Pulsed Quantum Cascade Laser Research Articles

Long-wave infrared pulsed external-cavity QCL spectrometer using a hollow waveguide gas cell

A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 – 13 µm (1,000 – 769 cm−1) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cell which has an internal volume of a few millilitres, a fast response time (∼1 s), and is advantageous when only low sample volumes, similar to the cell volume, are available. Propane is used as a test gas because it is easy to handle, and its spectral profile is comparable to VOCs of interest. Its absorption in the region is primarily within the ν21 band which spans from 10.55 – 11.16 µm (948 – 896 cm−1). Spectral measurements at a range of concentrations show good linearity and an Allan deviation of absorbance values recorded over a 100-minute period indicates a minimum detectable absorbance of 3.5×10−5 at an integration time of 75 s.

Optical and thermal simulation design of tapered waveguide for high-power pulsed quantum cascade laser

Optical and thermal simulation design of tapered waveguide for high-power pulsed quantum cascade laser

Active Region Overheating in Pulsed Quantum Cascade Lasers: Effects of Nonequilibrium Heat Dissipation on Laser Performance

Mid IR Quantum cascade lasers are of high interest for the scientific community due to their unique applications. However, the QCL designs require careful engineering to overcome some crucial disadvantages. One of them is active region (ARn) overheating, which significantly affects laser characteristics, even in the pulsed mode. In this work, we consider the effects related to the nonequilibrium temperature distribution when thermal resistance formalism is irrelevant. We employ the heat equation and discuss the possible limitations and structural features stemming from the chemical composition of the ARn. We show that the presence of solid solutions in the ARn structure fundamentally limits the heat dissipation in pulsed and CW regimes due to their low thermal conductivity compared with binary compounds. Also, the QCL postgrowths affect the thermal properties of a device closer to CW mode, while it is by far less important in the short-pulsed mode.

Rapid and non-invasive detection of high-thickness glucose solution concentrations using quantum cascade laser-based transmission infrared spectroscopy

Most existing spectroscopic methods measure the concentration of blood glucose using attenuated total reflection. However, the short penetration depth (in the range of 2–15 μm) is a severe limitation. In practice, this approach is difficult to use because the depth of blood vessels in the skin exceeds 0.2 mm. As a solution, this work introduces a transmission spectroscopic method to measure the concentration of glucose. A fast infrared spectroscopic glucose concentration measurement method using a mid-infrared wavelength-swept pulsed quantum cascade laser (QCL) is adopted. This method employs a transmission optics system that does not require complicated adjustments or maintenance, together with an asynchronous signal method developed to shorten the measurement time. The combination of the high power of the QCL with the proposed algorithm enables measurements of glucose solution samples with an optical path length of 0.2 mm, which is sufficient for obtaining absorbance spectra and calculating the concentration of glucose in these samples. A linear regression of the average absorbance of the different glucose concentrations yields R2 values as high as 0.999. Moreover, using the asynchronous signal–based method reduces the single measurement time to 20 ms, and a 30 × integration required only 0.6 s. The proposed transmission method not only enables detection on the skin but also exhibits potential for measuring tear film on the eyeball in the tangential direction. This has potential applications in the diagnosis and management of diabetes.

Observation of Long Turn-On Delay in Pulsed Quantum Cascade Lasers

We present an experimental study of the turn-on delay in pulsed mid-infrared quantum cascade lasers. We report the unexpectedly long delay time depending on the pumping current, which does not agree with conventional theoretical predictions for step-like excitation. Similar behavior has been observed in InP- and InAs-based QCLs emitting near 8${\mu}$m. Numerical simulations performed using a model based on rate equations for excitation by current pulses with non-zero rise time provide fair agreement with our observations.

