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Fast, real-time spectrometer based on a pulsed quantum-cascade laser
Optics Letters
Abstract
We describe a mid-infrared spectrometer that is based on the combination of a multiple-pass absorption cell and a submicrosecond pulsed quantum-cascade laser. The spectrometer is capable of both making sensitive measurements and providing a real-time display of the spectral fingerprint of molecular vapors. For a cell with a path length of 9.6 m, dilution measurements made of the nu9 band transitions of 1,1-difluoroethylene indicate a sensitivity of 500 parts in 10(9), corresponding to a fractional absorbance of 4 x 10(-4).
... effects, making the cells more suited to real world environments. The work that we describe in this paper differs from previous work in that it makes use of the intrapulse modulation technique for QCLs [36]. The principle advantage of this method is that it provides a full spectral scan within the submicrosecond timeframe of a single pulse. ...
... A mercury cadmium telluride [(HgCd) Te] thermoelectrically cooled photodetector with a detectivity of 2.6 × 10 9 cm Hz 1∕2 ∕W was used in conjunction with the laser system. The spectroscopic principle that the system utilizes is based on that described by Normand et al. [36]. The application of a top-hat current pulse to the QCL, with typical durations in the range from 100 to 1000 ns, results in a thermal variation across the laser chip. ...
... Its NEA was calculated from the pulse data and was found to be 5.9 × 10 −4 , which is slightly higher than the 4.1 × 10 −4 NEA measured for the 5 m path length HSW gas cell made from the 1000 μm bore diameter waveguide. These values are similar to the value of 4 × 10 −4 measured by Normand et al. [36], who used a similar spectrometer with a 10 m white cell. ...
Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1 × 10 − 4 . The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan–Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.
... It is well-known that driving a QCL in pulsed mode generates a down-chirp of the emission frequency of thermal origin, that can reach up to several tens of GHz. This effect can be exploited to detect in real time different gas species for applications in environmental and combustion monitoring, plasma diagnostic, or high-resolution spectroscopy [38][39][40][41][42][43][44]. ...
... One weak point of this technique is that the value of the QCL emission frequency at each instant of time during the pulse is not known, a fact that can be problematic, for instance for the determination of unknown transition lines. For sufficiently short driving pulses the frequency chirp is approximately linear, allowing an absolute frequency pre-calibration using a Fourier transform (FT) spectrometer [40]. The generation of wider frequency spans requires instead longer driving pulses, typically ranging from tens of s to several ms, during which the time dependence of the QCL frequency is highly non-linear, requiring the use of an etalon for real-time relative frequency calibration [41]. ...
Thanks to intrinsically short electronic relaxation on the ps time scale, III-V semiconductor unipolar devices are ideal candidates for ultrahigh-speed operation at mid-infrared frequencies. In this work, antenna-coupled, GaAs-based multi quantum-well photodetectors operating in the 10-11um range are demonstrated, with a responsivity of 0.3A/W and a 3dB-cutoff bandwidth of 100GHz at room-temperature. The frequency response is measured up to 220GHz: beyond 100GHz we find a roll-off dominated by the 2.5 ps-long recombination time of the photo-excited electrons. The potential of the detectors is illustrated by setting up an experiment where the time dependent emission frequency of a quantum cascade laser operated in pulsed mode is measured electronically and in real-time, over a frequency range >60GHz. By exploiting broadband electronics, and thanks to its high signal-to-noise ratio, this technique allows the acquisition, in a single-shot, of frequency-calibrated, mid-infrared molecular spectra spanning up to 100GHz and beyond, which is particularly attractive for fast, active remote sensing applications in fields such as environmental or combustion monitoring.
... It is well known that driving a QCL in pulsed mode generates a down-chirp of the emission frequency of thermal origin that can reach up to several tens of GHz. This effect can be exploited to detect in real time different gas species for applications in environmental and combustion monitoring, plasma diagnostic, or high-resolution spectroscopy [38][39][40][41][42][43][44]. ...
... One weak point of this technique is that the value of the QCL emission frequency at each instant of time during the pulse is not known, a fact that can be problematic, for instance, for the determination of unknown transition lines. For sufficiently short driving pulses the frequency chirp is approximately linear, allowing an absolute frequency pre-calibration using a Fourier transform (FT) spectrometer [40]. The generation of wider frequency spans requires instead longer driving pulses, typically ranging from tens of µs to several ms, during which the time dependence of the QCL frequency is highly non-linear, requiring the use of an etalon for real-time relative frequency calibration [41]. ...
Thanks to intrinsically short electronic relaxation on the ps time scale, III-V semiconductor unipolar devices are ideal candidates for ultrahigh-speed operation at mid-infrared frequencies. In this work, antenna-coupled, GaAs-based multi-quantum-well photodetectors operating in the 10–11 µm range are demonstrated, with a responsivity of 0.3 A/W and a 3-dB-cutoff bandwidth of 100 GHz at room temperature. The frequency response is measured up to 220 GHz: beyond 100 GHz we find a roll-off dominated by the 2.5-ps-long recombination time of the photo-excited electrons. The potential of the detectors is illustrated by setting up an experiment where the time dependent emission frequency of a quantum cascade laser operated in pulsed mode is measured electronically and in real time, over a frequency range $\gt 60\; {\rm GHz}$ > 60 G H z . By exploiting broadband electronics, and thanks to its high signal-to-noise ratio, this technique allows the acquisition, in a single-shot, of frequency-calibrated, mid-infrared molecular spectra spanning up to 100 GHz and beyond, which is particularly attractive for fast, active remote sensing applications in fields such as environmental or combustion monitoring.
... In the present paper, we report on a proof-of-concept study of a QCL-based absorption method that is optimized to measure temperature and CO 2 concentration of gas mixtures in a range close to room temperature on a nanosecond time scale. The method employs strong CO 2 absorption lines with opposite temperature dependence which are probed by a single nanosecond frequency down chirped IR pulse of a distributed feedback QCL operated in intrapulse mode [14] with scanning rates up to ∼40 kHz. We demonstrate measurements on simulated breath gas flow and discuss the method with respect to potential improvements. ...
... In order to record spectra, the thermal time-dependent frequency down-chirp of the QCL laser pulse is used (intrapulse mode, see, e.g., Refs. [14,16]). Unless noted otherwise, measurements were performed with a pulse width of 500 ns and 20 kHz pulse repetition rate. ...
