Continuous-wave Quantum Cascade Laser Research Articles

Infrared Photodissociation Spectroscopy of Water-Tagged Ions with a Widely Tunable Quantum Cascade Laser for Planetary Science Applications.

This work presents a benchtop method for collecting the room temperature gas phase infrared (IR) action spectra of protonated amino acids and their isomers. The adopted setup uses a minimally modified commercial electrospray ionization linear ion trap mass spectrometer (ESI-LIT-MS) coupled to a broadband continuous wave (cw) quantum cascade laser (QCL) source. This approach leverages messenger assisted action spectroscopic techniques using water-tagged molecular ions with complex formation, irradiation, and subsequent analysis, all taking place within a single linear ion trap stage. This configuration thus circumvents the use of multiple mass selection and analysis stages, cryogenic buffer cells, and complex high-power laser systems typically called upon to execute these techniques. The benchtop action spectrometer is used to collect the 935-1600 cm-1 (6.2-10.7 μm) IR action spectrum of a collection of amino acids and a dipeptide with results cross referenced against literature examples obtained with a free electron laser source. Recorded IR spectra are used for the analysis of binary mixture samples composed of constitutional isomers α-alanine and β-alanine with ratios determined to ∼4% measurement uncertainty without the aid of a front-end separation stage. This turn-key QCL-based approach is a major step in showing the viability of tag-based action spectroscopic techniques for use in future in situ planetary science sensors and general analytical applications.

New silicon-based micro-electro-mechanical systems for photo-acoustic trace-gas detection

The achievable sensitivity level of photo-acoustic trace-gas sensors essentially depends on the performances of the acoustic transducer. In this work, the mechanical response of different silicon-based micro-electro-mechanical systems (MEMS) is characterized, aiming at investigating both their mechanical properties, namely the resonance frequency and the quality factor, and the minimum detection limit (MDL) achievable when they are exploited as an acoustic-to-voltage transducer in a trace-gas photoacoustic setup. For this purpose, a 4.56µm Continuous-Wave (CW) quantum cascade laser (QCL) is used to excite a strong N2O roto-vibrational transition with a line strength of 2.14 × 10−19 cm/molecule, and the detection of MEMS oscillations is performed via an interferometric readout. As a general trend, the minimum detection limit decreases when the resonance frequency investigated increases, achieving a value of 15 parts per billion with a 3 dB cut-off lock-in bandwidth equal to 100 mHz, around 10 kHz.

Room Temperature, Continuous Wave Quantum Cascade Laser Grown Directly on a Si Wafer

We report the room temperature demonstration of a high power, continuous wave, LWIR quantum cascade laser grown directly on a Si substrate. A new wafer, based on a high efficiency, strain-balanced laser core was processed into a lateral injection buried heterostructure laser geometry. A pulsed efficiency of 11.1% was demonstrated at room temperature, with an emission wavelength of 8.35 μm. With low fidelity, epilayer-up packaging, CW emission up to 343 K was also demonstrated, with a maximum output power of >0.7 W near room temperature.

Examination of Mid-Infrared Photon Detection by Hot Electron Bolometer With Twin Slot Antenna

As a high-speed and sensitive mid-infrared (MIR) photon detector, we developed a MIR superconducting hot electron bolometer (MIR-HEB). The MIR-HEB consists of a twin-slot MIR-wavelength antenna and a small Nb/NbN superconducting strip located at the antenna's feeding point. To prevent the HEB from latching, a parallel shunt resistor was added. For detector evaluation, a continuous-wave quantum cascade laser with a wavelength of 4.57 μm was used as the source of MIR light. A constant bias current was applied near the HEB critical current during the detector operation. Upon illumination with MIR light, numerous voltage pulses with a width of approximately 4 ns were observed. Furthermore, it was demonstrated that the pulse count rates were nearly proportional to the incident power over a two-order magnitude range. We conclude that MIR-HEB achieved MIR photon detection based on this result. Also confirmed were the polarization dependences attributed to the antenna's operation.

Water vapor detection based on three-line TDLAS with a continuous wave quantum cascade laser

Water vapor is an important gas species in atmosphere. In this work, we present a three-line tunable diode laser absorption spectroscopy (TDLAS) gas sensor for water vapor detection based on direct absorption spectroscopy. A continuous wave quantum cascade laser (QCL) is employed as the light source with a wavelength near 7.15 µm. The output wavelength of the QCL is tuned to scan across three absorption lines of water vapor simultaneously in the detection. A detection limit of 10.7 ppm v (parts per million by volume) is obtained in 30-s integration time with a 40 cm gas cell. This detection limit could be further improved if wavelength modulation spectroscopy (WMS) or a multi-pass gas cell is introduced to the system.