High-speed mid-infrared spectrometer based on wavelength-swept quantum cascade laser using asynchronous-signal method

A high-speed mid-infrared spectrometer using a wavelength-swept, pulsed quantum cascade laser (QCL) as the light source is introduced in this study. The QCL uses a microelectromechanical-system (MEMS)-based scanning grating to sweep the wavelength range of the pulsed light. To obtain a continuous spectrum from a comb-shaped spectrum, we investigated an asynchronous-signal method that does not require phase-shift adjustment using a function generator. The measurement speed of the proposed system is improved and reaches a maximum of 20 ms, which is 100 times faster than that of the conventional synchronous-signal system. Thus, the QCL spectra can be obtained in almost real time, which is beneficial for analyzing several fast reaction processes. Compared with Fourier transform infrared (FTIR) systems, the proposed high-speed mid-infrared spectrometer is smaller and provides measurements faster. Owing to its advantages, the QCL system can be employed in several applications that require mid-infrared spectrum sensing technology to replace or complement FTIR systems.

Data Link with a High-Power Pulsed Quantum Cascade Laser Operating at the Wavelength of 4.5 µm.

This article is a short study of the application of high-power quantum cascade lasers and photodetectors in medium-infrared optical wireless communications (OWC). The link range is mainly determined by the transmitted beam parameters and the performance of the light sensor. The light power and the photodetector noise directly determine the signal-to-noise power ratio. This ratio could be maximized in the case of minimizing the radiation losses caused by atmospheric attenuation. It can be obtained by applying both radiation sources and sensors operated in the medium infrared range decreasing the effects of absorption, scattering or scintillation, beam spreading, and beam wandering. The development of a new class of laser sources based on quantum cascade structures becomes a prospective alternative. Regarding the literature, there are descriptions of some preliminary research applying these lasers in data transmission. To provide a high data transfer rate, continuous wave (cw) lasers are commonly used. However, they are characterized by low power (a few tens of mWatts) limiting their link range. Also, only a few high-power pulsed lasers (a few hundreds of mWatts) were tested. Due to their limited pulse duty cycle, the obtained modulation bandwidth was lower than 1 MHz. The main goal of this study is to experimentally determine the capabilities of the currently developed state-of-the-art high-power pulsed quantum cascade (QC) lasers and photodetectors in OWC systems. Finally, the data link range using optical pulses of a QC laser of ~2 W, operated at the wavelength of ~4.5 µm, is discussed.

Time-resolved measurements of light–current characteristic and mode competition in pulsed THz quantum cascade laser

Quantum cascade lasers (QCL) are widely adopted as prominent and easy-to-use solid-state sources of terahertz radiation. Yet some applications require generation and detection of very sharp and narrow terahertz-range pulses with a specific spectral composition. We have studied time-resolved light-current (L–I) characteristics of multimode THz QCL operated with a fast ramp of the injection current. Detection of THz pulses was carried out using an NbN superconducting hot-electron bolometer with the time constant of the order of 1 ns while the laser bias current was swept during a single driving pulse. A nonmonotonic behavior of the L–I characteristic with several visually separated subpeaks was found. This behavior is associated with the mode competition in THz QCL cavity, which we confirm by L–I measurements with use of an external Fabry–Perot interferometer for a discrete mode selection. We also have demonstrated the possibility to control the L–I shape with suppression of one of the subpeaks by simply adjusting the off-axis parabolic mirror for optimal optical alignment for one of the laser modes. The developed technique paves the way for rapid characterization of pulsed THz QCLs for further studies of the possibilities of using this approach in remote sensing.

High-sensitivity and low-interference gas analyzer with feature extraction from mid-infrared laser absorption-modulated signal

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 N2O 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.

