Wavelength Quantum Cascade Lasers Research Articles

Novel coding schemes for dual wavelength quantum cascade laser‐based free space optical links

AbstractDual wavelength quantum cascade lasers (DW‐QCLs) are a type of semiconductor laser that can emit two different wavelengths of light simultaneously. They are constructed using quantum cascade structures, consisting of multiple layers of semiconductor materials carefully engineered to produce the desired wavelengths of light. This research article reports the numerical simulation and evaluation of the maximum capacity of free space optical (FSO) link employing novel coding schemes for DW‐QCL‐based pulsed optical transmitters. A 48‐stage DW‐QCL emitting at 10.5 and 8.9 μm wavelengths and a Quantum‐Well based Infrared Photodetector based receiver are considered for the study. The performance of the link is analyzed under short pulse transmission from the device at both wavelengths for various scenarios. Optical pulses are obtained either at a single wavelength or two different wavelengths by proper choice of electrical input current amplitude. This feature enables the implementation of four different coding schemes. Among the various coding approaches proposed, the scheme, which operates on symbols, performs the best, providing a maximum link length of 1700m, for a BER of 10−9.

High Power, Room Temperature InP-Based Quantum Cascade Laser Grown on Si

We report on the realization of an InP-based long wavelength quantum cascade laser grown on top of a silicon substrate. This demonstration first required the development of an epitaxial template with a smooth surface, which combines two methods of dislocation filtering. Once wafer growth was complete, a lateral injection buried heterostructure laser geometry was employed for efficient current injection and low loss. The laser emits at a wavelength of ~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.8~\mu \text{m}$ </tex-math></inline-formula> and is capable of operation above 373 K, with a high peak power (>4 W) at room temperature. Laser threshold behavior with temperature is characterized by a T0 of 178 K. The far field beam shape is single lobed, showing fundamental transverse mode operation.

A Single Wavelength Mid-Infrared Photoacoustic Spectroscopy for Noninvasive Glucose Detection Using Machine Learning.

According to the International Diabetes Federation, 530 million people worldwide have diabetes, with more than 6.7 million reported deaths in 2021. Monitoring blood glucose levels is essential for individuals with diabetes, and developing noninvasive monitors has been a long-standing aspiration in diabetes management. The ideal method for monitoring diabetes is to obtain the glucose concentration level with a fast, accurate, and pain-free measurement that does not require blood drawing or a surgical operation. Multiple noninvasive glucose detection techniques have been developed, including bio-impedance spectroscopy, electromagnetic sensing, and metabolic heat conformation. Nevertheless, reliability and consistency challenges were reported for these methods due to ambient temperature and environmental condition sensitivity. Among all the noninvasive glucose detection techniques, optical spectroscopy has rapidly advanced. A photoacoustic system has been developed using a single wavelength quantum cascade laser, lasing at a glucose fingerprint of 1080 cm for noninvasive glucose monitoring. The system has been examined using artificial skin phantoms, covering the normal and hyperglycemia blood glucose ranges. The detection sensitivity of the system has been improved to mg/dL using a single wavelength for the entire range of blood glucose. Machine learning has been employed to detect glucose levels using photoacoustic spectroscopy in skin samples. Ensemble machine learning models have been developed to measure glucose concentration using classification techniques. The model has achieved a 90.4% prediction accuracy with 100% of the predicted data located in zones A and B of Clarke’s error grid analysis. This finding fulfills the US Food and Drug Administration requirements for glucose monitors.

Tunable single-mode chip-scale mid-infrared laser

Portable mid-infrared (mid-IR) spectroscopy and sensing applications require widely tunable, chip-scale, single-mode sources without sacrificing significant output power. However, no such lasers have been demonstrated beyond 3 μm due to the challenge of building tunable, high quality-factor (Q) on-chip cavities. Here we demonstrate a tunable, single-mode mid-IR laser at 3.4 μm using a tunable high-Q silicon microring cavity and a multi-mode Interband Cascade Laser. We achieve single-frequency lasing with 0.4 mW output power via self-injection locking and a wide tuning range of 54 nm with 3 dB output power variation. We further estimate an upper-bound effective linewidth of 9.1 MHz and a side mode suppression ratio of 25 dB from the locked laser using a scanning Fabry-Perot interferometer. Our laser platform based on a tunable high-Q microresonator can be expanded to higher wavelength quantum-cascade lasers and lead to the development of compact, high-performance mid-IR sensors for spectroscopic applications.

