WO1998057147A1 - Device for diagnosing combustion and flow processes using adjustable uv lasers - Google Patents

Device for diagnosing combustion and flow processes using adjustable uv lasers Download PDF

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Publication number
WO1998057147A1
WO1998057147A1 PCT/EP1998/003527 EP9803527W WO9857147A1 WO 1998057147 A1 WO1998057147 A1 WO 1998057147A1 EP 9803527 W EP9803527 W EP 9803527W WO 9857147 A1 WO9857147 A1 WO 9857147A1
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Prior art keywords
laser
tuned
etalon
radicals
excitation
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PCT/EP1998/003527
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German (de)
French (fr)
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Frank Christian Bormann
Peter Andresen
Heinrich Voges
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Lavision Gmbh
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Publication of WO1998057147A1 publication Critical patent/WO1998057147A1/en

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    • 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
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1062—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
    • 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
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G01N2021/653—Coherent methods [CARS]
    • G01N2021/655—Stimulated Raman
    • 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
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
    • G01N21/53—Scattering, i.e. diffuse reflection within a body or fluid within a flowing fluid, e.g. smoke
    • 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
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G01N21/6402—Atomic fluorescence; Laser induced fluorescence
    • 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
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3501—Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
    • G02F1/3507—Arrangements comprising two or more nonlinear optical devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2302/00—Amplification / lasing wavelength

Definitions

  • the invention relates to a device for the detection of molecules using a typically fixed-frequency, Q-switched solid-state laser (e.g. Nd: YAG, 1064.14 n).
  • a typically fixed-frequency, Q-switched solid-state laser e.g. Nd: YAG, 1064.14 n.
  • the bandwidth of the laser is limited by means of a specially selected intracavity etalon (ICE).
  • ICE intracavity etalon
  • the laser wavelength can then be tuned over a narrow spectral range, which, however, significantly extends the wavelength range of the solid-state laser without ICE.
  • the higher harmonic frequencies of the laser in the UV e.g. Nd: YAG 4th 266 nm, 5th 212 nm
  • the laser is also suitable for laser-induced fluorescence detection (LIF) of OH and NO molecules for the diagnosis of combustion and flow processes.
  • LIF laser-induced fluorescence detection
  • the fixed-frequency, Q-switched solid-state laser chosen as the starting point makes it possible, with the special tuning via an ICE, to set up a comparatively compact and inexpensive measuring system, which is characterized by a high measuring accuracy and spatial resolution due to the high pulse energies.
  • tunable lasers are suitable for the detection of molecules in gas dynamic processes.
  • OPO optical parametric oscillators
  • excimer lasers are suitable for two-dimensional LIF and Rayleigh measurements [Appl. Opt., 36, 3971 (1997)].
  • the tuning range is approx. 1 nm and, depending on the laser medium, allows selective single pulse detection of NO, O 2 and OH molecules.
  • tunable excimer lasers have been used successfully. With the latter measurements it is important to find gaps in the tuning range of the laser in which there is no resonant excitation of molecules.
  • All of the above-mentioned laser systems consist of at least two lasers (oscillator and amplifier) for generating the tunable ultraviolet light. Because of the complexity of the laser system, this results in the problem of the high cost of an optical measuring system for combustion or flow diagnostics.
  • the invention specified in claim 1 is based on the object of achieving a more cost-effective, more easily manageable measuring arrangement in that an Nd: YAG which normally emits at a fixed laser frequency by means of an intra-cavity etalon (ICE) over a small range (typically ⁇ 0.11 nm at 266 nm with 60 mJ pulse energy) is made narrow-band tunable.
  • Essential components of this invention are the narrowing of the spectral bandwidth by means of a special etalon and the tuning of the laser wavelength by controlled tilting of the same.
  • Fig. 1 Sketch of the device for the diagnosis of combustion and flow processes by means of tunable UV laser.
  • Fig. 2 Fig. 2 OH excitation spectrum taken from a flame with the fourth
  • the solid-state laser (Fig. 1) consists of an active medium 1, e.g. Neodymium-doped yttrium-aluminum-Garnet (Nd: YAG), the resonator 2, which consists of two mirrors highly reflecting for the laser transitions, and a Pockels cell 3, which is used for Q-switching the resonator.
  • the active medium 1 is excited via a pump light source 4 e.g. Flash lamps or diode lasers.
  • An etalon 5 within the resonator (intra-cavity) is used to generate narrow-band laser radiation. It can be reproducibly tilted via a stepper motor 6 and enables the frequency of the laser to be tuned. Tunable UV laser radiation on the 4th harmonic frequency in the UV at 266 nm is generated via two frequency doubling units 7. The spatial separation of the different laser frequencies takes place by means of one or more dispersion prisms or diochroic mirrors 10. The laser frequency to be used is guided into a sample 13 by means of beam shaping optics 12, while the laser frequencies that are not required are retained by beam stoppers 11. The light emitted from the sample volume is imaged onto a spectral filter 15 via an imaging optics 14.
  • the filtered light is generally recorded in a spatially resolved manner by means of a detector 16.
