Quantum Cascade Laser Array Research Articles

Compact Metasurface Terahertz Spectrometer

AbstractThe terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies and stars throughout the universe's history, as well as the process of planet formation. Detecting the unique spectral signatures of molecules and atoms requires terahertz spectrometers, which must be operated in space observatories due to water vapor absorption in the Earth's atmosphere. However, current terahertz spectrometers face challenges such as low resolution, limited bandwidth, large volume, and complexity. In this paper, the issues of size and weight are addressed by demonstrating a concept for a centimeter‐sized, low‐weight terahertz spectrometer using a metasurface. The design of the metasurface spectrometer is first discussed for the 1.85 to 2.4 THz range, followed by its fabrication. Next, an array of quantum cascade lasers operating at slightly different frequencies around 2.1 THz is utilized to characterize the spectrometer. Finally, a spectrum inversion method is applied to analyze the measured data, confirming a resolution R (λ/Δλ) of at least 273. This concept can be extended to other application areas, such as planetary observations and various wavelengths in the far‐infrared (FIR) and near‐infrared (NIR) ranges.

Phase-locked single-mode terahertz quantum cascade lasers array

We demonstrated a scheme of phase-locked terahertz quantum cascade lasers (THz QCLs) array, with a single-mode pulse power of 108 mW at 13 K. The device utilizes a Talbot cavity to achieve phase locking among five ridge lasers with first-order buried distributed feedback (DFB) grating, resulting in nearly five times amplification of the single-mode power. Due to the optimum length of Talbot cavity depends on wavelength, the combination of Talbot cavity with the DFB grating leads to better power amplification than the combination with multimode Fabry−Perot (F−P) cavities. The Talbot cavity facet reflects light back to the ridge array direction and achieves self-imaging in the array, enabling phase-locked operation of ridges. We set the spacing between adjacent elements to be 220 μm, much larger than the free-space wavelength, ensuring the operation of the fundamental supermode throughout the laser's dynamic range and obtaining a high-brightness far-field distribution. This scheme provides a new approach for enhancing the single-mode power of THz QCLs.

Monolithic beam combined quantum cascade laser arrays with integrated arrayed waveguide gratings.

Quantum cascade lasers (QCLs) are ubiquitous mid-infrared sources owing to their flexible designs and compact footprints. Manufacturing multiwavelength QCL chips with high power levels and good beam quality is highly desirable for many applications. In this study, we demonstrate an λ ∼ 4.9 µm monolithic, wavelength beam-combined (WBC) infrared laser source by integrating on a single chip array of five QCL gain sections with an arrayed waveguide grating (AWG). Optical feedback from the cleaved facets enables lasing, whereas the integrated AWG locks the emission spectrum of each gain section to its corresponding input channel wavelength and spatially combines their signals into a single-output waveguide. Our chip features high peak power from the common aperture exceeding 0.6 W for each input channel, with a side-mode suppression ratio (SMSR) of over 27 dB when operated in pulsed mode. Our active/passive integration approach allows for a seamless transition from the QCL ridges to the AWG without requiring regrowth or evanescent coupling schemes, leading to a robust design. These results pave the way for the development of highly compact mid-IR sources suitable for applications such as hyperspectral imaging.

Supersymmetric quantum cascade laser array

Electrically injected supersymmetric (SUSY) quantum cascade laser (QCL) array of the wavelength at around 13.4 μm has been demonstrated. By applying SUSY transformations to the main array with five laser elements, a lossy superpartner array with four laser elements is constructed. As a result, except for the fundamental supermode, which is confined in the main array, other high order supermodes penetrating into the superpartner array are suppressed. By selectively pumping the main array, the fabricated SUSY QCL array can emit light with a near-diffraction-limited (D.L.) beam over the whole dynamic range, with the full-width half-maximum up to 11.7° in the lateral direction and the output peak power up to 406 mW in pulsed operation at room temperature, which is 3.6 times the power of a single ridge laser of the same laser length. Furthermore, the far field tuning through the mode switching of fundamental and high order supermodes is realized experimentally, by electrically adjusting the gain and loss in the superpartner array, which agrees well with the designs. The electrically far field tunability has great potential to find applications in beam steering.

Long wavelength mid-infrared multi-gases spectroscopy using tunable single-mode slot waveguide quantum cascade laser.

