DE102019001738A1 - Device for interferometric evaluation of photoacoustic signals without moving components - Google Patents

Abstract

Photoacoustic measuring device comprising a photoacoustic measuring cell in which the species to be determined is located as a medium, an optical excitation path for generating acoustic waves in the medium and an optical microphone are used to detect the acoustic waves.

Description

Technical area

The present application relates to the field of optical measurement technology and relates in particular to the detection or concentration measurement of gases and liquids.

State of the art

Light source: A photoacoustic excitation of the measuring medium is implemented at the current time by means of black emitters, lasers or LEDs. The light sources are electrically pulsed or their intensity is modulated by means of a pulse diaphragm.

1. 1. Detektion bzw. Messung der Schallwellen mittels einer bzw. mehreren zur Schwingung angeregten Membranen mit kapazitiver oder induktiver Messung der Signale
2. 2. Detektion bzw. Messung der Schallwellen mittels einer zur Schwingung angeregten Membran und optischer Abstandsmessung (Triangulationsverfahren, Interferometrische Verfahren) der Membran
3. 3. Detektion bzw. Messung der Schallwellen mittels piezoresistiven Schwingquartz

Measuring cell and sound sensors: Photoacoustic measuring cells vary greatly in shape and size. The detection or measurement of the excited sound waves is based on the following principles, depending on the measuring medium:

1. 1. Detection or measurement of the sound waves by means of one or more membranes excited to vibrate with capacitive or inductive measurement of the signals
2. 2. Detection or measurement of the sound waves by means of a membrane excited to vibrate and optical distance measurement (triangulation method, interferometric method) of the membrane
3. 3. Detection or measurement of the sound waves by means of piezoresistive oscillating quartz

Presentation of the invention

1. 1. Frequenzabhängigkeit aufgrund der Massenträgheit der Membran bzw. des piezoresistiven Schwingquarzes
2. 2. Einfluss von Körperschall, der in die Membran eingekoppelt wird
3. 3. Parasitärer Einfluss von fluiden Strömungen, die eine Kraft auf die Sensorfläche ausüben Ein Messsystem ohne bewegliche Teile bzw. mit einer membranfreien Messung von Schallwellen ist somit von Vorteil aufgrund der:
   1. 1. Unabhängigkeit von Körperschall
   2. 2. Messmöglichkeit über eine größere Bandbreite
   3. 3. Messmöglichkeit in fluiden Strömungen
   4. 4. Einflussfreiheit von der Massenträgheit einer schallempfindlichen Membran
   5. 5. Reduzierung der lokalen EMV Empfindlichkeit

All the systems mentioned in the prior art are based on the fact that a sound wave hits a clamped surface and deflects it onto the surface as a function of the sound amplitude or the resulting force. As a result, the systems are subject to several disadvantages that lead to measurement errors. These are:

1. 1. Frequency dependence due to the inertia of the membrane or the piezoresistive quartz oscillator
2. 2. Influence of structure-borne noise that is coupled into the membrane
3. 3. Parasitic influence of fluid currents that exert a force on the sensor surface A measuring system without moving parts or with a membrane-free measurement of sound waves is therefore advantageous due to:
   1. 1. Independence from structure-borne noise
   2. 2. Measurement possibility over a larger bandwidth
   3. 3. Measurement possibility in fluid currents
   4. 4. Influence from the inertia of a sound-sensitive membrane
   5. 5. Reduction of the local EMC sensitivity

Description of the technical problem: In the industrial environment, technical systems are subject to special influencing factors such as vibrations. The use of photoacoustic methods is therefore only possible to a limited extent with the use of complex signal processing. This is e.g. B. associated with high costs, so that a widespread use of photoacoustic measurement technology has not yet established itself.
Another problem is the analysis of moving gases or liquids. Current photoacoustic systems are based on the fact that, for example, a bypass is placed in the flow of an analyte and concentration measurements can only be carried out sequentially. A direct measurement in the river leads, for example, to broadband noise and thus to a poor signal-to-noise ratio due to the interaction of the flow with the measuring surface of the sensor.

How was the technical problem solved?
Conventional photoacoustic detectors are based on the deflection of a clamped membrane or a piezoresistive oscillating crystal due to the sound pressure that acts on the component surface. Instead of using moving components, the sound is measured interferometrically between two statically fixed optical elements. The acoustic sound is interpreted as modulating the refractive index of the analyte. The optical path length and the phase of the electromagnetic wave (s) vary with the refractive index. Due to the optical interference, this phase shift varies the intensity of the (relative) transmission and (relative) reflection in the frequency of the modulating light source. An integration of this system into a photoacoustic measuring cell can lead to a completely static photoacoustic measuring system when using a pulsed light source.