Line positions and effective line strengths of trans-HONO near 1280 cm−1

The measurement of the line positions and effective line strengths of the ν3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm−1 (7.8 µm) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8–1282.2 cm−1 were determined using the HONO line strength at 1280.3841 cm−1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10−3 cm−1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO.

High bitrate data transmission in the 8-14 µm atmospheric window using an external Stark-effect modulator with digital equalization.

High bitrate mid-infrared links using simple (NRZ) and multi-level (PAM-4) data coding schemes have been realized in the 8 µm to 14 µm atmospheric transparency window. The free space optics system is composed of unipolar quantum optoelectronic devices, namely a continuous wave quantum cascade laser, an external Stark-effect modulator and a quantum cascade detector, all operating at room-temperature. Pre- and post-processing are implemented to get enhanced bitrates, especially for PAM-4 where inter-symbol interference and noise are particularly detrimental to symbol demodulation. By exploiting these equalization procedures, our system, with a full frequency cutoff of 2 GHz, has reached transmission bitrates of 12 Gbit/s NRZ and 11 Gbit/s PAM-4 fulfilling the 6.25 % overhead hard-decision forward error correction threshold, limited only by the low signal-to-noise ratio of our detector.

Mid-infrared absorption spectroscopy of ethylene at 10.5 µm using a chalcogenide hollow-core antiresonant fiber

We demonstrate the mid-infrared absorption spectroscopy with a chalcogenide glass IG3 hollow-core antiresonant fiber (HC-ARF) for gas sensing at 10.5 µm. A continuous-wave quantum cascade laser (CW-QCL) is adopted to detect the strong absorption line of ethylene (C2H4) centered at 949.5 cm−1 by coupling the laser beam into a chalcogenide HC-ARF of 22 cm in length. Both direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) are performed in this study for comparison. At the integration time of 0.1 s, the noise equivalent absorption (NEA) coefficient is determined to be 4.7 × 10-5 cm−1 for DAS and 5.1 × 10-6 cm−1 for WMS, respectively. Based on the Allan-Werle deviation analysis, the NEA coefficient of WMS can be further improved to 4.0 × 10-7 cm−1 by using a longer integration time of 80 s. The combination of QCLs and chalcogenide glass HC-ARFs provides a promising platform for mid-infrared gas sensing applications.

Measurement of nitric oxide spectral parameters: Considering the effects of CO2 and H2O

A continuous-wave quantum cascade laser (CW-QCL) based spectrometer is developed to investigate the high-resolution spectral parameters of nitric oxide (NO). Line strengths and nitrogen (N2)-, carbon dioxide (CO2)-, water vapor (H2O)-, broadening coefficients for NO R(6.5) are measured. The spectral region ranging from 1989 cm−1 to 1901 cm−1, which is suitable for the in situ laser sensing of trace NO, is investigated. Spectral parameters are determined by fitting absorption spectra with multi-peak Voigt profiles. The measured intensities are compared to the HITRAN2020 database to assess the performance of the system and a good agreement within ±5% is obtained for the established lines. The measurement results are useful for the design of a spectroscopic sensor for monitoring exhaled breath NO, vehicle exhaust, and industrial emissions.

Performance of a Mid-Infrared Sensor for Simultaneous Trace Detection of Atmospheric CO and N2O Based on PSO-KELM.

In this article, a field deployable sensor was developed using a self-developed 4.58-µm continuous wave quantum cascade laser (CW-QCL) for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O), both of which have strong fundamental absorption bands in this waveband. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) technology, which combined a multi-pass gas cell (MPGC) with a 41 m optical path length to achieve high-precision detection. Meanwhile, the particle swarm optimization-kernel extreme learning machine (PSO-KELM) algorithm was applied for CO and N2O concentration prediction. In addition, the self-designed board-level QCL driver circuit and harmonic signal demodulation circuit reduce the sensor cost and size. A series of validation experiments were conducted to verify the sensor performance, and experiments showed that the concentration prediction results of the PSO-KELM algorithm are better than those of the commonly used back propagation (BP) neural networks and partial least regression (PLS), with the smallest root mean square error (RMSE) and linear correlation coefficient closest to 1, which improves the detection precision of the sensor. The limit of detection (LoD) was assessed to be 0.25 parts per billion (ppb) for CO and 0.27 ppb for N2O at the averaging time of 24 and 38 s. Field deployment of the sensor was reported for simultaneous detection of CO and N2O in the air.