  • a computer with control electronics and software 17 is used to control the image acquisition and the tilting of the etalon 5 via the stepper motor 6.
  • the bandwidth of the laser transition is approximately 4 cm -1 (FWHM) [Laser Focus World, pp. 77, April 1992].
  • FWHM FWHM
  • Due to the competition of the different longitudinal modes, a line width of the fundamental emission (1st harmonic 1064.14 nm corresponding to 9397.3 cm " 1 ) results from a wavenumber of 1 cm -1 if no ICE is used.
  • the strongest laser transition at 1064.14 nm leads to an excitation Q 2 (13) line of the OH molecule at 266.05 nm (37586 cm -1 ) in the (2.0) band.
  • a much more efficient detection of the OH molecule via the P 1 (10) line is offered at the edge of an adjacent laser transition at 1064.6 nm (dashed line), which can be achieved by tilting the etalon.
  • FIG. 3 shows a single pulse recording of a laser light section at 266.13 nm from the exhaust gas of a flame, which makes it possible to determine the temperature distribution in the exhaust gas over a wide area.
  • Another measurement method with this system is the combinative measurement of the Rayleigh and Raman scattering for one-dimensional, spatially resolved measurements of temperature and gas composition.
  • a spherical lens with a long focal length is generally used for beam shaping 12 and the scatter signals are detected in a 1D location behind a spectrometer 14.
  • a commercially available Nd. ⁇ AG laser that emits without ICE at the fixed central wavelength of 1064.14 nm, it could be shown that the resonant excitation of OH molecules with the fourth harmonic frequency via the Q 2 (13) resonance led to interfering interference in the Raman measurements.
  • the fifth harmonic of the Nd: YAG laser is generated at 212 nm by expanding the laser structure (FIG. 1) with a frequency mixed crystal 8 (generally BBO).
  • a frequency mixed crystal 8 generally BBO
  • NO lines of the A 2 ⁇ ⁇ - X 2 Tl transition from the (1,0) and (2,1) band For example, hot NO can be detected in combustion processes. This is advantageous for the diagnosis of the thermally formed pollutant NO (claim 2).
  • the NO lines in the tuning range of the fifth harmonic are not suitable for the detection of cold NO, as it is added to the visualization as a tracer of a cold flow, since they consistently involve states with a high rotation quantum number.
  • the thermal occupation of these states lies at room temperature (300 K) in a range of 10 " 6 and thus about 3 orders of magnitude below the occupation at (2500 K). This leads to weak fluorescence signals.
  • Fig. 1 By expanding the laser structure (Fig. 1) with The 1st Stokes of the fifth harmonic is generated at 226 nm in a methane Raman shifter 9.
  • a methane Raman shifter 9 In the tuning range there are several NO lines of the A 2 ⁇ - X 2 Yl transition from the (0.0) band With a low rotation quantum number a high thermal occupation and are therefore suitable for NO detection.
  • the usually fixed-frequency, solid-state laser modified according to the invention offers an extremely simple solution for this purpose, since, for example, double-pulse Nd: YAG systems are already in use for diagnosing flow fields by means of particle imaging velocimetry measurements (PIV).
  • PIV particle imaging velocimetry measurements

Abstract

Known laser diagnostic detection methods use expensive adjustable laser systems in the UV spectral region. The new device consists of a compact, fixed frequency solid laser (1, 2, 3,4 ) which is adjusted so as to carry out special spectroscopic detection methods, at low cost and with high measuring precision and local resolution. An intracavity etalon (5) reduces the band width, and the laser wavelength is defined by tilting the etalon (5) and detuned in a way that can be reproduced (6). The higher harmonious (7, 8) and Raman shifted frequencies (9) are used to diagnose gas dynamic processes by different detection procedures. With this device, uni- or multidimensional measurements can be carried out of OH-radicals, of temperature fields, of gas compositions, and of NO as a pollutant and as a marking substance in hot and cold environments. A second identical device can be used to determine time-delayed speed fields from molecular distributions.

