Pulsed and modulated photoacoustic and transmission spectroscopy on trace gases employing continuously tunable CO₂-and diode-lasers
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Date
2007Type
- Doctoral Thesis
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Abstract
This thesis reports on laser-based spectroscopy for trace gas sensing in the mid- and near-infrared spectral region. The fundamental absorptionregion in the mid-infraredis accessed by a photoacousticspectrometer, which is based on a home-built,continuously tunable, high-pressure C02 laser employing periodically poled GaAs as non-linearmaterial in first-orderquasi-phase matching for second harmonic generation. Both, the fundamental 10 /xm- and the second harmonic 5 ^tm-radiations are used, either individually, or, as a novel approach to trace gas sensing, simultaneouslyto excite the same region of the sample (i.e. coaxial beams). The continuous tunability is a crucial feature for selective detection in multi-component mixtures and is not offered by the usual line tunable C02 lasers (10 ßm ränge) or CO lasers (5 ,um ränge). The characteristics of our source include wide tuning ranges and narrowlinewidths in both wavelength regions, i.e. the fundamental (9.2-10.7 ^m [1087-935cm-1], and 540MHz [0.018 cm"1]) and frequency-doubled (4.6-5.35^m [2174-1869cm-1], and 1050MHz [0.0315cm-1]), respectively. Fundamental pulse energies ränge from 10 to 80mJ, where as up to lmJ (typ. 0.04 to 0.4 mJ) can be achieved (external conversion efnciency up to 1.25%) in the frequency doubled regime (1 Hz repetition rate). The small photoacoustic cell (180 cm3 gas volume)features an 80-microphonearray and a built-in, battery-powered,low-noise preamplifier. The turbulence-free flow rate of up to 700 cm3/s allows real-time monitoring of samples at room temperatureand atmospheric pressures and below. Selected measurements, emphasizing on applications of the 5 fim wavelength extension of the C02 laser, are discussed. The nitric oxide 15NO/14NO isotope ratio was measured in good agreement with the literature in a mixture containingNO and traces of water vapor (H2O) as an impurity and interfering species. This demonstrates the high selectivity of the sensor. C02 was measured outside strong absorptionbands to show the good sensitivity. The detection limits (SNR=3) of these species are 42.2 ppmV for NO, 136 ppmV for H20, and 2.55% for C02. As a novelty, simultaneous detection of NO, H20, and C02 using both laser wavelengths has been investigated and found feasible,although signal normalizationissues remain. Overtone and combination bands in the near-infrared were investigated using spectrometersbased on a flber-coupled, continuous-wave, continuously tunable external cavity diode laser (ECDL) emitting in the telecommunication wavelength regionaround 1.6 //m. The source characteristics include a wide tuning ränge (1.54-1.66/zm, [6494-6024cm-1]) with a very narrow linewidth (<150kHz, [0.5xl0~6cm-1]) and 0.2-6.3mW average output power. Wavelengthmodulation (WM) with frequencies <10 kHz can causes a maximum carrier frequency shift of ±2 GHz. Detection schemes implemented include transmissionand resonant photoacoustic spectroscopy. Amplitudemodulation (AM) andWM are employed with both detection schemesusing a lock-in amplifier for demodulation. The transmissionspectrometer employs a White-type multipass cell (path length of 109 m at 80 passes and 4.31 sample volume)and home-madephotodetectors. Measurementsinclude the C02 concentrationin exhaled human breath, the 13C02/12C02 isotope ratio, and an exhaust sample from a motorcycle that demonstrates the analysis of a ulticomponentsample. Detected species include CO, C02, mediane (CH4), and acetylene (C2H2). Their detection limits (SNR^3) achieved with AM are 1112ppmV, 1390ppmV, 39ppmV, and 18ppmV, respectively. Using 1/ and 2/ WM techniques, the detection limits (SNR=3)improve by an average factor of 4.8 and 9.3, respectively. The absorption spectrometer is now used as a teaching device for laser-based spectroscopy of trace gases for undergraduate students at the physics department of the ETH Zürich. The resonant photoacoustic cell in Herriott-configuration (Q-factor of 70 at the resonance frequency of 1250 Hz, 2.31 sample volume, max. flow 1.51/min, flow mode time resolution of 5 min) employsa 16-microphonearray and has a total absorptionpath length in the photoacoustic part of 15m at 36 passes. An AM measurementof CH4 resulted in a detection limit of 11 ppmV (SNR-3).This can beimproved by employingWM techniquesfor which the optimal modulation depth is discussed. This thesis comprises four chapters. Chapter 1 gives a brief introduction to trace gas sensing. Chapter 2 summarizesthe theory relevant to the work presented. Chapter 3 reports on the frequency doubled C02 laser-based photoacoustic spectrometer and its applications. Finally, in chapter 4 discusses the ECDL-basedtransmission and photoacoustic spectrometers, as well as their Operationmodi and applications. Show more
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https://doi.org/10.3929/ethz-a-005461164Publication status
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ETHSubject
HALBLEITERLASER + LASERDIODEN (LASERTECHNIK); PHOTOAKUSTISCHE SPEKTROSKOPIE; KOHLENDIOXIDLASER + KOHLENMONOXIDLASER (LASERTECHNIK); CARBON-DIOXIDE LASERS + CARBON MONOXIDE LASERS (LASER ENGINEERING); PHOTO-ACUSTIC SPECTROSCOPY; INFRARED SPECTROSCOPY; INFRAROTSPEKTROSKOPIE; SEMICONDUCTOR LASERS + LASER DIODES (LASER ENGINEERING)Organisational unit
03599 - Esslinger, Tilman / Esslinger, Tilman
08775 - Sigrist, Markus (Tit.-Prof.)
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