A quantum cascade laser-based sensing technique is presented which allows for in situ high-precision temperature and/or CO2 concentration measurements of gases in the room temperature regime with sampling rates up to about 40 kHz. The method is based on Boltzmann-like thermally populated fundamental and hot-band rovibrational transitions of CO2 with opposite temperature dependence. Single absorption spectra at about 2350 to 2352 cm−1 are recorded by a nanosecond frequency down chirped IR pulse of a pulsed distributed feedback quantum cascade laser (intrapulse mode). The statistical uncertainty (1σ) in the temperature measurement within one laser pulse is about 1 K and can be further reduced down to about 0.1 K by time averaging over 100 ms. Online temperature and CO2 concentration measurements on a breath simulator controlled gas flow were performed to demonstrate response-time and sensitivity for an application-driven test system.
... In this contribution, we present simultaneous measurements of CO and CO 2 concentrations and flame temperatures with a QCL operating around 4.48 µm (2,232 cm −1 ) in laminar low-pressure flat flames. The QCL has been operated in intra-pulsed spectroscopy mode [18]. In this mode, the temperature-induced frequency down-chirp is used to scan the laser over about 2 cm −1 during the 500-nslong laser pulses. ...
... Intra-pulsed spectroscopy [18] has been used in this work. This has the advantage that a complete absorption spectrum can be measured within a single laser pulse. ...
... Here we are interested in the transmission properties of a pulsed QCL operating at 7.8 μm which is used to make spectroscopic measurements of methane concentrations via the intra-pulse technique [18]. In Sect. ...
... A mercury cadmium telluride (HgCd)Te detector with a detectivity of 2.6 × 10 9 cmHz 1/2 /W was used in conjunction with the laser system. Gas concentration measurements were made using the intra-pulse technique [18]. This is a form of tunable laser spectroscopy [28] whereby the wavelength tuning occurs across the laser pulse due to the thermal effect of the pulsing. ...
In this paper, the transmission characteristics of hollow silica waveguides with bore diameters of 300 and 1000 μm are investigated using a 7.8-μm quantum cascade laser system. We show that the bore diameter, coiling and launch conditions have an impact on the number of supported modes in the waveguide. Experimental verification of theoretical predictions is achieved using a thermal imaging camera to monitor output intensity distributions from waveguides under a range of conditions. The thermal imaging camera allowed for more detailed images than could be obtained with a conventionally used beam profiler. The results show that quasi-single-mode transmission is achievable under certain conditions although guided single-mode transmission in coiled waveguides requires a smaller bore diameter-to-wavelength ratio than is currently available. Assessment of mode population is made by investigating the spatial frequency content of images recorded at the waveguide output using Fourier transform techniques.
... At the same time, the spectral-to-time mapping appeared to bring benefits into absorption spectroscopy of gas mixtures and searching molecular fingerprints. Initially realized with a chirped pulsed quantum-cascade laser for 1,1-difluoroethylene spectroscopy at 10.26 µm [13], DFT spectroscopy has been effectively applied to investigate the absorption spectra of CH 4 , utilizing a supercontinuum source [14]. An unprecedented acquisition rate of DFT spectrometry has facilitated the real-time visualization of chemical reactions during the combustion process of acetylene [15]. ...
Dispersive Fourier transform (DFT) has emerged as a powerful technique, enabling the transformation of spectral information from an optical pulse into a temporal waveform. This advancement facilitates the implementation of absorption spectroscopy using a single-pixel photodetector and a pulsed laser, particularly effective when operating on wavelengths near the absorption lines of the gas under study. This paper introduces a DFT-spectrometer employing a mode-locked tunable fiber laser with the central wavelength of 1531.6 nm. We demonstrate fast acquisition NH3 absorption spectroscopy with a 0.2 nm spectral resolution, achieved through the utilization of the HITRAN database for estimating ammonia concentrations. Alongside the successful demonstration of NH3 absorption spectroscopy, we explore practical limiting factors influencing the system’s performance. Furthermore, we discuss potential avenues for enhancing sensitivity and spectral resolution, aiming to enable more robust and accurate gas sensing applications.
... Therefore, MIR spectroscopy systems have been extensively employed for various applications, such as air quality monitoring, health diagnostics, and scientific research [1][2][3][4][5][6]. In this context, the generation of MIR light sources emitting in the wavelength range of 2-10 µm has been effectively demonstrated using various techniques, such as quantum cascade lasers [7,8], optical parametric oscillators [9,10], and supercontinuum lasers [11][12][13][14]. Among them, the high-brightness supercontinuum light source, associated with a high-resolution diffractive grating spectrometer, shows high performance in the accurate detection of minute traces of multispecies gas molecules [5]. ...
We demonstrate the successful implementation of an artificial neural network (ANN) to eliminate detrimental spectral shifts imposed in the measurement of laser absorption spectrometers (LASs). Since LASs rely on the analysis of the spectral characteristics of biological and chemical molecules, their accuracy and precision is especially prone to the presence of unwanted spectral shift in the measured molecular absorption spectrum over the reference spectrum. In this paper, an ANN was applied to a scanning grating-based mid-infrared trace gas sensing system, which suffers from temperature-induced spectral shifts. Using the HITRAN database, we generated synthetic gas absorbance spectra with random spectral shifts for training and validation. The ANN was trained with these synthetic spectra to identify the occurrence of spectral shifts. Our experimental verification unambiguously proves that such an ANN can be an excellent tool to accurately retrieve the gas concentration from imprecise or distorted spectra of gas absorption. Due to the global shift of the measured gas absorption spectrum, the accuracy of the retrieved gas concentration using a typical least-mean-squares fitting algorithm was considerably degraded by 40.3%. However, when the gas concentration of the same measurement dataset was predicted by the proposed multilayer perceptron network, the sensing accuracy significantly improved by reducing the error to less than ±1% while preserving the sensing sensitivity.
... Multiple transition cells are also used in another type of TDLS called intra-pulse spectroscopy using pulsed QCL lasers operating at room temperature [62,63]. The duration of the laser pulse is usually a few microseconds, while the intensity of the laser supply current is several amperes greater than the threshold value. ...