Description

Beschreibungdescription
Vorrichtung zur Diagnostik von Verbrennungs- und Strömungsprozessen mittels abstimmbarer UV-LaserDevice for the diagnosis of combustion and flow processes by means of tunable UV lasers
Einleitungintroduction
Die Erfindung betrifft eine Vorrichtung zum Nachweis von Molekülen unter Einsatz eines typischerweise festfrequenten, gütegeschalteten Festkörperlasers (z.B. Nd:YAG, 1064.14 n ). Die Bandbreite des Lasers wird mittels eines speziell ausgesuchten Intracavity Etalon (ICE) eingeschränkt. Durch ein Verkippen des Etalons läßt sich die Laserwellenlänge dann über einen schmalen spektralen Bereich abstimmen, der allerdings den Wellenlängenbereich des Festkörperlasers ohne ICE entscheidend erweitert. Auf den höheren harmonischen Frequenzen des Lasers im UV (z.B. Nd:YAG 4.te 266 nm, 5.te 212 nm) bietet sich damit u.a. die Möglichkeit, interferenzfreie kombinative Rayleigh / Raman Messungen zur simultanen Bestimmung von Temperatur und Gaszusammensetzung durchzuführen. Aufgrund der Durchstimmbarkeit eignet der Laser sich auch für den laserinduzierte Fluoreszenz Nachweis (LIF) von OH und NO Molekülen zur Diagnostik von Verbrennungs- und Strömungsprozessen. Der als Ausgangspunkt gewählte festfrequente, gütegeschaltete Festkörperlaser ermöglicht es, mit der speziellen Durchstimmung über ein ICE ein vergleichsweis kompaktes und kostengünstiges Meßsystem aufzubauen, welches sich aufgrund der hohen Pulsenergien durch eine hohe Meßgenauigkeit und Ortsauflösung auszeichnet.The invention relates to a device for the detection of molecules using a typically fixed-frequency, Q-switched solid-state laser (e.g. Nd: YAG, 1064.14 n). The bandwidth of the laser is limited by means of a specially selected intracavity etalon (ICE). By tilting the etalon, the laser wavelength can then be tuned over a narrow spectral range, which, however, significantly extends the wavelength range of the solid-state laser without ICE. On the higher harmonic frequencies of the laser in the UV (e.g. Nd: YAG 4th 266 nm, 5th 212 nm), this means that the possibility to carry out interference-free combinative Rayleigh / Raman measurements for the simultaneous determination of temperature and gas composition. Due to the tunability, the laser is also suitable for laser-induced fluorescence detection (LIF) of OH and NO molecules for the diagnosis of combustion and flow processes. The fixed-frequency, Q-switched solid-state laser chosen as the starting point makes it possible, with the special tuning via an ICE, to set up a comparatively compact and inexpensive measuring system, which is characterized by a high measuring accuracy and spatial resolution due to the high pulse energies.
Stand der TechnikState of the art
Es ist bekannt, daß abstimmbare Laser sich zum Nachweis von Molekülen in gasdynamischen Prozessen eignen. Zur Erzeugung der schmalbandig, abstimmbarer Strahlung werden unter anderem Festkörperlaser-gepumpte optisch parametrische Oszillatoren (OPO) [PhotonicsIt is known that tunable lasers are suitable for the detection of molecules in gas dynamic processes. To generate the narrow-band, tunable radiation, solid-state laser-pumped optical parametric oscillators (OPO) [Photonics
Spectra, pp. 94, March 1997], Farbstofflaser [Laser Focus World, September 1992] oder abstimmbare Festkörperlaser [tm-Technisches Messen, 63, 1996, Seiten 278-281] verwendet.Spectra, pp. 94, March 1997], dye laser [Laser Focus World, September 1992] or tunable solid-state lasers [tm-Technischen Messen, 63, 1996, pages 278-281].
Diese Laser besitzen alle einen relativ großen Durchstimmbereich im UV von einigen 10 nm mit Pulsenergien von typischerweise wenigen 1 mJ. Die niedrige Pulsenergie limitiert dabei dieThese lasers all have a relatively large tuning range in the UV of a few 10 nm with pulse energies of typically a few 1 mJ. The low pulse energy limits the
Einzelpulsmeßgenauigkeit bzw. -ortsauflösung bei der Messungen mittels LIF und Rayleigh-Single pulse measurement accuracy or location resolution for measurements using LIF and Rayleigh
Streuung, da nur eine kleine Photonenzahl zur Verf gung steht.Scattering as there is only a small number of photons available.
Aufgrund der hohen Pulsenergien im ultravioletten Spektralbereich von einigen 100 mJ eignen sich Excimerlaser für zweidimensionale LIF und Rayleigh Messungen [Appl. Opt., 36, 3971 (1997)]. Der Durchstimmbereich beträgt ca. 1 nm und erlaubt je nach Lasermedium den selektiven Einzelpulsnachweis von NO, O2 und OH-Molekülen. Auch für kombinative eindimensionale Messungen von Temperatur und Gaszusammensetzung über spontane Rayleigh- und Ramanstreuung [Appl. Opt, 32, 907 (1993)] werden abstimmbare Excimerlaser erfolgreich eingesetzt. Bei letzteren Messungen ist es wichtig, daß man Lücken im Abstimmbereich des Lasers findet, in denen keine resonante Anregung von Molekülen vorliegt. Alle oben genannten Lasersysteme bestehen aus mindestens zwei Lasern (Oszillator und Verstärker) zur Erzeugung des durchstimmbaren ultravioletten Lichtes. Hieraus resultiert, aufgrund der Komplexität des Lasersystems, das Problem der hohen Kosten eines optischen Meßsystems zur Verbrennungs- oder Strömungsdiagnostik.Due to the high pulse energies in the ultraviolet spectral range of a few 100 mJ, excimer lasers are suitable for two-dimensional LIF and Rayleigh measurements [Appl. Opt., 36, 3971 (1997)]. The tuning range is approx. 1 nm and, depending on the laser medium, allows selective single pulse detection of NO, O 2 and OH molecules. Also for combinative one-dimensional measurements of temperature and gas composition via spontaneous Rayleigh and Raman scattering [Appl. Opt, 32, 907 (1993)], tunable excimer lasers have been used successfully. With the latter measurements it is important to find gaps in the tuning range of the laser in which there is no resonant excitation of molecules. All of the above-mentioned laser systems consist of at least two lasers (oscillator and amplifier) for generating the tunable ultraviolet light. Because of the complexity of the laser system, this results in the problem of the high cost of an optical measuring system for combustion or flow diagnostics.
