input of measurement parameters, etc. List of reference numbers

Abstract

Disclosed is a photoacoustic analysing system having a light source (1) designed to emit a broadband or band-limited light beam; an interferometer (2) designed to receive the light beam, to modulate the light and to transmit the light to a photoacoustic detector. The system differs from the state of the art, as the photoacoustic detector is a gas detector (3). Also disclosed is a method using the analysing system.

Description

Patent application: Photoacoustic Gas Analyser with lnterferometric Modulation
Applicant: Fraunhofer-Gesellschaft zur Fόrderung der angewandten Forschung e.V.
Technical Field
The invention concerns a photoacoustic gas analyser containing a light source, an interferometer for modulating the light intensity and a photoacoustic gas cell for detecting the photoacoustic signal generated by light absorption in the gas.
State of the art
Photoacoustic spectroscopy becomes increasingly accepted as a method for detecting the presence and measuring the concentration of substances that absorb light. Due to the absorbed light energy the material that contains the absorbing substance becomes warmer and expands. In the cases of periodic or pulsed illumination a pressure wave, that means a sound wave, is generated in the material, which can be detected by a pressure transducer. The received electric signal, that is the photoacoustic signal, can be regarded as a measure of the concentration of the substance that absorbs the light.
This principle has been successfully applied for measuring absorption spectra of solid, liquid and gaseous samples. The photoacoustic spectroscopy can be favourably applied in two cases:
a) for opaque, porous, powdered or other light scattering samples that are difficult to investigate by optical spectroscopy and
b) for trace gas detection.
In the case a) mostly a small closed photoacoustic cell is used. The solid or liquid sample is poured in a small cup. The sample is illuminated by an intensity modulated infrared light beam through a transparent window. The light energy absorbed in the sample generates a thermal wave in the sample, which causes a periodic temperature oscillation at the surface of the sample. The gas that fills the closed cell is periodically heated by the sample, thus a pressure oscillation is generated in the gas. This pressure oscillation is measured by a small microphone mounted into the PA cell, or connected to the PA cell by a capillary.
Besides the possibility to investigate opaque, or powdered, or other light scattering samples, the above described method has another advantage; the possibility of depth profiling. Since the information about the absorption of the light deep in the sample has to be brought to the sample surface for generating a PA signal in the gas, the penetration depth of the thermal wave plays an important role in PA spectroscopy. The thermal penetration depth, however, depends on the modulation frequency. Thus, a thicker layer of the sample contributes to the PA signal at a low modulation frequency, than at a higher modulation frequency. This effect can be used by depth profiling of the sample using a scan over an appropriate range of modulation frequencies. The frequencies are quite low; the range of ~5 Hz - 200 Hz is usually applied.
In case b) the high sensitivity of photoacoustic detection is used for detecting gas concentrations in the ppmV - ppbV (parts per million in volume - parts per billion in volume) range. Since the photoacoustic signal is proportional with the light power, mostly powerful tunable lasers are applied, while the photoacoustic cell is an acoustic resonator with high signal amplification. The light intensity is modulated at the resonance frequency, which is mostly in the kHz range.
There are photoacoustic gas detectors and analysers that utilize broadband light sources instead of tunable lasers. These kinds of devices are much simpler and cheaper than a system with tunable laser(s) or other tunable infrared light source. However, the sensitivity and selectivity of such devices is usually much smaller than that of photoacoustic detectors with laser excitation.
An important question in photoacoustic spectroscopy is the method of light modulation. The simplest way is the application of a chopper wheel. In this case the light beam is periodically blocked. This method is good for low frequencies. Modulation up to the kHz range can also be made by a mechanical chopper; however it produces acoustic noise, which is coherent with the PA signal. The level of this noise increases very rapidly with increasing frequency.
A much better method is the direct modulation of the light intensity. This can easily be done at diode lasers or quantum cascade lasers by modulating the laser current. Photoacoustic detectors with broadband light sources apply mostly a low frequency mechanical chopper.
A special modulation method is applied in the German patent application DE
10 2007 014 520.0. Here the modulation of the light intensity in the measuring PA cell is provided by two infrared beams that enter into the PA cell alternatively. One beam passes a gas cell filled with the target gas, where the light at the absorption lines of the gas is strongly absorbed, the other beam passes a reference cell filled with a nonabsorbing gas. The intensities of the two beams are adjusted to the same value. Thus, intensity modulation in the PA cell occurs only at those wavelengths, where the light is absorbed in the gas cell, i.e. at the absorption lines of the target gas. Consequently, the same gas produces the largest PA signal in the PA cell.
