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Laser-based Absorption Spectrometry Development of NICE-OHMS Towards Ultra-sensitive Trace Species Detection
... The project, which the work presented in this thesis is a part of, has so far resulted in the construction of a compact NICE-OHMS setup based on a fiber laser, operating at a wavelength of 1.53 µm, and a fiber-coupled electro-optic modulator [45]. The first realization of the fiber-laser-based NICE-OHMS has already been described in the doctoral thesis of Florian Schmidt [46]. ...
... The tuning of the cavity length was obtained with the help of ring PZT actuators glued between the spacer and the mirrors. For a detailed description of how the PZTs were mounted on the Zerodur bar see ref. [46]. The first two cavities were built with high voltage actuators (Physik Intrumente), while in the third low voltage actuators (Piezomechanik), shown in Figure 8.9, were used. ...
... The path length can be reduced by using wavelength modulation or balanced detection approaches, which furthermore improve the sensitivity of TDLAS sensors [17][18][19]. Although the detection of H 2 S concentrations of several ppb has been demonstrated by employing integrated cavity output spectroscopy approach (ICOS) [20], such sensitivities are difficult to obtain in field measurements as the robustness of this kind of systems remains limited [21,22]. Possible alternatives to bulky multipass absorption or delicate sensing schemes based on cavity spectroscopy are photoacoustic H 2 S measurement strategies [23][24][25][26]. ...
Sensitive detection of hydrogen sulfide (H2S) at different pressure levels using a cantilever-enhanced photoacoustic detector in combination with a telecom NIR L-band laser source is reported. Amplitude and wavelength modulation schemes for photoacoustic signal generation are compared. A detection limit (3σ) of 8 ppmv was achieved for amplitude modulation mode with a 50-s averaging time for the H2S absorption near 1.6 µm. As compared to simulated spectra, the cantilever-enhanced photoacoustic detection approach in combination with the sufficiently stable and narrow bandwidth NIR laser is able to reproduce the rotationally resolved H2S spectrum at low pressures of 300 mbar.
... When measuring H 2 S by IR spectroscopy high sensitivities are difficult to achieve due to the intrinsically weak linestrenghts of the H 2 S ro-vibrational features within the spectral range covered by diode lasers. Although detection of H 2 S concentrations levels of several ppbv has been demonstrated by employing integrated cavity output spectroscopy approaches [47], such sensitivities are difficult to obtain in field measurements as the robustness of these kind of systems remain limited [48,49]. A fully developed and industry tailored H 2 S sensor based on photoacoustic spectroscopy and a LOD of 0.5 ppmv is described in [50]. ...
The present work reports on the first application of a ring-cavity-surface-emitting quantum-cascade laser (RCSE-QCL) for sensitive gas measurements. RCSE-QCLs are promising candidates for optical gas-sensing due to their single-mode, mode-hop-free and narrow-band emission characteristics along with their broad spectral coverage. The time resolved down-chirp of the RCSE-QCL in the 1227-1236 cm⁻¹ (8.15-8.09 µm) spectral range was investigated using a step-scan FT-IR spectrometer (Bruker Vertex 80v) with 2 ns time and 0.1 cm⁻¹ spectral resolution. The pulse repetition rate was set between 20 and 200 kHz and the laser device was cooled to 15-17°C. Employing 300 ns pulses a spectrum of ~1.5 cm⁻¹ could be recorded. Under these laser operation conditions and a gas pressure of 1000 mbar a limit of detection (3σ) of 1.5 ppmv for hydrogen sulfide (H2S) in nitrogen was achieved using a 100 m Herriott cell and a thermoelectric cooled MCT detector for absorption measurements. Using 3 µs long pulses enabled to further extend the spectral bandwidth to 8.5 cm⁻¹. Based on this increased spectral coverage and employing reduced pressure conditions (50 mbar) multiple peaks of the target analyte H2S as well as methane (CH4) could be examined within one single pulse.
