WO2007054751A1

WO2007054751A1 - Method and apparatus for the selective photoacoustic detection of contaminants in a gaseous sample by making use of spectrally distant exciting wavelengths

Info

Publication number: WO2007054751A1
Application number: PCT/HU2006/000098
Authority: WO
Inventors: Zoltán Bozóki; Árpád MOHÁCSI; Gábor SZABÓ; Anikó HEGEDÜS VERES
Original assignee: Szegedi Tudományegyetem
Priority date: 2005-11-14
Filing date: 2006-11-14
Publication date: 2007-05-18
Also published as: HU0501060D0; HUP0501060A2; HU226449B1

Abstract

In accordance with the method according to the invention, a laser light (220) is introduced into a light path crossing at least one measuring station realized by a photoacoustic chamber (280) containing the gaseous sample, by subjecting the incident laser light (220), as fundamental harmonic, to frequency multiplication at least one higher order harmonics thereof is produced, by directing the fundamental harmonic through a measuring station the impurity components of the gaseous sample are excited, by directing the at least one higher order harmonies obtained by frequency multiplication through a measuring station one by one the impurity components of the gaseous sample are excited, the photoacoustic signals obtained at the wavelengths, as exciting wavelengths, of the fundamental and the higher order harmonics are detected, and by evaluating the photoacoustic signals obtained the impurity components of the gaseous sample are identified one by one.

Description

METHOD AND APPARATUS FOR THE SFXECTIVE PHOTOACOUSTIC DETECTION OF

CONTAMINANTS IN A GASEOUS SAMPLE BY MAKING USE OF SPECTRALLY DISTANT

EXCITINGWAVELENGTHS

The invention relates to a method and apparatus for the selective detection of impurities, espe- cially of those with a broad absorption spectrum being of the aerosol-type in a gaseous sample by utilizing photoacoustic principles and applying exciting wavelengths spreading over a broad wavelength range and being distant from each other.

Selective and accurate determination of the amount of impurities in different gaseous samples is an ever more and more important task. For example, to measure selectively (i.e. for each component) the amount of solid particles (powders, particles) or liquid drops - further on summarized as aerosols — in air, i.e. the aerosol impurities of air is indispensable for the targeted elimination of harmful effects of the above materials on the human organism, as the extent of the harm of aerosols on health depends on the composition of the aerosol impurities, as well as on their physical (e.g. size, mass) or/and chemical (e.g. adsorption capacity) properties. Characteristically, dust in the aerosols of environmental air is less harmful for human health, whereas carbon black (originating mainly from burning processes of fuels or the exhaust gases of cars) being part of it is very dangerous. The harmful effect of carbon black is even stronger if its surface contains further adsorbed harmful materials (such as polycyclic aromatic hydrocarbons, PAH), or if the characteristic size of the carbon black particles falls in the region un- der 10^"6 m.

Presently used methods for detecting the aerosol impurities of gases work partly on optical absorption principles, i.e. on measuring the amount of light energy absorbed by the gaseous sample studied. This is based on the Beer-Lambert law, according to which the extent of light absorption by a sample is proportional to the concentration(s) of the absorbing component(s). In contrast to the narrow absorption bands characteristic of small gas molecules, such as water vapour, methane, carbon-dioxide, or ammonia, which can easily be identified, the optical absorption spectra of aerosols usually do not contain characteristic, readily identifiable absorption bands. The optical absorption of aerosols can be characterized by a slow change in a broad wavelength range. Consequently, the measurement of a gaseous sample containing aero- sol cannot be solved reliably by using light sources emitting light only in a narrow wavelength range: measurements carried out by using such light sources do not allow for qualitative and quantitative differentiation between the different aerosol types. However, the use of light sources emitting in a narrow wavelength range has also an advantage: a light source emitting in a narrow wavelength range can be tuned between the well determined emission bands of small molecules also present in the gaseous sample, i.e. such wavelengths can be chosen for performing the optical measurement at which the small molecules present in the gaseous sample do not absorb, or absorb only to a small extent.

A significant part of optical measurements are indirect methods. In such cases, the extent of absorbed light is determined from the difference between the energies of the incident and the leaving beam. This difference is caused partly by the absorption of the light, and partly by its scattering in the sample. However, the ratio of absorption and scattering processes taking place cannot be determined easily, and in most of the cases, only inaccurately. This means that the indirect optical measurements can only estimate the amount of aerosols in the sample with some uncertainty. Results thus obtained are therefore not reliable enough.

For eliminating this problem, experts started to apply spectroscopic methods measuring optical absorption by a direct way, e.g. the so-called photoacoustic spectroscopic methods. The essence of the photoacoustic measuring principle is that a gaseous sample illuminated by a suitable light source transforms the absorbed energy into acoustic energy, which can be recorded by an appropriate detector. The light power emitted by the light source consists of pulses characteristically (mostly periodically) modulated or of series of them. It is also characteristic of the wavelength of the emitted light that it overlaps fully or partly with one or more optical absorption band(s) of the components of the gaseous sample to be measured. If the light of the light source is absorbed only by one component of the gaseous sample, the concentration of the component to be determined is relatively simple to calculate from the photoacoustic signal measured. In such cases the measurement is selective. Taking air as the gaseous sample, the photoacoustic measurement of aerosols contained therein is made much more difficult and less selective by the fact that the aerosol components have spectrally broad, overlapping, uncharacteristic absorption bands in almost the whole spectral range, and this makes very difficult to differentiate between them.

U.S. Patent No. 6,662,627 discloses a photoacoustic sensor used for determining the particle content of exhaust gases. The laser source applied in the sensor for excitation works at one single wavelength, either at 1047 or at 532 nm, thus the measurement is performed also at one of these wavelengths. The result of the measurement is the total particle content of the gaseous sample, but it does provide no information about its composition.

