Optical facet coatings for high-performance LWIR quantum cascade lasers atλ∼8.5μm

We report on the performance improvement of long-wave infrared quantum cascade lasers(LWIR QCLs)by studying and optimizing the anti-reflection(AR)optical facet coating.Compared to the Al2O3 AR coat⁃ing,the Y_(2)O_(3)AR coating exhibits higher catastrophic optical mirror damage(COMD)level,and the optical facet coatings of both material systems have no beam steering effect.A 3-mm-long,9.5-μm-wide buried-heterostruc⁃ture(BH)LWIR QCL ofλ~8.5μm with Y_(2)O_(3)metallic high-reflection(HR)and AR of~0.2%reflectivity coating demonstrates a maximum pulsed peak power of 2.19 W at 298 K,which is 149%higher than that of the uncoated device.For continuous-wave(CW)operation,by optimizing the reflectivity of the Y_(2)O_(3)AR coating,the maximum output power reaches 0.73 W,which is 91%higher than that of the uncoated device.

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 220μm,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.

Quantum cascade lasers grown by MOCVD

Sharing the advantages of high optical power,high efficiency and design flexibility in a compact size,quantum cascade lasers(QCLs)are excellent mid-to-far infrared laser sources for gas sensing,infrared spectroscopic,medical diagnosis,and defense applications.Metalorganic chemical vapor deposition(MOCVD)is an important technology for growing high quality semiconductor materials,and has achieved great success in the semiconductor industry due to its advantages of high efficiency,short maintenance cycles,and high stability and repeatability.The utilization of MOCVD for the growth of QCL materials holds a significant meaning for promoting the large batch production and industrial application of QCL devices.This review summarizes the recent progress of QCLs grown by MOCVD.Material quality and the structure design together determine the device performance.Research progress on the performance improvement of MOCVD-grown QCLs based on the optimization of material quality and active region structure are mainly reviewed.

Low Power Consumption Distributed-Feedback Quantum Cascade Lasers Operating in Continuous-Wave Mode above 90℃ at λ～7.2μm

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

Very low threshold operation of quantum cascade lasers

A strain-compensated InP-based quantum cascade laser（QCL） structure emitting at 4.6 μm is demonstrated,based on a two-phonon resonant design and grown by solid-source molecular beam epitaxy（MBE）.By optimizing the growth parameters,a very high quality heterostructure with the lowest threshold current densities ever reported for QCLs was fabricated.Threshold current densities as low as 0.47 kA/cm^2 in pulsed operation and 0.56 kA/cm^2 in continuous-wave（cw） operation at 293 K were achieved for this state-of-the-art QCL.A minimum power consumption of 3.65 W was measured for the QCL,uncooled,with a high-reflectivity（HR） coating on its rear facet.

Periodic and chaotic oscillations in mutual-coupled mid-infrared quantum cascade lasers

Dynamic states in mutual-coupled mid-infrared quantum cascade lasers(QCLs) were numerically investigated in the parameter space of injection strength and detuning frequency based on the Lang-Kobayashi equations model. Three types of period-one states were found, with different periods of injection time delay τ_(inj), 2τ_(inj), and reciprocal of the detuning frequency. Besides, square-wave, quasi-period, pulse-burst and chaotic oscillations were also observed. It is concluded that external-cavity periodic dynamics and optical modes beating are the mainly periodic dynamics. The interaction of the two periodic dynamics and the high-frequency dynamics stimulated by strong injection induces the dynamic states evolution.This work helps to understand the dynamic behaviors in QCLs and shows a new way to mid-infrared wide-band chaotic laser.

High-Power and High-Efficiency Operation of Terahertz Quantum Cascade Lasers at 3.3 THz

A high-power and high-effciency GaAs/A1GaAs-based terahertz （THz） quantum cascade laser structure emitting at 3.3 THz is presented. The structure is based on a hybrid bound-to-continuum transition and resonant-phonon extraction active region combined with a semi-insulating surface-plasmon waveguide. By optimizing material structure and device processing, the peak optical output power of 758mW with a threshold current density of 120 A/cm2 and a wall-plug effciency of 0.92% at 10K and 404mW at 77K are obtained in pulsed operation. The maximum operating temperature is as high as llS K. In the cw mode, a record optical output power of 160roW with a threshold current density of 178A/cm2 and a wall-plug efficiency of 1.32% is achieved at 1OK.

Room Temperature Operation of Strain-Compensated 5.5μm Quantum Cascade Lasers

Room temperature operation is an important criterion for high performance of quantum cascade lasers. A strain-compensated quantum cascade laser（λ≈5.5μm） with optimized waveguide structure lasing at room temperature is reported. Accurate control of layer thickness and strain-compensated material composition is demonstrated using X-ray diffraction. An output power of at least 45mW per facet is realized for a 20μm-wide and 2mm-long laser at room temperature.

Single-Mode Fabry-Pérot Quantum Cascade Lasers at λ ~10.5 μm

In this paper, we report a single-mode Fabry-Pérot long wave infrared quantum cascade lasers based on the double phonon resonance active region design. For room temperature CW operation, the wafer with 35 stages was processed into buried heterostructure lasers. For a 4 mm long and 13 μm wide laser with high-reflectivity (HR) coating on the rear facet, continuous wave output power of 43 mW at 288 K and 5 mW at 303 K is obtained with threshold current densities of 2.17 and 2.7 kA/cm2. The lasing wavelength is around 10.5 μm. Single mode emission was observed for this particular device over the whole investigated current and temperature range.