Der im Patentanspruch 1 angegebenen Erfindung liegt die Aufgabe zugrunde, ein kostengünstigere, einfacher handhabbare Meßanordnung dadurch zu erreichen, daß ein normalerweise auf einer festen Laserfrequenz emittierender Nd:YAG durch ein Intra-Cavity Etalon (ICE) über eine kleinen Bereich (typ. ±0.11 nm bei 266 nm mit 60 mJ Pulsenergie) schmalbandig abstimmbar gemacht wird. Wesentliche Bestandteile dieser Erfindung sind die Einengung der spektralen Bandbreite mittels eines speziellen Etalons und die Durchstimmung der Laserwellenlänge durch kontrollierte Verkippung desselben. Der Einsatzbereich von Festkörperlaserstrahlung und deren höheren harmonischen Frequenzen wird für die Laserdiagnostik (Rayleigh, Raman, LIF-Techniken) von Verbrennungs- und Strömungsprozessen entscheiden erweitert (Patentanspruch 1) da mit Hilfe des speziell ausgewählten Etalons ein Nebenübergang des Nd:YAG Lasers genutzt werden kann . Die im Vergleich zu Meßsystemen, die mit den oben aufgeführten Lasern ausgestattet sind erzielte Vorteile liegen in der größeren Kompaktheit, den geringeren Kosten und der dennoch, aufgrund der relativ hohen Pulsenergien im UV, erzielbaren hohen Meßgenauigkeit und Ortsauflösung. Die konkrete Anwendung der mit dem speziellen ICE erzeugten abstimmbaren UV- Laserstrahlung, und der mit diesen über stimmulierte Ramanstreuung erzeugten Stokeslinien für die Laserdiagnostik ist in den Patentansprüchen 1 bis 4 ausformuliert.The invention specified in claim 1 is based on the object of achieving a more cost-effective, more easily manageable measuring arrangement in that an Nd: YAG which normally emits at a fixed laser frequency by means of an intra-cavity etalon (ICE) over a small range (typically ± 0.11 nm at 266 nm with 60 mJ pulse energy) is made narrow-band tunable. Essential components of this invention are the narrowing of the spectral bandwidth by means of a special etalon and the tuning of the laser wavelength by controlled tilting of the same. The area of application of solid-state laser radiation and its higher harmonic frequencies is decisively expanded for laser diagnostics (Rayleigh, Raman, LIF techniques) of combustion and flow processes (claim 1) because with the help of the specially selected etalon a side transition of the Nd: YAG laser can be used . The advantages achieved compared to measuring systems equipped with the lasers listed above are the greater compactness, the lower costs and the high measuring accuracy and spatial resolution that can be achieved due to the relatively high pulse energies in UV. The specific application of the tunable UV laser radiation generated with the special ICE, and the Stokes lines generated with it via stimulated Raman scattering for laser diagnostics is formulated in patent claims 1 to 4.
Ausführungs- und Anwendungsbeispiele der Erfindung sind in den Zeichnungen dargestellt, die im folgenden näher beschrieben werden. Es zeigen :Execution and application examples of the invention are shown in the drawings, which are described in more detail below. Show it :
Fig. 1 : Skizze der Vorrichtung zur Diagnostik von Verbrennungs- und Strömungsprozessen mittels abstimmbarer UV-Laser. Fig. 2 Fig. 2 OH-Anregungsspektrum aus einer Flamme aufgenommen mit der viertenFig. 1: Sketch of the device for the diagnosis of combustion and flow processes by means of tunable UV laser. Fig. 2 Fig. 2 OH excitation spectrum taken from a flame with the fourth
Harmonischen des Nd.YAG Lasers um 266 nm. Fig. 3 Zweidimensionaler Laserlichtschnitt durch eine Flamme bei Abstimmung derHarmonics of the Nd.YAG laser around 266 nm. Fig. 3 Two-dimensional laser light section through a flame when the
Laserwellenlänge der vierten Harmonischen auf 266.13 nm (37575 cm"1). Beispiel für eine interferenzfreie, Einzelpuls Rayleighmessung zur Temperaturbestimmung im heißenLaser wavelength of the fourth harmonic at 266.13 nm (37575 cm "1 ). Example of an interference-free, single-pulse Rayleigh measurement for hot temperature determination
Flammenabgas. Intensität in Graustufen. Höhere Rayleighintensität = HöhereFlame exhaust. Intensity in grayscale. Higher Rayleigh intensity = higher
Teilchendichte = kleinerer Temperatur. Fig. 4 Zweidimensionaler Laserlichtschnitt durch eine Flamme bei Abstimmung derParticle density = lower temperature. Fig. 4 Two-dimensional laser light section through a flame when the
Laserwellenlänge der vierten Harmonischen auf 266.19 nm (37567 cm"1). Beispiel für eine Einzelpulsmessung der der Fluoreszenzverteilung des OH-Moleküls nach Anregung des Pι(10) Übergangs in der (2,0)-Bande.Laser wavelength of the fourth harmonic at 266.19 nm (37567 cm "1 ). Example of a single pulse measurement of the fluorescence distribution of the OH molecule after excitation of the Pι (10) transition in the (2.0) band.