Intra-pulse H2O absorption diagnostic for temperature sensing in a rapid compression machine

An in-situ temperature diagnostic based on intra-pulse absorption spectroscopy has been developed using two pulsed quantum cascade lasers (QCLs) centered at 5.46 and 5.60 μm for rapid compression machine (RCM) experiments. Pulsed mode operation of the QCLs yielded a broad spectral tuning range (1.8–2.3 cm−1), through which spectral line-shapes of two H2O ro-vibrational transitions were resolved at high pressure conditions in the RCM (15–20 bar). Based on the resolved line-shapes, a calibration-free two-line thermometry method was used to determine the gas temperature. A high temporal resolution of 10 μs was achieved through a pulse repetition frequency of 100 kHz. The diagnostic was validated through measurements of temperature rise during the first-stage ignition of n-pentane/air mixtures. Thereafter, temperature rise during the first-stage ignition of iso-octane/air mixtures was quantified for the first time and compared with the calculated temperature rise using a chemical kinetic model.

Terahertz optical scanning imaging of motionless polyurethane insulation materials

The authors demonstrate a terahertz optical scanning imaging system which utilises a combined 3D translation platform. Three mirrors are installed on the three sliders of the platform to realise the terahertz beam scanning of the motionless object. A pulsed quantum-cascade laser at the frequency of 4.3 THz and a fast quantum-well photodetector are employed in this terahertz imaging system to research the polyurethane insulation materials. A Fourier transform spectrometer is applied to measure the transmittance of the polyurethane insulation materials from 2 to 8 THz. The terahertz images of the polyurethane foams with a badge or three drops of water are obtained with great contrast.

Photoacoustic remote sensing of suspicious objects for defence and forensic applications.

Quantum cascade laser (QCL) based photoacoustic spectroscopic technique has been developed for detection of hazardous molecules contaminants/adsorbed on surfaces such as plastic and cloth from short standoff distances. The laser source and detection system is integrated together in a single unit. Spectra were recorded for traces of various molecules in mid-infrared spectral band (1130-1430 cm-1) from distance of 0.5 m. Pulsed quantum cascade laser source, modulated at 25 kHz and 42 kHz frequency was used to detect molecular species adsorbed on surfaces of cloth and plastic. Ultrasonic microphones operating at 25 and 42 kHz resonant frequencies were used as detectors. The photoacoustic spectra of hazardous chemicals, explosive materials and bio-chemicals such as acetic acid, PETN (pentaerythritol tetranitrate), DPA (dipicolinic acid) in very low quantities were recorded. Sensitivities of 5 and 10 μg/cm2 of these analytes were achieved with frequencies of 25 and 42 kHz respectively at distance of 0.5 m. In the present technique there was no interference of audio frequency (20 Hz to 20 kHz) and bright sunlight. The technique can be applied for screening of suspicious objects for homeland security and forensic applications. The present spectroscopic technique can be developed in man portable standalone product.

Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information.

For the first time, a standard rapid-scan Fourier-transform infrared (FT-IR) spectrometer was coupled with quantum cascade lasers (QCLs) tunable within the 1876-905 cm-1 spectral range, within one single setup, by keeping one single sample compartment. The aim was to extend the time resolution of absorption measurements by several orders of magnitude thanks to the fast pulsed QCL technology without losing the spectral information provided by standard FT-IR spectroscopy, both probing the same sample. By slightly modifying the optical bench arrangement, the spectrometer now enables a fast and easy switch between the standard FT-IR mode, used for classical broadband scans from 6000 to 650 cm-1, and the new QCL-irradiation mode, used for ultrafast recording at specific wavenumbers (the two diagnostics have superimposed beam paths). So, one can study a sample (in condensed or gaseous state) during a physical or chemical transformation first as a whole in a broadband configuration and then immediately switch to the QCL mode to monitor a selected absorption feature (associated with an intermediate, a structural change, a diffusing substance, etc., for example) versus time. The QCL mode then drastically boosts the time resolution from tens of milliseconds (in rapid-scan FT-IR) to a few microseconds, as demonstrated here in the case of ammonia diffusion into a commercial zeolite ZSM-5.