The registration is based on the combination of a pulsed light source and a photoacoustic measuring cell with an integrated static, membrane-free optical microphone and an evaluation unit.
The physical quantity to be determined is the concentration of an analyte.
The analyte is measured by a light source pulsed with the frequency f ( 11 ) stimulated. This creates a frequency-dependent change in density and thus in the refractive index, resulting in a sound wave. Phenomena of acoustic resonance should be used to amplify the sound quantity to be detected. This is achieved by the Frequency f is chosen to be identical to an acoustic room resonance frequency of the measuring cell. The measuring cell has different, geometrically defined room modes, which are differently pronounced. The sound wave propagates in the medium and passes the mirrors of the interferometric system. If the measuring cell is operated in acoustic resonance, standing waves are formed. The sensitivity of the sound detection increases with the amplitude of the standing sound wave in the system. At the same time, the amplitude of this standing sound wave is therefore location-dependent. The sensitivity of the measurement system is low when sound is detected at sound nodes and high at sound bellies, since the change in the density of the analyte and thus also the change in the refractive index is greatest at sound bellies (or sound pressure observation or sound velocity). The change in the refractive index leads to a phase shift of the light in the interferometer and to a change in the reflected and transmitted light intensity. The light intensity also changes due to the absorption of the analyte in proportion to its density. Reflected and / or transmitted light intensities are measured using light-sensitive sensors. DC signals are compensated for using suitable electrical filters and the filtered signal is amplified.

Ways of Carrying Out the Invention

One possible embodiment of an amplifier is in 4th shown: with transimpedance amplifier ( 201 ), Non-inverting amplifier ( 202 ), Non-inverting integrator ( 203 ) and power supply ( 204 ). In this, with a transimpedance amplifier, the light-sensitive sensor ( 114/115 ) incoming signal is amplified. The constant component of the signal, which corresponds to the operating point of the interferometer, is compensated with the aid of a second amplifier stage. This generates an opposite current that is applied to the input of the first amplifier stage, parallel to the light-sensitive sensor ( 114/115 ), is coupled. Even high levels of constant light are compensated for with the control loop that is created in this way. The combination of the two amplifier stages leads to a frequency response which z. B. has a high-pass characteristic or band-pass characteristic or can also "cut off the diagram appropriately". The DC component is eliminated, and low-frequency ambient noise and low-frequency signals (e.g. at 20 dB / dec) are suppressed. Above the cutoff frequency of the high pass filter up to the cutoff frequency of the transimpedance converter (TIA), the signal is amplified with the amplification of the TIA. In a further stage, the now filtered signal is further amplified.

In the following, several possibilities for realizing the abstractly described system are presented. These can be combined with one another in their individual components.
The pulsed light source ( 103 ) has coherent or non-coherent properties. The light falls through an optic ( 110 ) collimated or uncollimated into the measuring cell. Optionally, it is made using an optic ( 110 ) and a light-sensitive sensor ( 116 ) or reflected back into the cell by a concave or plane mirror (113). In order to avoid a solid-state photoacoustic effect, the part of the cell wall that is illuminated can be replaced by a window.
The measuring cell ( 101 ) contains the analyte. The analyte is fed through an inlet ( 105 ) introduced into the measuring cell and through a drain ( 106 ) derived. The connections are located, for example, on the side walls of the measuring cell and at the same time in the nodes of the standing sound waves so that the sound modes do not collapse. Sound nodes can be displaced by agitation of the analyte. By cleverly positioning the connections, undesired modes can be caused to collapse.
The interferometer consists of a light source ( 107 ) with an optical imaging unit ( 108 ) for coupling the light into an optical fiber. The light is passed through an Nx1 coupler or optical circulator. The fiber ends ( 107 ) are not or partially mirrored. They can have the same or different reflectivities / refractive indices in order to maximize the measurement signal, but with increasing reflectivities of the fiber end faces, the working point of the system in the form of the constant component of the light output / phase shift plays an increasingly important role. For an optimal evaluation it is advantageous that the working point is at a turning point with regard to the transmittance and reflectance of the transfer functions (see 2 ) of the Fabry-Perot interferometer. For higher reflectivities of the fiber ends, steeper edges result, which can significantly amplify the measurement signal, but at the same time this leads to a decrease in the half-width of the interference peaks. As a result, the operating point should be set to a rising or falling edge, for example by means of a wavelength variation for a given geometry of the measurement setup. The changes in the refractive index over time can then be mapped to periodic intensity fluctuations with a high alternating signal amplitude for each transmission and reflection (see 3 ) The optical collimators ( 110 ) are not or partially mirrored. The collimators can have the same or different reflectivities. The light passes through the measuring cell and is generated by the collimator ( 110 ) coupled into the fiber end and to a light-sensitive sensor ( 114 ) directed. The reflected light is converted to a light-sensitive sensor by means of a circulator or Nx1 coupler ( 115 ) directed. The signal is high-pass filtered and electrically amplified.