Der Festkörperlaser (Fig. 1) besteht aus einem aktiven Medium 1, wie z.B. Neodymium dotierten Yttrium-Aluminium-Garnet (Nd:YAG), dem Resonator 2, der aus zwei für die Laserübergänge hochreflektierenden Spiegeln besteht und einer Pockelszelle 3, die zur Güteschaltung des Resonators (Q-Switching) verwendet wird. Die Anregung des aktiven Mediums 1 erfolgt über eine Pumplichtquelle 4 z.B. Blitzlampen oder Diodenlaser.The solid-state laser (Fig. 1) consists of an active medium 1, e.g. Neodymium-doped yttrium-aluminum-Garnet (Nd: YAG), the resonator 2, which consists of two mirrors highly reflecting for the laser transitions, and a Pockels cell 3, which is used for Q-switching the resonator. The active medium 1 is excited via a pump light source 4 e.g. Flash lamps or diode lasers.
Zur Erzeugung schmalbandiger Laserstrahlung wird ein Etalon 5 innerhalb des Resonators verwendet (Intra Cavity). Es läßt sich über einen Schrittmotor 6 reproduzierbar verkippen und ermöglicht es, die Frequenz des Lasers abzustimmen. Über zwei Frequenzverdopplungseinheiten 7 wird abstimmbare UV-Laserstrahlung auf der 4. ten Harmonischen Frequenz im UV bei 266 nm erzeugt. Die räumliche Trennung der verschiedenen Laserfrequenzen erfolgt durch ein oder mehrere Dispersionsprismen oder diochroitische Spiegel 10. Die zu verwendende Laserfrequenz wird mittels einer Strahlformungsoptik 12 in eine Probe 13 geleitet während die nicht benötigten Laserfrequenzen über Strahlstopper 11 zurückgehalten werden. Das aus dem Probevolumen emittierte Licht wird über eine Abbildungsoptik 14 auf ein spektrales Filter 15 abgebildet. Mittels eines Detektors 16 wird das gefilterte Licht im allgemeinen ortsaufgelöst aufgenommen. Ein Computer mit Steuerungselektronik und Software 17 dient zur Kontrolle der Bildaufnahme und der Verkippung des Etalons 5 über den Schrittmotor 6. Für den stärksten Übergang im Nd.ΥAG bei 1064.14 nm beträgt die Bandbreite des Laserübergangs ungefähr 4 cm-1 (FWHM) [Laser Focus World, pp. 77, April 1992]. Aufgrund des Wettkampfs der verschiedenen longitudinalen Moden resultiert eine Linienbreite der fundamentalen Emission (1. Harmonische = 1064.14 nm entsprechend 9397.3 cm"1) von einer Wellenzahl 1 cm-1, wenn kein ICE verwendet wird. Durch Einsatz eines speziellen ICE kann man erreichen, daß der Laser innherhalb seiner gesamten Bandbreite von 4 cm"1 schmalbandig (0.5 cm"1) anschwingt. Es ist zudem möglich ein Anschwingen auf dem stärksten Laserübergang zu unterdrücken und gleichzeitig die stimulierte Emission auf benachbarten Laserübergängen zu ermöglichen [IEEE Journal of Quantum Electronics, Vol. QE-14, No. 1, January 1978]. Im Rahmen dieser Erfindung wurde dies erstmalig mit nur einem Etalon 5 bei einem gepulsten (gütegeschalteten) Nd:YAG Laser erreicht.An etalon 5 within the resonator (intra-cavity) is used to generate narrow-band laser radiation. It can be reproducibly tilted via a stepper motor 6 and enables the frequency of the laser to be tuned. Tunable UV laser radiation on the 4th harmonic frequency in the UV at 266 nm is generated via two frequency doubling units 7. The spatial separation of the different laser frequencies takes place by means of one or more dispersion prisms or diochroic mirrors 10. The laser frequency to be used is guided into a sample 13 by means of beam shaping optics 12, while the laser frequencies that are not required are retained by beam stoppers 11. The light emitted from the sample volume is imaged onto a spectral filter 15 via an imaging optics 14. The filtered light is generally recorded in a spatially resolved manner by means of a detector 16. A computer with control electronics and software 17 is used to control the image acquisition and the tilting of the etalon 5 via the stepper motor 6. For the strongest transition in the Nd.ΥAG at 1064.14 nm, the bandwidth of the laser transition is approximately 4 cm -1 (FWHM) [Laser Focus World, pp. 77, April 1992]. Due to the competition of the different longitudinal modes, a line width of the fundamental emission (1st harmonic = 1064.14 nm corresponding to 9397.3 cm " 1 ) results from a wavenumber of 1 cm -1 if no ICE is used. By using a special ICE one can achieve that the laser oscillates within its entire bandwidth of 4 cm " 1 narrowband (0.5 cm" 1 ). It is also possible to suppress oscillation on the strongest laser transition and at the same time to enable the stimulated emission on neighboring laser transitions [IEEE Journal of Quantum Electronics, Vol. QE-14, No. 1, January 1978]. In the context of this invention, this was achieved for the first time with only one etalon 5 with a pulsed (Q-switched) Nd: YAG laser.