Single-mode tunable quantum cascade lasers (QCLs) are promising for high-resolution and highly sensitive trace gases sensing across the mid-infrared (MIR) region. We report on the development of a tunable single-mode slot waveguide QCL array in the long wavelength part of the MIR regime (>12 µm). This laser array exhibits a tuning range of around 12 cm-1, from 735.3 to 747.3 cm-1. Using this developed single-mode tunable QCL, we demonstrate individual gas sensing, yielding the detection limit of 940 ppb and 470 ppb for acetylene and o-xylene, respectively. To verify the potential of the developed QCL array in multi-species gas detection, laser absorption measurements of two mixed gases of acetylene and o-xylene were conducted, showing the absorption features of the corresponding gases agree well with the theoretical predictions.

Stable single-mode operation of distributed feedback quantum cascade laser under high current via a grating reflector

Abstract We report on an index-coupled distributed feedback (DFB) quantum cascade lasers (QCLs) of high single-mode stability by using of a grating reflector (GR) to reflect effectively light of a specific wavelength. Fabrications of the QCLs were performed with λ∕4 equivalent phase shift (EPS) or sampled Moiré grating (SMG) structure, which regulates the light intensity distribution in the cavity and coupling strength. Both EPS-GR and SMG-GR QCLs demonstrate a high emission wavelength precision and a side-mode-suppression-ratio (SMSR) of 24.5 dB at the injection current of 5 A and the pulse width of 5 μs, offering considerable output power even at the edge of the gain spectrum. Furthermore, we investigated the arrays of EPS-GR and SMG-GR QCLs with lower threshold current and higher output power by introducing semi-insulated InP (Fe) ranging from 8.25 to 8.67 μm and 8.16–8.63 μm, respectively. The present QCLs via a GR are prospective for applications requiring single-mode stability and wide tunability.

Phase-locked terahertz quantum cascade laser array integrated with a Talbot cavity.

Increasing the power of a quantum cascade laser by widening laser ridges will lead to the degradation of the beam quality because of the operation of high-order transverse modes. We report on a phase-locked array scheme of terahertz quantum cascade laser (THz QCL) utilizing Talbot effect. By adjusting the absorbing boundary width of each ridge in the array, stable operation of the fundamental supermode is realized. A five-element array shows 4 times power amplification than that of a single ridge device. Due to the large power amplification efficiency, stable mode selection, and simple fabricating process, the phase-locked array scheme is very promising to further improve the performance of THz QCL.

Beam combining of a broadly and continuously tunable quantum cascade laser.

We report a cost-efficient method to demonstrate the beam combining of five laser elements in an array of tunable slot waveguide quantum cascade lasers in the mid-infrared region at around 10 µm. An aspherical lens with five fine-tuned mini mirrors was employed to collimate the individual beams from the laser array. To verify the feasibility of this beam combining approach, the combined beams were coupled into a hollow-core fiber gas cell with a low numerical aperture (N.A.) of 0.03 and a coupling efficiency >= 0.82, for gas sensing of binary compound gases of ammonia and ethylene simultaneously.

Anti-symmetric sampled grating quantum cascade laser for mode selection

For mode selection in a quantum cascade laser (QCL), we demonstrate an anti-symmetric sampled grating (ASG). The wavelength of the –1-th mode of this laser has been blue-shifted more than 75 nm (∼ 10 cm−1) compared with that of an ordinary sampled grating laser with an emission wavelength of approximately 8.6 μm, when the periodicities within both the base grating and the sample grating are kept constant. Under this condition, an improvement in the continuous tuning capability of the QCL array is ensured. The ASG structure is fabricated in holographic exposure and optical photolithography, thereby enhancing its flexibility, repeatability, and cost-effectiveness. The wavelength modulation capability of the two channels of the grating is insensitive to the variations in channel size, assuming that the overall waveguide width remains constant. The output wavelength can be tailored freely within a certain range by adjusting the width of the ridge and the material of the cladding layer.

Widely tunable single-mode slot waveguide quantum cascade laser array.

We report designs and experimental demonstrations of a widely tunable single-mode quantum cascade laser array based on slot waveguide structures in the mid-infrared region. The laser array device realized a continuous tuning range of 71 cm-1 from 9.66 μm to 10.37 μm at 300K only using the current tuning without any external heatsink temperature adjustments, in good agreement with the design. Stable single-mode operations free of undesired mode-hops have been obtained over the whole tuning range. Another slot waveguide QCL array with a 41 cm-1 continuous tuning range around 7.3 μm has also been realized with the same design principle, demonstrating the universal applicability of the array design. The broadly continuous tuning with simple processing makes the array device a suitable candidate for mid-infrared sensing and spectroscopy application.