High-brightness LWIR quantum cascade lasers.

Long-wave infrared (LWIR, λ≈8-12µm) quantum cascade lasers (QCLs) are drawing increasing interest, as they provide the possibility of long-distance transmission of light through the atmosphere owing to the reduced water absorption. However, their development has been lagging behind the shorter wavelength QCLs due to much bigger technological challenges. In this Letter, through band structure engineering based on a highly localized diagonal laser transition strategy and out-coupler design using an electrically isolated taper structure, we demonstrate high beam-quality single-mode LWIR QCLs with high-brightness (2.0MWcm-2sr-1 for λ≈10µm, 2.2MWcm-2sr-1 for λ≈9µm, 5.0MWcm-2sr-1 for λ≈8µm) light extraction from a single facet in continuous-wave operation at 15°C. These results mark an important milestone in exploring the lighting capability of inter-sub-band semiconductor lasers in the LWIR spectral range.

Long wavelength (λ &amp;gt; 13 μm) quantum cascade laser based on diagonal transition and three-phonon-resonance design

An InP-based quantum cascade laser structure emitting at a wavelength of 13.6 μm is proposed and demonstrated. The active region is based on a diagonal transition and three-phonon-resonance design. A 5 mm long, 30 μm wide high-reflection coated device with a double channel ridge waveguide structure has shown a threshold current density of 3.0 kA/cm2, a dynamic range of 4.4 kA/cm2, a peak output power close to 1 W, and an average optical power up to 11.7 mW at 3% duty cycle, at 293 K. The laser shows a characteristic temperature T0 of 314 K and T1 of 336 K over a temperature range from 283 to 313 K.

A Sensitive Carbon Dioxide Sensor Based on Photoacoustic Spectroscopy with a Fixed Wavelength Quantum Cascade Laser.

A photoacoustic spectroscopy (PAS) based carbon dioxide (CO2) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 μm in the mid-infrared spectral region matched a fundamental CO2 absorption line. Amplitude modulation of the laser intensity was used to match the resonant photoacoustic (PA) cell. The noise from the background was reduced with the correlation demodulation technique. The experimental results showed that the sensor had excellent signal stability and a concentration linear response. When the integration time was 1 s, a 1σ minimum detection limit (MDL) of 2.84 parts per million (ppm) for CO2 detection was achieved. The long-term stability of the sensor was evaluated by means of an Allan deviation analysis. With an integration time of ~100 s, the MDL was improved to 1 ppm. This sensor was also used to measure the CO2 concentration from some common emission sources, such as cigarette smoking, automobile exhaust, and the combustion of some carbon-containing materials, which confirmed the stability and robustness of the reported FW-QCL based CO2-PAS sensor system.

Low-Loss Regrowth-Free Long Wavelength Quantum Cascade Lasers

Optical power output is the most sought-after quantity in laser engineering. This is also true for quantum cascade lasers operating especially at long wavelengths. Buried heterostructure cascade lasers with epitaxial regrowth have typically shown the lowest loss due to high current confinement as well as superior lateral thermal conductivity at the expense of complexity and cost. Among the many factors affecting optical output are the widely used passivating materials such as Si3N4 and SiO2. These materials have substantial optical absorption in the long wavelength infrared, which results in optical loss reducing the output of the laser. In this letter, we report on quantum cascade lasers with various waveguide widths and cavity lengths using both PECVD grown Si3N4 and e-beam evaporated HfO2 as passivating material on the same structure. Their slope efficiency was measured, and the cavity losses for the two lasers were calculated. We show that HfO2 passivated lasers have approximately 5.5 cm−1 lower cavity loss compared to Si3N4 passivated lasers. We observe up to 38% reduction in lasing threshold current, for lasers with HfO2 passivation. We model the losses of the cavity due to both insulator and metal contacts of the lasers using Comsol Multiphysics for various widths. We find that the loss due to absorption in the dielectric is a significant effect for Si3N4 passivated lasers and lasers in the 8–12- $\mu \text{m}$ range may benefit from low loss passivation materials such as HfO2. Our results suggest that low-loss long wavelength quantum cascade lasers can be realized without epitaxial overgrowth.