... In the case of breast cancer, it can reach even 1.2 ppm [15]. In Table 1, some representative human breath biomarkers are listed [9,16]. ...
The potential of Quantum Cascade Laser technology has been recently harnessed in industry, medicine and military to create a range of original infrared gas sensors. These sensors have opened up many new applications due to compact size, excellent sensitivity, robust construction and low power requirements. They rely on infrared absorption spectroscopy to determine identity and quantity of gases. The measurement of these gases has relied on different technologies including multi-pass spectroscopy, photoacoustic spectroscopy, cavity ring down spectroscopy, and their various modifications. In this review paper some technologies are described in terms of its advantages/disadvantages in many application. The results of own works about methane, ammonia, nitric oxide, nitrous oxide, and carbonyl sulfide detection are presented as well
... Comparison of selected absorption spectroscopy (AS) techniques according to their typically achieved minimum detectable absorption coefficients, k min , their robustness and complexity. Abbreviations which are not explained in the text can be found in appendix A.1 (adapted from [42]). ...
The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.
A continuous-wave, mid-infrared, distributed feedback, interband cascade laser was used to detect and quantify formaldehyde (H2CO) using off axis, integrated cavity output spectroscopy in gas mixtures ranging from 1-25 ppmV in H2CO.
© 2005 Optical Society of America
OCIS codes: (120.3940) Metrology (300.6340) Spectroscopy, infrared
1
The sensitivity of FM spectroscopy can be dramatically enhanced by location of the sample in a high-finesse cavity, for example, ∼5 orders of magnitude in this study. To avoid conversion of laser frequency noise into amplitude noise by the cavity, we choose the rf modulation frequency to match the cavity’s free spectral range. In this way small frequency fluctuations produce no additional noise, and a pure FM dispersion signal is recovered in transmission. We present a systematic study of the detection sensitivity, signal line shape and size, and slope at the central tuning. Experimentally, using a weakly absorbing gas such as C2H2 or C2HD placed inside an external high-finesse resonator, we obtained an integrated absorption sensitivity of 5×10−13 (1×10−14/cm) for the gas’s weak near-IR molecular overtone transitions. As an interesting application, a Nd:YAG laser was well stabilized on the P(5) line of the C2HD (ν2+3ν3) band by this technique. The high attainable sensitivity permitted selection of slow molecules with low power and gas pressure to give a linewidth 13 times below the room-temperature transit-time limit.
We propose a new scheme for locking the frequency of a laser to a resonant reference cavity. A linear polarizer or Brewster plate is placed inside the reference cavity, so that the reflected light acquires a frequency-dependent elliptical polarization. A simple polarization analyzer detects dispersion shaped resonances which can provide the error signal for electronic frequency stabilization without any need for modulation techniques.
We describe and demonstrate a new method to reduce actively the amplitude modulation of a phase-modulated laser to the shot-noise limit. This theoretical limit of ultrahigh sensitivity of FM spectroscopy is achieved in a linear absorption experiment with iodine.
A high-finesse optical cavity was employed to perform highly sensitive spectroscopy of molecular oxygen at wavelengths near 763 nm. An equivalent absorption length of ∼1 km was obtained by a 26-cm-long optical cavity with a finesse of 6000. An extended cavity diode laser was frequency locked to the cavity, and pure absorption profiles were recovered by monitoring of the cavity transmission during continuous scans of the cavity resonance through O2 rotational lines, allowing a detailed investigation of the line shapes. Phase modulation of the laser at a frequency equal to the cavity free-spectral-range frequency was employed for detection of weak absorption signals inside the cavity. A minimum detectable absorption coefficient of 6.9×10−11 cm−1 Hz−1/2 was measured. Finally, a test of the symmetrization postulate in 16O nuclei was demonstrated.