Methane is the most frequently analyzed gas with different concentrations ranging from single ppm or ppb to 100%. There are a wide range of applications for gas sensors including urban uses, industrial uses, rural measurements, and environment monitoring. The most important applications include the measurement of anthropogenic greenhouse gases in the atmosphere and methane leak detection. In this review, we discuss common optical methods used for detecting methane such as non-dispersive infrared (NIR) technology, direct tunable diode spectroscopy (TDLS), cavity ring-down spectroscopy (CRDS), cavity-enhanced absorption spectroscopy (CEAS), lidar techniques, and laser photoacoustic spectroscopy. We also present our own designs of laser methane analyzers for various applications (DIAL, TDLS, NIR).
... The main advantage in operating the QC laser in inter pulse mode is its simplicity with regard to technical prerequisites for the detection system. Intrapulse mode operation [24,25] or continuous-wavelength QC lasers could provide a significantly lower noise level but require sophisticated fast signal detection and data acquisiton equipment. ...
A non-invasive CO2 gas temperature sensing technique at or close to the room temperature range based on quantum cascade laser absorption spectroscopy is presented. The method probes thermally populated ground state and hot-band rotational-vibrational transitions of CO2 in the frequency range from 2349 to 2351 cm−1 from which the gas temperature is obtained from Boltzmann statistics. Transmission spectra are recorded by injection-current driven frequency-scans, the so-called inter pulse mode, of a pulsed distributed feedback quantum cascade laser. The statistical uncertainty (1σ) in temperature for single frequency scans with time resolution of 10 ms is 4 K and can be further reduced down to ∼50 mK by long-time averaging of about 1 min. The technique is evaluated with particular emphasis on implementation, data acquisition, data analysis and potential improvements.
... This wavelength was selected due in part to demonstrated utility for LAS measurements of spectral transitions in the fundamental band of carbon monoxide for numerous sensing applications [7,8,[21][22][23]. Note that this continuous-wave laser must be differentiated from pulsed DFB lasers used in other high-speed sensing techniques [24][25][26][27][28][29][30][31], and typically offers better scan-to-scan repeatability in output intensity. The laser temperature and mean injection current are set using an Arroyo 6310-QCL controller. ...
Variations in injection-current waveform are examined using diplexed RF-modulation with continuous-wave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-to-noise ratio for MHz-rate laser absorption spectroscopy. Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leading-edge ramp rate are further examined. A spectral scan depth on the order of 1 cm ⁻¹ at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 µm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via laser absorption spectroscopy at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work.
... Due to the influence of current on the refractive index of a cavity, the frequency of the laser will change sharply. Some studies have used this phenomenon to regulate the output wavelengths of lasers [11,17], while others have researched how to suppress its impact on optical communications systems [18,19]. The thermal chirp is caused by the thermal effect of injected current on the active layer. ...
We present a high-precision, low-cost demodulation method for the fiber Bragg grating (FBG) using a thermal-induced chirp and a shallow neural network. The thermal-induced chirp of a semiconductor laser generates the different wavelength components in a single pulse, which will form an exponential function echo signal after being reflected by the FBG. By learning the shape of the reflected light, the back-propagation neural network can simultaneously demodulate the sensing temperature and laser power. The whole detection system has only a few basic detection devices, which makes it low cost. The experimental results show that the multivision demodulation (MVD) method can reach a high demodulation precision of 0.35°C. We believe these results indicate the MVD method is an outstanding scheme in the field of FBG interrogation.
... Nowadays, pulsed distributed feedback (DFB) QCLs provide the most practical spectroscopic technique for atmospheric gas monitoring or gas analysis sensors. [10][11][12][13][14] The pulsed current driving leads to the transient self-heating process in the active region of the DFB QCL. This is the key mechanism for spectral tuning. ...
We present a method to produce a fast frequency swept laser emission from a monolithic mid-infrared laser. A commercially available Fabry–Pérot cavity quantum cascade laser (QCL) operating at a wavelength of 8.15 μm was electrically driven by a current pulse with a 10 μs duration and a slow front rising time of ∼2 μs. Due to the switching of the lasing emission from the vertical to the diagonal transition in the QCL and a strong quantum-confined Stark effect energy shift of the diagonal transition, the frequency of the emitted light was blue-shifting as the injection current continues to raise above the threshold. The temporal evolution of the laser spectrum was measured by a high-resolution step-scan Fourier transform infrared spectrometer. The blue-chirped emission was strongly influenced by the heatsink temperature due to the high thermal sensitivity of the threshold current and slope efficiency. By optimizing carefully the QCL operating temperature and the amplitude of the current pulse, we demonstrate a high-speed self-sweeping laser emission under room temperature operation conditions, reaching the spectral tuning range of ∼25 cm⁻¹ within 1.8 μs.
... Another method called intra-pulse modulation (IPM) is a unique absorption spectroscopy approach using QCLs [23,24]. When a QCL is operating in pulsed mode, transient temperature changes in the laser chip cause a wavelength chirp within the pulse duration. ...
In this paper, we propose a gas analyzer that uses a quantum cascade laser (QCL) and achieves high sensitivity and gas selectivity with simple configuration and signal processing. Feature quantities are extracted from an absorption-modulated signal, which is obtained by the logarithmic conversion of a detector signal receiving a wavelength-modulated laser light. The extracted feature quantities are used for the determination of target and interfering gas concentration with simple simultaneous linear equations. As a result of the demonstration of CO gas measurement with a gas analyzer consisting of a 4.6 μm pulsed QCL, a small-volume Herriott cell with a path length of 5 m and a thermopile as a photodetector, it is shown that the limit of detection is 2.0 ppb at the integration time of 1 s and that interference by N 2 O can be eliminated. It is also shown that various disturbances such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure change of coexisting gases can be corrected by extracting appropriate additional feature quantities.
... Since the spacing is much narrower than the κ/n eff π = 10 cm −1 photonic bandgap of the DFB grating, the possibility of simultaneous lasing on both sides of the bandgap is excluded. We believe the slightly dual-peak behavior is caused by intra-pulse heating, as has been observed in ref. 17 , and can be removed once continuous wave (CW) operation of the OPA is realized. In all current conditions, the side-mode suppression ratio (SMSR) is more than 25 dB. ...