Particle Swarm Optimization approach to identify optimum electrical pulse characteristics for efficient Gain Switching in Dual Wavelength Quantum Cascade Lasers

Numerical simulation of Gain Switching in Dual Wavelength QCLs (DW-QCLs) for short pulse generation in the mid-Infrared region is reported. A four-level rate equation model is used to investigate the QCL which consists of 48 periods of injector and active regions, emitting at 10.5 μm and 8.9 μm. The device is simulated for various inputs such as square, haversine and tangential hyperbolic electrical pulses. The optimum parameters of electrical pulse pertaining to generation of short pulses with maximum power and minimum pulse width for single wavelength and simultaneously at both wavelengths are determined using Particle Swarm Optimization (PSO) technique. The physical nature of drive current and its time period dictates the duration and peak power in both the wavelengths. It is found that square pulse of amplitude 11.12 mA and 0.1 ns produces short optical pulses of width 15.89 ps with peak power of 2.2 mW at 10.5 μm. Similarly, short optical pulses with peak power 1.8 mW and width 15.92 ps are generated by square pulse of amplitude 13.12 mA at 8.9 μm. It is also observed that tangential hyperbolic pulse of amplitude 10.79 mA and 8.47 mA produces short optical pulses with equal power and equal pulse width respectively in both the wavelengths at 0.1 ns.

Time-of-Flow Micromechanical Mass Spectrometry and Micromechanical Infrared Spectroscopy Using Microfluidic Cantilever

A microfabricated, high resonance frequency cantilever operating in vacuum with a high quality factor can detect changes in its mass with unprecedented sensitivity into sub-femtogram and beyond. However, severe damping of a resonating cantilever in a liquid media reduces it mass sensitivity when operated in a liquid restricting its use as a mass sensor for biomolecules in their native states. Recent developments in the fabrication of cantilevers, with embedded microfluidic channels, however, enable their operation in vacuum conditions. Since this microfluidic cantilever is operated in vacuum, it allows high sensitivity mass-based detection of biomolecules in the confined liquid inside the fluidic channel. Although biomolecular detection based on mass using microfluidic cantilever is extremely sensitive, it cannot provide any molecular selectivity. Therefore, separation of biomolecules prior to detection is an essential condition for this sensing approach. Separation of biomolecules by capillary gel electrophoresis is well-established method, but it is technically challenging for micron-sized systems such as microfluidic cantilever. For example, uniformly immobilizing the stationary phase inside cantilever channel is a challenge due to its extreme small size. In addition, ultra-low ionic conductivity of integrated channels makes it difficult to realize charge-based separation of mixed molecules even at the time scale of several hours. However by coupling a microfluidic cantilever with capillary electrophoresis, it is possible to develop a time-of-flow mass spectrometer that can detect separated biomolecules in their native state. Recently we demonstrated such an electrophoresis assisted time-of-flow mass spectrometer using ‘U’ shaped hollow nanomechanical resonators (HNR). By combining an external capillary electrophoresis arrangement with the HNR based mass detection, it is possible to overcome the low ionic conductivity of channels embedded in the HNR preventing direct in-situ electrophoretic separation. The flow of separated biomolecules through the HNR was achieved by balancing the hydrodynamic pressure to override the electromotive force and inhibit the motion of analytes towards the anode for capillary electrophoresis. As the separated biomolecules go through the HNR, the resonance frequency changes sensitively, providing a frequency-time plot (mass-time plot). With a resonance frequency of around 1.5 MHz, the HNR could detect complex samples, such as egg white proteins, in the molecular weight range of 14-250 kDa. Through integrated separation and detection mechanism, this method has the potential to provide precise and fast detection of separated biomolecules in their native state compared to conventional mass spectrometry. This technique also eliminates the need for staining used in conventional polyacrylamide gel electrophoresis. Finally, since the microfluidic cantilevers can be vacuum packaged as a chip, this technique eliminate the need for vacuum pumps, which has been the main bottleneck in the miniaturization of mass spectrometers. These cantilevers also can be fabricated as bi-material microfluidic cantilevers (BMC) to detect extremely small changes in their temperature. Illuminating the BMC with a tunable wavelength quantum cascade laser (operating in the mid-infrared region) can selectively excite the molecules in the channel. Non-radiative decay of the excited molecules results in the generation of thermal energy, which can be observed as the cantilever bending. A plot of the cantilever bending as a function of the illuminating wavelength shows the IR absorption peaks of the molecules. These nanomechanical IR spectra can be used for unique identification of molecules. Therefore, by combining the liquid-based micromechanical mass spectrometry with the mid-infrared micromechanical spectroscopy offers a novel platform for molecular recognition of biomolecules in their native state. This concept paves the way towards the development of low-cost and on-chip mass spectrometers with ultra-miniaturized dimensions for field applications where the sample consumption is just under a nanoliter.