An historical overview of laser-based, spectroscopic methods that employ high-finesse optical resonators is presented. The overview begins with the early work in atomic absorption (1962) and optical cavities (1974) that led to the first mirror reflectivity measurements in 1980. This paper concludes with very recent extensions of cavity-enhanced methods for the study of condensed-phase media and biological systems. Methods described here include cavity ring-down spectroscopy, integrated cavity output spectroscopy, and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Given the explosive growth of the field over the past decade, this review does not attempt to present a comprehensive bibliography of all work published in cavity-enhanced spectroscopy, but rather strives to illustrate the rich history, creative diversity, and broad applications potential of these methods.
The combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is
used to suppress time dependent background signals, which in general limit spectrometer performance during measurements. A
rapid background subtraction scheme, in which the external electric field was turned off on alternate scans, as well as a
double modulation experiment show drift free, white noise limited characteristics up to integration times of more than 1000s.
This exceeds the generally obtained spectrometer stability by about one order of magnitude.
A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm−1) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼510−10 cm−1 for 1 s averaging over scans, with a dynamic range of four orders of magnitude.
An ambient trace-ammonia sensor that uses resonant photoacoustic spectroscopy and a line-tunable CO2 laser has been developed. This system achieves a 1 replicate precision of 32 parts-per-trillion (ppt) with an averaging time of 5s and a total measurement time of 40s. This 32-ppt precision corresponds to a minimum detectable fractional absorbance of 8.810-9, a minimum detectable absorption coefficient of 9.610-10cm-1, and a minimum path-length-normalized detectivity of 1.110-8Wcm-1/Hz1/2. Background interference from CO2, H2O, and cell-window absorption were subtracted by switching to a neighboring off-resonance laser wavelength .
A new sensor has been developed for measuring atomic mercury using absorption spectroscopy with 254-nm radiation generated
from two sum-frequency-mixed diode lasers. Beams from a 375-nm external-cavity diode laser and a 784-nm distributed feedback
diode laser are mixed in a beta-barium-borate crystal to generate approximately 4nW of ultraviolet radiation. The development
of the sensor is described along with extensive characterization experiments in a mercury vapor cell in the laboratory. An
accuracy of ±6% in the absolute concentration of atomic mercury has been demonstrated by comparison with equilibrium vapor
pressure calculations. The detection limit is approximately 0.1 parts per billion of atomic mercury in a meter path length
for 300-K gas and a 10-s integration time. The insensitivity of the sensor to broadband attenuation is demonstrated. Measurements
of collision-broadening coefficients for air, N2, Ar, and CO2 are reported, and implementation of wavelength-modulation spectroscopy with the sensor is demonstrated. Finally, results
are presented from measurements with the sensor in situ in the exhaust stream of an actual coal-fired combustor.
A continuous-wave, mid-infrared, distributed feedback, interband cascade laser was used to detect and quantify formaldehyde
(H2CO) using off-axis, integrated cavity output spectroscopy in gas mixtures containing ≈1–25 parts in 106 by volume (ppmV) of H2CO. Analysis of the spectral measurements indicates that a H2CO concentration of 150 parts in 109 by volume (ppbV) would produce a spectrum with a signal to noise ratio of 3 for a data acquisition time of 3s. This is a
relevant sensitivity level for formaldehyde monitoring of indoor air, occupational settings, and on board spacecraft in long
duration missions in particular as the detection sensitivity improves with the square root of the data acquisition time.
We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b<sup>1</sup>Σ g -X<sup>3</sup>Σ g transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 10<sup>6</sup> can be detected.
In this paper, a new method to produce high-contrast Doppler-free optical feedback was described and preliminary experiments carried out on the D2 line of Rb were presented. Two original schemes of pure collinear polarization spectroscopy were proposed. It was also show that they were indeed qualified to optically locking of the diode laser frequency onto an atomic resonance. The principle of each scheme was that a polarized laser beam experiences different polarization alterations and after retroreflection is normally obstructed by the initial polarizer. To perform spectroscopy with a tunable laser, a semiconductor laser was frequency locked to an external tunable Fabry-Perot cavity based on standard techniques of optical feedback. The sharpest feedback peaks were observed in the scheme employing a Faraday rotator, and in the presence of magnetic coils compensating for the Earth's field. The ability of the two schemes to provide total frequency stabilization of a diode laser was also investigated.
