Distributed Feedback Quantum Cascade Laser Research Articles

Optical feedback in dfb quantum cascade laser for mid-infrared cavity ring-down spectroscopy

A simple external optical feedback system has been applied to a distributed feedback quantum cascade laser (DFB QCL) for cavity ring-down spectroscopy (CRDS) and a clear effect of feedback was observed. A long external feedback path length of up to 4m can decrease the QCL linewidth to around 50kHz, which is of the order of the transmission linewidth of our high finesse ring-down cavity. The power spectral density of the transmission signal from high finesse cavity reveals that the noise at frequencies above 20kHz is reduced dramatically.

A CO trace gas detection system based on continuous wave DFB-QCL

A compact and mobile system was demonstrated for the detection of carbon monoxide (CO) at trace level. This system adopted a high-power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at ∼22°C as excitation source. Wavelength modulation spectroscopy (WMS) as well as second harmonic detection was used to isolate complex, overlapping spectral absorption features typical of ambient pressures and to achieve excellent specificity and high detection sensitivity. For the selected P(11) absorption line of CO molecule, located at 2099.083cm−1, a limit of detection (LoD) of 26ppb by volume (ppbv) at atmospheric pressure was achieved with a 1s acquisition time. Allan deviation analysis was performed to investigate the long term performance of the CO detection system, and a measurement precision of 3.4ppbv was observed with an optimal integration time of approximate 114s, which verified the reliable and robust operation of the developed system.

Low Power Consumption Distributed-Feedback Quantum Cascade Lasers Operating in Continuous-Wave Mode above 90°C at λ ∼ 7.2 μm**Supported by the National Basic Research Program of China under Grant No 2013CB632801, the National Key Research and Development Program under Grant No 2016YFB0402303, the National Natural Science Foundation of China under Grant Nos 61435014, 61627822, 61574136 and 61306058, the Key Projects of

We report on the design and fabrication of λ ∼ 7.2 μm distributed feedback quantum cascade lasers for very high temperature cw operation and low electrical power consumption. The cw operation is reported above 90°C. For a 2-mm-long and 10-μm-wide laser coated with high-reflectivity on the rear facet, more than 170 mW of output power is obtained at 20°C with a threshold power consumption of 2.4 W, corresponding to 30mW with a threshold power consumption of 3.9 W at 90°C. Robust single-mode emission with a side-mode suppression ratio above 25 dB is continuously tunable by the heat sink temperature or injection current.

Implementation of a quantum cascade laser-based gas sensor prototype for sub-ppmv H2S measurements in a petrochemical process gas stream

The implementation of a sensitive and selective as well as industrial fit gas sensor prototype based on wavelength modulation spectroscopy with second harmonic detection (2f-WMS) employing an 8-μm continuous-wave distributed feedback quantum cascade laser (CW-DFB-QCL) for monitoring hydrogen sulfide (H2S) at sub-ppm levels is reported. Regarding the applicability for analytical and industrial process purposes aimed at petrochemical environments, a synthetic methane (CH4) matrix of up to 1000 ppmv together with a varying H2S content was chosen as the model environment for the laboratory-based performance evaluation performed at TU Wien. A noise-equivalent absorption sensitivity (NEAS) for H2S targeting the absorption line at 1247.2 cm−1 was found to be 8.419 × 10−10 cm−1 Hz−1/2, and a limit of detection (LOD) of 150 ppbv H2S could be achieved. The sensor prototype was then deployed for on-site measurements at the petrochemical research hydrogenation platform of the industrial partner OMV AG. In order to meet the company’s on-site safety regulations, the H2S sensor platform was installed in an industry rack and equipped with the required safety infrastructure for protected operation in hazardous and explosive environments. The work reports the suitability of the sensor prototype for simultaneous monitoring of H2S and CH4 content in the process streams of a research hydrodesulfurization (HDS) unit. Concentration readings were obtained every 15 s and revealed process dynamics not observed previously.