W. Patrick Arnott et al. teach {Atmospheric Environment, 33, p.2845-2852, 1999) a photo- acoustic spectrometer measuring the light absorption of aerosols in air as a function of time. As exciting unit, a diode laser emitting at 685 ran and a Nd:YAG laser of doubled frequency operating at 532 ran were used. Altering the wavelengths is realized by interrupting the measurement and changing the light source, thus the data do not relate to the same time, correspondingly, the data are registered under different circumstances. In addition, as the measurements are carried out only at two different, relatively near wavelengths, the composition of aerosols cannot be determined accurately from the spectra obtained.

German Utility Model No. DE 200 17 795 Ul describes a carbon black particle sensor based on photoacoustic principles which enables the separation of the absorptions of gaseous and solid impurities equally present in a gaseous sample. Separation is performed mechanically: the gaseous sample to be studied, previously to its feeding into the photoacoustic chamber, is periodically led through a filter element filtering out the solid impurities of the sample. The difference between the photoacoustic spectra of filtered and unfiltered samples characterizes the total solid material content of the sample. The carbon black sensor described is suitable for measuring only at one wavelength, thus it is not applicable for the selective photoacoustic detection of the composition of the solid material content. For determining this, other methods (e.g. mass spectrometry) have to be used for analyzing the filtrate.

A.H. Veres et al. {Spectroscopy Letters, 38, p.377-388, 2005) introduce a measuring device by which the concentration of one of the gaseous components of a gaseous sample, namely that of ozone, can be measured photoacoustically. In the measuring system shown, excitation is performed by pulsed or quasi-continuous lasers. To excite ozone strongly absorbing in the UV range (at λ =266 nm), a laser light produced by the so-called frequency multiplication method is applied. In particular, for the measurement a frequency-quadrupled laser light of 1064 nm in wavelength emitted by a Nd: YAG laser is used. Since in this UV region the aerosol components have also significant absorption, in case of gaseous samples containing aerosols, the measurement of the ozone concentration takes place at a significant background signal, which makes a reliable evaluation of the data extremely difficult. Another disadvantage of the meas- uring system is that no selective determination of the aerosol content of a gaseous sample can be performed.

One of the common disadvantages of all the above methods and devices is that by using them, it cannot be determined that in the impurities present in the gaseous samples, specifically aero- sols, to what extent the individual components participate in the absorption, thus to what extent they contribute to the photoacoustic signal measured. In other words, the methods and devices used at present are not suitable for detecting the impurity components of gaseous samples usually having broad and uncharacteristic absorption spectra, such as the aerosols and their components, i.e. the selectivity of the measurement for these components is not guaran- teed.

Our investigations led us to the conclusion that selectivity could be ensured if the measurements on a gaseous sample could be carried out in a broad wavelength range, or at least at several wavelengths located far from each other in a broad wavelength range without interrupting the measurement for changing the exciting light source or disjoining the device.

According to this, our present aim is to provide a method and an apparatus for the selective photoacoustic detection of the components of a given gaseous sample, especially its impurities, specifically aerosol components, one component by one component. Another aim of the invention is to provide a method and an apparatus ensuring possibility for the simultaneous measurement of the optical absorption (absorptance) of an arbitrary gaseous sample (contain- ing eventually solid and/or gaseous impurities) at several distant wavelengths in a relatively broad wavelength range.

Our aims were achieved by developing a method wherein a laser light is introduced into a light path crossing at least one measuring station realized by a photoacoustic chamber containing the gaseous sample, by subjecting the incident laser light, as fundamental harmonic, to fre- quency multiplication at least one higher order harmonics thereof is produced, by directing the fundamental harmonic through a measuring station the impurity components of the gaseous sample are excited, by directing the at least one higher order harmonics obtained by frequency multiplication through a measuring station one by one the impurity components of the gaseous sample are excited, the photoacoustic signals obtained at the wavelengths, as exciting wave- lengths, of the fundamental and the higher order harmonics are detected, and by evaluating the photoacoustic signals obtained the impurity components of the gaseous sample are identified one by one. Preferred embodiments of the method according to the invention are identified by claims 2 to 20.

In a further aspect, the above aims are achieved by accomplishing a photoacoustic measuring apparatus comprising an exciting unit emitting a laser light; at least one measuring station with a photoacoustic chamber containing the gaseous sample and suitable for detecting the photo- acoustic signal generated in the gaseous sample by laser light; a light path for directing laser light formed from the exciting unit to the at least one measuring station, the light path passing through the measuring station; at least one frequency converting element arranged in the light path and capable of generating higher order harmonics of the incident laser light; and at least one wavelength selective element arranged in the light path and capable of separating the incident mixture of laser lights with respect to the wavelengths of the component laser lights, wherein the section of the light path passing through the measuring station is formed to be capable of directing a laser light of a single wavelength through the gaseous sample at a time. Preferred embodiments of the apparatus according to the invention are described in claims 22 to 32.

The invention is described further in detail on basis of the attached figures, wherein

- Figure 1 shows the block scheme of a possible embodiment of the measuring apparatus according to the invention, wherein the measurement can be carried out simultaneously at three wavelengths (at a wavelength of the fundamental harmonic and at those of the frequency- doubled second harmonic obtained by frequency-doubling and the fourth harmonic obtained by frequency-quadrupling) by applying three individual photoacoustic chambers;

- Figure 2 is the block scheme of another possible embodiment of the measuring apparatus according to the invention performing measurements at two wavelengths (at that of the fundamental harmonic and of the second harmonic obtained by frequency-doubling) by applying one single photoacoustic chamber;

- Figure 3 shows the block scheme of another possible embodiment of the measuring apparatus according to the invention, in which the measurement is carried out at three wavelengths (at that of the fundamental harmonic and at those of the second and fourth harmonics obtained by frequency-doubling and frequency-quadrupling, respectively) by applying one single photoacoustic chamber;

- Figure 4 is the block scheme of an even further possible embodiment of the measuring apparatus according to the invention performing measurements at four wavelengths (at that of the fundamental harmonic and at those of frequency-doubled, frequency-tripled and frequency- quadrupled harmonics) by applying one single photoacoustic chamber; and - Figure 5 shows the absorption coefficients for two carbon blacks of different origin and nature as a function of the wavelength (λ) from the UV region to the near IR region.

