激光应用及光谱学 Laser Spectroscopy

Department of Electronic and Electrical EngineeringEE986 Assignment and Professional StudiesGroup Project Interim ReportName: Kuo SunZhe ZhangNan ZhouContentAbstract (2)1. Introduction (3)2. Background (3)2.1 Principle of LASER (3)2.1.1 Deconstructing the LASER (3)2.1.2 Underlying physics of LASER (4)2.1.3 LASER Oscillation (4)2.2 Principle of TDLS (5)2.2.1 TDLS with Direct Detection (6)2.2.2 TDLS with Wavelength Modulation Spectroscopy (6)2.3 Fundamentals of Cavity Ring-down Spectroscopy (7)2.3.1 Basic CRDS Set-Up (8)2.3.2 Principle of CRDS (8)2.3.3 Technical Characteristics of CRDS (9)2.4 Principle of photoacoustic spectroscopy (9)2.4.1 Foundational Principle (9)2.4.2 Physical Process (10)2.4.3 Advantages of Photoacoustic Spectroscopy (10)3. Industrial Application (11)3.1 Industry applications of TDLS (11)3.2 Cavity Ring-Down Spectroscopy for Combustion Studies (12)3.2.1 Experimental Set-up (12)3.2.2 Methodology for CRD Flame Experiment (12)4. Further work (14)5. References (15)AbstractModern spectroscopy which has the advantages of immunity to electromagnetic interference, resistance to chemical corrosion, high sensitivity, large bandwidth, and remote operation has become the preferred option for industrial gas monitoring.This paper presents the basic principle of laser operation which involved to the spectroscopy discuss in the next section. Then the paper discusses the fundamental of three different types of spectroscopy--Tunable Diode Laser Spectroscopy, Cavity Ring-down Spectroscopy and Photo acoustic Spectroscopy. Furthermore, the applications in industrial of the spectroscopy mentioned before are given to provide a further explanation. Finally, the future work is shown to make the project plan clear.1.IntroductionThe title of this group project is ‘Optical Fibre Gas Sensors’. Gas sensing has been an important issue since the existence of mankind. This project involves a number of optical gas sensing techniques especially different kinds of spectroscopy and how each technique can be used in industrial applications. In the project each member will mainly do the research on one specific kind of spectroscopy and study the knowledge of its industry applications. Under the permit conditions an experimental system will be used to measure the characteristics of spectroscopy.2.Background2.1 Principle of LASERLASER, which is the short name for Light Amplification by Stimulated Emission of Radiation, has already explained its main process in the full name.2.1.1 Deconstructing the LASERAs an optical oscillator, a laser can be deconstructed into three essential elements —pump, gain medium and resonator. Figure 1 below could show the detailed structure of a laser.Figure 1: Structure of a laser [1]Pump acts as source of energy, providing external energy to gain medium, allowing it to amplify light. Generally, it can be specified into different types of optical pump, thermal pump, electrical pump and chemical pump. The types utilized should be determined by gain medium.Gain medium is the key element of a laser that creates and amplifies light at appropriate wavelength. It absorbs energy from pump and produces light at the required frequency. Types of gain medium can be classified according to their material, such as insulating solids, semi-conductors, gas and liquid.Resonator is made up by two mirrors, one of which is partially reflective and the other is 100% reflective. This is used to re-circulates light. Resonator has three effects: keeping the emission of radiation continuous, accelerating the photons and confining the direction of emitted light.2.1.2 Underlying physics of LASERIn 1917, Albert Einstein published the paper On the Quantum Theory of Radiation that explained the absorption, spontaneous emission and stimulated emission of photons. For an electron, the nearer the electron orbits to the Bohr atom, the lower energy it has; vice versa. Therefore, the electron orbit near to the atom is defined as low energy level.For a system, if photons transit from low energy level E1to high energy level E2, they will absorbe energy E=E2-E1=hν=hc/λ from external system. Absorption occurs with a rate of N1ρνB12, where N1 is the photon population at E1, ρν is the photon energy density, and B12 is the probability of absorption