High-Speed Mid-Infrared Frequency Modulation Spectroscopy Based on Quantum Cascade Laser

We demonstrated a high-speed mid-infrared frequency modulation spectroscopy scheme based on a distributed-feedback quantum Cascade laser for the application in trace gas sensing by means of all-optical frequency modulation. With this method, the modulation frequency spectrum of a gas sample can be obtained within the mid-infrared pulse duration of $\sim 200$ ns. A frequency modulation of the middle infrared lasing optical frequency was achieved in quantum Cascade laser with a modulation frequency of 200 MHz by illuminating with a 1550-nm near-infrared laser. For CO gas, with 2-mW near-infrared illumination, the noise-equivalent sensitivity of the frequency modulation spectroscopy was estimated to be 0.12 ppmv for an absorption length of 6.1 m, indicating an improvement by a factor of 7 compared with Voigt-fitted direct absorption spectroscopy (0.84 ppmv). The frequency modulation signal was found to be proportional to the incident near-infrared power, and therefore, the noise-equivalent sensitivity is expected to be further improved by increasing the near-infrared illumination power. This high-speed frequency modulation spectroscopy based on the distributed-feedback quantum Cascade laser has promising applications in high-speed and high-sensitivity gas sensing.

Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

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.

Open-path cavity ring-down spectroscopy sensor for atmospheric ammonia

An open-path cavity ring-down spectroscopy (CRDS) sensor has been developed for measurement of atmospheric ammonia (NH3) and represents the first use of open-path CRDS in the mid-infrared region. The sensor uses a continuous-wave distributed feedback quantum cascade laser at 10.33 μm to target strong absorption features. The optical cavity is constructed with two high-reflectivity mirrors (R = 0.9995). The open-path configuration removes inlet effects, which are very challenging for closed-path instruments, and can be enabling for compact, low-power designs. Sensor performance was validated in the laboratory by measuring known concentrations in a closed-path configuration. The open-path configuration was validated by comparison against a commercial closed-path CRDS instrument for outdoor measurements at a small feed lot. Ammonia concentrations from the two instruments showed good agreement with slope of 0.990 (R 2 = 0.92), for 5-min averages. The precision of the open-path instrument was found from Allan variance studies as 1.2 ppb (2-σ) for 3-s measurement durations.

Heterogeneously Integrated Distributed Feedback Quantum Cascade Lasers on Silicon

Silicon integration of mid-infrared (MIR) photonic devices promises to enable low-cost, compact sensing and detection capabilities that are compatible with existing silicon photonic and silicon electronic technologies. Heterogeneous integration by bonding III-V wafers to silicon waveguides has been employed previously to build integrated diode lasers for wavelengths from 1310 to 2010 nm. Recently, Fabry-Pérot Quantum Cascade Lasers integrated on silicon provided a 4800 nm light source for mid-infrared (MIR) silicon photonic applications. Distributed feedback (DFB) lasers are appealing for many high-sensitivity chemical spectroscopic sensing applications that require a single frequency, narrow-linewidth MIR source. While heterogeneously integrated 1550 nm DFB lasers have been demonstrated by introducing a shallow surface grating on a silicon waveguide within the active region, no mid-infrared DFB laser on silicon has been reported to date. Here we demonstrate quantum cascade DFB lasers heterogeneously integrated with silicon-on-nitride-on-insulator (SONOI) waveguides. These lasers emit over 200 mW of pulsed power at room temperature and operate up to 100 °C. Although the output is not single mode, the DFB grating nonetheless imposes wavelength selectivity with 22 nm of thermal tuning.

Quantum Cascade Laser Measurements of Line Intensities, N2-, O2- and Ar- Collisional Broadening Coefficients of N2O in the ν3 Band Near 4.5 µm.

This study deals with precise measurements of absolute line intensities, N2-, O2- and Ar- collisional broadening coefficients of N2O in the P-branch of the ν3 vibrational band near 4.5 µm. Collisional broadening coefficients of N2O-air are derived from the N2- and O2- broadening contributions by considering an ideal atmospheric composition. Studies are performed at room temperature for 10 rotational transitions over 2190-2202 cm(-1) spectral range using a distributed-feedback quantum cascade laser. To retrieve spectroscopic parameters for each individual transition, measured absorption line shape is simulated within Voigt and Galatry profiles. The obtained results compare well with previous experimental data available in the literature: the discrepancies being less than 4% for most of the probed transitions. The spectroscopic data reported here are very useful for the design of sensors used to monitor the abundance of N2O in earth's atmosphere.

Compact quantum cascade laser based quartz-enhanced photoacoustic spectroscopy sensor system for detection of carbon disulfide.