Figur 2 zeigt ein OH-Anregungsspektrum (durchgezogene Linie) welches mit der vierten Harmonischen Frequenz (um 266 nm = 37594 cm-1) aus einer Flamme aufgenommen wurde. Der stärkste Laserübergang bei 1064.14 nm führt zu einer Anregung Q2(13)-Linie des OHMoleküls bei 266.05 nm (37586 cm-1) in der (2,0) Bande. Ein weitaus effizienterer Nachweis des OH-Moleküls über die P1(10)-Linie bietet sich am Rande eines benachbarten Laserübergangs bei 1064.6 nm (gestrichelte Linie) der sich durch ein Verkippen des Etalons erreichen läßt. Figur 4 zeigt eine zweidimensionale Fluoreszenzverteilung des OH-Moleküls aus der Flamme nach Einzelpuls Anregung auf der Pι(10)-Linie in der (2,0) Bande bei 266.19 nm (37567 cm"1). Die Daten zur Lage der Molekülresonanzen findet man im SRI report No. MP 96-001 (1996). Die Möglichkeit der Visualisierung von Turbulenzen und Flammenfront mit einem einzelnen Laserpuls über den flächigen OH-Nachweis in Verbrennungsprozessen ist ein wichtiger Punkt bei der Beurteilung der gewerblichen Anwendbarkeit der Erfindung (Patentanspruch 1). Ein weiterer Vorteil, der durch das Abstimmen des Lasers auf den benachbarten Laserübergang bei 1064.6 nm erreicht wird, stellt die Möglichkeit dar bei 266.13 nm (37575 cm"1) entfernt von jeder OH-Resonanz interferenzfrei, Messungen der Rayleighstreuung der Moleküle durchzuführen. Die Formulierung "bei 266.13 nm" ist so zuverstehen, daß alle Wellenlängen im Maximum der Energieverteilung des Laser auf der vierten Harmonischen des benachbarten Laserübergangs bei 1064.6 nm verwendet werden. Figur 3 zeigt eine Einzelpulsaufhahme eines Laserlichtschnitts bei 266.13 nm aus dem Abgas einer Flamme, die es erlaubt flächig die Temperaturverteilung im Abgas zu bestimmen. Ein weiteres in Patentanspruch 1) angemeldetes Meßverfahren mit diesem System ist die kombinative Messung der Rayleigh- und Raman-Streuung für eindimensionale, ortsaufgelöste Messungen von Temperatur und Gaszusammmensetzung. Hierzu wird zur Strahlformung 12 in der Regel eine langbrennweitige sphärische Linse verwendet und die Streusignale werden 1D- ortsaufgelöst hinter einem Spektrometer 14 detektiert. Mit einem handelsüblichen Nd.Υ AG- Laser der ohne ICE bei der festen Zentralwellenlänge 1064.14 nm emittiert konnte gezeigt werden, daß gerade in heißen Flammen die resonante Anregung von OH-Molekülen mit der vierten Harmonischen Frequenz über die Q2(13)-Resonanz zu störenden Interferenzen bei den Raman-Messungen führte.Figure 2 shows an OH excitation spectrum (solid line) which was recorded with the fourth harmonic frequency (around 266 nm = 37594 cm -1 ) from a flame. The strongest laser transition at 1064.14 nm leads to an excitation Q 2 (13) line of the OH molecule at 266.05 nm (37586 cm -1 ) in the (2.0) band. A much more efficient detection of the OH molecule via the P 1 (10) line is offered at the edge of an adjacent laser transition at 1064.6 nm (dashed line), which can be achieved by tilting the etalon. 4 shows a two-dimensional fluorescence distribution of the OH molecule from the flame after single-pulse excitation on the PI (10) line in the (2.0) band at 266.19 nm (37567 cm "1 ). The data on the position of the molecular resonances can be found in SRI report No. MP 96-001 (1996) The possibility of visualizing turbulence and flame front with a single laser pulse via the flat OH detection in combustion processes is an important point when assessing the commercial applicability of the invention (claim 1). Another advantage, which is achieved by tuning the laser to the neighboring laser transition at 1064.6 nm, is the possibility at 266.13 nm (37575 cm " 1 ) away from any OH resonance without interference to carry out measurements of the Rayleigh scattering of the molecules. The phrase "at 266.13 nm" is to be understood so that all wavelengths in the maximum of the energy distribution of the laser are used on the fourth harmonic of the adjacent laser transition at 1064.6 nm. FIG. 3 shows a single pulse recording of a laser light section at 266.13 nm from the exhaust gas of a flame, which makes it possible to determine the temperature distribution in the exhaust gas over a wide area. Another measurement method with this system, which is registered in claim 1), is the combinative measurement of the Rayleigh and Raman scattering for one-dimensional, spatially resolved measurements of temperature and gas composition. For this purpose, a spherical lens with a long focal length is generally used for beam shaping 12 and the scatter signals are detected in a 1D location behind a spectrometer 14. Using a commercially available Nd.Υ AG laser that emits without ICE at the fixed central wavelength of 1064.14 nm, it could be shown that the resonant excitation of OH molecules with the fourth harmonic frequency via the Q 2 (13) resonance led to interfering interference in the Raman measurements.