Another possibility for modulating light intensity is utilized by the known method of FTIR spectroscopy, where a Michelson-type interferometer equipped with a mirror moving by velocity u modulates the intensity of a light beam of wavelength λ by a frequency of f = 2u/λ. Thus, the different components of the broadband infrared light are modulated at different frequencies. In a FTIR spectrometer the light transmitted through a sample is detected by an infrared photodetector, and the spectrum of the detector signal is calculated by Fourier transformation. Since each frequency components of the spectrum correspond to a given wavelength, an optical transmission spectrum can be obtained. From the difference of the transmission spectra with and without sample the optical absorption spectrum of the sample can be determined.
FTIR spectrometers can also be used with photoacoustic detection (FT-IR Photoacoustic Spectroscopy). In this case a photoacoustic detector is used instead of the infrared photodetector. The US company MTEC Photoacoustics, Inc. (www.mtecpas.com) produces a photoacoustic detector (Model 300) that can be used by several commercially available FTIR spectrometers. This detector is similar to that described in case a) above. It can be placed into the sample compartment of the FTIR spectrometer. The IR beam is then focused to the sample inside the PA cell. The different frequency components of the microphone signal are proportional with the optical absorption of the sample at the corresponding wavelength. Thus, the absorption spectrum of the sample can be determined.
However, the different frequency components of the PA signal represent the absorption in unequally thin layers of the sample. Therefore, the photoacoustic detector is mostly used with the so-called "step-scan" mode of the FTIR spectrometer. In this case the movable mirror of the interferometer is moved stepwise. Constructive interference enhances the light intensity at a wavelength, where the path difference corresponds to an integer multiple of that wavelength. Then the mirror is brought to oscillation around that position. The amplitude of the oscillation is much smaller than the half of the wavelength. Thus, mostly the phase of the light will be modulated. This phase modulation is used for generating the photoacoustic signal in the PA detector. It is possible to make a wavelength scan stepwise with the same oscillation frequency. In this case the thermal penetration depth in the sample is the same for each wavelength components. By repeating the wavelength scan by different oscillation frequencies, absorption properties of the sample in different depths can be investigated.
Because of the depth- profiling potential of FTIR-PAS this method is often applied for studying material properties. Several patents can also be found by searching after the word combination of "FTIR and Photoacoustic". However, these patents extend the application the above described method and instrumentation for diverse, very specific fields.
The FT-IR photoacoustic spectroscopy is mostly used for depth profiling of solid samples (thin films, layered structures, biological samples, etc.). This method is not applied for measuring gas concentrations.
As mentioned above, an interferometer can modulate the light intensity. The US patent US2006/0262316 A1 describes a "System and method for interferometric photoacoustic spectroscopy". The system is essentially a photoacoustic gas detector with a laser source and a photoacoustic detector cell. However, the intensity of the laser beam is modulated by means of a Michelson-type interferometer. The modulated beam is then used for generating the photoacoustic signal and also for determining the wavelength of the laser accurately.
Sepcification
The main task of the presented invention is to overcome the disadvantages of the State of the art and to provide a low-priced (reasonably priced) method and an associated device for gas analysis.
This task is resolved by the independent claims of the invention. The dependent claims specify advantageous embodiments of the invention.
It was recognized that the modulation of broadband or band limited light by interferometric technique and using this light in a photoacoustic gas detector results to a superior photoacoustic analysing system. Until know the advantages of modulating light by an interferometer in the field of photoacoustic was only used for solid and/or fluid samples.
The photoacoustic analysing system according to the invention has a light source designed to emit a broadband or band limited light beam. The system has further an interferometer, designed to receive the light beam, to modulate the light and to transmit the light to the photoacoustic gas detector.
The basic principle of using modulated light is known from the FTIR spectroscopy. In an interferometer a light beam will be divided in a first and second light beam. The light beams have different path lengths and will then be brought together. If the difference is equal λ or an integer multiple of λ the power of the two beams will be added due to constructive interference. If the difference is equal λ/2 or an integer multiple λ/2 due to destructive interference there is no resulting light beam.