As it is obvious from the above, by using the invention, we want to perform selective photo- acoustic measurements on a gaseous sample of a given composition in a relatively broad wavelength range spreading from the UV to the near IR region, at given wavelengths (relatively distant from each other), and from the results of these measurements conclusions concerning the composition of the gaseous sample are to be drawn.

Figure 1 shows the block scheme of an embodiment of a photoacoustic apparatus 100 which is developed for performing measurements on gaseous samples at more (in the embodiment three) measuring stations spatially separated from each other. Apparatus 100 contains an exciting unit 110 basically in form of lasers, frequency converting elements 140, 145, wavelength selective elements 150, 155, photoacoustic chambers 180, 182, 184 constituting the measuring stations and containing the gaseous samples, and optical power meters 190, 192, 194. The frequency converting elements 140, 145, the wavelength selective elements 150, 155 and the photoacoustic chambers 180, 182, 184 are arranged in a single multiply-branched light path. As to the exciting laser lights in connection with the frequency converting elements discussed later, the individual branches of the light path spread from the laser outlet opening of the excit- ing unit 110 to the sensoring surfaces of the power meters 190, 192 and 194. The branching points of the light path coincide with the wavelength selective elements 150, 155, i.e. the number of branching points is identical with the number of wavelength selective elements. The photoacoustic chamber 180 is positioned before power meter 190 and after wavelength selective element 150, whereas photoacoustic chambers 182 and 184 are situated in the correspond- ing branches of the light path before power meters 192 and 194, and after wavelength selective element 155, respectively. In the light path, before individual wavelength selective elements 150 and 155, frequency converting elements 140 and 145 are placed, respectively. When needed, additional optical elements 130, 132, 134 (e.g. collective or dispersive lenses) can also be situated in the light path arranged preferably before wavelength selective elements 150, 155. The frequency converting element 140 and wavelength selective element 150 (in the given case completed with optical element 132) together placed into the light path form the first beam split unit. The frequency converting element 145 and wavelength selective element 155 in the light path (complemented in the given case by optical element 134) form together a second beam split unit. In apparatus 100 shown in the Figure, the first beam split unit is placed in the light path before the second beam split unit. Here, and further on, ,,before" and ,,after" mean the position of a given element in the light path relative to another one in the direction of light propagation.

The exciting laser 110 is preferably a pulsed laser or a continuous laser the emission of which is modulated in a known way (e.g. periodically). In one of the preferred embodiments of apparatus 100 the exciting unit 110 is preferably a Nd: YAG pulsed laser operating at 1064 nm as fundamental wavelength. In another possible preferred embodiment of apparatus 100, the ex- citing unit 110 is preferably a Ti:saphhire laser tunable between about 700 and 1000 nm (operated generally at about 800 nm). Naturally, other exciting lasers 110 can also be used.

Frequency converting elements are optical elements producing, from the incident laser light of a given wavelength, in addition to the incident laser beam a further laser light of different wavelength. Correspondingly, each of the frequency converting elements 140, 145 emit two laser lights of different wavelengths. The frequency converting elements 140, 145 are preferably per se known crystals of non-linear optical properties oriented in an optically suitable way.

In the embodiment of apparatus 100 according to the invention shown in Fig. 1 the frequency converting element 140 is preferably a non-linear crystal which e.g. doubles the frequency of the infrared laser light 120 (fundamental harmonic) of 1064 nm of the Nd: YAG laser, thus in addition to transmitting a given portion of the laser light 120, also produces a second laser light 122 in the visible region at 532 nm (second harmonic). Here, the frequency converting element 140 is provided preferably e.g. by crystals of lithium triborate (LiB₃O₅, in what follows LBO), of potassium dihydrophosphate (KH₂PO₄, in what follows KDP), of potassium dideuterium phosphate (KD₂PO₄, in what follows DKDP), of ammonium hydrophosphate (NH₄H₂PO₄, in what follows ADP) or of β-barium borate (β-Ba₂O₄, in what follows BBO). In the embodiment of the apparatus 100 according to the invention shown in Fig. 1 the frequency converting element 145 is preferably a non-linear crystal which, in addition to transmitting a given portion of the incident second harmonic laser light 122 of 532 nm, also produces a laser light 124 with a wavelength of 266 nm (fourth harmonic) falling into the UV region by dou- bling the frequency of the laser light 122. Here, the frequency converting element 145 is preferably formed from crystals of cesium lithium borate (CsLiB₆O₁₀, in what follows CLBO) or KDP. It should be noted that by using suitable non-linear crystals as frequency converting elements 140, 145, by coupling simultaneously two harmonics of the same laser beam into the given crystal with a certain phase difference, the crystal can produce further harmonics of the fundamental harmonic laser light. For example, as it is known by the skilled person in the art, by coupling the fundamental harmonic and the second harmonic of a Nd:YAG laser into the LBO crystal under suitable conditions, the third harmonic with a wavelength of 355 nm of the Nd:YAG laser can be produced in a simple manner. Similarly, e.g. by coupling the fundamental harmonic and the second harmonic of a Ti:sapphire laser into a CLBO crystal at suitable phase locking, the third harmonic of the Ti: sapphire laser can also be produced in a simple manner. It is also noted here that the frequency converting elements applied develop their frequency multiplying effect only if the incident laser light has a polarization plane harmonizing with the crystal lattice of the non-linear optical crystal.

The wavelength selective elements 150, 155 are optical elements diffracting the individual components of the incident laser lights of different wavelengths to different extents depending on the wavelengths of the components, directing thereby the component laser lights to the appropriate different branches of the light path. Wavelength selective elements 150, 155 are preferably dichroic mirrors or prisms known per se in an optically appropriate orientation.