Until now, applications of cavity ring down spectroscopy (CRDS) employed pulsed laser sources. Here, we demonstrate that a commercial single-frequency CW laser can also be conveniently employed, allowing to gain in spectral resolution, signal intensity and data acquisition rate. As a demonstration, we measured a section of the weak HCCH overtone transition near 570 nm, and compare to existing photoacoustic data. Our high quality and reproducible Doppler-limited spectra display a (rms) noise-equivalent absorption of 10−9/cm or 5×10−8 per pass through the sample. Most interesting applications of CW-CRDS include high resolution spectroscopy at low pressure, sub-Doppler absorption spectroscopy in a supersonic jet, and trace-gas detection using compact diode laser sources.
After the brief survey of atomic and molecular energy structures in Chapters 2 and 3 we will now consider radiation and scattering processes by which atoms and molecules change their energy state. The processes are accompanied by the absorption or release of radiation giving rise to spectra. These spectra, can be used to clarify the structure of atoms and molecules and for a wealth of analytical purposes. We will first consider the case of transitions at a frequency corresponding to given energy separations (resonance radiation) and then discuss Rayleigh, Raman and Mie scattering. A detailed presentation of the theory of radiation and scattering processes can be found in [4.1–11]. Several of the books on atomic, molecular and quantum mechanics, earlier cited, also discuss this topic in more detail.
We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shot-noise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-m Nd:YAG laser, we obtained sub-Doppler overtone resonances of C 2 HD, C 2 H 2 , and CO 2 molecules. A detection sensitivity of 5 10 13 of integrated absorption (1 10 14 /cm) over 1-s averaging time has been achieved.
In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.
In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.
A compact noise-immune cavity-enhanced optical heterodyne molecular
spectrometry (NICE-OHMS) spectrometer, based on a narrowband
erbium-doped fiber laser and an integrated optics electro-optic
modulator, has been developed for trace species detection. A general
theoretical description of NICE-OHMS signals demodulated at an arbitrary
FM detection phase is provided in terms of the analyte concentration.
Explicit expressions for Doppler-broadened NICE-OHMS line shapes, which
are in excellent agreement with the measurements, are given. In a first
demonstration, using a cavity with a finesse of 1400, acetylene has been
detected on a Doppler-broadened transition at ˜1531 nm. A limit of
detection of 130 ppt, corresponding to an absorption of
2.4×10-9 cm-1, has been
obtained.
We report a visible laser with a subhertz linewidth for use in precision spectroscopy and as a local oscillator for an optical frequency standard. The laser derives its stability from a well-isolated, high-finesse, Fabry-Pérot cavity. For a 563 nm laser beam locked to our stable cavity, we measure a linewidth of 0.6 Hz for averaging times up to 32 s. The fractional frequency instability for the light locked to the cavity is typically 3×10-16 at 1 s. Both the linewidth and fractional frequency instability are approximately an order of magnitude less than previously published results for stabilized lasers.
This chapter discusses the issues involved in diode laser locking. The chapter describes in detail the various steps needed to lock the laser to a cavity resonance: (1) Derivetion of the error (locking) signal, (2) design of the electronic feedback circuitry, (3) initial locking of the laser, (4) adjustment of the feedback design, and (5) evaluation of the lock performance. The chapter illustrates this discussion by frequency locking an extended-cavity diode laser, reducing the linewidth to a few hertz relative to the cavity. The chapter concludes with an example in which the locking apparatus is modified for a cavity ring-down demonstration. Included are results showing the laser repetitively locking and unlocking to the cavity.