If the path length of the first or the second light beam is dependent on the position of a mirror, moving the mirror changes the power of the light beam produced by interfering the first light beam and the second light beam. This changing the power is important for producing a photoacoustic signal. If the power is constant over the time, the gas will be heated until a stationary state, where the heating by the incident light is equal to the cooling losses to the ambience. If the power of the incident light changes, the gas will be heated temporary. The resulting expansion leads to acoustic pressure, which can be detected.
So a Michelson-type interferometer equipped with a mirror moving by velocity u modulates the intensity of a light beam of wavelength λ by a frequency of f = 2u/λ.
In a preferred embodiment of the invention the acoustic resonance frequency oft the photoacoustic resonator can be excited. If the resonance frequency is fR, then the mirror of a Michelson Interferometer has to be moved with the velocity u = fR * λ/2 for resonant excitation. Exciting the photoacoustic detector at the resonance frequency yields to a high acoustic pressure and thus to a high photoacoustic signal. So by adjusting a desired velocity of the mirror it is possible to excite the resonance frequency for different absorption wavelengths. This is also possible using a broadband light source, as only the light of a specific wavelength is modulated at the resonance frequency of the photoacoustic gas detector.
For the reasons described above the photoacoustic gas detector includes in a preferred embodiment of the invention at least one acoustic resonator for enhancing the photoacoustic signal.
The light source can be a broadband source, like a blackbody resonator, or a broadband IR-emitter, which emits broadband infrared light in the wavelength range of 2μm - 20 μm. Also possible are incandescent lamps. Alternative, maybe also in addition, a source with limited bandwidth can be used. This can be for example light emitting diodes (LEDs) and/or superluminescent LEDs and/or an IR-emitter with limited bandwidth.
In a preferred embodiment of the invention a simplified interferometer design is applied, which is easier and cheaper to realise. The interferometer includes a reflective beam divider. Light coming from the light source can be reflected in the beam divider for providing two coherent light beams. Further there are means for guiding the light beams to a double mirror. The means for guiding the light can be one mirror for each light beam. The guided light beams reaching the double mirror will be reflected, each light beam at the related side of the double mirror. The first and the second light beam, reflected on the double mirror, will be guided to a common beam. Dependent on the difference of the path length of the first beam and of the second beam there is destructive or constructive difference as described above with respect to the Michelson interferometer.
If the position of the double mirror is changed, it is possible to increase the path length of the first beam and to decrease the path length of the second beam. So a displacement of the double mirror for dx can increase the path length of one of the beams by dx * 2. This is due to the fact, that the path length to reach the double mirror is increased and also the path length to come back from the double mirror. If the double mirror is positioned in an appropriate way, the path length of the other light beam is decreased by the same amount. So moving the double mirror by dx, changes the path length difference by 4* dx. With respect to the explanations concerning the Michelson interferometer, it will be clear, that this interferometer doubles the modulation frequency at given moving velocity of the mirror to f = 4u/λ.
It should not be neglected, that arrangements of the double mirror are possible, where the calculation above is not quite correct. But nearly in every case moving the double mirror leads to an increase of the path length of one light beam and to a decrease of the path length of the other light beam. For modulating the light beam in every case it is necessary to move the double mirror.
The above described interferometer is especially useful for photoacoustic detection, but not limited to photoacoustic detection. It is also useful for other applications, where wavelength-selective modulation of light is required.
As described above means for moving the double mirror with a required velocity are useful to modulate the light. An appropriate way is to provide a rotating profile disc. So a constant, but adjustable mirror velocity can be provided.
For example the rotating disk can have a profile that allows a linear increase of the radius with the angle from RO to RO+L over a range of 3π/2 and a smooth return to RO in the remaining range of π/2. The mirror can be mounted on a holder that can move along a straight line determined by two linear bearing. It is pressed by a spring to the perimeter of the rotating disk, thus the mirror holder is forced to scan back and forth along a path of L. Because of the profile of the disk only the movement when the mirror holder is pushed away from the rotation axis can be used, where the radial velocity u = dR/dt of the disk is constant over the 0 - 3π/2 range: u = ωL/(3π/2) = 4wL/3, where ω and w = ω/2π are the angular velocity of the rotation and rotation speed of the disk, respectively. During the last quarter of the rotation the speed changes quite rapidly; the mirror holder slows down, turns back, slides back, turns again and speeds up to the constant u velocity at RO. Both the radius and the velocity change smoothly at the beginning and end of the linear scan range.