Abstract We demonstrate single-mode, 16-channel, optical phased arrays based on quantum cascade laser technology, with emission wavelengths around 4.8 µm. The integrated device consists of a distributed feedback seed section, a highly-efficient tree array multi-mode interferometer power splitter, and a 16-channel amplifier array with a 4° angled facet termination. With a single layer Y2O3 coating, the angled facet reflectivity is estimated to be less than 0.1% for suppressing amplifier self-lasing. A peak output power of 30 W is achieved with an emission spectrum narrower than 11 nm and a side mode suppression ratio over 25 dB. Far field distribution measurement result indicates a uniform phase distribution across the array output. Using the same phased array architecture, we also demonstrate single-mode 3.8 µm QCL amplifier arrays with up to 20 W output power.
... Many substances can be identified by their spectral features associated with fundamental rotational and vibrational absorption bands in the infrared spectral region. This has spurred much attention to infrared (IR) technologies relating to applications, such as infrared spectroscopy [1][2][3][4] and infrared hyperspectral imaging [5][6][7][8]. Infrared spectroscopy has found applications within diverse fields like environmental gas monitoring [9,10] and medical applications e.g. ...
Broadly tunable upconversion is demonstrated for long-wave infrared (LWIR) detection. The upconversion system is evaluated by the detection of 50 ns pulses from a narrow linewidth tunable quantum cascade laser (QCL) in the 9.4 to 12 µm range. The LWIR signal is mixed with a 1064 nm laser beam in a silver gallium sulfide (AgGaS2) crystal, resulting in an upconverted signal in the 956 to 977 nm range, using angle tuning for optimal phase-matching. This allows for efficient, high speed detection using a standard silicon detector. A theoretical model including absorption and diffraction shows qualitative agreement with experimental data.
... Besides their applications in optical free-space communications where low-noise sources are desirable due to bandwidth, transmission range, and detection limitations arising from noisy sources [9,10], QCLs are also suitable sources for gas spectroscopy [13,14]. Intensity noise of free-running lasers stemmed from random carrier generation and recombination processes as well as intensity and phase noise induced by the injected light contribute to total relative intensity noise (RIN) of the slave laser and affect the emission characteristics [13]. ...
In this paper, we develop an analytical approach by linearization of small-signal rate equations in the presence of the Langevin noise sources to investigate the noise properties of mid-IR optically injected locked quantum cascade lasers. Excellent agreement between the reported numerical results and our developed analytical method have been obtained with the advantage of drastic calculation time reduction and deeper system performance knowledge. Then, a comprehensive study has been performed which reveals notable noise reduction, especially near the threshold current. It is shown that increasing the phase difference between the slave and master lasers enhances the corresponding noise term and makes this term the dominant noise-generating process near the locking edges. Additionally, by introducing relative intensity noise (RIN) maps, we demonstrate that increasing the bias current leads to lower RIN and frequency detuning range, while increasing the optical injection power eventuates wider frequency range and also, lower RIN. It has also been discussed that higher linewidth enhancement factors enhance the locking range and decreases the RIN near the positive frequency detuning boundaries. RIN reduction efficiency versus total injected power to the slave laser has been also introduced as a measure of effectiveness of optical injection locking.
... This was later advantageously exploited by using single long laser pulses (i.e., some hundred nanoseconds) to acquire entire absorption spectra in a very short time scale. This quantum cascade laser absorption spectroscopy (QCLAS) method is known as the intra pulse mode [71,72]. From the middle of the last decade QCLAS has been successfully implemented for a variety of plasma diagnostics purposes (see [3] and references therein), although specific obstacles such as pulse-to-pulse fluctuations inherent to pulsed operation had to be overcome. ...
The considerably higher power and wider frequency coverage available from quantum cascade lasers (QCLs) in comparison to lead salt diode lasers has led to substantial advances when QCLs are used in pure and applied infrared spectroscopy. Furthermore, they can be used in both pulsed and continuous wave (cw) operation, opening up new possibilities in quantitative time resolved applications in plasmas both in the laboratory and in industry as shown in this article. However, in order to determine absolute concentrations accurately using pulsed QCLs, careful attention has to be paid to features like power saturation phenomena. Hence, we begin with a discussion of the non-linear effects which must be considered when using short or long pulse mode operation. More recently, cw QCLs have been introduced which have the advantage of higher power, better spectral resolution and lower fluctuations in light intensity compared to pulsed devices. They have proved particularly useful in sensing applications in plasmas when very low concentrations have to be monitored. Finally, the use of cw external cavity QCLs (EC-QCLs) for multi species detection is described, using a diagnostics study of a methane/nitrogen plasma as an example. The wide frequency coverage of this type of QCL laser, which is significantly broader than from a distributed feedback QCL (DFB-QCL), is a substantial advantage for multi species detection. Therefore, cw EC-QCLs are state of the art devices and have enormous potential for future plasma diagnostic studies.
... In order to measure an absorption spectrum, it uses the intra-pulse frequency chirp of a pulsed DFB-QCL. The performance and reliability of intra-pulse spectroscopy using chirped QCLs have been established and demonstrated for gas concentration measurement such as ozone [41], ammonia and ethylene [42], methane and nitrous oxide [38], carbon dioxide and water vapor [43], difluoroethylene [44], nitric oxide [29,30,45], and methane isotopic composition [46] to name a few. As the laser is emitting the pulse for a few 100 ns, the substrate temperature varies causing a change in its refractive index. ...
Fugitive gas emissions from agricultural or industrial plants and gas pipelines are an important environmental concern as they can contribute to the global increase of greenhouse gas concentration. Moreover, they are also a security and safety concern because of possible risk of fire/explosion or toxicity. This study presents gas concentration measurements using a quantum cascade laser open path system (QCLOPS). The system retrieves the pathaveraged concentration of N2O and CH4 by collecting the backscattered light from a scattering target. The gas concentration measurements have a high temporal resolution (68 ms) and are achieved at sufficient range (up to 40 m, ~ 130 feet) with a detection limit of 2.6 ppm CH4 and 0.4 ppm for N2O. Given these characteristics, this system is promising for mobile/multidirectional remote detection and evaluation of gas leaks. The instrument is monostatic with a tunable QCL emitting at ~ 7.7 μm wavelength range. The backscattered radiation is collected by a Newtonian telescope and focused on an infrared light detector. Puffs of N2O and CH4 are released along the optical path to simulate a gas leak. The measured absorption spectrum is obtained using the thermal intra-pulse frequency chirped DFB QCL and is analyzed to obtain path averaged gas concentrations.