We present a theoretical description of the ultrasensitive cavity-enhanced spectroscopic technique called noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE OHMS) for the case of transitions described by a Voigt line shape. The two levels of modulation used in NICE OHMS are treated with the standard theory for frequency modulation spectroscopy and a Fourier description of wavelength modulation spectroscopy. We compare predicted line shapes with experimental results for pressure-broadened transitions in molecular oxygen and show that our description can be used to determine the spectroscopic parameters. A key aspect of this research is the application of NICE OHMS to broad absorption features across a range of wavelengths, and etalon effects are shown to limit the detection sensitivity.
A long optical path has been folded between two 7.5-cm diam spherical or aspherical mirrors to provide an output beam which can be well separated from previous reflections with 1000 or more passes between the mirrors. The 3000-m path provides 10 µsec of delay. This system can be used as a dispersionless optical delay line for use in filtering or storage of information modulated onto the light beam. The pattern of beams between the two mirrors is obtained in one of two ways. A small perturbing mirror may be inserted to give a series of offset ellipses, or one or both of the mirrors can be made astigmatic to give a Lissajous pattern of spots on each mirror. The output beam can be separated from others by discriminating in both angle and position. The diffraction losses of the system are much lower than those for an open beam because of the periodic focusing of the spherical mirrors. The extreme dependence of the loss of the delay line upon the absorption and scattering loss of the mirrors makes the system depeentupon very low loss mirrors and also makes the system a suitable method for measuring mirror loss.Block diagrams are shown for some possible filtering and storage applications.
Two different multiple traversal optical systems are described; one gives the longest paths, the other the best compensation for vibration and misalignment problems. In the first, seven mirrors in a near confocal arrangement permit a large aperture beam of light to pass through a restricted volume for a discrete and very large number of times. A rectangular array of images corresponding to different numbers of passes appears on four mirrors at one end of the system. At the other end, three mirrors form the array and illuminate each image in it from one or more different directions. The possible numbers of passes are (4 mn - 2) k + 2, where m and n are any integers representing, respectively, the number of columns and half the number of rows in the array. k is the number of different directions from which the array is illuminated. Geometrically, the beam may be isolated after thousands of passes; practically, the number is limited by reflection losses. In the second system the addition of four diagonal mirrors to a White cell converts the two lines of images on the single mirror to a rectangular array of images, almost squaring the maximum possible number of passes. With multiples of four rows of images in the array, the position of the output image is invariant to small errors in alignment of the mirrors.
Sensitive sub-Doppler resolution molecular spectroscopy developed by Hall and co-workers [J. Opt. Soc. Am. B 15 (1998) 6] has been applied to the 1.66 µm region using a widely tunable external-cavity diode laser. We have recorded saturated absorption spectra of the 2nu3 vibrational overtone band of methane with a sensitivity of 9.5×10-11 cm-1 at a time constant of 1.25 s and a spectral linewidth of 320 kHz (full width at half maximun; FWHM) over the tunable range of 1.63 to 1.68 µm.
An instrumentation for detection of nitric oxide
(
NO
)
by direct absorption spectrometry in the parts in
10
9
(ppb) range on its electronic
X
2
Π
(
ν
″
=
0
)
−
A
2
Σ
+
(
ν
′
=
0
)
transition has been constructed around a commercially available fully diode-laser-based laser system producing milliwatts powers of ultraviolet light at
∼
226.6
nm
, and its analytical performance has been evaluated. It is shown that the system is capable of detecting
NO
down to
3
ppb
∙
m
under low-pressure conditions (at a signal-to-noise ratio of 3 for a signal averaging of
5
s
), which is 2 orders of magnitude below that of any other diode-laser-based absorption technique. The combined line strength of the targeted lines was assessed to
3.1
×
10
−
18
cm
−
1
∕
(
molecule
cm
−
2
)
, which supersedes typical line strengths of the fundamental vibrational band and the first and second overtone bands of
NO
by
∼
2
,
∼
4
, and
∼
5
orders of magnitude, respectively. Also the collision broadening and shift of the targeted lines in
NO
by
N
2
have been assessed.