If the rotating disk is driven by a DC micromotor, the rotation speed can be changed easily by the supply voltage. Since the required rotation speed is small compared to the usual ones of DC micromotors a gearbox is inserted between the motor and rotating disk to reduce the speed of rotation into the required range.
In order to adjust and control the rotation speed an appropriate device, for example a magnetic or optical encoder, can be attached to the DC motor. This device provides an output signal, whose frequency is proportional to the rotation speed of the motor. The periodic output signal of the encoder can also be used at the later explained AD conversion of the photoacoustic signal.
In a further embodiment of the invention the photoacoustic detector has at least one acoustic pressure sensor, preferably a microphone, designed to transform the sound pressure signal to an electrical signal. Microphones are convenient pressure sensors, widely known and commercial available.
The photoacoustic gas detector can also include two identical acoustic resonators, especially in a geometrically symmetric arrangement, so that a difference between the sound pressure signal produced in one resonator and the sound pressure signal produced in the other resonator can be determined. So acoustic noise and vibrations from the environment can be suppressed very effectively. Also absorption for example caused by the entrance windows and by gas constituents not to be investigated can be suppressed, since this absorption is equal in both resonators.
in many cases it is useful to construct the acoustic resonator such a way, that incident light is reflected several times in the acoustic resonator. So the incident light beam will be reflected back and forth several times in the resonator and more light will be absorbed. This enhances the acoustic signal.
By changing the velocity of the mirror of the interferometer the wavelength of the light which is modulated at a given frequency can be changed. So in an easy way measurements with different wavelengths are possible at a desired modulation frequency, normally at the resonance frequency of the acoustic resonator.
Preferably there are means for determining the analogue acoustic signal, for example an alternating current voltmeter. This voltmeter can be connected to the at least one acoustic pressure sensor, normally the microphone. So a signal, which can be processed more easily, is available. So means for preamplifying and filtering the analogue acoustic signal can be used. Furthermore means for calculating the rms (root mean square) value of the preamplified and filtered acoustic signal should be provided to determine the photoacoustic signal.
To determine the root mean square (RMS) value of the microphone signal is the simplest method. In this case the wavelength resolution of the system is determined by the halfwidth of the acoustic resonance. This method can be applied, when no other components absorb in the wavelength range selected by the acoustic resonance, or the absorption of the substance under investigation is much stronger than the absorption of other substances present in the gaseous sample under investigation.
It is also possible to provide means for converting the preamplified and filtered acoustic signal to digital data. This can be done by a widely known analog-to digital converter (ADC). For storing the digital data a digital memory has to be provided. Also necessary are means for evaluating the digital data. This can be a personal computer. In this case all the presently mentioned evaluation methods can be applied by using appropriate evaluation software. So besides the above mentioned determining the rms-value also the following methods can be used:
It is possible to pick up a single wavelength within the range given by the resonance curve by applying a lock-in detection method. In this case a reference signal is required, whose frequency corresponds to the wavelength of the absorption line to be measured. This frequency should be provided by an independent source (for example the internal reference of the lock-in amplifier). The selectivity of the method can be significantly (about one order of magnitude) increased, but the sensitivity will be reduced.
Another method is the Fourier-analysis of the microphone signal. This analysis results the absorption spectrum weighted by the frequency response of the acoustic resonator. Thus only one wavelength range is amplified from the spectrum; the range that corresponds to the frequency range of the acoustic resonance.
Strong absorption at wavelengths outside the wavelength range determined by the acoustic resonance may disturb the measurement. Therefore, it is advantageous to apply a band-pass electronic filter to suppress frequency components of the PA signal outside the resonance range.