Durch Erweiterung des Laseraufbaus (Fig. 1) mit einem Frequenzmischkristall 8 (i.a. BBO) wird die fünfte Harmonische des Nd:YAG Laser bei 212 nm erzeugt. Im Abstimmbereich liegen mehrere NO-Linie des A2Σ <- X2Tl Übergangs aus der (1,0) und (2,1) Bande. Es läßt sich z.B. heißes NO in Verbrennungsprozessen nachweisen. Dies ist für die Diagnostik des thermisch gebildeten Schadstoffs NO von Vorteil (Patentanspruch 2). Zum Nachweis von kaltem NO, wie es zur Visualisierung als Tracer einer kalten Strömung beigemengt wird, eignen sich die NO- Linien im Abstimmbereich der fünften Harmonischen nicht, da es sich durchgehend um Zustände mit hoher Rotationsquantenzahl handelt. Die thermische Besetzung dieser Zustände liegt bei Zimmertemperatur (300 K) in einem Bereich von 10"6 und damit ca. 3 Größenordnungen unter der Besetzung bei (2500 K). Dies führt zu schwachen Fluoreszenzsignalen. Durch Erweiterung des Laseraufbaus (Fig. 1) mit einem Methan Raman-Shifter 9 wird die 1. Stokes der fünften Harmonischen bei 226 nm erzeugt. Im Abstimmbereich liegen mehrere NO- Linien des A2Σ — X2Yl Übergangs aus der (0,0) Bande. Auch bei Raumtemperatur weisen mehrere Zustände mit niedriger Rotationsquantenzahl eine hohe thermische Besetzung auf und eignen sich damit für den NO-Nachweis. Eine besondere Bedeutung kommt der simultanen Anregung der 0^(19) und O12(3) Linie bei 40069 cm-1 zu, die es gestattet, NO-Verteilungen über einen großen Temperaturbereich mit starken Fluoreszenzsignalen zu messen. Dies ist zum Beispiel wichtig für die Visualisierung reaktiver Strömungen mit kalten und heißen Bereichen, denen NO als Markierungssubstanz (Tracer) beigemengt wird (Patentanspruch 3). Im Patentanspruch 4) wird der Einsatz zweier identischer Meßsysteme nach Anpruch 1 , 2 oder 3 angemeldet, mit dem Doppelpulsmessungen der laserinduzierten Fluoreszenz am OH- oder NO- Molekül durchgeführt werden können. Aus der zeitlichen Entwicklung der zweidimensionale OH oder NO Verteilung kann man mit speziellen Algorithmen (GIV = Gas Imaging Velocimetry) auf das Geschwindigkeitsfeld einer Gaströmung zurückschliessen. Der dazu erfindungsgemäß modifizierte, üblicherweise festfrequente, Festkörperlaser bietet hierfür eine äußerst einfache Lösung an, da beispielsweise Doppelspuls Nd:YAG System zur Zeit schon für Diagnostik von Strömungsfeldern mittels Particle Imaging Velocimetry Messungen (PIV) im Einsatz sind. The fifth harmonic of the Nd: YAG laser is generated at 212 nm by expanding the laser structure (FIG. 1) with a frequency mixed crystal 8 (generally BBO). In the tuning area there are several NO lines of the A 2 Σ <- X 2 Tl transition from the (1,0) and (2,1) band. For example, hot NO can be detected in combustion processes. This is advantageous for the diagnosis of the thermally formed pollutant NO (claim 2). The NO lines in the tuning range of the fifth harmonic are not suitable for the detection of cold NO, as it is added to the visualization as a tracer of a cold flow, since they consistently involve states with a high rotation quantum number. The thermal occupation of these states lies at room temperature (300 K) in a range of 10 " 6 and thus about 3 orders of magnitude below the occupation at (2500 K). This leads to weak fluorescence signals. By expanding the laser structure (Fig. 1) with The 1st Stokes of the fifth harmonic is generated at 226 nm in a methane Raman shifter 9. In the tuning range there are several NO lines of the A 2 Σ - X 2 Yl transition from the (0.0) band With a low rotation quantum number a high thermal occupation and are therefore suitable for NO detection. Of particular importance is the simultaneous excitation of the 0 ^ (19) and O 12 (3) line at 40069 cm -1 , which allows NO - Measuring distributions over a wide temperature range with strong fluorescence signals, for example, this is important for the visualization of reactive flows with cold and hot areas, to which NO is added as a tracer Is ngt (claim 3). In claim 4) the use of two identical measuring systems according to claim 1, 2 or 3 is registered, with which double-pulse measurements of the laser-induced fluorescence on the OH or NO molecule can be carried out. From the temporal development of the two-dimensional OH or NO distribution, special algorithms (GIV = Gas Imaging Velocimetry) can be used to infer the velocity field of a gas flow. The usually fixed-frequency, solid-state laser modified according to the invention offers an extremely simple solution for this purpose, since, for example, double-pulse Nd: YAG systems are already in use for diagnosing flow fields by means of particle imaging velocimetry measurements (PIV).