A polarization control method is applied to suppress the etalon fringe, and experimental results are described. It is found that this method can suppress an etalon fringe of the order of 10-6 in terms of the equivalent absolute absorption value, 1/100 of the off-controlled condition. This technique enables our tunable diode laser absorption spectrometry (TDLAS) system to measure atmospheric methane of about 2 ppmv concentration with ambiguity within 10-4 ppmv (0.1 ppbv). This scheme is useful for the portable TDLAS systems because of its simplicity.
The room temperature absorption spectrum of formaldehyde, H2CO, from 6547 to 6804 cm-1 (1527 1470 nm) is reported with a spectral resolution of 0.001 cm-1. The spectrum was measured using cavity-enhanced absorption spectroscopy (CEAS) and absorption cross-sections were calculated after calibrating the system using known absorption lines of H2O and CO2. Several vibrational combination bands occur in this region and give rise to a congested spectrum with over 8000 lines observed. Pressure broadening coefficients in N2, O2, and H2CO are reported for an absorption line at 6780.871 cm-1, and in N2 for an absorption line at 6684.053 cm-1.
With a diode-laser spectrometer, we have measured self-broadening coefficients of 42 lines of the P-, Q- and R-branches in the ν4 fundamental band of CH4 at room temperature, with J values ranging between 1 and 12. For the determination of self-broadening parameters, we have fitted to the experimental lineshape two theoretical line profiles: the Voigt profile taking into account Doppler and collisional broadenings, and the hard collision model developed by Rautian and Sobel’man incorporating Dicke narrowing.
It has been at least five years since a textbook devoted to optical spectroscopy was available, and this text admirably fills the void. It unquestionably fulfills its stated goal of providing a thorough treatment of the fundamental principles, methodology, and instrumentation common to analytical optical methods. The first six chapters provide a detailed description of the instrumental aspects of optical spectroscopy. Much of this material cannot be found in more general textbooks on instrumental analytical chemistry, and what can be found elsewhere is generally covered at a less advanced level. The remainder of the book is divided largely into atomic and molecular spectroscopy. The introductory chapters on each area (Chapters 7 and 12) are comprehensive and lucid. The chapters on atomic spectroscopy (Chapters 8-11) and molecular (Chapters, 13, 15, and 16) spectroscopy in the UV-vis regions of the spectrum cover all the important techniques at a rigorous level, testifying to the authors experience in these areas.
A novel optical interferometric technique is proposed for stabilizing and measuring a laser frequency in terms of an rf standard. In preliminary studies, a sensitive optical dual-frequency modulation scheme allows locking a laser to an optical cavity and the cavity in turn to a radio frequency reference with a noise level of 60 × 10-3 Hz or 2 parts in 1010. In principle, the laser frequency ω0 can acquire the stability of the rf standard and being locked to a high-order multiple n of the rf frequency ω1 facilitates the optical-rf division ω0/n=ω1.
The signal generation process in the wavelength modulation absorption spectrometry (WMAS) technique, mainly applied to diode lasers, is scrutinized in detail. The basic foundations of the technique, including its partly unique nomenclature and its relation to conventional absorption spectrometry, are reviewed. Most of the description of the signal generation process is made in terms of a newly developed formalism based on Fourier series. It is shown that the nth harmonic output of the lock-in amplifier in a WMAS instrument is given by the nth Fourier coefficient of the detector signal. This implies that many of the intricate characteristics of the WMAS technique can be derived from various properties of Fourier analysis. Analytical expressions for an arbitrary harmonics of the WMAS signals from Lorentzian, Gaussian and Voigt absorption profiles are given. It is shown how an associated laser-power modulation affects the WMAS signals and how multiple reflections, so-called etalons, give rise to background signals that often limits the applicability of the technique. It is furthermore shown that the traditional description of the WMAS technique, applicable only when small frequency-modulation amplitudes are used and often referred to as derivative spectroscopy, is a subset of the new formalism. Additional features covered are: WMAS signals from multiline transitions; the temperature dependence of the signal; WMAS under optically thick conditions (including the concept of an extended dynamic range); the advantages of multi-harmonic detection; WMAS background signals from frequency-doubled diode laser light; and double modulation techniques. It is also demonstrated that the new formalism can be used to predict the shift of zero crossings of odd harmonics, which is a feature often used for frequency-locking of lasers. Although some of these topics have been discussed previously in the literature, this work presents much new information. It also constitutes the first review of the WMAS technique based upon the new Fourier series-based formalism. (C) 2001 Elsevier Science B.V. All rights reserved.