... In order to measure an absorption spectrum, it uses the intra-pulse frequency chirp of a pulsed DFB-QCL. The performance and reliability of intra-pulse spectroscopy using chirped QCLs have been established and demonstrated for gas concentration measurement such as ozone [41], ammonia and ethylene [42], methane and nitrous oxide [38], carbon dioxide and water vapor [43], difluoroethylene [44], nitric oxide [29,30,45], and methane isotopic composition [46] to name a few. As the laser is emitting the pulse for a few 100 ns, the substrate temperature varies causing a change in its refractive index. ...
Fugitive gas emissions from agricultural or industrial plants and gas pipelines are an important environmental concern as they contribute to the global increase of greenhouse gas concentrations. Moreover, they are also a security and safety concern because of possible risk of fire/explosion or toxicity. This study presents standoff detection of CH4 and N2O leaks using a quantum cascade laser open-path system that retrieves path-averaged concentrations by collecting the backscattered light from a remote hard target. It is a true standoff system and differs from other open-path systems that are deployed as point samplers or long-path transmission systems that use retroreflectors. The measured absorption spectra are obtained using a thermal intra-pulse frequency chirped DFB quantum cascade laser at ~7.7 µm wavelength range with ~200 ns pulse width. Making fast time resolved observations, the system simultaneously realizes high spectral resolution and range to the target, resulting in path-averaged concentration retrieval. The system performs measurements at high speed ~15 Hz and sufficient range (up to 45 m, ~148 feet) achieving an uncertainty of 3.1 % and normalized sensitivity of 3.3 ppm m Hz−1/2 for N2O and 9.3 % and normalized sensitivity of 30 ppm m Hz−1/2 for CH4 with a 0.31 mW average power QCL. Given these characteristics, this system is promising for mobile or multidirectional search and remote detection of gas leaks.
... The sub-threshold ramp signal heats the QCL so that each QCL pulse is generated at a different wavelength. This technique is called inter-pulse modulation [30,31]. Such a design makes it possible to develop compact sensors employing PAS, QEPAS, and wavelength modulation spectroscopy (WMS). ...
The paper presents the possibility of using laser absorption spectroscopy in medical diagnostics. Some detection techniques of disease markers existed in exhaled air are described. There is also discussed performance of quantum cascade lasers (QCL's) in view of their applications in infrared spectroscopy. In the main part the work, some methods of QCL's spectral tuning are analyzed. In summary, preliminary test results of the special QCL driving system for the wavelength tuning are discussed.
... A detailed comparison was not conducted in [4] because, on the one hand, the frequency sweeping rates of classical injection lasers were already at the limit, and, on the other hand, the speed of recording fast signals in the IR spectral range was also limited. With the advent of fast frequency swept quantum-cascade lasers and modern means of recording spectra, many researchers returned to investigation of these affects [6][7][8][9][10]. In general, the results obtained ...
Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k0z ~ 100) is proposed.
... There were no detailed quantitative comparisons in [4] since the rate of the frequency tuning of 'classical' diode lasers and the detection speed of fast signals in the IR region were limited. Later the development of fast-tuning quantum cascade lasers and modern techniques of spectra recording stimulated the new studies of these effects [6][7][8][9][10]. The results obtained at a higher technical Laser Physics Self-radiation of an absorbing medium induced by a fast frequency-tuning laser level were qualitatively similar to those described in [4] and allowed the quantitative analysis. ...
The self-radiation of an extended absorbing medium under the action of a laser field with a fast-changing frequency is studied. The oscillations in the time-dependent intensity of the medium radiation are found. These oscillations are caused by the beatings between the vibrations of the oscillator at the fast-changing exciting and constant resonance frequencies. The time duration between local peaks of the oscillations is inversely proportional to the difference between the resonance frequency of the oscillator and the instantaneous laserfrequency.
... Applications can be satisfied in a narrow scan and use much faster tuning rates as suggested by Tsai and Wysocki, 25 who offer a 5 KHz scan rate over 7 cm −1 and a narrow linewidth, or an even narrower scan using a cheaper DFB-QCL and intrapulse mode. 26,27 Improving scanning rates can also improve detection levels by providing more data enabling more powerful averaging to reduce SNR, and simultaneous operation of several LOS (by splitting and encoding each beamlet) can be considered to provide useful information for safety and security applications. These developments, together with the growing availability of these lasers, encourage further research and development of similar systems for various applications such as environmental monitoring. ...
An open-path spectrometer for fast spatial detection and identification of gaseous plumes in a realistic
environmental conditions is presented. Gases are released in a 500 m3 hall; detection and identification is performed by spectroscopic means—measuring the light spectral absorption (at 8 to 10 μm) by shining an external cavity quantum cascade laser beam through the inspected volume. Real-time identification is demonstrated for gas plumes of CH2FCF3 (R134a) and CHF3 at a distance of 30 m round trip with a minimum identification level of 0.2 ppm (response times of 2 to 10 s). The relatively wide spectral coverage allows a high probability of detection (PD) and low probability for a false alarm to be obtained in these realistic conditions. It is also demonstrated that the use of several lines-of-sight improves PD as gas spreading in the hall in these conditions is slow and unpredictable.
... An appropriate theory was constructed. Later, in the 2000s, numerous experimental studies were stimulated by the invention of tunable quantum cascade lasers (see, e.g., [5][6][7][8][9][10]). ...
We consider the possibility of using spectroscopy for the diagnostics of optically dense media by recording the time-dependent intensity of transmitted light under the action of a laser field with fast-changing frequency. We compare advantages of this approach with the traditional technique based on the Beer-Bouguer-Lambert law. Reasonably accurate extinction measurements of the order of k(ω 0)l ≈ 102 and greater are realizable with the help of tunable diode lasers.
... On profite ensuite de ce balayage en fréquence pour enregistrer un spectre à l'intérieur même de l'impulsion. Cette idée a été utilisée en 2003 par Normand et al.[99] et Beyer et al.[100]. ...