Coupled‐Waveguides for Dispersion Compensation in Semiconductor Lasers

AbstractA dual waveguide for intracavity dispersion compensation in semiconductor lasers is presented and applied to a short mid‐infrared wavelength quantum cascade laser operating in the first atmospheric window ( 4.6 μm). As a result, stable comb operation on the full multi‐mode dynamical range is achieved in this device. Unlike previously proposed schemes, the dual waveguide approach can be applied to various types of semiconductor lasers and material systems. In particular, it could enable efficient group velocity dispersion compensation at near‐infrared wavelengths where semiconductor materials exhibit a large value of that parameter.

Frequency stability of a dual wavelength quantum cascade laser.

We characterized the dual wavelength operation of a distributed Bragg reflector (DBR) quantum cascade laser (QCL) operating at 4.5 μm using two independent optical frequency discriminators. The QCL emits up to 150 mW fairly evenly distributed between two adjacent Fabry-Perot modes separated by ≈11.6 GHz. We show a strong correlation between the instantaneous optical frequencies of the two lasing modes, characterized by a Pearson correlation coefficient of 0.96. As a result, we stabilized one laser mode of the QCL to a N2O transition using a side-of-fringe locking technique, reducing its linewidth by a factor 6.2, from 406 kHz in free-running operation down to 65 kHz (at 1-ms observation time), and observed a simultaneous reduction of the frequency fluctuations of the second mode by a similar amount, resulting in a linewidth narrowing by a factor 5.4, from 380 kHz to 70 kHz. This proof-of-principle demonstration was performed with a standard DBR QCL that was not deliberately designed for dual-mode operation. These promising results open the door to the fabrication of more flexible dual-mode QCLs with the use of specifically designed gratings in the future.

Quantum Cascade Lasers in the InAs/AlSb Material System

We review the current state of the InAs/AlSb quantum cascade laser (QCL) technology. These materials have brought significant progress in short wavelength QCL due to the high-conduction band offset. The first QCLs emitting below 3 μm have been demonstrated in this system. The short wavelength limit of QCL operation has further been moved down to 2.6 μm. This system is also well suited for far-infrared QCLs because high intersubband gain can be achieved at low transition energies due to the small electron effective mass in InAs. Room temperature operation has been demonstrated in InAs-based QCLs to a wavelength of 21 μm, for the first time for any semiconductor laser emitting above 16 μm. Both short- and long-wavelength single-frequency distributed feedback InAs/AlSb QCLs have been realized, as well as widely tunable external cavity sources for the 3–3.5 μm spectral range.

Multiwatt long wavelength quantum cascade lasers based on high strain composition with 70% injection efficiency

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission near 9 µm, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. Maximum pulsed and continuous wave room temperature optical power of 4.5 and 2 W and wallplug efficiency of 16% and 10%, respectively, were demonstrated for a 3mm by 10 µm laser mounted epi-side down on an AlN/SiC composite submount. Pulsed laser characteristics were shown to be self-consistently described by a simple model based on rate equations using measured 70% injection efficiency for the upper laser level.