per unit time.Spontaneous emission occurs when photons transit from high energy level E2 to low energy level E1. This is a spontaneous active due to the lifetime for a photon at upper state and emit energy E=E2-E1=hν=hc/λ with a random direction. The rate is N2A21, where is the probability of spontaneous decay per unit time from state 2 to state 1.Stimulated emission is what we expect. The emission is stimulated by an incoming photon with energy E=E2-E1=hν=hc/λ incidenting on an atom to stimulate an electron transiting from low energy level to high energy level. The emitted photon has the same direction and energy with the incident photon. The rate of stimulated emission is N2ρνB21.2.1.3 LASER OscillationThe operational process of laser is self-sustaining optical oscillation. When the system is powered with pump, spontaneous emission occur and emit radiates in all directions. With the increasing of pump power, the provided energy will achieve a threshold value to exceed the losses in the optical cavity and compensate for the loss to spontaneous emission. At thismoment, the spontaneous emission reflected by the mirror becomes the incident photons to start the stimulated emission and part of the stimulated emission get out while the others reflected and repeat this process, which finishes the build-up process of laser oscillation. See in figure 2.Figure 2: Build-up laser oscillation [2]With further increase of the stimulated emission, circulating light extracts more energy and output more power until the population reaches the upper state and the amplifier gain becomes saturated. At this time, the energy produced equals to the output and losses. Then the laser oscillation steps to steady state. It is shown in figure 3.Figure 3: Steady state of laser oscillation [2]2.2 Principle of TDLSCompare with many other optical gas sensing approaches, tunable diode laser spectroscopy (TDLS) is widely used based on its specific advantages, particularly in industrial gas monitoring area. TDLS was firstly developed in 1970’s. Up until that time conventional spectrometers and gas lasers had been used to obtain gas absorption data, and the technique of using the unique capabilities of diode lasers was a novel approach to providing high resolution measurements of absorption spectra.[3]The technique used mid-infrared (midIR) lasers at first. It need cooled detectors and cooled lasers which will have some difficulties with deployment. Later, distributed feedback (DFB) lasers and InGaAs photodiode detectors which use near-IR technology for communication applications were available at low cost and high performance. There are two common forms for the TDLS. One is called TDLS with direct detection and the other one is TDLS with wavelength modulationspectroscopy (WMS).2.2.1 TDLS with Direct DetectionFigure 4: System diagram of direct TDLS [4]The basic system diagram of tunable diode laser spectroscopy with direct detection is shown above. TDLS with direct detection involves temperature tuning of the wavelength of a laser at DFB laser part to overlap with that of a particular target absorption line and applying a repetitive ramp signal to the laser's injection current to sweep its output wavelength across the entire absorption profile. [5]To make sure the intensity variations do not have a bad influence on the measurement results, a part of main laser beam is isolated and then monitored at a reference photoreceiver. A ratio to make up the difference between the main beam and reference beam is taken at a point away from the gas absorption. This is called ‘off-lin e’ measurement. The ratio is known as ‘zero-point reference’. However, the sensitivity of TDLS with direct detection is limited by the noise across a large bandwidth. What is more, further issues will occur when the measurement is made at high temperature.2.2.2 TDLS with Wavelength Modulation SpectroscopyFigure 5: System diagram of TDLS/WMS [6]The basic system diagram of TDLS with WMS is shown above. There is a low frequency ramp (tens of HZ) at ramp generator with the direction version. Also, one small amplitude, high frequency (tens of kHZ) sinusoidal dither is produced by sinusoidal generator to test the line shape. The amplifier of the frequency deviation is defined in terms of the modulation index m where