Pulsed Quantum Cascade Laser Research Articles

Sensitive detection of acrolein and acrylonitrile with a pulsed quantum-cascade laser

We report on spectroscopic measurements of acrolein and acrylonitrile at atmospheric pressure using a pulsed distributed feedback quantum-cascade laser in combination with intra- and inter-pulse techniques and compare the results. The measurements were done in the frequency region around 957 cm−1. In the inter-pulse technique, the laser is excited with short current pulses (5–10 ns), and the pulse amplitude is modulated with an external current ramp resulting in a ∼2.3 cm−1 frequency scan. In the intra-pulse technique, a linear frequency down-chirp during the pulse is used for sweeping across the absorption line. Long current pulses up to 500 ns were used for these measurements which resulted in a spectral window of ∼2.2 cm−1 during the down-chirp. These comparatively wide spectral windows facilitated the measurements of the relatively broad absorption lines (∼1 cm−1) of acrolein and acrylonitrile. The use of a room-temperature mercury-cadmium-telluride detector resulted in a completely cryogen-free spectrometer. We demonstrate ppb level detection limits within a data acquisition time of ∼10 s with these methodologies.

An effective TDLS setup using homemade driving modules for evaluation of pulsed QCL

Integrated, compact, and economical pulse driving and TE control modules for pulsed quantum cascade lasers (QCL) have been developed. Based on those modules, an effective tunable diode laser spectroscopy (TDLS) setup has been constructed for the evaluation of pulsed QCL and demonstration of TDLS concepts. Connecting our mid-infrared QCL module to the driving module using cable of 10 Ω characteristic impedance results in down chirp light pulses suitable for intrapulse scheme of TDLS; a usable down chirp of about 0.78 cm−1 is achieved for current pulse with 240-ns pulse width. Connecting the QCL module to the driving module using stripline of characteristic impedance <1 Ω results in narrow Gaussian-like light pulses that are suitable for interpulse scheme of TDLS; the light pulse width of 15 ns is achieved. The absorption features of N2O at 1289.86 cm−1 have been measured using a homemade distributed feedback QCL around about 7.75 μm adopting intrapulse scheme; the measurement limit below 1 ppmv is achieved with a 10-m white cell.

Studying the frequency tuning of pulsed terahertz quantum cascade lasers

Radiation spectra of pulsed quantum cascade lasers operated at about 2.6 and 3.2 THz have been studied using the Fourier-transform technique. Tuning of the laser radiation frequency by 5.4 GHz in a temperature variation range of 8 to 86 K for the 2.6 THz laser and by 2.7 GHz in a range of 10 to 60 K for the 3.2 THz laser, resulting mainly from the temperature dependence of the effective refractive index of the active region, has been demonstrated. Frequency tuning by 420 MHz during the radiation pulse was shown for the first time in a laser with the active region designed on the basis of fast removal of the carriers from the lower operating level due to the phonon scattering.

Experimental study on spectral characteristics of room temperature operated pulsed quantum cascade laser

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.

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents (CWAs) are recognized as serious threats of terrorist acts against the civilian population. Minimizing the impact of these threats requires early detection of the presence of CWAs. Cavity ring-down spectroscopy (CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species. In this letter, the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate (DMMP). A limit of DMMP detection of approximately 77 ppb is achieved. The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2 x 10(-7) cm(-1).

Inter-Pulse Spectroscopy Based on Room-Temperature Pulsed Quantum-Cascade Laser for N&lt;sub&gt;2&lt;/sub&gt;O Detection

Mid-infrared lasers are very suitable for high-sensitive trace-gases detection for their wavelengths cover the fundamental absorption lines of most gases. Quantum-cascade (QC) lasers have been demonstrated to be ideal light sources with its special power, tuning and capability of operating in room-temperature. All these merits make it appropriate for the high resolution spectrum analysis. The absorption spectrum monitoring technology based on the QC laser pulsed operating in the room temperature, combining with the strong absorption of the gas molecule in the basic frequency, has become an effective way to monitor the trace gas with the characteristic of high sensitivity, good selectivity and fast response. In this paper, the inter-pulse spectroscopy based on a room-temperature distributed-feedback pulsed QC laser was introduced. Our approach to trace gas monitoring with QC lasers relies on short current pulses which are designed to produce even shorter light pulses. Each pulse corresponds to a single point in a spectrum. The N2O absorption spectrum centered at 2178.2cm-1was also obtained.

Measurements of NO and CO in Shanghai urban atmosphere by using quantum cascade lasers

Nitric oxide (NO) and carbon monoxide (CO) in Shanghai urban atmosphere have been measured during the EXPO 2010 by an optical trace gas monitoring system based on room-temperature pulsed quantum cascade lasers (QCL). The results showed obvious diurnal variation in their concentrations. A great correlation was found by analyzing the diurnal variation, indicating common emission sources for both NO and CO. However analysis of the data from street canyon measurements showed that on a vehicle-by-vehicle basis NO emissions correlated only weakly with CO emissions. A possible explanation to this issue has been given.