Ridge-width dependence of the threshold of long wavelength (λ ≈14 µm) quantum cascade lasers with sloped and vertical sidewalls

We investigate the ridge-width dependence of the threshold of Quantum Cascade lasers fabricated by wet and dry etching, respectively. The sloped sidewalls resulting from wet etching affect the threshold in two ways as the ridge gets narrower. First, the transverse modes are deeper in the substrate, hence reducing the optical confinement factor. Second, more important, a non-negligible field exists in the lossy SiO2 insulation layer, as a result of transverse magnetic mode coupling to the surface plamon mode at the insulator/metal surface, which increases the waveguide loss. By contrast, dry etching is anisotropic and leads to waveguides with vertical sidewalls, which avoids the shift of the modes to the substrate layer and coupling to the surface plasmons, resulting in improved threshold compared with wet-etched lasers, e.g., for narrow ridge widths below 20 µm, the threshold of a 14 µm wide λ ≈ 14 µm laser by dry etching is ~60% lower than that of a wet-etched laser of the same width, at 80 K.

Room Temperature CW Operation of Short Wavelength Quantum Cascade Lasers Made of Strain Balanced Ga $_{\bm x}$In$_{\bm {1-x}}$As/Al$_{\bm y}$ In$_{\bm {1-y}}$As Material on InP Substrates

We present our recent development of short wavelength quantum cascade lasers (QCLs) made of strain balanced GaxIn1-xAs/AlyIn1-yAs material on InP substrates. We demonstrate room temperature continuous-wave (CW) lasing of the fundamental lateral mode at four wavelengths of 4.6, 4.0, 3.8, and 3.5 μm. We obtained 60-mW CW output power at 10 °C and 3.55-μm wavelength, which is the shortest CW lasing wavelength demonstrated at room temperature by a QCL, to the best of our knowledge. We also performed a life test on λ = 4.6 μm QCL chips. To date, we have accumulated the life test data for more than 11 000 and 4100 h under two aging conditions, 20°C and 0.85-A constant current, and 60°C and 1-A constant current, respectively.

Gain competition in dual wavelength quantum cascade lasers

We investigated dual wavelength mid-infrared quantum cascade lasers based on heterogeneous cascades. We found that due to gain competition laser action tends to start in higher order lateral modes. The mid-infrared mode with the lower threshold current reduces population inversion for the second laser with the higher threshold current due to stimulated emission. We developed a rate equation model to quantitatively describe mode interactions due to mutual gain depletion.

Design of InxGa1−xAs1−yNy∕AlAs quantum cascade structures for 3.4μm intersubband emission

We report the design of an active region of InxGa1−xAs1−yNy∕AlAs quantum cascade laser structure emitting in the near infrared wavelength range based on an eight-band k∙p model. The InxGa1−xAs1−yNy∕AlAs heterostructure system is of significant interest for the development of short wavelength quantum cascade lasers due to its large conduction band discontinuity (∼1.5eV) and compatibility with the mature GaAs fabrication process. A detailed analysis of the intersubband transition energy within the conduction band as a function of layer thickness, composition, electric field, and temperature has been carried out. Finally, an optimized combination of In0.2Ga0.8As0.97N0.03∕AlAs three-coupled-well structure has been obtained. Under an applied field of 100kV∕cm and at room temperature, a shortest wavelength of 3.4μm has been achieved by making use of this system without introducing an upper lasing level beyond the X minima of the AlAs barrier.

In As ∕ Al Sb quantum cascade lasers emitting below 3μm

Quantum cascade lasers emitting below 3μm are demonstrated. The lasers based on the InAs∕AlSb material system emit at 2.95–2.97μm in pulsed mode with threshold current densities near 3kA∕cm2 at 84K and operate up to room temperature. No degradation has been observed in laser performances compared with InAs∕AlSb quantum cascade lasers emitting at 3.14–3.35μm. The obtained results show no influence of the L valley in InAs on operation of these short wavelength quantum cascade lasers.

In Ga As ∕ Al As Sb ∕ In P quantum cascade lasers operating at wavelengths close to 3μm

The authors report the realization of short wavelength (3.05μm⩽λ⩽3.6μm) InP lattice-matched In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum cascade lasers (QCLs). The highest-performance device displays pulsed laser action at wavelengths between 3.4 and 3.6μm, for temperatures up to 300K, with a low temperature (80K) threshold current density of approximately 2.6kA∕cm2, and a characteristic temperature of T0∼130K. The shortest wavelength QCL (λ≈3.05μm) has a higher threshold current density (∼12kA∕cm2 at T=20K) and operates in pulsed mode at temperatures up to 110K.