In the embodiment of apparatus 100 according to the invention shown in Fig. 1, wavelength selective element 150 is preferably a dichroic mirror separating the incident beam formed by the fundamental harmonic laser light 120 (λ = 1064 nm) and the second harmonic laser light 122 (λ = 532 nm) according to the wavelengths of the components, i.e. for example transmits laser light 120 , whereas changes the direction of propagation of laser light 122 by deflecting thereby laser light 122. Similarly, the wavelength selective element 145 is preferably a dichroic mirror, separating the incident beam consisting of the second harmonic laser light 122 (λ = 532 nm) and the fourth harmonic laser light 124 (λ = 266 nm) with respect to their wavelengths, e.g. it transmits laser light 122, whereas deflects laser light 124 by changing its propagation direction. The wavelength selective element 150 is placed in the light path so that the laser light transmitted by it gets into the photoacoustic chamber 180, whereas the deflected (specifically reflected) laser light 122 gets into the frequency converting element 145. The wavelength selective element 155 is placed in the light path so that laser light 122 transmitted by it gets into the photoacoustic chamber 182, whereas the deflected (specifically reflected) laser light 124 gets into the photoacoustic chamber 184. The construction, structure and geometry of one of the possible embodiments of photoacoustic chambers 180, 182, 184 usable in apparatus 100 according to the invention are described in detail e.g. in Rev. ScL Instrum., Tl, p.1937-1955, 2001 by A. Miklόs et al, and hence are not discussed here. Spatially separated photoacoustic chambers 180, 182, 184 operating as meas- uring stations of apparatus 100 are preferably identical, though different chambers (e.g. having different acoustic resonance frequencies) can also be used. It should be noted that in this latter case, processing and evaluation of the results are significantly more complicated. Further on, the individual photoacoustic chambers 180, 182, 184 can be operated in the resonant working mode (i.e. when the modulation frequency of the incident laser light and the acoustic reso- nance frequency of the chamber are identical), or in working modes differing from this.

As it is shown in Fig. 1, photoacoustic chamber 180 is provided with an inlet stub 180a for directing the gaseous sample into the chamber, and with an outlet stub 180b for removing the gaseous sample from the chamber. Similarly, photoacoustic chamber 182 is provided with an inlet stub 182a, and an outlet stub 182b, whereas photoacoustic chamber 184 is provided with an inlet stub 184a and an outlet stub 184b.

The power meters 190, 192, 194 are optical power meters known per se that are capable of measuring continuously the optical power of the incident laser light falling onto their sensor- ing surfaces and forwarding these electric signals to the single purpose device for further processing. As it is obvious to the skilled person in the relevant field, the role of power meters 190, 192, 194 can be taken over by common photodiodes sensitized to the wavelength to be detected and/or to its neighbourhood, or by photoacoustic chambers filled with a known amount of some absorptive material. Further on, by applying excitation with a steady, in time stable power, the power meters 190, 192, 194 may even be left out; they are needed only if the power of the exciting laser source may vary during the measurement.

The role of power meters 190, 192, 194 is to eliminate the part of the fluctuation of the photoacoustic signal caused not by the change in the concentration of the components to be measured, but only by the fluctuation of the exciting laser beam's power. One of the possible solutions for this is e.g. if the photoacoustic signal measured is divided by the optical power measured by units 190, 192, 194, then the concentration of the components studied is determined from the ratio thus formed by using the calibration procedure and multicomponent analysis method described (e.g. in Infrared Phys. TechnoL, 36, p.585-615, 1995) by A. Thony et al. At calibrating or applying apparatus 100, after scavenging the photoacoustic chambers 180,182,184, the sample to be studied is fed into photoacoustic chambers 180, 182, 184. Measurements can be carried out in a closed or in a flow-through working mode of the chambers 180, 182, 184. After filling up the photoacoustic chambers 180, 182, 184, the exciting unit 110, in this case a Nd: YAG laser, is put to operation for emitting the fundamental harmonic (λ = 1064 nm) laser light 120. The laser light 120 gets via optical element 130 arranged in the light path into the first beam split unit, where the frequency converting element 140 produces the frequency- doubled (in this case at 532 nm) laser light 122, besides the laser light 120. After that, laser lights 120, 122 are passed through the optical element 132 (e.g. focus- sing) to wavelength selective element 150. Wavelength selective element 150 transmits laser light 120 in the original propagation direction, whereas simultaneously deflects laser light 122 separating thus the two laser lights 120, 122. The laser light 120, at leaving the first beam split unit enters photoacoustic chamber 180 where it may excite the impurities in the gaseous sample at the absorption wavelength corresponding to its own wavelength, producing thereby a suitable photoacoustic signal. This photoacousic signal is then fed into a central evaluation unit (not shown in the Figure) for further processing, after the necessary conditioning/amplifying. The not absorbed part of laser light 120 passing through the photoacoustic chamber 180, leaves at the opposite side of the inlet, and in the embodiment shown in Fig. 1, gets into power meter 190, by means of which its power is measured. The electric signal ob- tained as the result of power measurement is also forwarded to the evaluation unit.

Laser light 122 leaving the first beam split unit gets into the second beam split unit, where the frequency converting element 145 tuned to the wavelength of laser light 122 produces the frequency-quadrupled (in this case 266 nm) laser light 124. Laser lights 122, 124 when leaving the frequency converting element 145 get through optical element 134 applied in the given case (e.g. for focussing) to wavelength selective element 155. The wavelength selective element 155 separates laser lights 122 and 124 by transmitting laser light 122 in the original propagation direction and simultaneously deflecting laser light 124. Laser light 122, at leaving the second beam split unit, gets into photoacoustic chamber 182, where it generates photoacoustic signal by exciting at its wavelength the impurities of the gaseous sample which have a broad absorption region. The photoacoustic signal thus obtained is fed into the evaluation unit in the earlier described way. Part of laser light 122 leaving the photoacoustic chamber 182 is captured by power meter 192, and the electric signal obtained as a result is also forwarded to the evaluation unit. The laser light 124 leaving the second beam split unit gets into photoacoustic chamber 184, in which it generates a photoacoustic signal by exciting the impurities of broad absorption region in the gaseous sample at the wavelength corresponding to the laser light 124. The photoacoustic signal thus obtained is also fed into the evaluation unit in the known way. The part of laser light 124 not absorbed in the photoacoustic chamber 184, after its leaving the chamber, is captured by power meter 194, and the electric signal obtained as the result of power measurement is also forwarded to the evaluation unit.