m=δv/γ. Here δv is the frequency deviation and γ is the half-width-half-maximum (HWHM) linewidth. [6] The interaction of the frequency modulation (FM) generates detected amplitude-modulation (AM) signals at the modulation frequency f and its higher harmonics. The first harmonic AM signal at the receiver is defined as the residual-AM (RAM) signal.The transmitted intensity I out at a spec ific optical frequency v is given by Beer’s law [7]:I out =I in e −α(v)Cl =I in (1−α(v)Cl)Where,I in is the incident intensity on the gas volume;α(v) is the absorption coefficient at frequency v;l is the length through which the beam and gas interact;C is the gas concentration expressed as C =N/N 0.For the gas absorption line, the absorption coefficient α(v ) is defined by Lorentzian profile:α(v )=α0{1+(v −v 0γ)2}=α02 Where,v 0 and α0 are the frequency and absorption coefficient at the line center;γ is the half line width;α0= N 0S/(γπ), S is the line strength. 2.3 Fundamentals of Cavity Ring-down SpectroscopyCavity Ring-Down Spectroscopy (CRDS) is a direct absorption technique which has outstanding sensitivity than conventional absorption spectroscopy. The principle of CRDS is to determine the rate of absorption, rather than the magnitude of absorption, by measuring the decay time. Decay time, also called cavity ring-down time, is the total time that the light circulating in the cavity.As CRDS measuring the ring down time which independent on the incident light intensity, CRDS has the advantages such as high sensitivity, high Signal-Noise-Ratio and resist to interference on the laser pulse.2.3.1 Basic CRDS Set-UpAs the figures shows below the ring-down cavity consisting of two highly reflective mirrors (Reflectivity R>99%, R ≈1). The pulse produced by the laser travels forth and back in the cavity.A fast detector measures the output light intensity as a function of time.Figure 6: Basic CRDS Set-UpAs the reflectivity of the ring-down cavity mirrors is sufficient high, the pulse decay time in the cavity can be extremely long, that is to say the gas total absorption path is pretty long which can significantly promote the measuring result.2.3.2 Principle of CRDSFrom the Beers Law, the output light intensity against the decay time can be expressed:I(t)=I 0exp (tc L(lnR −αCL)) Where I 0is the incident light, c is light speed, L is the cavity length, C is the concentration of the gas and α is the frequency-dependent absorption coefficient. As the reflectivity R ≈1, lnR ≈-(1-R), the expression can be rewritten as,I (t )=I 0exp (−tc (1−R +αCL)) The ring down time τ which defined as the 1/e decay time of the exponential expression can be expressed asτ=L As the expression shows the ring down time depends on the cavity length L, the absorption coefficient α and the reflectivity R. For the vacuum condition, the ring down time can be expressed asτ0=L So the vacuum ring down time only depends on the reflectivity R. If the cavity reflectivity R is constant, it is possible to determine the absorption loss αCL by measuring the ring down time ταCL=L(1−1)For a particular wavelength the absorption coefficient is constant which makes it possible to determine the gas concentration by measuring the ring down time.C=1cα(1τ−1τ0)2.3.3 Technical Characteristics of CRDSi.Incident light intensity independent.As mentioned before the concentration of the gas which to be determined is only depends on the ring down time in gas and vacuum (τ and τ0). Thus, it is possible to increase the SNR by measuring the 1/e decay time.ii.Long absorption path.The absorption path in the cavity is the product of decay time and the light speed which can be expressed asL eff=LSince αCL≪(1−R)L eff≈LAs the reflectivity of the cavity mirrors is sufficient high, the absorption path can be pretty long. For example, if L=60cm, R=99 %(it is much higher in reality which could be99.7%, 99.9%) the L eff>60m [8] [9]iii.High sensitivity.CRDS has been applied at wavelengths between 197nm and 3.2 mm. A sensitivity of 10−6cm−1can easily obtained.2.4 Principle of photoacoustic spectroscopy2.4.1 Foundational PrinciplePhotoacoustic spectroscopy is a new developed spectroscopic analysis technique based on photoacoustic effect. This technique is developed in recent decades, however, the foundational principle of it was discovered