Cavity-enhanced absorption sensor for carbon monoxide in a rapid compression machine

Abstract A sensor based on cavity-enhanced absorption spectroscopy (CEAS) was implemented for the first time in a rapid compression machine (RCM) for carbon monoxide concentration measurements. The sensor consisted of a pulsed quantum cascade laser (QCL) coupled to a low-finesse cavity in the RCM using an off-axis alignment. The QCL was tuned near 4.89 µm to probe the P(23) ro-vibrational line of CO. The pulsed mode operation resulted in rapid frequency down-chirp (6.52 cm−1/µs) within the pulse as well as a high time resolution (10 µs). The combination of rapid frequency down-chirp and off-axis cavity alignment enabled a near complete suppression of the cavity coupling noise. A CEAS gain factor of 133 was demonstrated in experiments, resulting in a much lower noise-equivalent detection limit than a single-pass arrangement. The sensor thus presents many opportunities for measuring CO formation at low temperatures and for studying kinetics using dilute reactive environments; one such application is demonstrated in this work using dilute n-heptane/air mixtures in the RCM. The formation of CO during first-stage ignition of n-heptane was measured over 802–899 K at a nominal pressure of 10 bar. These conditions correspond to the NTC region of n-heptane and such results provide useful metrics to test and compare the predictions of low-temperature heat release by different kinetic models.

Ultrasonic Standoff Photoacoustic Sensor for the Detection of Explosive and Hazardous Molecules

<p>Standoff photoacoustic spectroscopic technique has been studied for the detection of hazardous molecules adsorbed on surfaces and in vapour/aerosols form in open air. Detection and identification of components in explosive mixtures in trace amounts is very challenging by any point or standoff spectroscopic detection technique. Discusses detection and identification of such components using standoff laser photoacoustic spectroscopic technique. Laser photoacoustic spectra of various trace molecules in the mid-infrared spectral band 7 μm - 9 μm have been recorded in vapor, aerosol, liquid forms as well as samples adsorbed on surfaces such as plastic and cloth. Pulsed quantum cascade laser is modulated at a frequency of 42 kHz resonant with that of microphone. Hazardous chemicals/explosives adsorbed on plastic and cloths surfaces were detected from a standoff distance up to 1.5 m. The sensitivities were found to be 20 μg/cm2, 20 μl liquid and 1.0 ppm corresponding to solid, liquid and vapour phases respectively. The chemicals/explosives used in the study were PETN, DNT, Acetone, and DMMP. Our study suggests that the photoacoustic technique has high selectivity and sensitivity for the trace detection and be used for screening of suspicious objects for security applications as a handy product.</p>

Intra-pulse laser absorption sensor with cavity enhancement for oxidation experiments in a rapid compression machine.

A sensor based on a mid-IR pulsed quantum cascade laser (QCL) and off-axis cavity enhanced absorption spectroscopy (OA-CEAS) has been developed for highly sensitive concentration measurements of carbon monoxide (CO) in a rapid compression machine. The duty cycle and the pulse repetition rate of the laser were optimized for increased tuning range, high chirp rate, and small line width to achieve effective laser-cavity coupling. This enabled spectrally resolved CO line-shape measurements at high pressures (P ~10 bar). A gain factor of 133 and a time resolution of 10 μs were demonstrated. CO concentration-time profiles during the oxidation of highly dilute n-octane/air mixtures were recorded, illustrating new opportunities in RCM experiments for chemical kinetics.

Measurement of automobile N2O from bag and continuous stream by quantum cascade laser spectroscopy