Measurements performed at the individual wavelengths are carried out in the present case in photoacoustic chambers 180, 182, 184 spatially separated, then the photoacoustic signals be- longing to different exciting wavelengths are evaluated in a central unit, in a PC developed for this purpose with suitable software/hardware. Sometimes, in certain cases, they can also be presented numerically or graphically. In the evaluation, the concentrations of impurities present in the gaseous sample can be determined by using the photoacoustic signals at different wavelengths, and if needed, also by using the light power measured, according to the calibra- tion procedure and multicomponent analysis described in detail in the work of Thony et al, already cited.

Figures 2 to 4 illustrate further possible embodiments of the apparatus according to the invention realized with one single measuring station. Apparatus 200 shown in Fig. 2 makes the carrying out of the measurement at two different wavelengths possible, whereas apparatus 200' in Fig. 3 and apparatus 200" in Fig. 4 make measurements at three and four different wavelengths possible, respectively. We note that elements in Fig. 1 and Fig. 2 denoted by similar reference signals refer to elements of similar functions. Further on, elements of the apparatus according to the invention, in embodiments shown in Figs. 2 and 3-4 denoted by identical reference signals also refer to identical elements. Consequently, in what follows, only elements of apparatuses 200, 200' and 200" are discussed in detail which are not part of apparatus 100 or have different properties relative to those of apparatus 100.

Apparatus 200 is provided with an exciting unit 210 emitting laser light 220, a polarization switching element 260 corresponding to the wavelength of the incident beam 220, a polarization selective element 265, a frequency converting element 240, wavelength selective elements 250, 252, a photoacoustic chamber 280 functioning as the measuring station and having inlet stub 280a and outlet stub 280b, and optical power meters 290, 292. It should be noted that in case of a timely stable excitation, power meters 290, 292 can be left out.

Polarization switching element 260, polarization selective element 265, frequency converting element 240, wavelength selective elements 250, 252 and photoacoustic chamber 280 are aligned in the branching light path spreading from the laser beam outlet of the exciting unit 210 to the sensor surfaces of the power meters 290, 292 arranged with their optical axis correspondingly to their function. In the light path, the polarization switching element 260 is positioned after the exciting element 210 and before the polarization selective element 265. The branching point of the light path coincides with the polarization selective element 265. The po- larization selective element 265 is situated in the light path before wavelength selective elements 250, 252 so that the wavelength selective elements 250, 252 are arranged in different branches of the light path. The photoacoustic chamber 280 is situated before the power meters 290, 292 and after the wavelength selective elements 250, 252 in branches of the light path defined by the polarization selective element 265 so that both branches of the light path are crossing it. Further on, the frequency converting element 240 is arranged in one of the sections of light path between the wavelength selective elements 250, 252 and the polarization selective element 265. In this light path additional optical elements assisting the propagation of light can also be positioned (e.g. in Figs. 2 to 4 condensing and/or dispersing lenses, not shown in the figures).

Here, the wavelength selective element 250 is preferably a dichroic mirror, which deflects the laser light 222 by changing the propagation direction of the frequency-doubled laser light 222, whereas other laser lights of different wavelengths, thus also the fundamental harmonic laser light 220 among them, are transmitted in the original direction of propagation. To the contrary, the wavelength selective element 252 in the present case is preferably a dichroic mirror modi- fying the direction of propagation of laser light 220, whereas other laser lights of different wavelengths, among them the frequency-doubled laser light 222, are transmitted in the original propagation direction.

In one of the preferred embodiments, the polarization switching element 260 known per se comprises a half-wavelength plate (λ/2 -plate) made of a birefringent material. In another pos- sible embodiment of the polarization switching element 260, it contains a nematic liquid crys- tal which can change the position of its optical axis (and thereby its light refracting capacity) by coupling a given electric voltage to the liquid crystal.

The task of polarization switching element consists in modifying the polarization plane of the incident laser light 220 when required, and producing a laser light 220 at its outlet with a po- larization plane identical or just perpendicular to the incident laser light 220. Corresponding to this, polarization switching element 260 is arranged in the light path so that the angle between its optical axis and the polarization plane of incident beam 220 can be changed automatically or manually between 0° and 45° (in the given embodiments e.g. by rotating the λ/2 -plate around an axis or by coupling the required electric voltage to the liquid crystal). When the an- gle between the optical axis of the polarization switching element 260 and the polarization plane of laser light 220 is 0°, the polarization plane of laser light 220 leaving the polarization switching element 260 does not change relative to the incident laser light 220, whereas when this angle is 45°, the polarization planes of the laser beam falling to polarization switching element 260 and that of leaving it are perpendicular to each other.

The polarization selective element 265 modifies the propagation direction of the incident laser light 220 depending on the polarization plane of the laser light 220 (i.e. on the position of polarization switching element 260). Specifically, the polarization selective element 265 is formed of a polarization dependent beam divider which transmits laser light 220 of a certain polarization plane position in form of laser light 220A and directs it into one branch of the light path, e.g. into the one containing frequency converting element 240, whereas the laser light 220 having another polarization plane position is reflected in the form of laser light 220B and is directed into the other branch of the light path. Thus, in the apparatus 200 according to the invention, the polarization selective element 265 chooses the branch of the light path via which the laser light exciting the gaseous sample passes through.