Les travaux présentés dans ce manuscrit concernent l’utilisation des lasers à cascade quantique dans l’infrarouge moyen en vue d’applications atmosphériques. Les deux premières parties du manuscrit présentent le contexte général de la thèse, le principe des lasers à cascade quantique (QCL), ainsi que l’étude de la bande _1 du SO2 autour de 9 μm avec un QCL en régime continu. Au cours de la troisième partie, nous présentons la première méthode que nous avons utilisée en régime pulsé qui est également la plus répandue utilisant des impulsions de courte durée (_10 ns). Nous avons pu alors mettre en évidence, dans le cadre de l’étude de la molécule de NH3, des problèmes liés à l’utilisation de cette technique qui en limite l’intérêt pour des applications en spectroscopie. Dans la quatrième partie, nous présentons la commande des QCL par impulsions longues (> 500 ns) permettant d’obtenir des spectres intra-impulsionnels avec des temps d’enregistrement de l’ordre de la microseconde. Nous avons pu appliquer cette technique pour l’étude du SO2 (à 9 μm) et comparer avec les résultats obtenus en régime continu. La comparaison des deux modes de fonctionnement précédents et leurs limitations nous a amené à proposer un nouveau mode de fonctionnement, utilisant des impulsions de durée intermédiaire (< 100 ns), présenté dans la cinquième partie du manuscrit. Cette méthode nous a permis de solutionner une grande partie des restrictions usuelles de ce type de laser et au cours de cette étude, nous avons également pu mettre en avant un certain nombre de points importants en ce qui concerne les phénomènes de rapid passage
... 27) Another method relies on the linear frequency chirp obtained from a QC laser when relatively long ( > 100 ns) current pulses are applied to map the spectral information into the temporal domain. 29) This approach requires an IR detector and a an analog digital converter (ADC) with a temporal resolution of ~ 1 ns or better. The ultimate spectral resolution is given by the equation ∆ν = C(dν/dt) 1/2 , where C~1 is a dimensionless constant and dn/dt is the chirp rate. ...
There is an increasing need in many chemical sensing applications ranging from environmental science to industrial process control as well as medical diagnostics for fast, sensitive, and selective trace gas detection based on laser spectroscopy. The recent availability of novel pulsed and continuous wave (cw) quantum and interband cascade distributed feedback (QC and IC DFB) lasers as mid-infrared spectroscopic sources ad-dresses this need. A number of spectroscopic techniques have been demonstrated worldwide. For example, the authors have employed infrared DFB QC and IC lasers for the detection and quantification of trace gases and isotopic species in ambient air at ppmv, ppbv and even sub-ppbv levels by means of direct absorption, cavity enhanced, photoacoustic and wavelength modulation spectroscopy.
We applied an artificial neural network to a mid-infrared trace gas sensing system to completely compensate the detrimental thermally-induced spectral shift of the spectrometer, improving the accuracy of the retrieved gas concentration.
Distributed or multipoint gas detection is crucial for several industrial and civil applications. Due to various losses, the traditional series-parallel method has a restricted number of measurement points. In this paper, an innovative cascaded connection method has been introduced for measuring a large number of locations. It is basically a branching route that contains a gas cell and a reflecting mirror. A small amount of light is coupled to the branch channel, leading to limited effects on transmission power. Using a light source that corresponds to the gas absorption line, an optical time domain reflectometer (OTDR) device interrogates each cell by branching. After establishing a length of delay fiber at one of the branch locations, the signals coming from each air cell are recognized individually at the receiving end. In this experiment, 99:1 couplers are used, and only 1% of the total light is transferred to the branch route gas cell. The signal-to-noise ratio increases dramatically compared to previous studies based on Rayleigh scattering. This is because the amplitude of the reflected light is one-half order more than that of the scattered light. A 1530 nm laser for acetylene and a 1653 nm laser for methane with a short pulse width are used for evaluating the novel multipoint gas detection technique. There is an analysis and discussion of the number of measurement points. The proposed method is capable of optimally laying 220 cells at a maximum monitoring distance of 8.8 kilometers within a dynamic range of ∼13.56 dB. Using the present wavelength power calibration method, the minimum detection limits for methane and acetylene along the 1% branching path are 0.093 low explosion level (LEL) and 0.321 LEL, respectively.
We present an innovative approach for a mid-IR spectrometer comprising a commercial Fabry-Perot cavity quantum cascade laser. Utilization of the transient blue-chirped emission of such a laser the spectral fingerprint of methane at 8.15μm was clearly observed in absorption.
We present a digital platform-based lock-in detection scheme to improve the gas sensing performance of a spectrometer. A numerically-controlled oscillator serves as reference clock to trigger the supercontinuum source and to demodulate the detector signal.
The spectral measurement of the blue chirped emission in FP cavity QCL operating at 8.15 pm has been successfully demonstrated. The output frequency of the laser was swept over the spectral range of 25 cm-1 within 2 ps.
Using rapidly-tunable quantum cascade laser arrays, we demonstrate acquisition of absorption spectra over 50cm-1 with < 0.1cm−1 resolution and a minimum detectable absorbance of 1 × 10−4 over the full spectrum in < 0.5s.
Methods of analytical chemistry are used to separate, identify, and quantify chemical components with respect to chemical structure and composition. This chapter explains the two most important techniques to measure CO2 analytically using physical methods: spectroscopy and gas chromatography. It describes the analytical determination of dissolved carbon dioxide in water with chemical methods. Spectroscopy is a term used to refer to the measurement of radiation intensity as a function of wavelength. Gas spectroscopy – as it is used also for the detection of CO2 concentrations in gases – exploits the behaviour of gas molecules to act as mechanical resonators. Laser absorption spectroscopy is the suitable method to exploit the narrow absorption lines of the ro‐vibrational transitions of molecular gases. Gas chromatography is a technique that is used in analytical chemistry for separating and analysing compounds that can be vaporized without decomposition.
A comprehensive study on the output power, the modulation response, and the relative intensity noise (RIN) behavior of an optically injection‐locked mid‐infrared quantum‐cascade laser reveals that the modulation bandwidth and the output power are enhanced in the stable locking range, while the RIN of the slave laser is a superposition of the master and slave noise sources. Since the RIN level of the master laser can even take the lead, a design procedure is introduced to improve the main characteristics of a free‐running laser, including the RIN, the photon lifetime, the modulation bandwidth, and the bias current, using facet reflectivity tailoring. A figure of merit is defined and the RIN reduction of about 20 dB Hz⁻¹ is obtained for very low injection powers compared with the injection‐locked system before the design of master laser.