Another embodiment of the diaphragmless optical microphone is shown in FIG 5 shown. shown with the measuring cell ( 101 ), the inlet ( 105 ), the process ( 106 ), the interferometer ( 111 ) and the example of a wave to be detected ( 102 ) within the measuring cell. The interferometer consists of a light source ( 107 ). z. B. coherent and narrow band, with an optical imaging unit ( 108 ) for coupling the light into an optical fiber. The light can be passed through an optical circulator in order to avoid modulation due to back reflection into the light source. The light is coupled out of the fiber. The fiber end is low in reflection and not mirrored. A collimator collimates the light beam. The beam is through an optical element, such as. B. a beam splitter cube ( 112 ) divided into two partial beams. A partial beam is reflected by a mirror ( 113 ) aligned parallel to the other beam. Both parallel beams are guided through the measuring chamber and on the rear side through mirrors ( 113 ) reflected. These can be combined as a mirror. The reflected partial beams are reflected by a mirror or prism ( 113 ) and beam splitter ( 112 ) brought into congruence and using a collimator ( 110 ) into an optical fiber ( 109 ) coupled. This leads to the detector, e.g. B. a photodiode with amplifier, (114), which separates and amplifies the intensity fluctuations. The pulsed light source ( 103 ) is used for photoacoustic stimulation. For example, a mirror ( 104 ) is used.

$\text{T=}\frac{\left(1-R1\right)\left(1-R2\right)}{{\left(1-\sqrt{R1R2}\right)}^2+4\sqrt{R1R2}\text{ sin}{\left(\Delta \varphi \right)}^2}.$

2 shows an example of the transmission behavior in transmission and reflection for the same reflectivities ( R1 or. R2 ) of the two fiber ends that make up the Fabry-Perot interferometer 1 form: left: transmittance T (y-axis, 0 to 1) as a function of the phase change Δ x (x-axis, 0 to 2.5 π), shown in diagrams from bottom to top R1 = R2 = 90%, 50% , 20% and 4%; right: reflectance R (y-axis, 0 to 1) depending on the phase change Δϕ (x-axis, 0 to 2.5 π), seen from bottom to top in the diagram R1 = R2 = 4%, 20%, 50 % and 90%. For example, $\text{R =}\frac{{\left(\sqrt{\mathrm{R.}1}-\sqrt{\mathrm{R.}2}\right)}^2+\mathrm{4th}\sqrt{\mathrm{R.}1\mathrm{R.}2}\text{sin}{\left(\Delta \varphi \right)}^2}{{\left(1-\sqrt{\mathrm{R.}1\mathrm{R.}2}\right)}^2+\mathrm{4th}\sqrt{\mathrm{R.}1\mathrm{R.}2}\text{sin}{\left(\Delta \varphi \right)}^{2\text{'}}}$

$\text{T =}\frac{\left(1-\mathrm{R.}1\right)\left(1-\mathrm{R.}2\right)}{{\left(1-\sqrt{\mathrm{R.}1\mathrm{R.}2}\right)}^2+\mathrm{4th}\sqrt{\mathrm{R.}1\mathrm{R.}2}\text{sin}{\left(\Delta \varphi \right)}^2}.$

3 shows the transformation of the time-dependent phase difference to periodic intensity fluctuations due to external influences on the Fabry-Pérot interferometer: top left: transmittance T (y-axis, 0 to 1) as a function of the phase change Δϕ (x-axis, 0 to 2.5 π) for R1 = R2 = 90%. 50%, 20% and 4% (from bottom to top in the diagram); top right: Intensity I (t) (y-axis) as a function of time (x-axis. 0 s to 0.04 s), the optical constant component, product of T and I ₀ , is shown in dashed lines; bottom: time plotted on the y-axis (0 s (top) to 0.04 s (bottom)), phase change Δϕ plotted on the x-axis.

Claims (8)

Photoacoustic measuring device, comprising - a photoacoustic measuring cell designed to accommodate a gaseous medium to be determined, - an optical excitation path for generating acoustic waves in the medium, - An optical detection unit for detecting the acoustic waves modified by the gaseous medium. Device according to Claim 1 , comprising - a light excitation device with a periodically (frequency f) pulsed light source which, with the aid of optics, generates an optical excitation path along a first direction within the medium of the photoacoustic measuring cell. Device according to Claim 2 , comprising - a detection unit designed as an optical microphone, which is formed from a coherent light source, a first and a second interferometer arm and a detector detecting the interference signal, one of the two interferometer arms being arranged parallel to the first direction within the medium of the photoacoustic measuring cell, in such a way that both arms interact with the acoustic wave via the medium. Device according to one of the preceding claims, wherein - The first and the second interferometer arm are arranged at a distance from one another. Device according to one of the preceding claims, wherein - An interferometer arm being designed for multiple reflection via partially reflective mirrors, in particular in the case of a fiber optic interferometer structure the fiber ends are designed to be partially reflective, not anti-reflective. Device according to one of the preceding claims, wherein - the photoacoustic measuring cell is designed as a flow measuring cell for the species to be determined, an input and an output being arranged opposite one another and their connecting line defining a flow direction which simultaneously is arranged within the optical excitation path. Device according to one of the preceding claims, wherein - The paths of the interferometer are formed by means of a beam part. Apparatus according to any preceding claim, further comprising - a beam splitter for coupling out the light to be measured.

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