A review is given of how measurements of intensive thermodynamic properties of a gas, such as of refractivity and permittivity, can enable single-parameter monitoring of gas density. The last decade has seen the introduction of a number of new and improved optical and dielectric measurement techniques as well as significant advances in atomic structure calculations. Such advances may find application, for example, in the determination of primary gas mixtures and fundamental constants, buoyancy compensation in precision mass metrology and in dynamic pressure metrology.
Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance.
The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.
The advantages of infrared laser monitoring in terms of sensitivity, selectivity and the ability of non-intrusive detection of gases are reviewed. Emphasis is laid on direct absorption spectroscopy and evanescent-field spectroscopy. The performance of the latter for gas detection in the near-infrared is demonstrated for the analysis of volcanic gases. For industrial process control, direct mid-infrared absorption spectroscopy is used to detect CO in the high-temperature atmosphere of a glass melting furnace. For both applications portable, stable, rugged and easy-to-handle laser systems are needed. Mid-infrared absorption spectroscopy is also applied to detect different explosives. Material evaporation is achieved by plasma generation with a pulsed laser at high repetition rate. Energetic materials contain high concentrations of nitrogen; therefore NO is present in the generated plasma. However, the rate at which NO is produced varies in a highly characteristic manner for different energetic materials. This enables the distinction between different types of explosives.
We present a method for recovering Doppler broadened absorption line shapes from frequency modulated ~FM! line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen ~NCCN!, probed on the A -X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal-to-noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics. © 1996 American Institute of Physics.@S0021-9606~96!00406-0#
Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy.
Instrumental improvements result in a new high-performance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity
of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low-adsorption flow cell,
and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing.
Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ∼5×10-10cm-1Hz-1/2, independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56μm
yield sub-ppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO2, CO, H2O, NH3, C2H2, and other hydrocarbons). By measuring spectroscopic features in the 1.525μm band of C2H2 gas, we realise detection limits of 19nTorr (2.5×10-11atm) of neat C2H2 (Doppler-limited at low pressure) and 0.37ppbv of C2H2 in air (pressure-broadened at 1atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost,
compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.
We performed an experiment to investigate modulation transfer spectroscopy in two-photon transition of Na2 using a CW dye laser. Comparing with UV fluorescence detection, the signals obtained by modulation transfer spectroscopy have a very high S/N without the Doppler background problem and its line shape is in agreement with theoretical calculations.
A novel highly sensitive method to detect the concentration of trace gas for environmental purposes is proposed. This method includes the application of frequency modulation for stabilization of laser frequency, together with optical heterodyne measurement. The shift of resonance frequency of the Fabry-Perot cavity resonator is due to the change of refractive index, accompanied by the dispersion effect due to the presence of a target gas (e.g., methane, carbon dioxide or nitrogen dioxide) inside the Fabry-Perot cavity.
A simple, economic diode laser based cavity ringdown system for trace-gas applications in the petrochemical industry is presented.