Apparatus 200 becomes suitable for selective concentration measurement at two wavelengths after the calibration previously described. Photoacoustic chamber 280 is filled up with the gaseous sample to be measured. Laser light 220 emitted by exciting unit 210 (e.g. the fundamental harmonic of a Nd: YAG laser with the frequency of 1064 nm) passes through the polarization switching element 260. During this, depending on the angle between the optical axis of the plane and the polarization plane of laser light 220, laser light 220 either keeps its original (first) polarization plane, or it takes a different (second) polarization plane which is rotated by 90°. Then, laser light 220 gets into the polarization selective element 265 which directs laser light 220 into one branch of the light path either as laser light 220A, or as laser light 220B.

The laser light 220A of a first polarization plane passes to the frequency converting element

240, at the output of which a mixture of the fundamental harmonic 220A and the second har- monic (in the present case at 532 nm) laser lights 222. This mixture gets then into wavelength selective element 250. The wavelength selective element 250 transmits laser light 220A, whereas it deflects laser light 222 in the light path to the direction of photoacoustic chamber

280. Laser light 222 generates a photoacoustic signal in photoacoustic chamber 280, and the part not absorbed during excitation leaves for the power meter 290 (in the present case via the wavelength selective element 252 letting it through). The photoacoustic signal measured and the signal of the power meter 290 are forwarded to a central evaluation unit for processing.

At the same time, the laser light 220B of a second polarization plane from the polarization selective element 265 passes directly to wavelength selective element 252, from which it leaves for photoacoustic chamber 280 after having changed its propagation direction. It generates a photoacoustic signal in photoacoustic chamber 280, and its not absorbed part leaves for power meter 292 (in present case via wavelength selective element 250). The photoacoustic signal measured and the signal of the power meter 292 are forwarded to a central evaluation unit. Only one single laser light of a certain wavelength passes through the photoacoustic chamber 280 of apparatus 200 at a given time. The choice of the exciting wavelength occurs by the co- operation of the switch between the two positions of polarization switching element 260 and one of the wavelength selective elements 250, 252..

The gaseous sample is measured by apparatus 200 preferably in a continuous mode for a given time. In the course of this, switching between the two positions of polarization switching element 260 is timed so that the excitation at individual wavelengths should be long enough for averaging in time the photoacoustic signals.

Apparatus 200' shown in Fig. 3 makes selective measurements of gaseous sample at three wavelengths possible so that in the branch of light path containing frequency converting element 240 of apparatus 200 in Fig. 2, between the frequency converting element 240 and the (first) wavelength selective element 250, further directing and modifying optical elements are inserted for treating laser light 222 leaving frequency converting element 240. Specifically, after frequency converting element 240 a polarization switching element and after that a fire- quency converting element 245, then a wavelength selective element 255, a polarizer 298 an at the end, the wavelength selective element 250 is coupled. In the present case, the polarization switching element 261 is fitted to the second harmonic of the Nd: YAG laser at λ = 532 nm, i.e. it chooses one from the two perpendicular polarization planes of laser light 222 depending on the state whether the angle between the polarization plane of laser light 222 and the axis of polarization switch 261 is 0° or 45°. The frequency converting element 245 is preferably a nonlinear optical crystal producing also a fourth harmonic (λ = 266 nm) falling into the UV region by doubling the frequency of the second harmonic incident laser light 222 (λ = 532 nm). The wavelength selective element 255 is preferably a dichroic mirror selecting from the incident beam band consisting of fundamental harmonic laser light 220A, second harmonic laser light 222 and fourth harmonic laser light 224 laser light 224 and deflecting it to photoacoustic chamber 280, whereas the other components of the band are transmitted via polarizer 298. Polarizer 298 serves for filtering out laser light 222. Polarizer 298 is oriented in the light path so that it filters out totally the part of transmitted laser light 222 provided by polarization switch 261 with the first polarization plane, whereas it transmits totally the part perpendicular to this polarization plane (second).

By using apparatus 200', measurements are carried out several times in the way described for apparatus 200 at the three wavelengths (near IR, visible and UV). For choosing the actual exciting wavelength, following capacities are utilized: the capacity of polarization switching elements 260, 261 situated in the light path for setting/switching of the polarization plane; the difference in the light transmitting capacity of the polarization selective element 265 depending on the polarization plane; the property of frequency converting element 245 that frequency multiplication occurs exclusively for incident laser lights of a suitable polarization plane; and the filtering/transmitting property of the polarizer 298 depending on the polarization plane po- sition of the incident laser beam.

Apparatus 200" shown in Fig. 4 makes the selective determination of the impurities in the gaseous sample at wavelengths of the fundamental harmonic, and the first three following harmonic laser lights obtained by frequency multiplication emitted by the excitation source 210 possible. The third harmonic wavelength, i.e. in the given case the λ = 355 nm wavelength of the Nd:YAG laser is generated so that in the branch of the light path of apparatus 200' in Fig. 3 containing the frequency converting element 240, between frequency converting element 240 and polarization switching element 261, in the direction of the propagation of the mixture of laser lights 220A and 222 a further, second polarization switching element 261, after this a frequency converting element 248 generating from the incident mixture of laser lights 220A and 222 a further harmonic laser light 226, and a further wavelength selective element 254 are introduced. Wavelength selective element 254 is preferably a dichroic mirror selecting laser light 226 from the beam band consisting of the incident fundamental harmonic 220A, second harmonic laser light 222 and third harmonic laser light 226 and deflecting it to the direction of photoacoustic chamber 280, transmitting at the same time the other beams to frequency converting element 245. After frequency converting element 245, at the position of the polarization plane provided to the photoacoustic chamber 280, first optical elements direct- ing back second harmonic laser light 222 (specifically two wavelength selective elements 250 and a polarizer 298 between them), then optical elements directing back fourth harmonic laser light 224 (namely two wavelength selective elements 255) are arranged. By setting and harmonizing the elements suitable for selecting the polarization plane, in the photoacoustic chamber 280 of apparatus 200", excitation occurs only at one wavelength in a given moment, and corresponding to this, only one photoacoustic signal can be measured. Measurements are repeated several times at the four wavelengths by utilizing the polarization plane modifying effect of the appropriate polarization plane modifying elements. The results obtained in apparatus 200" at the different exciting wavelengths are then forwarded into a central evaluation unit. We note that it is obvious for the expert that it is possible to complement apparatus 200" with further (similar or identical to those shown) optical elements so that for performing the selective photoacoustic measurement the fifth harmonic of e.g. the Nd: YAG laser at λ = 213 nm can also be utilized.