Laser spectroscopy provides the basis of instrumentation developed for the diagnosis of infectious disease, via quantification of organic biomarkers that are produced by associated bacteria. The technology is centred on a multichannel pulsed quantum cascade laser system that allows multiple lasers with different wavelengths to be used simultaneously, each selected to monitor a different diagnostic biomarker. The instrument also utilizes a hollow silica waveguide (HSW) gas cell which has a very high ratio of interaction pathlength to internal volume. This allows sensitive detection of low volume gas species from small volume biological samples. The spectroscopic performance of a range of HSW gas cells with different lengths and bore diameters has been assessed using methane as a test gas and a best-case limit of detection of 0.26 ppm was determined. The response time of this cell was measured as a 1,000 sccm flow of methane passed through it and was found to be 0.75 s. These results are compared with those obtained using a multi-pass Herriot cell. A prototype instrument has been built and approved for clinical trials for detection of lung infection in acute-care patients via analysis of ventilator breath. Demonstration of the instrument for headspace gas analysis is made by monitoring the methane emission from bovine faeces. The manufacture of a hospital-ready device for monitoring biomarkers of infection in the exhaled breath of intensive care ventilator patients is also presented.
We present here an open-path analyser, initially intended for security applications, specifically for the detection of gas plumes from illicit improvised explosive device (IED) manufacturing. Subsequently, the analysers were adapted for methane measurement and used to investigate its applicability for leak detection in different scenarios (e.g. unconventional gas extraction sites). Preliminary results showed consistent measurements of gas plumes in the open path.
Widely tunable quantum cascade lasers (QCLs) spanning the long-wave infrared (LWIR) atmospheric transmission window and an HgCdTe detector were incorporated into a transceiver having a 50-mm-diameter transmit/receive aperture. The transceiver was used in combination with a 50-mm-diameter hollow retro-reflector for the open-path detection of chemical clouds. Two rapidly tunable external-cavity QCLs spanned the wavelength range of 7.5 to 12.8 m. Open-path transmission measurements were made over round-trip path-lengths of up to 562 meters. Freon-132a and other gases were sprayed into the beam path and the concentration-length (CL) product was measured as a function of time. The system exhibited a noise-equivalent concentration (NEC) of 3 ppb for Freon-132a given a round-trip path of 310 meters. Algorithms based on correlation methods were used to both identify the gases and determine their CLproducts as a function of time.
The technological advances of multi component analysers which utilise Process Analytical Tuneable Laser Spectroscopy (PATLS) provide significant benefits in the areas of industrial process control. These benefits allow on-line multi component measurements, across a number of process streams. These benefits cover technical advantages and improved capability as well as reduced cost/resource requirements benefits due to •Simultaneous multiple component measurement capability •Fully modular platform •Significantly lower total cost of ownership, particularly when compared to gas chromatographic (GC) based methods in the same service. •Faster analysis speeds (real time) when compared to existing technologies The speed of analysis advantage of mid infra-red (MIR) PATLS over traditional GC has resulted in applications savings, in the order of several $100,000's in credits for some end users. This paper reports on a number of Cascade Technologies Industrial Process application projects and will show how the requirements for on-line impurity and process composition analysis of industrial process gas analyser applications arc being met using mid IR PATLS.
Process Analytical Tunable Laser Spectroscopy (PATLS) using mid IR Quantum Cascade laser spectrometers has realized considerable success in on-line measurement for automotive, industrial, and environmental applications. More recently they have begun to receive attention for process gas monitoring on actual process streams in the refining and petrochemical manufacturing sectors as alternatives to the more traditional Gas Chromatographs (GCs) and Non-Dispersive Infrared (NDIR) in part due to their inherently lower cost of ownership. In an effort to be competitive with these other technologies there are certain commercial and technical hurdles that must be addressed: • Competitive equipment cost • Multiple component measurements • Sensitivity • Efficient mechanisms for developing new applications. This presentation looks at how these and other requirements necessary for process gas analyzers are addressed using a compact mid IR multi-laser QCL product platform, developed to function as an on-line process gas analyzer.
Mid-infrared lasers are very effective for the spectroscopic measuring of the fundamental absorption lines for most gases. The quantum cascade laser (QCL), with the advantages of high power, wide tuning range and room-temperature operation, is one of the ideal mid-IR sources for trace gas detection. In this paper, the spectral characteristics of room temperature operated pulsed QCL with a cental wavelength of 1274 cm??1 is presented. By using high-bandwidth HgCdTe detector and high-speed data acquisition devices, and combining with a high-resolution etalon, the features of the spectrum in the frequency domain can be acquired. Analysis of the measured spectral data gives the information about spectral shape, tuning characteristics and resolution variation with time, also about the influences of voltage and temperature on spectrum. The effective way to improve the spectral resolution and optimize the spectral quality is obtained, and it can be used to improve the accuracy of spectrum quantitative analysis.
The intra-pulse spectroscopy based on room-temperature pulsed quantum cascade(QC) lasers was introduced. With a room-temperature pulsed QC laser centered at 1904 cm-1, the sample NO gas has been detected. The least-squared algorithm of baseline fitting and concentration retrieving method were introduced for the single line spectroscopy detection, and the gas concentration can be calculated with scan integration and the corresponding values from HITRAN04 database by direct-absorption technique, without calibration. The detection limit was obtained by analyzing the residual of the absorption spectroscopy as 3.4 ×10-6m.
Quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy is reported. Key technical issues of mobile laser remote sensing are discussed through simulating leak detection. For quantitative remote sensing of trace leakage, the signals with counter-phase and isovalue of residual amplitude modulation are introduced. Then the offset is compensated, the influence of harmonic signal is reduced and sensitivity is improved. The improved soft-threshold denoising method is proposed by analyzing the characteristics of echo absorption spectrum. The results show that the signal-to-noise ratio is improved two times as great as that by the traditional soft-threshold denoising method and the shape of second harmonic (2f) signal is well preserved. The minimum remote sensitivity will be 80 ppm/m under the mobile remote sensing.