A compact gas sensor system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a continuous wave (CW) distributed feedback quantum cascade laser (DFB-QCL) operating at 4.59 µm was developed for detection of carbon disulfide (CS2) in air at trace concentration. The influence of water vapor on monitored QEPAS signal was investigated to enable compensation of this dependence by independent moisture sensing. A 1 σ limit of detection of 28 parts per billion by volume (ppbv) for a 1 s lock-in amplifier time constant was obtained for the CS2 line centered at 2178.69 cm-1 when the gas sample was moisturized with 2.3 vol% H2O. The work reports the suitability of the system for monitoring CS2 with high selectivity and sensitivity, as well as low sample gas volume requirements and fast sensor response for applications such as workplace air and process monitoring at industry.

Sensitive CH4 detection applying quantum cascade laser based optical feedback cavity-enhanced absorption spectroscopy.

We report on sensitive detection of atmospheric methane employing quantum cascade laser based optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS). An instrument has been built utilizing a continuous-wave distributed feedback quantum cascade laser (cw-QCL) with a V-shaped cavity, a common arrangement that reduces feedback to the laser from non-resonant reflections. The spectrometer has a noise equivalent absorption coefficient of 3.6 × 10-9 cm-1 Hz-1/2 for a spectral scan of CH4 at 7.39 μm. From an Allan-Werle analysis a detection limit of 39 parts per trillion of CH4 at atmospheric pressure within 50 s acquisition time was found.

Quartz-enhanced photoacoustic detection of ethylene using a 10.5 μm quantum cascade laser.

A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor has been developed for the sensitive detection of ethylene (C2H4) at 10.5 µm using a continuous-wave distributed-feedback quantum cascade laser. At this long-wavelength infrared, the key acoustic elements of quartz tuning fork and micro-resonators were optimized to improve the detection signal-to-noise ratio by a factor of >4. The sensor calibration demonstrated an excellent linear response (R2>0.999) to C2H4 concentration at the selected operating pressure of 500 and 760 Torr. With a minimum detection limit of 50 parts per billion (ppb) achieved at an averaging time of 70 s, the sensor has been deployed for measuring the C2H4 efflux during the respiration of biological samples in an agronomic environment.

Rf-modulation of mid-infrared distributed feedback quantum cascade lasers

We present the electrical and optical characterization and theoretical modeling of the transient behavior of regular 4.5-μm single-mode emitting distributed feedback (DFB) quantum cascade lasers (QCLs). Low residual capacitance together with a high-frequency optimized three-terminal coplanar waveguide configuration leads to modulation frequencies up to 23.5 GHz (optical) and 26.5 GHz (electrical), respectively. A maximum 3-dB cut-off value of 6.6 GHz in a microwave rectification scheme is obtained, with a significant increase in electrical modulation bandwidth when increasing the DC-current for the entire current range of the devices. Optical measurements by means of FTIR-spectroscopy and a heterodyne beating experiment reveal the presence of a resonance peak, due to coupling of the lasing DFB- with its neighboring below-threshold Fabry-Pérot-(FP-)mode, when modulating around the cavity roundtrip frequency. This resonance is modeled by a 2-mode Maxwell-Bloch formalism. It enhances only one sideband and consequently leads to the first experimental observation of the single-sideband regime in such kind of devices.

Chaotic light at mid-infrared wavelength.

The onset of nonlinear dynamics and chaos is evidenced in a mid-infrared distributed feedback quantum cascade laser both in the temporal and frequency domains. As opposed to the commonly observed route to chaos in semiconductor lasers, which involves undamping of the laser relaxation oscillations, quantum cascade lasers first exhibit regular self-pulsation at the external cavity frequency before entering into a chaotic low-frequency fluctuation regime. The bifurcation sequence, similar to that already observed in class A gas lasers under optical feedback, results from the fast carrier relaxation dynamics occurring in quantum cascade lasers, as confirmed by numerical simulations. Such chaotic behavior can impact various practical applications including spectroscopy, which requires stable single-mode operation. It also allows the development of novel mid-infrared high-power chaotic light sources, thus enabling secure free-space high bit-rate optical communications based on chaos synchronization.

Linewidth measurement of mid infrared quantum cascade laser by optical feedback interferometry

We estimated experimentally the linewidth (0.28 MHz) of a distributed feedback quantum cascade laser emitting at 6.2 μm using optical feedback interferometry. The method is simply based on the analysis of the histogram of laser self-mixing fringe periods measured directly as voltage variation across the laser terminals. We assessed the optimal experimental conditions estimating the influence of the optical feedback strength on the interferometric phase noise and compared our results with those reported using conventional interferometric methods based on the analysis of the frequency noise power spectral density.