With the apparatus according to the invention, we can draw unambiguous conclusions from the absorption values at the individual wavelengths and from its comparison with each other concerning the types of impurities in the gaseous sample by performing measurements in a broad wavelength region, as is shown in Fig. 5. Figure 5 shows the absorption curves of two carbon blacks of different origin and properties in the function of wavelength in a range from UV to near IR. In Figure 5 the significant difference between the two carbon black particles in the absorbance in the near infrared is striking, on the basis of which these two kinds of carbon blacks can be identified unambiguously and selectively. By utilizing the method of multicom- ponent analysis, from the absorption values found by the method according to the invention, the concentrations of the two carbon blacks in the gaseous sample can also be determined from the absorption values at different wavelengths. The apparatus according to the invention can preferably used for the selective determination of aerosols (such as carbon black and dust particles) in a gaseous sample (e.g. air). For this purpose, the property is utilized that e.g. dust hardly absorbs at the excitation wavelength of 1064 nm, whereas it shows significant absorption at 532 ran.. To the contrary, the absorption of car- bon black is nearly identical at these two wavelengths. Thus if the photoacoustic signals are nearly identical at these two wavelengths, the gaseous sample contains much carbon black. If, however, the photoacoustic signals of the gaseous sample are identical only at the excitation wavelength of 532 nm, the gaseous sample contains significant amount of dust.

It should be noted that by applying the apparatus according to the invention the composition of gaseous samples containing mainly gaseous impurities can be determined simply, accurately and selectively as well. In this case, in order to eliminate the interfering effect of an eventual aerosol impurity, the gaseous sample should be led through appropriate filter element(s) for filtering out the aerosol particles before feeding the sample into the photoacoustic chambers.

The selectivity of the apparatus according to the invention for aerosols can be increased so that the photoacoustic signal obtained by measuring the gaseous sample stripped totally from its aerosol content characteristic of the gaseous impurities of the gaseous sample is subtracted from the signal of the not filtered gaseous sample.

We also note that the method and apparatus according to the invention are also suitable for detecting gaseous substances having an absorption wavelength coinciding with the fundamental harmonic, or one of the higher order harmonics of the exciting laser light. By using e.g. Nd:YAG laser as exciting unit, and removing previously the aerosol content of the gaseous sample in order to increase selectivity, the method and apparatus according to the invention can be used for the detection of nitrogen dioxide absorbing at λ = 532 nm, and/or ozone absorbing at λ = 266 nm.

Claims

1. Method for the selective photoacoustic detection of impurity components in a gaseous sample by making use of spectrally distant exciting wavelengths, characterized in that a laser light (120; 220) is introduced into a light path crossing at least one measuring station re- alized by a photoacoustic chamber (180, 182, 184; 280) containing the gaseous sample, by subjecting the incident laser light (120; 220), as fundamental harmonic, to frequency multiplication at least one higher order harmonics thereof is produced, by directing the fundamental harmonic through a measuring station the impurity components of the gaseous sample are excited, by directing the at least one higher order harmonics obtained by frequency multiplica- tion through a measuring station one by one the impurity components of the gaseous sample are excited, the photoacoustic signals obtained at the wavelengths, as exciting wavelengths, of the fundamental and the higher order harmonics are detected, and by evaluating the photoacoustic signals obtained the impurity components of the gaseous sample are identified one by one.

2. The method according to claim 1, characterized in that non-absorbed portions of the laser lights (120, 122, 124; 220B, 222, 224, 226) exciting the impurity components of the gaseous sample which leave the measuring stations are subjected to power measurement.

3. The method according to claim 2, characterized in that through dividing the photoacoustic signals by the respective power values measured, the fluctuation in the photoacoustic signals due to the light power variation is eliminated.

4. The method according to claim 1 or 3, characterized in that the evaluation is carried out by performing multicomponent analysis of the photoacoustic signals measured at the exciting wavelengths.

5. The method according to claim 4, characterized in that during the evaluation the concentrations of the impurity components in the gaseous sample are determined.

6. The method according to claim 5, characterized in that frequency multiplication is performed by using at least one crystal of non-linear optical properties, wherein the at least one crystal is arranged with a crystal lattice orientation harmonizing with the polarization plane of the incident laser light.

7. The method according to claim 6, characterized in that the number of measuring stations provided in the light path is at least equal to the total number of the fundamental harmonic and the higher order harmonics obtained by frequency multiplication, said measuring stations are arranged in different sections of the light path and one and only one harmonic from the fundamental harmonic and the higher order harmonics is directed through each measuring station.

8. The method according to claim 7, characterized in that directing the fundamental harmonic and the harmonic(s) obtained by frequency multiplication to different measuring stations is carried out by the insertion of at least one wavelength selective element (150, 155) into the light path at site(s) located before the measuring station(s).

9. The method according to claim 6, characterized in that only one measuring station is provided in the light path, via which the fundamental and the at least one higher order harmonics obtained by frequency multiplication are directed through in such a manner that only one of the harmonics is present in said measuring station at a time.

10. The method according to claim 9, characterized in that selecting one of the fundamental harmonic and the at least one higher order harmonics obtained by frequency multiplication is carried out by the insertion of at least one polarization switching element (260, 261), at least one polarization selective element (265) and at least one wavelength selective element (250, 252, 254, 255) into the light path at site(s) located before the measuring station.