Coupled-ridge waveguide quantum cascade laser array lasing at λ ~ 5 µm

In this work, we demonstrated high-power quantum cascade laser (QCL) arrays lasing at λ ~ 5 µm by employing an optimized coupled-ridge waveguide (CRW) structure. Five-element QCL arrays were simulated and fabricated through a two-step etching method to extend the CRW structure to a mid-wave infrared regime. A lateral far-field with the main peak near a diffraction-limited intensity curve of about 10° was observed by properly designing a geometric shape of the ridges and interspaces. By introducing a buried 2nd-order distributed feedback (DFB) grating, substrate emission with a radiation power above 1 W at 25 °C is achieved. Single longitudinal mode operation is obtained by changing the temperature of the heatsink with a good linear wavelength tuning coefficient of –0.2 cm–1/K.

Terahertz quantum cascade laser array with spatially-separated beams

A lens-coupled terahertz (THz) quantum cascade laser (QCL) array with multiple spatially-separated beams is presented. The beam divergence and focusing efficiency after lens are both investigated by simulations and experiments. By integrating an appropriate hyper-hemispherical silicon lens with a well-designed six-element distributed feedback (DFB) QCL array inside the cryostat window, excellent emission characteristics of small divergence angles from 4.1°×3.5° to 7.2°×5.8° and multi-wavelength spectra are realized at 77 K. The deflection angle between adjacent beams is ~ 10°. The output intensity after lens is 113 to 25 times that of original beams with a focusing efficiency ranging from ~ 58% to ~ 37% for lasers at different positions. This method provides a simple solution to form a multi-beam THz source with high intensity and low far-field divergence.

Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors.

Spectroscopic techniques based on Distributed FeedBack (DFB) Quantum Cascade Lasers (QCL) provide good results for gas detection in the mid-infrared region in terms of sensibility and selectivity. The main limitation is the QCL relatively low tuning range (~10 cm−1) that prevents from monitoring complex species with broad absorption spectra in the infrared region or performing multi-gas sensing. To obtain a wider tuning range, the first solution presented in this paper consists of the use of a DFB QCL array. Tuning ranges from 1335 to 1387 cm−1 and from 2190 to 2220 cm−1 have been demonstrated. A more common technique that will be presented in a second part is to implement a Fabry–Perot QCL chip in an external-cavity (EC) system so that the laser could be tuned on its whole gain curve. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy, where the gas sample is placed within the laser resonator. Moreover, a technique only using the QCL compliance voltage technique can be used to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed. Finally, a specific scheme using an EC coherent QCL array can be developed. All these widely-tunable Quantum Cascade-based sources can be used to demonstrate the development of optical gas sensors.

Portable broadband photoacoustic spectroscopy for trace gas detection by quantum cascade laser arrays.

We report a portable broadband photoacoustic spectroscopic system for trace gas detection using distributed feedback quantum cascade laser arrays. By sequentially firing 128 lasers, our system acquires a photoacoustic spectrum covering 565cm-1 (935-1500cm-1) with a normalized-noise-equivalent-absorption coefficient of 2.5×10-9cm-1WHz-1/2. The firing sequence that determines when and which laser to activate is programmable, which enables frequency-multiplexing excitation. For demonstration, 12 lasers are modulated simultaneously at distinct frequencies, and a photoacoustic spectrum is acquired within 13 ms. The compactness (28cm×17cm×13cm, 3.5 kg) and low power consumption enable convenient installation for on-site monitoring.

Broadband detection of methane and nitrous oxide using a distributed-feedback quantum cascade laser array and quartz-enhanced photoacoustic sensing

Here we report on the broadband detection of nitrous oxide (N2O) and methane (CH4) mixtures in dry nitrogen by using a quartz-enhanced photoacoustic (QEPAS) sensor exploiting an array of 32 distributed-feedback quantum cascade lasers, within a spectral emission range of 1190−1340 cm−1 as the excitation source. Methane detection down to a minimum detection limit of 200 ppb at 10 s lock-in integration time was achieved. The sensor demonstrated a linear response in the range of 200−1000 ppm. Three different mixtures of N2O and CH4 in nitrogen at atmospheric pressure have been analyzed. The capability of the developed QEPAS sensor to selectively determine the N2O and CH4 concentrations was demonstrated, in spite of significant overlap in their respective absorption spectra in the investigated spectral range.

Optically mutual-injected terahertz quantum cascade lasers for self-mixing velocity measurements.