Fast-sweeping continuous wave quantum cascade laser operating in an external cavity with polygon mirror.

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm-1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm-1. The linewidth of the emitting modes is less than ∼0.05 cm-1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

Enhancement of photoacoustic spectroscopy with sorption enrichment for ppt-level benzene detection.

A real-time trace gas detector for benzene is demonstrated. The measurement system takes advantage of modest enrichment through short adsorption periods to reach a ppt-level detection limit with a sampling cycle of 90 s, which includes sample adsorption, desorption, and a spectroscopic measurement. Benzene is collected on Tenax TA sorbent for 30 s and then detected from the enriched samples with photoacoustic spectroscopy. High sensitivity is achieved using cantilever-enhanced photoacoustic spectroscopy and a continuous-wave quantum cascade laser emitting at 14.8 µm wavelength, which corresponds to the absorption wavelength of the strongest benzene infrared band. We reach a detection limit of 150 ppt of benzene, over one sampling cycle. Interference from humidity and other common petrochemicals is evaluated.

Application of continuous wave quantum cascade laser in combination with CIVP spectroscopy for investigation of large organic and organometallic ions.

Rapidly developing mid-infrared quantum cascade laser (QCL) technology gives easy access to broadly tunable mid-IR laser radiation at a modest cost. Despite several applications of QCL in the industry, its usage for spectroscopic investigation of synthetically relevant organic compounds has been limited. Here, we report the application of an external cavity, continuous wave, mid-IR QCL to cryogenic ion vibrational predissociation spectroscopy to analyze a set of large organic molecules, organometallic complexes, and isotopically labeled compounds. The obtained spectra of test molecules are characterized by a high signal-to-noise ratio and low full width at half-maximum-values, allowing the assignment of two compounds with just a few wavenumber difference. Data generated by cw-QCL and spectra produced by another standard Nd:YAG difference-frequency generation system are compared and discussed.

Time evolution of CO2 ro-vibrational excitation in a nanosecond discharge measured with laser absorption spectroscopy

CO2 dissociation stimulated by vibrational excitation in non-equilibrium discharges has drawn lots of attention. Nanosecond (ns) discharges are known for their highly non-equilibrium conditions. It is therefore of interest to investigate the CO2 excitation in such discharges. In this paper, we demonstrate the ability for monitoring the time evolution of CO2 ro-vibrational excitation with a well-selected wavelength window around 2289.0 cm−1 and a single continuous-wave quantum cascade laser with both high accuracy and temporal resolution. The rotational and vibrational temperatures for both the symmetric and the asymmetric modes of CO2 in the afterglow of CO2 + He ns-discharge were measured with a temporal resolution of 1.5 μs. The non-thermal feature and the preferential excitation of the asymmetric stretch mode of CO2 were experimentally observed, with a peak temperature of Tv 3, max = 966 ± 1.5 K, Tv 1,2, max = 438.4 ± 1.2 K and T rot = 334.6 ± 0.6 K reached at 3 μs after the nanosecond pulse. In the following relaxation process, an exponential decay with a time constant of 69 μs was observed for the asymmetric stretch (001) state, consistent with the dominant deexcitation mechanism due to VT transfer with He and deexcitation on the wall. Furthermore, a synchronous oscillation of the gas temperature and the total pressure was also observed and can be explained by a two-line thermometry and an adiabatic process. The period of the oscillation and its dependence on the gas components is consistent with a standing acoustic wave excited by the ns-discharge.

High-repetition-rate mid-IR femtosecond pulse synthesis from two mid-IR CW QCL-seeded OPAs

Coherent pulse synthesis in the mid-infrared (mid-IR) domain is of great interest to achieve broadband sources from parent pulses, motivated by the advantages of optical frequency properties for molecular spectroscopy and quantum dynamics. We demonstrate a simple mid-IR coherent synthesizer based on two high-repetition-rate optical parametric amplifiers (OPAs) at nJ-level pump energy. The relative carrier envelope phase between the two OPAs was passively stable for a shared continuous wave (CW) quantum cascade laser (QCL) seed. Lastly, we synthesized mid-IR pulses with a duration of 105 fs ranging from 3.4 to 4.0 µm. The scheme demonstrated the potential to obtain broader mid-IR sources by coherent synthesis from multiple CW QCL-seeded OPAs.