Infrared spectroscopic analysis of human interstitial fluid in vitro and in vivo using FT-IR spectroscopy and pulsed quantum cascade lasers (QCL): Establishing a new approach to non invasive glucose measurement

Interstitial fluid, i.e. the liquid present in the outermost layer of living cells of the skin between the Stratum corneum and the Stratum spinosum, was analyzed by Fourier transform infrared spectroscopy and by infrared spectroscopy using pulsed quantum cascade infrared lasers with photoacoustic detection. IR spectra of simulated interstitial fluid samples and of real samples from volunteers in the 850–1800 cm −1 range revealed that the major components of interstitial fluid are albumin and glucose within the physiological range, with only traces of sodium lactate if at all. The IR absorbance of glucose in interstitial fluid in vivo was probed in healthy volunteers using a setup with quantum cascade lasers and photoacoustic detection previously described [11]. A variation of blood glucose between approx. 80 mg/dl and 250 mg/dl in the volunteers was obtained using the standard oral glucose tolerance test (OGT). At two IR wavelengths, 1054 cm −1 and 1084 cm −1, a reasonable correlation between the photoacoustic signal from the skin and the blood glucose value as determined by conventional glucose test sticks using blood from the finger tip was obtained. The infrared photoacoustic glucose signal (PAGS) may serve as the key for a non-invasive glucose measurement, since the glucose content in interstitial fluid closely follows blood glucose in the time course and in the level (a delay of some minutes and a level of approx. 80–90% of the glucose level in blood). Interstitial fluid is present in skin layers at a depth of only 15–50 μm and is thus within the reach of mid-IR energy in an absorbance measurement. A non-invasive glucose measurement for diabetes patients based on mid-infrared quantum cascade lasers and photoacoustic detection could replace the conventional measurement using enzymatic test stripes and a drop of blood from the finger tip, thus reducing pain and being a cost-efficient alternative for millions of diabetes patients.

Wavelength modulation spectroscopy with a pulsed quantum cascade laser for the sensitive detection of acrylonitrile

A pulsed, distributed feedback (DFB) quantum cascade laser centered at 957 cm−1 was used in combination with a wavelength modulation spectroscopic technique for the detection of acrylonitrile. The laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with a linear subthreshold current ramp at 20 Hz resulting in a ∼2.5 cm−1 frequency scan. This allowed the measurement of spectroscopic features of acrylonitrile with absorption line widths of ∼1 cm−1. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We then superimposed a 10 kHz sine wave current modulation on top of the 20 Hz current ramp. The resulting high frequency temperature modulation of the DFB structure results in wavelength modulation. A minimum detectable absorbance of ∼10−5, corresponding to the sub 109 levels of acrylonitrile, was achieved with less than a minute averaging time.

Measurement of CO amount fractions using a pulsed quantum-cascade laser operated in the intrapulse mode

Carbon monoxide (CO) is an important molecule for environmental monitoring, industrial process control, and a biomarker in exhaled human breath. Obtaining reliable and traceable data is indispensable. We employed direct absorption spectroscopy-based absolute amount fraction measurements of CO in a gravimetrically prepared gas mixture. A quantum-cascade laser operated in the intrapulse mode was used to probe the P(1) line of CO at 2139.4 cm−1. The spectrometrically determined CO amount fraction agrees perfectly with the gravimetric reference value. We focused on the method, the uncertainty analysis of the spectrometry-based data retrieval and the respective traceability of input parameters to the SI. An uncertainty budget is presented. Our reproducibility is better than 1%. The relative deviation of the spectrometric CO amount fractions from the gravimetric reference value reads minus 1.8%, which is covered by a 4% relative expanded uncertainty of single measurements (k=2).

Applications of midinfrared quantum cascade lasers to spectroscopy

We review the use of both pulsed and continuous wave quantum cascade lasers in high-resolution spectroscopic studies of gas phase species. In particular, the application of pulsed systems for probing kinetic processes and the inherent rapid passage structure that accompanies observations of low-pressure samples using these rapidly chirped devices are highlighted. Broadband absorber spectroscopy and time-resolved concentration measurements of short-lived species, respectively exploiting the wide intrapulse tuning range and the pulse temporal resolution, are also mentioned. For comparison, we also present recent sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using a continuous wave external cavity quantum cascade laser system. Using this methodology the stability and resolution of this source is quantified. We find that the laser linewidth as measured via the Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates, but decreases to 1.3 MHz as the laser scan rate is increased such that the transition is observed at 30 kHz. Using this source, wavelength modulation spectroscopy of NO is presented.

High-resolution emission spectra of pulsed terahertz quantum-cascade lasers

The spectra of pulsed terahertz quantum-cascade lasers were measured with high spectral resolution. The characteristic line width at half maximum was 0.01 cm−1; it is controlled by laser temperature variations during the supply voltage pulse. It was shown that an increase in the laser temperature leads to a decrease in the emission frequency, which is caused by an increase in the effective refractive index of the active region. It was also found that a decrease in the supply voltage results in a decrease in the emission frequency, which is caused by a change in the energy of diagonal transitions between lasing levels.