In a preferred embodiment the sampling rate of converting to digital data is synchronised to the rotation speed of the rotating disc. So it can be adjusted, that the number of the digital samples during a scan is a power of two and is independent on the rotation speed. Assume that the encoder produces M pulses for one rotation of the motor and the gearbox reduces the rotation by a factor of C. Then the rotation speed w of the scanner disk can be given as w = fE/M/C, where fE is the frequency of the encoder output signal. The scanner velocity u can be written as u = 4LfE/(3MC). At this scanner velocity the wavelength component λ = 16L fE/(3MCfR) will be measured by the highest sensitivity. Since M, C and fR are constants, the wavelength is proportional with the encoder frequency. Thus, fE can be used for adjusting (tuning) the system to the strongest absorption wavelength of the substance to be measured.
For the AD conversion a sampling rate derived from the encoder frequency may provide significant advantages. The duration of a single scan is determined by the time needed for 3A rotations: T = 3/4w = 3MC/4fE. The number of the digital data sampled at a rate of fs during this time is N = Tfs = (3MC/4)* (fs/fE). If the ratio of fs/fE is constant, the number N is independent on the speed of rotation. Moreover, it is possible to choose this ratio such a way that the number of the data is equal to a power of two: N - 2k, which allows the application of a fast FFT routine for the calculation of the spectrum.
In order to increase the sensitivity several scans can be averaged, similarly to the averaging used in FTIR spectroscopy. The digital data can be recorded over several rotations of the scanner disc, and the linear scan domains can be selected, averaged and evaluated by the evaluation software after completing the recording.
For efficient operations of the photoacoustic analysing system an electronic processor is provided to analyse the photoacoustic signal for determining the characteristic properties of the investigated gas and/or to control the analysing system.
Also disclosed is a method for photoacoustic gas detection using a gas analysing system as described above.
Example to execute the invention
In the following the invention will be presented, without loss of generality, in an exemplary embodiment. In Figure 1 the main parts of the photoacoustic gas analyzer are shown; The light comes from the light source 1 to the interferometer 2 for modulating the intensity of the light coming from the light source 1. The modulated light is guided to the photoacoustic detector 3. The electronic unit 4 controls the system.
The light source 1 is a broadband blackbody IR-emitter equipped with a parabolic mirror. The output IR power is confined to cylindrical domain of about 8 mm diameter.
The construction of the interferometer 2 is shown in Figure 2. The incident light beam is divided to two beams by a reflective beam divider 5. This device is much cheaper than the single crystal beam splitters used in infrared FTIR spectroscopy. Both beams are reflected by mirrors 6 and directed to the moving double mirror 7, whose both surfaces are reflecting. The beams are reflected back by the moving mirror 7 through mirrors 6 to the reflective beam divider 5 where they will be united into a common beam. The interference of both partial beams in the united beam leads to the modulation of the light intensity. Input and output beams of the interferometer are separated by applying a smail angle deviation of the incident IR beam from 90° incidence in the plane that is perpendicular to the plane of Figure 2.
The mirror is scanned by a rotating disk 8 having a profile for constant velocity scan. Such a profile is shown in Figure 3. The disk 8 is mounted to the output axis of a planetary gearbox 9 attached directly to the DC micromotor 10. A magnetic encoder 1 1 is attached to the back side of the motor. The output of the encoder provides the TTL signals for controlling the rotation speed of the motor, thus selecting the wavelength of the measurement.
The photoacoustic detector 3, as shown in Figure 4 consists of at least one acoustic resonator 12, at least one microphone 13 to measure the sound pressure and arrangements 14 for driving the gas to be measured through the acoustic resonator.
The main parts of the electronic unit 4 are shown in Figure 5. The power supply 15 provides the electric supply for the light source, for the DC motor and for the electronics. The motor controller 16 measures and regulates the rotation speed of the DC motor, and provides the sampling signal for AD conversion. The microphone electronics 17 filters and amplifies the microphone signal(s) and converts it (them) to digital data. The main controller 18 collects and evaluates the measurement data, controls the entire measurement procedure, displays the results, performs self tests of the different measurement functions, allows the input of measurement parameters, etc. List of reference numbers
1 IR light source unit
2 Interferometer unit 3 Photoacoustic detector unit
4 Electronic unit
5 Reflective beam divider
6 Mirrors
7 Moving mirror holder + double mirror 8 Disk equipped with a profile for linear scan
9 Planetary gearbox
10 DC micromotor
1 1 Encoder
12 Acoustic resonator 13 Microphone
14 Gas handling
15 Power supply
16 Motor controller
17 Microphone electronics 18 Main controller+software