by Alexander Graham Bell early in 1988 [10]. Photoacoustic effect is an interaction between light and materials, is a physical process of materials absorbing light and turning it to acoustic energy. Using intense laser to illuminate the sample which is enclosed in a cylindrical chamber, the sample absorbs energy from the light and turns it into thermal energy. The thermal energy heats the sample and mediumaround it with a modulated frequency of light, and therefore the medium produce a periodic pressure wave. This wave can be detected by sensitive microphones and amplified through lock-in amplifiers, which is called photoacoustic effect. If the wavelength of incident monochromatic light is variable, then people could get photoacoustic signal spectrums in a variable ranges. This is the principle of photoacoustic spectroscopy.2.4.2 Physical ProcessThe cylindrical chamber used to cause photoacoustic effect can be divided into three spaces: sample, backing and gas (medium), shown in figure 4.Figure 7: Model of cylindrical chamberConsidering the incident light is only absorbed and decayed on the surface of samples, solve the thermal diffusion equations in these three spaces, and we can get the change of periodic pressure produced by incident light with a modulated frequency of ω:δP =Q cosωtwhere Q is the amplitude of pressure changing. The expression for Q is quite complicated:Q=BIγp[(r−1)(b−1)(eσL−(r+1)(b−1)e−σL+2(b−r)e−βL)] 2√2TKL′α(β2−α2)[(g+1)(b+1)eσL−(g−1)(b−1)e−σL]where βis the light absorption coefficient of material; I is the strength of incident light; P is the pressure of gas in the chamber; γis the ratio of isobaric heat capacity to constant volume heat capacity of gas in the chamber; T is the temperature in the chamber; L is the thickness of sample; L′is the length of space for gas; K,K′,K′′are density of sample, gas and backing respectively; C,C′,C′′are the specific heat of sample, gas and backing respectively. In additionb=√K′′ρ′′C′′/KρC; g=√K′ρ′C′/KρC;α=√ωρC/2K; σ=(1+i)√ωρC/2K;r=(1+i)β/2√ωρC/2K; i=√−1.2.4.3 Advantages of Photoacoustic SpectroscopyPhotoacoustic spectroscopy is an effective backup to the traditional spectroscopy. The maindifference between them is that photoacoustic spectroscopy does not directly detect the emitted photons of the material after illumination, but measures the energy absorbed by the interaction between light and materials. Due to this, photoacoustic spectroscopy could overcome the difficulties of traditional spectroscopy [11]:1.For slight absorption materials, the transmission signal hardly has any decay. It isextremely difficult to measure the tiny difference of the transmission light.2.For intense scattering materials, the absorption method cannot tell if the photons arereally absorbed.3.For the materials that don’t allo w light passing through or too thick for light to passthrough, the absorption method can never detect the transmission light.3. Industrial Application3.1 Industry applications of TDLSFigure 8: Location of the NIR system ALTO and the MID-IR lead-salt TDL COLD on M55 Geophysica [12]Gas sensing act as an important part in industry is to protect from harmful gases and optimize the production processes. During the production process, Tunable diode laser sensors can be used to make the measurement effectively. It was pointed out that Tunable Diode Laser Spectroscopy (TDLS) has managed to turn from a “promising technology” into an “established technology” in the industry in the session held at the 4t h International Conference. In 2004 the first company sold the 1,000th TDLS instrument by using NIR wavelengths which mainly used O2, NH3, CH4 and water vapour. Also the laser producers cooperate with each other to develop and market VCSELs (vertical-cavity surface-emitting laser), DFB-lasers and QC-lasers (Quantum cascade lasers). Besides, the TDLS systems areused for medical diagnostics and breath analysis in healthcare and novel trace gas analyzer for environmental measurements.3.2 Cavity Ring-Down Spectroscopy for Combustion StudiesThe first flame experiment using the CRDS technic was reported by Meijer etal in 1994 which provided a new way in measuring the reactive species in flames.3.2.1 Experimental Set-upFlames can be classified into two parts, laminar and turbulent. CRD method only deals