Claims

Patentansprüche claims
1. Meßsystem zum ortsausfgelösten Nachweis von OH-Radikalen, Temperaturfeldern und der Gaszusammensetzung von Majoritätenspezies in Strömungs- und Verbrennungsprozessen mit einem durchstimmbaren Laser zur Anregung der OH-Radikale und zur Erzeugung von1. Measuring system for the locally triggered detection of OH radicals, temperature fields and the gas composition of majority species in flow and combustion processes with a tunable laser to excite the OH radicals and to generate
Rayleigh- und/ oder Raman-Streuung, einer UV-empfindlichen intensivierten Kamera und einer Auswerteeinheit, dadurch gekennzeichnet, daß der durchstimmbare Laser ein gütegeschalteter Nd.YAG Laser ist, der vom Hauptübergang bei 1064.14 nm auf den Nebenübergang bei 1064.6 nm mittels eines Intracavity-Etalons abgestimmt wird und innherhalb der Bandbreite dieser Übergänge schmalbandig durchgestimmt werden kann, wobei durch nachgeschaltete Erzeugung der vierten harmonischen Frequenz der Laserstrahlung, durch nichtlineare Optik, der ortsaufgelöste Nachweis der OH-Radikale nach Anregung der Pj(10) Linie in der (2,0) Bande bei 266.19 nm erfolgt und die Temperaturfelder über Rayleigh-Streuung und die Gaszusammensetzung über Raman- Streuung bei 266.13 nm interferenzfrei gemessen werden können.Rayleigh and / or Raman scattering, a UV-sensitive intensified camera and an evaluation unit, characterized in that the tunable laser is a Q-switched Nd.YAG laser that moves from the main transition at 1064.14 nm to the secondary transition at 1064.6 nm by means of an intracavity Etalons is tuned and can be tuned within the bandwidth of these transitions in narrowband, with the generation of the fourth harmonic frequency of the laser radiation, through nonlinear optics, the spatially resolved detection of the OH radicals after excitation of the P j (10) line in the (2, 0) band occurs at 266.19 nm and the temperature fields can be measured without interference using Rayleigh scattering and the gas composition via Raman scattering at 266.13 nm.
2. Meßsystem nach Anspruch 1 dadurch gekennzeichnet, daß durch nichtlinearen Optik die fünften harmonischen Frequenz des Nd:YAG Lasers erzeugt und mit dem Intracavity Etalon die Laserwellenlänge auf 212.85 nm abgestimmt wird, um ortsaufgelöst das NO Molekül über die Qι(40) Anregung in der (1,0) Bande nachzuweisen.2. Measuring system according to claim 1, characterized in that the fifth harmonic frequency of the Nd: YAG laser is generated by nonlinear optics and the laser wavelength is tuned to 212.85 nm with the intracavity etalon in order to have the NO molecule spatially resolved via the Qι (40) excitation in the (1,0) Band to prove.
3. Meßsystem nach Anspruch 2, dadurch gekennzeichnet, daß die fünfte harmonische Frequenz über stimulierte Raman-Streuung in Methan auf die erste Stokes Wellenlänge bei 226 nm verschoben wird und mit den Intracavity Etalon die Laserwellenlänge auf 226.92 nm abgestimmt wird, um ortsaufgelöst und temperaturunempfindlich das NO Molekül über die simultane O12(19) und Oι2(3) Anregungen in der (0,0) Bande nachzuweisen.3. Measuring system according to claim 2, characterized in that the fifth harmonic frequency via stimulated Raman scattering in methane is shifted to the first Stokes wavelength at 226 nm and the laser wavelength is tuned to 226.92 nm with the intracavity etalon in order to be spatially resolved and insensitive to temperature NO molecule via the simultaneous O 12 (19) and Oι 2 (3) excitations in the (0.0) band.
4. Meßsystem nach Anspruch 1,2 und 3, dadurch gekennzeichnet, daß zwei identische Meßsysteme zur Messung von Geschwindigkeitsfeldern verwendet werden wobei durch zeitliche versetzte Laseranregung und Fluoreszenzdetektion die Bewegung der entsprechenden Moleküleverteilung innerhalb der vorgegebenen Zeitspanne in der Ebene des Laserlichtschnitts verfolgt wird und mit Hilfe der Methoden der Gas Imaging Velocimetry (GIV) das Geschwindigkeitsfeld der Molekülverteilung bestimmt wird. 4. Measuring system according to claim 1, 2 and 3, characterized in that two identical measuring systems are used for measuring speed fields, the movement of the corresponding molecular distribution being tracked within the predetermined period of time in the plane of the laser light section and with the aid of temporally offset laser excitation and fluorescence detection the methods of gas imaging velocimetry (GIV) determine the velocity field of the molecular distribution.
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