An equivalent circuit model for terahertz quantum cascade lasers:Modeling and experiments

Terahertz quantum cascade lasers（THz QCLs） emitted at 4.4 THz are fabricated and characterized. An equivalent circuit model is established based on the five-level rate equations to describe their characteristics. In order to illustrate the capability of the model, the steady and dynamic performances of the fabricated THz QCLs are simulated by the model.Compared to the sophisticated numerical methods, the presented model has advantages of fast calculation and good compatibility with circuit simulation for system-level designs and optimizations. The validity of the model is verified by the experimental and numerical results.

Beam steering characteristics in high-power quantum-cascade lasers emitting at∼4.6µm

A beam steering effect of high-power quantum cascade(QC) lasers emitting at 4.6 μm was investigated. The continuous wave(CW) output power of an uncoated, 6-mm-long, 7.5-μm-wide buried-heterostructure QC laser at 25℃ was as high as 854.2 m W. The maximum beam steering angle was offset by ±14.2° from the facet normal(0°) in pulsed mode. The phenomenon was judged explicitly by combining the diffraction limit theory and Fourier transform of the spectra. It was also verified by finite element method software simulation and the calculation of two-dimensional(2 D)effective-index model. The observed steering is consistent with a theory for coherence between the two lowest order lateral modes. Therefore, we have established an intrinsic linkage between the spectral instabilities and the beam steering by using the Fourier transform of the spectra, and further presented an extremely valid method to judge the beam steering. The content of this method includes both three equidistant peak positions in the Fourier transform of the spectra and the beam quality located between once the diffraction limit(DL) and twice the DL.

Extreme events in quantum cascade lasers

We demonstrate experimentally that mid-infrared quantum cascade lasers(QCLs)operating under external optical feedback exhibit extreme pulses.These events can be triggered by adding small amplitude periodic modulation,with the highest success rate for the case of a pulse-up excitation.These findings broaden the potential applications for QCLs,which have already been proven to be a semiconductor laser of interest for spectroscopic applications and countermeasure systems.The ability to trigger extreme events paves the way for optical neuron-like systems where information propagates as a result of high intensity bursts.

Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

We demonstrate the electrical beat note analysis and radio frequency （RF） injection locking of a continuous wave （cw） terahertz quantum cascade laser （QCL） emitting around 3 THz （-100 μm）. In free running the beat note frequency of the QCL shows a shift of ~180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes Ⅰ, Ⅱ, and Ⅲ are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more ef- ficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.

Tapered quantum cascade lasers operating at 9.0μm

The tapered quantum cascade lasers operating at about 9.0/zm are reported. In contrast to the common ridge waveguide laser, tapered devices give rather small horizontal beam divergence. Performances of devices with identical 11 μm ridge waveguide sections and different tapered gain sections are comparatively studied. The optimal taper angle of 3° leads to a relative high output power and a very small horizontal beam divergence of 7.1 °.

High efciency beam combination of 4.6-μm quantum cascade lasers

The quantum cascade laser(QCL),a potential laser source for mid-infrared applications,has all of the advantages of a semiconductor laser,such as small volume and light weight,and is driven by electric power.However,the optical power of a single QCL is limited by serious self-heating efects.Therefore,beam combination technology is essential to achieve higher laser powers.In this letter,we demonstrate a simple beam combination scheme using two QCLs to extend the output peak power of the lasers to 2.3 W.A high beam combination efciency of 89% and beam quality factor of less than 5 are also achieved.

Quantum cascade lasers operating from 1.4 to 4 THz

The development of teranertz （THz） quantum cascade lasers （QCLs） has progressed considerably since their advent almost a decade ago.THz QCLs operating in a frequency range from 1.4 to 4 THz with electron-phonon scattering mediated depopulation schemes are described.Several different types of GaAs/AlGaAs superlattice designs are reviewed.Some of the best temperature performances are obtained by the so-called resonant-phonon designs that are described.Operation above a temperature of 160 K has been obtained across the spectrum for THz QCLs operating at ν 〉 1.8 THz.The maximum operating temperature of previously reported THz QCLs has empirically been limited to a value of ～ω/k B.A new design scheme for THz QCLs with scattering-assisted injection is shown to surpass this empirical temperature barrier,and is promising to improve the maximum operating temperatures of THz QCLs even further.

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 8.6μm,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.

Holographic fabricated continuous wave operation of distributed feedback quantum cascade lasers atλ≈8.5μm

The fabrication and characterization of distributed feedback（DFB） quantum cascade lasers emitting atλ≈8.5μm are reported.The first-order DFB grating structure was defined using the holographic lithography technique.Reliable dynamic single-mode emission with a side-mode suppression ratio of 20 dB and a tuning coefficient of-0.277 cm^（-1）/K from 93 to 210 K is obtained in continuous wave mode by using high-reflectivity coating on the rear facet.The output power is over 100 mW at a temperature of 80 K.

Broad area quantum cascade lasers operating in pulsed mode above 100℃ at λ ～ 4.7 μm

We demonstrate a broad area（400 μm） high power quantum cascade laser（QCL）. A total peak power of 62 W operating at room temperature is achieved at λ~ 4.7 μm. The temperature dependence of the peak power characteristic is given in the experiment, and also the temperature of the active zone is simulated by a finite-elementmethod（FEM）. We find that the interface roughness of the active core has a great effect on the temperature of the active zone and can be enormously improved using the solid source molecular beam epitaxy（MBE） growth system.

Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation

In this paper,we presented single mode terahertz quantum cascade lasers(THz QCLs)with sampled lateral grating emitting approximately 3.4 THz.Due to strong mode selection,the implementation of sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB.The measured peak power of the grating laser is improved by about 11.8%compared to the power of devices with uniform distributed feedback gratings.Furthermore,the far-field pattern of the presented device is uninfluenced by grating structures.