As acetylene (C2H2) is sometimes present as an interfering contaminant in the gas flow of ethylene (ethene, C2H4) in a polyethylene production process, an on-line monitoring of such traces is essential. We investigated C2H2–C2H4 mixtures in a gas-flow configuration in real time. The experimental setup consists of a near-infrared external cavity diode
laser with an output power of a few mW, standard telecommunication fibers and a home-made gas cell providing a user-friendly
cavity alignment. A noise-equivalent detection sensitivity of 4.5×10-8cm-1 Hz-1/2 was achieved, corresponding to a detection limit of 20ppbV C2H2 in synthetic air at 100mbar. In an actual C2H2–C2H4 gas-flow measurement the minimum detectable concentration of C2H2 added to the C2H4 gas stream (which may already contain an unknown C2H2 contamination) increased to 160ppbV. Moreover, stepwise C2H2 concentration increments of 500ppbV were resolved with a 1-min time resolution and an excellent linear relationship between
the absorption coefficient and the concentration was found.
Precision spectroscopy on molecular tellurium is performed by measuring the frequency difference between the observed lines and an eigenfrequency of a high-finesse cavity mode. The mode frequency is derived from a measurement of the cavity's free spectral range taking into account the cavity dispersion due to phase shifts in the dielectric mirror coatings. The experimental technique is based on dual frequency modulation and is applied to determine the transition wavenumbers of several lines in130Te2 near 467 nm with an uncertainty of 2 10–8.
A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements
of gas mixing ratios using near-infrared diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis
trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant
unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4×10-5 over an effective optical path of 4.2km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of
approximately 3.1×10-11cm-1 Hz-1/2. The instrument has been used for sensitive measurements of CO, CH4, C2H2 and NH3.
Clinical breath analysis remains in its infancy, despite the fact that its potential has been recognized for centuries and
that blood, urine, and other bodily fluids and tissues are routinely analyzed to diagnose disease or to monitor therapy. This
review discusses the present status of clinical breath analysis and suggests reasons why breath analysis has not received
similar widespread clinical use. Currently, a number of marker molecules have been identified in breath that could be used
to identify disease, disease progression, or to monitor therapeutic intervention and this list is expected increase dramatically
since the analysis of breath is ideally suited for population-based studies in the developed and underdeveloped world. Recent
advances in analytical instrumentation have suggested that the use of exhaled breath in medicine should now be re-examined.
In particular, the availability of real-time, portable monitors will represent a breakthrough for clinical diagnosis. Progress
in clinical breath analysis will require collaboration amongst device makers, experts in breath analysis, and clinicians.
The feasibility of laser-photoacoustic measurements for the detection and the analysis of different isolated doping agents
in the vapour phase is discussed. To the best of our knowledge, this is the first time that photoacoustic vapour-phase measurements
of doping substances have been presented. Spectra of different doping classes (stimulants, anabolica, diuretica, and beta
blockers) are shown and discussed in terms of their detection sensitivity and selectivity. The potential of laser spectroscopy
for detecting the intake of prohibited substances by athletes is explored.
We discuss a refined, hybrid rf/optical technique for studying sub‐Doppler saturated absorption/dispersion resonances with excellent precision and symmetry. Sensitivity is limited mainly by fundamental noise in the signal. Resonance profiles obtained in I 2 are in remarkable agreement with theory. The method promises a new level of accuracy for laser locking to an optical resonance.
A highly sensitive cavity-enhanced frequency modulation spectroscopy technique has been used to measure ultraweak transitions in molecular oxygen that had not previously been characterized. The self-broadened half-width and line intensity of the measured transitions are reported. We include 12 high J transitions in the band of 16O2 (the so-called A band), 59 transitions in the hot band of 16O2, and 17 high J transitions in the band of 16O18O. Our measurements of line positions of the 16O18O transitions are used to determine improved molecular constants for the excited state of 16O18O.
A new approach is described in which continuous, narrow band laser sources are employed with the recently developed integrated cavity output spectroscopy technique to obtain sensitive, quantitative absorption spectra in a simple experimental configuration. Absorption data obtained with cw-ICOS are related to the classical Fabry–Perot intracavity absorption model, which describes why the intracavity absorption is enhanced. A method of continuously injecting cw laser light into the cavity is described, as is a simple means of interpreting the ICOS data to extract accurate absorption intensities. Absorption spectra of vibrational combination bands of CO2 and H2O in the 1.3 μm region are presented.