11. The method according to claim 8 or 10, characterized in that the wavelength selective element (150, 155; 250, 252, 254, 255) is a dichroic mirror.

12. The method according to claim 11, characterized in that the crystal of non-linear optical properties is at least one of lithium triborate (LBO), potassium dihydrophosphate (KDP), potassium dideuterium phosphate (DKDP), ammonium dihydrophosphate (ADP), β- barium borate (BBO) and cesium lithium borate (CLBO) crystals.

13. The method according to claim 12, characterized in that the impurity components of the gaseous sample are aerosols with broad, uncharacteristic, overlapping absorption bands.

14. The method according to claim 13, characterized in that previously to directing the gaseous sample to the measuring station, at least part of the solid impurity components of the gaseous sample is removed by filtering, and then by subtracting the photoacoustic signal obtained for the gaseous sample and characteristic of the gaseous impurity components from the photoacoustic signal measured for an unfiltered sample originating from the gas under study, selectivity of the method for identifying the solid impurity components is increased.

15. The method according to claim 12, characterized in that the impurity components of the gaseous sample are gaseous substances absorbing at given wavelengths.

16. The method according to claim 15, characterized in that the gaseous substance is ozone and/or nitrogen dioxide.

17. The method according to any of claims 13, 14 or 16, characterized in that the Ia- ser light fed into the light path is the fundamental harmonic of a Nd: YAG laser having a wavelength of 1064 run.

18. The method according to claim 17, characterized in that the harmonic(s) obtained by frequency multiplication is(are) at least one of the second, third, fourth and fifth harmonic of the Nd: YAG laser with a wavelength of 532 ran, 355 nm, 266 nm and 213 nm, respectively.

19. The method according to any of claims 13, 14 or 15, characterized in that the laser light fed into the light path is the fundamental harmonic of a Ti:sapphire laser tuned in the wavelength range of 700-1000 nm.

20. The method according to claim 19, characterized in that the harmonic(s) obtained by frequency multiplication is(are) at least one of the second and third harmonics of the Ti:sapphire laser tuned in the wavelength ranges of 350-500 nm and 233-333 nm, respectively.

21. Apparatus for the selective photoacoustic detection of impurity components in a gaseous sample by making use of spectrally distant exciting wavelengths, characterized in that it comprises an exciting unit (110, 210) emitting a laser light (120, 220); at least one measuring station with a photoacoustic chamber (180, 182, 184, 280) containing the gaseous sample and suitable for detecting the photoacoustic signal generated in the gaseous sample by laser light; a light path for directing laser light formed from the exciting unit (110, 210) to the at least one measuring station, the light path passing through the measuring station; at least one frequency converting element (140, 145; 240, 245,248) arranged in the light path and capable of generating higher order harmonics of the incident laser light; and at least one wave- length selective element (150, 155; 250, 252, 254, 255) arranged in the light path and capable of separating the incident mixture of laser lights with respect to the wavelengths of the component laser lights, wherein the section of the light path passing through the measuring station is formed to be capable of directing a laser light of a single wavelength through the gaseous sample at a time.

22. The apparatus according to claim 21, characterized in that each measuring station is provided with a power meter (190, 192, 194; 292, 290) for measuring the power of the laser light (120, 122, 124; 220B, 222, 224, 226) leaving the respective photoacoustic chamber (180, 182, 184; 280).

23. The apparatus according to claim 21 or 22, characterized in that the frequency converting element(s) (140, 145; 240, 245, 248) is(are) crystal(s) of non-linear optical properties arranged in the light path with crystal lattice orientation harmonizing with the polarization plane of the incident laser light.

24. The apparatus according to claim 23, characterized in that the crystal(s) of non- linear optical properties is(are) selected from lithium triborate (LBO), potassium dihydrophos- phate (KDP), potassium dideuterium phosphate (DKDP), ammonium dihydrophosphate (ADP), β-barium borate (BBO) and cesium lithium borate (CLBO) crystals.

25. The apparatus according to claim 23 or 24, characterized in that the wavelength selective element(s) (150, 155; 250, 252, 254, 255) is(are) dichroic mirror(s).

26. The apparatus according to claim 25, characterized in that the number of measuring stations provided in the light path is at least equal to the total number of the fundamental harmonic and the higher order harmonics obtained by frequency multiplication, said measuring stations are arranged in different sections of the light path and each measuring station is capable of receiving a laser light of a definite wavelength.

27. The apparatus according to claim 26, characterized in that as far as the propagation of the laser light is concerned, at least one wavelength selective element (150, 155) is capable of selecting one of the different sections of the light path containing the measuring stations.

28. The apparatus according to claim 25, characterized in that it is provided with a single measuring station being capable of receiving the laser light (220) emitted by the exciting unit (210) and the higher order harmonic(s) generated by the frequency converting elements) (240, 245, 248) one after the other.

29. The apparatus according to claim 28, characterized in that it comprises at least one polarization switching element (260, 261) arranged in the light path and capable of switching between the polarization states of the incident laser light characterized by perpendicular polarization planes, and at least one polarization selective element (265) capable of selecting the direction of propagation of the laser light depending on the polarization state thereof, wherein said elements are inserted into the light path at site(s) located before the measuring station(s).

30. The apparatus according to claim 29, characterized in that the polarization switching element (260, 261) comprises at least one of a half- wavelength plate (λ/2 -plate) or a nematic liquid crystal.

31. The apparatus according to claim 28, characterized in that as far as the propagation of the laser light is concerned, the at least one wavelength selective element (250, 252, 254, 255) is capable of performing the selection of one of the different sections of the light path containing the measuring stations in cooperation with the at least one polarization switching element (260, 261) and the at least one polarization selective element (265).

32. The apparatus according to claim 27 or 31, characterized in that the exciting unit

(110, 210) is one of a Nd: YAG laser and a Ti:sapphire laser.