Output facet heating mechanism for uncoated high power long wave infrared quantum cascade lasers

Output facet temperatures of an uncoated high power continuous-wave quantum cascade laser (QCL) emitting at 8.5 μm were measured by using micro-Raman thermometry. The rate of the measured temperature changes with the injected electrical power increased from 6.5 K/W below the laser threshold to 12.3 K/W above the threshold. In addition, the measured temperature rise exceeded 220 K at an optical power of 0.9 W, well above the model projections based only on Joule heating. Facet oxidation was characterized via x-ray photoelectron spectroscopy measurements at incremental etch depths. While the oxidation reactions of InP and Ga were observed only at the surface level, the measured penetration of native Al2O3 was ∼24 nm. COMSOL thermal modeling demonstrated that light reabsorption by the native Al2O3 layer could well explain the additional temperature rise above the threshold. These results suggest that facet oxidation must be addressed to ensure the reliability of high-power long wave infrared QCLs.

Continuous wave quantum cascade lasers with 5.6 W output power at room temperature and 41% wall-plug efficiency in cryogenic operation

In this paper, we report a post-polishing technique to achieve nearly complete surface planarization for the buried ridge regrowth processing of quantum cascade lasers. The planarized device geometry improves the thermal conduction and reliability and, most importantly, enhances the power and efficiency in continuous wave operation. With this technique, we demonstrate a high continuous wave wall-plug efficiency of an InP-based quantum cascade laser reaching ∼41% with an output power of ∼12 W from a single facet operating at liquid nitrogen temperature. At room temperature, the continuous wave output power exceeds the previous record, reaching ∼5.6 W.

Detection of nitric oxide with Faraday rotation spectroscopy at 5.33 µm

We report the development of a static magnetic field Faraday rotation spectrometer for NO detection. A 5.33 µm continuous-wave quantum cascade laser was used as the probing laser. Line absorption at 1875.81 cm−1 (2Π3/2Q(3/2), υ=1←0) was chosen for the detection. By using a Chernin type multipass cell, a detection precision of 1.15 ppbv (1σ, 1s) was achieved with an absorption pathlength of 108 m. This value was reduced to 0.43 ppbv by increasing the data-acquisition time to 15 s.

High-repetition-rate femtosecond mid-infrared pulses generated by nonlinear optical modulation of continuous-wave QCLs and ICLs.

We demonstrate an effective method to obtain high-repetition-rate femtosecond mid-infrared (mid-IR) pulses by nonlinear optical modulation of mid-IR continuous-wave (CW) quantum and interband cascade lasers (ICLs and QCLs). In the experiment, a high-repetition-rate femtosecond ytterbium-doped fiber laser with nanojoule-level pulse energy was used as the pump source of optical parametric amplifiers to modulate and amplify the mid-IR CW laser. Near transform-limited 84 fs duration (7.3 cycles) mid-IR pulses were generated with above 200 mW average power and a repetition rate of 160 MHz. Moreover, the spectral tunability was demonstrated using CW QCL at different wavelengths. The scheme offered a simple method to produce high-repetition-rate ultrashort pulses and that can be flexibly adopted in other mid-IR regions.

CO2-broadening coefficients for the NO2 transitions at 6.2 µm measured by mid-infrared absorption spectroscopy

A continuous-wave (CW) quantum cascade laser (QCL)-based spectroscopic sensor is demonstrated to investigate the high-resolution spectra of 35 spectral lines of NO2 ranging from 1630.2 cm−1 to 1632.5 cm−1. The air-broadened half widths of two NO2 lines are compared with the HITRAN database to assess the performance of the system. The absorption spectra of the NO2 and CO2 mixtures at pressures of 10–90 mbar are measured to obtain the CO2-broadening coefficients. To the best of our knowledge, no measured CO2-broadening coefficients for the NO2 transitions in this region have been published previously. For signal processing of unresolved lines, a self-established spectral analysis algorithm integrated with the continuous wavelet transform is developed to calculate the second derivative for determining the number and position of individual peaks. The measurement results are useful for the design of a spectroscopic sensor for monitoring carbonated beverages production, tobacco puffing treatment, and food preservation.