Off-axis cavity enhanced spectroscopy based on a pulsed quantum cascade laser for sensitive detection of ammonia and ethylene

A pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm(-1) was used in combination with an off-axis cavity enhanced absorption (CEA) spectroscopic technique for the detection of ammonia and ethylene. Here, the laser is coupled into a high-finesse cavity with an optical path length of ∼76 m. The cavity is installed into a 53 cm long sample cell with a volume of 0.12 L. The laser is excited with short current pulses (5-10 ns), and the pulse amplitude is modulated with an external current ramp, resulting in a ∼0.3 cm(-1) frequency scan. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We demonstrated detection limits of ~15 ppb and ∼20 ppb for ammonia and ethylene, respectively, with less than 5 s averaging time.

Real-time monitoring of nitric oxide in diesel exhaust gas by mid-infrared cavity ring-down spectroscopy

We report the accurate and precise measurement of nitric oxide (NO) in automotive exhaust gas by cavity ring-down spectroscopy (CRDS) using a thermoelectrically cooled, pulsed quantum cascade laser (QCL) as a light source. A mid-infrared QCL with a 5.26 μm wavelength was used to detect fundamental vibrational transitions of NO. An effective optical path length of 2.1 km was achieved in a 50 cm long cell using high-reflectivity mirrors. In combination with a particle filter and a membrane gas dryer, stable and sensitive measurement of NO in exhaust gas was achieved for more than 30 minutes with a time resolution of 1 s. The results of this work indicate that a laser based NO sensor can be used to measure NO in exhaust gas over a dynamic range of three orders of magnitude.

Quantum cascade laser absorption spectroscopy as a plasma diagnostic tool: an overview.

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.

Non-symmetrical line broadening effects using short-pulse QCL spectrometers as determined with sub-nanosecond time-resolution

Quantum cascade lasers (QCLs) have attracted considerable interest as an alternative tuneable narrow bandwidth light source in the mid-infrared spectral range for chemical sensing. Pulsed QCL spectrometers are often used with short laser pulses and a bias current ramp similar to diode laser spectroscopy. Artefacts in the recorded spectra such as disturbed line shapes or underestimated absorption coefficients have been reported. A detailed time-resolved high-bandwidth analysis of individual pulses during a laser sweep has been performed. Quantitative results for CH4 absorption features around 1347 cm−1 (7.42 μm) fell short of the expected values for reasonable operating conditions of the QCL. The origin of the artefacts using short pulses was identified to be partly of the same nature as in the case of long laser pulses. A complex combination with the tuning principle was found, leading to an apparently increased instrumental broadening (effective line width) and underestimated concentrations at low-pressure conditions.

QEPAS detector for rapid spectral measurements

A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combina- tion with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196- 1281 cm −1 spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon- 125), acetone, and ethanol both individually and in com- bined mixtures. The precision achieved for freon-125 con- centration in a single 1.1 s long spectral scan is 13 ppbv.

Photoacoustic investigation of QCL modulation techniques

High detection sensitivity and spectral selectivity is important for gas analysers to identify the measured compound and to detect low concentrations. We investigated three different modulation methods – pulse gate modulation, pulse frequency modulation and chopper modulation – for a new pulsed quantum cascade laser based photoacoustic sensor. The spectral selectivity and the detection limit for the three modulation methods are compared by measuring nitric oxide absorption lines and different concentrations. The highest detection sensitivity of 70 ppb was achieved with pulse gate modulation but at the lowest spectral resolution. The highest spectral resolution was achieved with chopper modulation but at the lowest detection sensitivity. It is demonstrated that for the three modulation methods a compromise has to be made between selectivity and sensitivity for each measuring task.

Spectroscopy processing for the NO measurement based on the room-temperature pulsed quantum cascade laser

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.

Tunable Mid-IR lasers: A new avenue to robust and versatile physical chemosensors

A novel approach for a physical chemosensor for liquids based on mid-infrared (mid-IR) laser spectroscopy is presented. Mid-infrared sensor systems access chemical information directly through measuring vibrational spectra of the samples under investigation. The introduction of mid-IR Quantum Cascade Lasers (QCLs) with their high spectral power density opens up new possibilities in liquid phase sensing. This concept is predestined for transmission measurements in highly absorbing matrices, such as aqueous phases. In a first step a pulsed ExternalCavity QCL (EC-QCL), tunable from 1030–1230cm−1, was employed to perform transmission measurements of physiological solutions of path lengths >125μm, thus significantly improving measurement capabilities of mid-IR sensors employing thermal light sources. Furthermore, the signal-to-noise ratios achieved compete with those obtainable with state-of-the-art FT-IR spectrometers. Results on direct determination of glucose and lactate in aqueous solutions are reported.