with the laminar flame which provides a stable and thin (<1mm0) [13]flame front. The figure shows below is a simple low-pressure burner with ring down cavity. Two pinholes are introduced to maintain the cavity match to the TEM00mode. The burner is positioned at the center of the cavity. Wavelength selected laser beam provides the incident light. A fast photomultiplier tube detector connected with a digital oscilloscope.Figure 9: Low-pressure burner with ring down cavity3.2.2 Methodology for CRD Flame Experimenti.Integrated AbsorptionTwo different methods are being widely used in literal in CRD flame experiment. The first one is based on measuring the integrated absorption (cm−1) as a function of laser frequency in wavenumbers (cm−1).Make the assumption that the optically thin limit which means a weak absorption.(i.e. α(ω,T)l s≪1). [14]The integrated absorption A can be expressed as a function ofabsolute molecules population N iA I=πe2e2×N i f if l sπe2m e c2=1.13×1012cm−1, f if is the oscillator strength for the absorption, l s the absorption sample path. From the definition of temperature dependent Boltzmannfactor f Bf B =N i TThe expression can be rewritten asN T =A I s ×m e c 22×1if ×1BWhere N T is the total molecules numbers. The oscillator strength f if can be direct provided by the literature.ii. Peak AbsorptionThe second method is based on peak absorption measurement. Thetemperature-dependent absorption cross-section is defined by Derzy et al.σ(ω,T )=√4ln2×11/2×πe 2e 2×f if f B Again, πe 2m e c =1.13×1012cm −1. ∆ω1/2 is the full width at half maximum.As σ(ω,T )=α(ω,T ), the equation can be rewritten asN T =α(ω,T)peak ×∆ω1/2×m e c 22×1if ×1B4.Further workFigure 10: Gantt chartIn the future firstly each member will keep on the study of the principle and industry applications for the spectroscopy. Then find a way to develop the experiment to measure the characteristics of spectroscopy, test the experimental results and make a conclusion for the research.5. References[1] Michael Lengden, “Lecture Note: Introduction to Lasers” from class EE473 PhotonicSystem, Department of Electronic & Electrical Engineering, University of Strathclyde. [2] Cinan Wu, Sigeng Yang, “Photoacoustic Spectroscopy and Its Application” from Journalof Guizhou Normal University (Nature Science), No.1 Vol.16 1998.[3] Kevin Duffin, “Wavelength Modulation Spectroscopy with Tunable Diode Lasers: aCalibration-Free Approach to the Recovery of Absolute Gas Absorption Line-Shapes,”pp23, Apr4l 2007.[4] Kevin Duffin, “Wavelength Modulation Spectroscopy with Tunable Diode Lasers: aCalibration-Free Approach to the Recovery of Absolute Gas Absorption Line-Shapes,”pp17, April 2007.[5] Andrew J McGettrick, Walt er Johnstone, Robert Cunningham and John Black, “TunableDiode Laser Spectroscopy With Wavelength Modulation: Calibration-Free Measurement of Gas Compositions at Elevated Temperatures and Varying Pressure,” Journal of Lightwave Technology, vol. 27, no.15, August 2009.[6] Kevin Duffin, “Wavelength Modulation Spectroscopy with Tunable Diode Lasers: aCalibration-Free Approach to the Recovery of Absolute Gas Absorption Line-Shapes,”pp21, April 2007.[7] Kevin Duffin, Andrew James McGettrick, Walter Johnstone, George Stewart and David G.Moodie, “Tunable Diode-Laser Spectroscopy with Wavelength Modulation: a Calibration-Free Approach to the Recovery of Absolute Gas Absorption Line Shapes,” vol.25, no.10, October 2007.[8] Ml Yunpitg ,WANG Xiaopirig, Review on cavity ring down spectroscopy technology andits application, 2007[9] Sneep, M.; Hannemann, S, Cavity Ring-Down Spectroscopy, Express 2008, 16,15013-15023[10] Cinan Wu, Sigeng Yang, “Photoacoustic Spectroscopy and Its Application” from Jour nalof Guizhou Normal University (Nature Science), No.1 Vol.16 1998.[11] “Photo-Acoustic Spectroscopy Application for Dissolved Gas Analysis”, BaiduDocuments, [Online]. 27th January, 2013. Available:/view/29490c6da45177232f60a263.html[12] M. Pantani, F. Castagnoli, F. D´Amato, M. De Rosa, P. Mazzinghi, P. Werle, "Twoinfrared laser spectrometers for the in-situ measurement of stratospheric gas concentration", Infrared Physics & Technology 46, 109-113 (2004).[13] FLAMES AND FLAME STRUCTURE, /eee/cpe630/comfun3.html[14] GIEL BERDEN, Cavity Ring-Down Spectroscopy Techniques and Applications, 2009。