Claims

Claims
1. Photoacoustic analysing system having a light source (1) designed to emit a broadband or band-limited light beam; an interferometer (2) designed to receive the light beam, to modulate the light and to transmit the light to a photoacoustic detector, characterized in, that the photoacoustic detector is a gas detector (3).
2. Photoacoustic analysing system according to claim 1 , characterized in, that the resonance frequency of the photoacoustic gas detector (3) can be excited.
3. Photoacoustic analysing system according to claim 1 or claim 2, characterized in, that the photoacoustic gas detector (3) includes at least one acoustic resonator (12) for enhancing the photoacoustic signal at the resonance frequency.
4. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the light source (1) is a broadband source, like a blackbody resonator, or a broadband IR-emitter, and/or a source with limited bandwidth, preferably a light emitting diode or a superluminescent light emitting diode or an IR-emitter with limited bandwidth.
5. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the interferometer (2) includes a reflective beam divider (5) for providing two coherent light beams, means for guiding (6) the light beams to a double mirror (7), means for guiding (6) the light beams reflected at the double mirror (7) to a common beam.
6. Photoacoustic analysing system according to one of the preceding claims, characterized in, that there are means provided to move the double mirror (7) with a required velocity, preferably a rotating profile disc (8).
7. Photoacoustic analysing system according to claim 6, characterized in, that the rotating profile disc (8) can be driven by a micromotor (10), preferably through a gearbox (9).
8. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the photoacoustic detector (3) has at least one acoustic pressure sensor, preferably a microphone (13), designed to transform the sound pressure signal to an electrical signal.
9. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the photoacoustic gas detector (3) includes two identical acoustic resonators (12), especially in a geometrically symmetric arrangement, so that a difference between the sound pressure signal produced in one resonator and the sound pressure signal produced in the other resonator can be determined.
10. Photoacoustic analysing system according to one of the claims 5 to 9, characterized in, that by changing the velocity of the double mirror (7) the wavelength of the light which is modulated at a given frequency can be changed.
1 1. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the system includes: means for determining the analogue acoustic signal, for example an alternating current voltmeter; means for preamplifying and filtering the analogue acoustic signal; means for calculating the rms (root mean square) value of the preamplified and filtered acoustic signal (18).
12. Photoacoustic analysing system according to one of the preceding claims, characterized in, that the system includes: means for determining the analogue acoustic signal, for example an alternating current voltmeter; means for preamplifying and filtering the analogue acoustic signal; means for converting the preamplified and filtered acoustic signal to digital data; a digital memory for storing the digital data; means for evaluating the digital data (18).
13. Photoacoustic analysing system according to claim 12, characterized in, that the sampling rate of converting to digital data is synchronized to the rotation speed of the rotating profile disc (8).
14. Photoacoustic analysing system according to one of the preceding claims, characterized in, that an electronic processor (18) is provided to analyse the photoacoustic signal for determining the characteristic properties of the investigated gas and/or to control the analysing system.
15. A method for photoacoustic gas detection using a gas analysing system according to one of the preceding claims.
PCT/EP2010/000255 2009-01-16 2010-01-18 Photoacoustic gas analyser with interferometric modulation WO2010081739A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP09000625.5 2009-01-16
EP09000625 2009-01-16

Publications (1)

Publication Number Publication Date
WO2010081739A1 true WO2010081739A1 (en) 2010-07-22

Family

ID=41821852

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/EP2010/000255 WO2010081739A1 (en) 2009-01-16 2010-01-18 Photoacoustic gas analyser with interferometric modulation

Country Status (1)

Country Link
WO (1) WO2010081739A1 (en)