Absorption spectroscopy in the ultraviolet (UV) and mid-infrared (MIR) spectral region has been used in a comparative study for the detection of formaldehyde in laminar low pressure flames of dimethyl ether (DME) and methane. Both spectral regions were tested to explore respective advantages and limitations, especially for the detection of stable molecules in flames. In the UV, cavity ring-down spectroscopy (CRDS), a highly sensitive multi-pass absorption technique, has been used for the detection of formaldehyde in the
The broadband electroluminescence of a quantum cascade device based on a multi-color active region covering the wavelengths 5.9μm – 7.2μm was measured. Anti-reflection coatings were applied on both cleaved facets to remove the Fabry-Pérot cavity and prevent the device from lasing. This allows the latter to be studied either as a superluminescent diode or a single-pass amplifier in order to determine its suitability as a source for low speckle imaging applications. At 243 K, the amplified spontaneous emission has a peak power of 38μW that agrees well with a simple model of spontaneous emission intensity. The light of a similar structure could be modulated up to 1 GHz, limited by the RC constant of the device. The peak gain was measured from high-resolution luminescence spectra and determined to be 6.3 cm⁻¹, corresponding to a single-pass gain of 1.89.
An experimental setup for the simultaneous detection of CO and \(\hbox {CO}_2\) and the temperature in low-pressure flames using a pulsed quantum cascade laser at 4.48 μm is presented. This comparatively new type of laser offers good output energies and beam quality in the mid-infrared, where the strong fundamental transitions of many molecules of interest can be accessed. A single-pass absorption setup was sufficient to obtain good accuracy for the stable species investigated here. Due to the high repetition rate of the laser and the speed of the data acquisition, measurement of two-dimensional absorption spectra and subsequent tomographic reconstruction was feasible. As demonstration of this technique, two-dimensional CO and \(\hbox {CO}_2\) concentrations have been measured in two fuel-rich methane flames with different coflow gases (nitrogen and air). The influence of the coflow gas on the flame center concentration profiles was investigated and compared with one-dimensional model simulations.
Quantum Cascade Lasers (QCL’s) have been successfully used to monitor atmospheric pollutants in the mid-infrared
(mid-IR) region. However, their use for multiple gases in ambient conditions is less familiar. This paper explores the
performance of a novel field deployable open path system based on a chirped single distributed-feedback QCL. In
particular, we report both laboratory and open path measurements for simultaneous detection of two greenhouse gases
(GHG) methane (CH4) and nitrous oxide. (N2O). We focused on CH4 and N2O because they are long-lived greenhouse
gases in the atmosphere with significant global warming effects.
Gas spectra were recorded by tuning the QC laser wavelength using a thermal down chirp technique over 1297–1300
cm-1 optimal spectral window with 0.008 cm-1 spectral resolution. Based on careful optimization of the spectral window
for absorption features of CH4 and N2O, a dual-species, cost-effective, robust and rapid response open-path laser based
monitor has been developed for ambient trace gas monitoring.
Theoretical signal to noise ratio (SNR) analysis of the system based on an ideal system is briefly discussed in the paper
but our main focus is on actual system performance, long term stability and systemic errors. Finally, preliminary results
of the open path system are reported.
A simultaneously time-resolved and calibration-free sensor has been demonstrated to measure temperature at the nanosecond timescale at repetition rates of 1.0 MHz. The sensor benefits from relying on a single laser, is intuitive and straightforward to implement, and can sweep across spectral ranges in excess of 1 cm − 1 . The sensor can fully resolve rovibrational features of the CO molecule, native to combustion environments, in the mid-infrared range near λ = 4.85 μm at typical combustion temperatures (800–2500 K) and pressures (1–3 atm). All of this is possible through the exploitation of chirp in a quantum cascade laser, operating at a duty cycle of 50%, and by using high bandwidth (500 MHz) photodetection. Here, we showcase uncluttered, spectrally-pure Voigt profile fitting with accompanying peak SNRs of 150, resulting in a typical temperature precision of 0.9% ( 1 σ ) at an effective time-resolution of 1.0 MHz. Our sensor is applicable to other species, and can be integrated into commercial technologies.
We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm . The noise equivalent absorbance for our measurement was 5 parts in 10(5), limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N(2)O in 10(9) parts N(2) in a 1-m path length.
There is an increasing need in many chemical sensing applications
ranging from industrial process control to environmental science and
medical diagnostics for fast, sensitive, and selective gas detection
based on laser spectroscopy. The recent availability of novel pulsed and
CW quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared
spectroscopic sources address this need. A number of spectroscopic
techniques have been demonstrated worldwide by several groups. For
example, the authors have employed QC-DFB lasers for the monitoring and
quantification of several trace gases and isotopic species in ambient
air at ppmv and ppbv levels by means of direct absorption, wavelength
modulation, and cavity enhanced and cavity ringdown spectroscopy
Lasing characteristics were evaluated for distributed-feedback quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the QC lasers' suitability for use in high-resolution spectroscopic applications, including Doppler-limited molecular absorption and pressure-limited lidar applications. By use of a rapid-scan technique, direct absorbance measurements of nitric oxide (NO) and ammonia (NH>3) were performed with several QC lasers, operating at either 5.2 or 8.5 textmum. Results include time-averaged linewidths of better than 40 MHz and long-term laser frequency reproducibility, even after numerous temperature cycles, of 80 MHz or better. Tuning rates of 2.5 cm-1 in 0.6 ms can be easily achieved. Noise-equivalent absorbance of 3 texttimes 10-6 was also obtained without optimizing the optical arrangement.
We present results that describe the evolution of the spectrum of a pulsed quantum cascade (QC) laser. By mapping the temporal characteristics of the light pulse into the wave number domain, we show how the spectral evolution depends on the duration and the quality of the current pulses used to excite the QC laser.
A pulsed quantum-cascade distributed feedback laser operating at near room temperature was used for sensitive high-resolution IR absorption spectroscopy of ambient air at a wavelength of approximately 8 microm. Near-transform-limited laser pulses were obtained owing to short (approximately 5-ns) current pulse excitation and optimized electrical coupling. Fast and slow computer-controlled frequency scanning techniques were implemented and characterized. Fast computer-controlled laser wavelength switching was used to acquire second-derivative absorption spectra. The minimum detectable absorption was found to be 3 x 10(-4) with 10(5) laser pulses (20-kHz repetition rate), and 1.7 x 10(-4) for 5 x 10(5) pulses, based on the standard deviation of the linear regression analysis.