3.36 µm single-mode quantum cascade laser with a dissipation below 250 mW.

We present 3.36 µm buried heterostructure distributed-feedback quantum cascade lasers with a power dissipation at threshold below 250 mW and operation temperatures as high as 130 °C. Threshold values below 20 mA at -10 °C in pulsed operation and 30 mA at -20 °C in continuous-wave operation are reported. Optical power above 130 mW and 13 mW are achieved at -20 °C in pulsed and continuous-wave operation, respectively. Continuous-wave operation occurs until 15 °C. We show single-mode emission in pulsed and continuous-wave operation. Single-mode performance is demonstrated in long pulse (5.56 µs) operation. The laser far-field exhibits a single lobe emission with full-width-half-max of 27 ° × 34 °.

Measurements of the linewidth enhancement factor of mid-infrared quantum cascade lasers by different optical feedback techniques

Precise knowledge of the linewidth enhancement factor of a semiconductor laser under actual operating conditions is of prime importance since this parameter dictates various phenomena such as linewidth broadening or optical nonlinearities enhancement. The above-threshold linewidth enhancement factor of a mid-infrared quantum cascade laser structure operated at 10∘C is determined experimentally using two different methods based on optical feedback. Both Fabry-Perot and distributed feedback quantum cascade lasers based on the same active area design are studied, the former by following the wavelength shift as a function of the feedback strength and the latter by self-mixing interferometry. The results are consistent and unveil a clear pump current dependence of the linewidth enhancement factor, with values ranging from 0.8 to about 3.

Efficient dielectric metasurface collimating lenses for mid-infrared quantum cascade lasers.

Light emitted from single-mode semiconductor lasers generally has large divergence angles, and high numerical aperture lenses are required for beam collimation. Visible and near infrared lasers are collimated using aspheric glass or plastic lenses, yet collimation of mid-infrared quantum cascade lasers typically requires more costly aspheric lenses made of germanium, chalcogenide compounds, or other infrared-transparent materials. Here we report mid-infrared dielectric metasurface flat lenses that efficiently collimate the output beam of single-mode quantum cascade lasers. The metasurface lenses are composed of amorphous silicon posts on a flat sapphire substrate and can be fabricated at low cost using a single step conventional UV binary lithography. Mid-infrared radiation from a 4.8 μm distributed-feedback quantum cascade laser is collimated using a polarization insensitive metasurface lens with 0.86 numerical aperture and 79% transmission efficiency. The collimated beam has a half divergence angle of 0.36° and beam quality factor of M2=1.02.

Development of Low Power Consumption DFB Quantum Cascade Lasers

We report on the development of $\lambda \sim 4.97~\mu \text{m}$ continuous-wave operation of substrate emitting distributed feedback quantum cascade lasers. A low threshold power consumption of 1.27 W is obtained at 20 °C, along with an ultralow threshold current density of 0.81 kA/cm2 by depositing the high reflection coatings on both facets. Single-lobed far-field radiation pattern with a low divergence angle of about 0.24° $\times $ 20.34° is obtained. Reliable single-mode emission with a side-mode suppression ratio about 25 dB is observed among the working temperature range of 10 °C–30 °C.

Frequency Ageing and Noise Evolution in a Distributed Feedback Quantum Cascade Laser Measured Over a Two-Month Period

We report on the evaluation of the frequency stability of a distributed feedback quantum cascade laser (QCL) at 8 μm that was continuously monitored over a 2-month period using a spectroscopic setup. The QCL was operated in continuous wave mode at room temperature (21.4 °C). It was driven by a home-made low-noise controller at a nominal current of ∼600 mA located in the middle of its operation range. Two distinctive behaviors were observed. A monotonous frequency drift of about 1.8 GHz was observed during slightly more than the first month, followed by a stable regime in the second month where the frequency remained within a 100 MHz range. In addition, the electrical flicker noise of the QCL was regularly measured during the same period, and similarly showed two different regimes. The noise regularly decreased at a small rate of about 0.3%/day during the first month, whereas it tends to stabilize during the second month. We believe that an improvement of the QCL contacts over time is responsible for this behavior. After the initial one-month period, the QCL showed a remarkably stable behavior that is beneficial for many applications that require stable long-term operation.