N2O emission from automobile has been regulated by the United States Environment Protection Agency due to its higher global warming potential compared to CO2. Responding to this, an instrument for fast sensing of ultra-low concentration of N2O from automobile has become an urgent need. In this study, an instrument based on the quantum cascade laser spectroscopy has been developed and applied for certification testing of ultra-low N2O in automobile exhaust. The pulsed quantum cascade laser can emit coherent lights in the mid-infrared region where N2O shows strong absorption and better control of wavelength. Therefore, a very low detection limit can be achieved and interference of co-existing gases can be avoided using a super fine resolution of the mid-infrared spectrum. The US Code of Federal Regulations Parts 1065/1066 allows measuring N2O from sample storage bags, from a continuous dilute stream or a raw exhaust stream. Typically, batch (bag) sampling has better accuracy and repeatability, but continuous sampling is more efficient in terms of test cell running time and provides test-mode emissions. In this study, the quantum cascade laser analyzer has been applied to investigate correlations between bag sampling and continuous dilute exhaust sampling using a fleet of vehicles with a wide range of N2O emission levels all meeting United States Environment Protection Agency emission standards. Very good correlation between these two sampling methods was observed for the majority of tests conducted and in best case the difference was less than ±1%. The direct injection gasoline vehicle emits higher N2O than conventional port injection gasoline vehicles. The quantum cascade laser analyzer has been successfully applied for United States Environment Protection Agency N2O certification test.

Homogeneous spectral broadening of pulsed terahertz quantum cascade lasers by radio frequency modulation.

The authors present an experimental investigation of radio frequency modulation on pulsed terahertz quantum cascade lasers (QCLs) emitting around 4.3 THz. The QCL chip used in this work is based on a resonant phonon design which is able to generate a 1.2 W peak power at 10 K from a 400-µm-wide and 4-mm-long laser with a single plasmon waveguide. To enhance the radio frequency modulation efficiency and significantly broaden the terahertz spectra, the QCLs are also processed into a double-metal waveguide geometry with a Silicon lens out-coupler to improve the far-field beam quality. The measured beam patterns of the double-metal QCL show a record low divergence of 2.6° in vertical direction and 2.4° in horizontal direction. Finally we perform the inter-mode beat note and terahertz spectra measurements for both single plasmon and double-metal QCLs working in pulsed mode. Since the double-metal waveguide is more suitable for microwave signal transmission, the radio frequency modulation shows stronger effects on the spectral broadening for the double-metal QCL. Although we are not able to achieve comb operation in this work for the pulsed lasers due to the large phase noise, the homogeneous spectral broadening resulted from the radio frequency modulation can be potentially used for spectroscopic applications.

Multi-spectral terahertz sensing: proposal for a coupled-cavity quantum cascade laser based optical feedback interferometer.

We propose a laser feedback interferometer operating at multiple terahertz (THz) frequency bands by using a pulsed coupled-cavity THz quantum cascade laser (QCL) under optical feedback. A theoretical model that contains multi-mode reduced rate equations and thermal equations is presented, which captures the interplay between electro-optical, thermal, and feedback effects. By using the self-heating effect in both active and passive cavities, self-mixing signal responses at three different THz frequency bands are predicted. A multi-spectral laser feedback interferometry system based on such a coupled-cavity THz QCL will permit ultra-high-speed sensing and spectroscopic applications including material identification.

Broadband Mid-Infrared Stand-Off Reflection-Absorption Spectroscopy Using a Pulsed External Cavity Quantum Cascade Laser.

Broadband mid-infrared molecular spectroscopy is essential for detection and identification of many chemicals and materials. In this report, we present stand-off mid-infrared spectra of 1,3,5-trinitro-1,3,5-triazine or cyclotrimethylene trinitramine (RDX) residues on a stainless-steel surface measured by a broadband external cavity quantum cascade laser (QCL) system. The pulsed QCL is continuously scanned over 800 cm-1 in the molecular fingerprint region and the amplitude of the reflection signal is measured by either a boxcar-averager-based scheme or a lock-in-amplifier-based scheme with 1 MHz and 100 kHz quartz crystal oscillators. The main background noise is due to the laser source instability and is around 0.1% of normalized intensity. The direct absorption spectra have linewidth resolution around 0.1 cm-1 and peak height sensitivity around 10-2 due to baseline interference fringes. Stand-off detection of 5-50 µg/cm2 of RDX trace adsorbed on a stainless steel surface at the distance of 5 m is presented.