Self-mixing velocity sensor based on a mutual-injected two-element terahertz quantum cascade laser (THz QCL) array is studied theoretically. The working characteristics of mutual-injected THz QCL array with different frequency detunings and self-mixing feedback strengths, as well as their influences on the self-mixing measurements are discussed in detail. Within the phase-locked range, each laser in the array reaches a stable state rapidly and can be used as a self-mixing detector due to the mutual injection coupling. The array will no longer be phase-locked when the frequency detuning of the lasers is too large, and only the laser that receives the feedback light can still be used for self-mixing velocity measurements. It is also found that even for the case of strong feedback, the THz QCLs will not be completely unstable and the self-mixing velocity measurements could also be possible. In addition, the simulation also shows that the array could measure two independent moving targets simultaneously. These results provide the theoretical support for the future applications of THz QCL arrays in self-mixing sensors.

Analysis of Mode Competition in Resonant Leaky-Wave Coupled Phase-Locked Arrays of Mid-IR Quantum Cascade Lasers

A numerical model is developed for a phase-locked array of quantum cascade lasers. The population density is derived from rate equations. The temperature distribution for stationary generation is produced by ohmic heating. It is shown that the results of above-threshold operation modeling by the semi-vectorial beam propagation method are in good agreement with the modal analysis provided by the vectorial-COMSOL solver supplemented with the Rigrod’s model estimations. The wall-plug efficiency and the limits of the single-mode lasing are found. Thermal lensing is shown to be the main reason limiting the single-spatial-mode power under CW-operating conditions.

High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm.

We report a room-temperature eight-element phase-locked quantum cascade laser array emitting at 8 µm with a high continuous-wave power of 8.2 W and wall plug efficiency of 9.5%. The laser array operates primarily via the in-phase supermode and has single-mode emission with a side-mode suppression ratio of ~20 dB. The quantum cascade laser active region is based on a high differential gain (8.7 cm/kA) and low voltage defect (90 meV) design. A record high wall plug efficiency of 20.4% is achieved from a low loss buried ridge type single-element Fabry-Perot laser operating in pulsed mode at 20 °C.

Basic phase-locking, noise, and modulation properties of optically mutual-injected terahertz quantum cascade lasers.

The phase-locking, noise, and modulation properties of two face-to-face optically mutual-injected terahertz (THz) quantum cascade lasers (QCLs) are analyzed theoretically. In the phase-locking range, the two THz QCLs are in stationary states working at the same frequency. Outside the phase-locking range, the amplitude and the instantaneous frequency of the optical field oscillate with time, and the power spectrum shows a series of discrete peaks. For strong mutual injection, the optical field of the THz QCL array also exhibits oscillatory behavior. Coherent collapse or chaotic behavior is not observed within the range of the parameters used in this simulation. The spontaneous emission noise of phase-locked THz QCLs is higher than that of THz QCLs at free-running operation, and mutual injection may introduce additional modulation peaks in the noise spectrum. The modulation response of the mutual-injected THz QCLs to an individual modulation is investigated. It is found that the modulation bandwidth and the phase difference are significantly dependent on the modulation parameters. These results are helpful for further understanding the nonlinear dynamic behaviors of THz QCLs under optical injection and provide theoretical support for the development of THz QCL phase-locked arrays.

Thermal imaging of buried heterostructure quantum cascade lasers (QCLs) and QCL arrays using CCD-based thermoreflectance microscopy

Thermal profiles of a single-element quantum cascade laser (QCL) and a five-element QCL array at different bias currents, under quasi-continuous-wave (QCW) conditions, are obtained using the charge-coupled device (CCD)-based thermoreflectance imaging technique. Peak temperature changes of 55 K and 105 K are measured on the single-element QCL (operating at 1.2 A) and the central element of the QCL array (operating at 4.2 A), respectively. The average facet temperature of the single QCL device shows a linear relationship with the dissipated power, indicating an effective thermal resistance of Rth = 3.0 ± 0.2 K/W (7%) for the device. The thermal transient behavior of the single QCL device, in response to a 35 μs-wide heating pulse, is also measured. From the transient curve, an effective thermal time constant of τth = 9.5 ± 0.4 μs (4%) is obtained. Experimental results are compared to the results obtained from heat-transfer models for both the single-element and array devices. Thermal profiles show a thermal lensing effect at the facet of the single-element QCL. In the array device, a more pronounced heating is observed at the center of the device while the temperature gradually decreases away from the central element.