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4457162A (en) * 1982-09-17 1984-07-03 Institute Of Gas Technology Multi-frequency photo-acoustic detector
US5675412A (en) * 1995-11-24 1997-10-07 On-Line Technologies, Inc. System including unified beamsplitter and parallel reflecting element, and retroreflecting component
US20060262316A1 (en) 2005-05-20 2006-11-23 Baney Douglas M System and method for interferometric laser photoacoustic spectroscopy
US20080204757A1 (en) * 2006-08-17 2008-08-28 Christopher Manning Handheld FT-IR spectrometer
EP2149785A1 (en) * 2008-07-30 2010-02-03 Honeywell International Inc. A photoacoustic spectroscopy system
EP2157419A1 (en) * 2008-08-21 2010-02-24 Honeywell International Inc. Photoacoustic sensor

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4457162A (en) * 1982-09-17 1984-07-03 Institute Of Gas Technology Multi-frequency photo-acoustic detector
US5675412A (en) * 1995-11-24 1997-10-07 On-Line Technologies, Inc. System including unified beamsplitter and parallel reflecting element, and retroreflecting component
US20060262316A1 (en) 2005-05-20 2006-11-23 Baney Douglas M System and method for interferometric laser photoacoustic spectroscopy
US20080204757A1 (en) * 2006-08-17 2008-08-28 Christopher Manning Handheld FT-IR spectrometer
EP2149785A1 (en) * 2008-07-30 2010-02-03 Honeywell International Inc. A photoacoustic spectroscopy system
EP2157419A1 (en) * 2008-08-21 2010-02-24 Honeywell International Inc. Photoacoustic sensor

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
CHARLES DE IZARRA ET AL: "An infrared laser interferometer with photoacoustic detection", MEASUREMENT SCIENCE AND TECHNOLOGY, IOP, BRISTOL, GB, vol. 15, no. 1, 1 January 2004 (2004-01-01), pages 271 - 274, XP020066361, ISSN: 0957-0233 *
STEEL W H: "On Mobius-band Interferometer", OPTICA ACTA, XX, XX, 1 August 1964 (1964-08-01), pages 211 - 212,214, XP002974588, ISSN: 0030-3909 *

Similar Documents

Publication Publication Date Title
JP4431622B2 (en) Photoacoustic spectroscopic gas detection method and detector for improving measurement accuracy by quartz crystal
US8594507B2 (en) Method and apparatus for measuring gas concentrations
CN110672554B (en) Random vibration drive ring-down cavity calibration-free gas concentration measurement system
Mordmüller et al. An electrically and optically cooperated QEPAS device for highly integrated gas sensors
WO2013011253A1 (en) Method and apparatus for gas monitoring and detection
JP2013506838A (en) Gas sensor based on photoacoustic detection
EP2342544A2 (en) Wavelength-modulation spectroscopy method
WO2006092874A1 (en) High-resolution high-speed terahertz spectrometer
JP2008517252A (en) Heat selective multivariate optical computing
Liu et al. Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy
WO2008074442A1 (en) Differential photoacoustic detection of gases
Lindley et al. A sensitivity comparison of three photoacoustic cells containing a single microphone, a differential dual microphone or a cantilever pressure sensor
JP4164164B2 (en) DSP technology for photoacoustic spectroscopy (PAS) sample pulse response for depth profile detection
JP2004533620A (en) Gas identification device
JP5241274B2 (en) Detection method of detected substance
CN114018829A (en) Tuning fork resonance enhanced double-optical comb multi-component gas detection system
Palmer et al. Step-scan FT-IR photothermal spectral depth profiling of polymer films
WO2010081739A1 (en) Photoacoustic gas analyser with interferometric modulation
WO2016083358A1 (en) Interferometer with an oscillating reflector provided by an outer surface of a sonotrode and fourier transform infrared spectrometer
Bayrakli A portable N2O sensor based on quartz-enhanced photoacoustic spectroscopy with a distributed-feedback quantum cascade laser for medical and atmospheric applications
Weber et al. Novel approach for efficient resonance tracking in photoacoustic gas sensor systems based on a light-induced wall signal
JPH0694602A (en) Spectral photographing device for detecting absorption of modulated electromagnetic wave based on ultrasonic wave
Tortschanoff et al. Improved MOEMS-based ultra-rapid Fourier transform infrared spectrometer
WO2001040773A2 (en) System and method for frequency domain interferometric second harmonic spectroscopy
Briand et al. Gas detection using a micromachined FTIR spectrometer

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 10701204

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 10701204

Country of ref document: EP

Kind code of ref document: A1