Quantum cascade lasers (QCLs) have received enormous attention from the scientific community due to their broad range of applications in a wide variety of industries, agriculture, healthcare, environmental protection, and many other scientific and technical fields. In this article, in addition to a review of the main applications and the state of research and development of high -power QCLs in the mid -infrared range, we consider the features of their manufacturing technology that make it possible to obtain a high peak power and discuss the effect of overheating of the active region on the output optical power and spectral characteristics. A comparison is made of the characteristics of QCLs with the same cavity parameters but with different active regions made on the basis of substrate -matched or strained heteropairs, which provides a different energy barrier between the upper laser level and the continuum. It is shown that the use of strained heteropairs in the active region of a QCL provides an almost twofold increase in the characteristic temperature T0 as well as a significantly higher efficiency and an increase in the maximum output optical power to over 21 W, which is a world record for a single stripe QCL with a 8 pm spectral range.

A series of low-voltage thyristor current switches based on (Al)GaAs/GaAs homo- and heterostructures with a volume charge region formed in the lightly doped p-GaAs base layer have been developed. The transient processes characteristics in pulse generation mode of nanosecond duration have been studied. It has been shown that the use of a wide-bandgap barrier based on AlGaAs at the n‑emitter/p-base junction allows reducing the minimum control current amplitude from 30 to 3 mA, and the turn-on delay time can be shortened to 6 ns. For the developed thyristor switches, a minimum transition time of 3.7–3.9 ns was demonstrated when operating in a circuit with a 1 nF capacitive load. In a circuit with a nominal 1 Ω resistive load, the thyristor switches provided a peak current of 17.5 A with a pulse duration of 3.7 ns.

A comparison is made of conventional pin photodetectors and modern uni-traveling-carrier photodetectors based on InGaAs/InP heterostructures. The differences in the designs of heterostructures of these photodetectors are considered and the resulting features of their functioning are analyzed. It is shown that with comparable photosensitivity, the saturation photocurrent of uni-traveling-carrier photodetectors is approximately four times greater than that in conventional pin photodetectors. Devices with a photosensitive area 40 mu m in diameter reach the saturation current at an input optical power of 10-12 and 60-70 mW for pin and uni-traveling-carrier photodetectors, respectively (reverse bias voltage of 5 V).

The development of high-power mid-infrared laser sources is highly desired for a number of applications in free-space optical communication, laser imaging, detection, and ranging (LIDAR) and environmental monitoring. Quantum cascade lasers (QCLs) hold solid position among these technologies, however up to now their highest powers are demonstrated in 4.5 - 5 $\mu \mathrm{m}$ spectral range [1], while the results at other mid-IR wavelength may differ by an order of magnitude. In this work, we consider three different designs of high-power QCLs grown by a two-stage MBE and MOCVD epitaxy. The quality of all fabricated heterostructures is similar to that of structures produced solely by the MBE technique. The active region remains the same as in [2] in all three types of structures, while the main difference lies in the design of the upper cladding and contact layer. In particular, we discuss two structures with thick uniformly doped InP upper cladding together with InP or InGaAs contact layer (Types I and II correspondingly), and design with gradient doping of the InP upper cladding accompanied by InGaAs contact layer (Type III). All three structures were subjected to post-growth processing and fabrication of QCL chips with 40 and 60 $\mu \mathrm{m}$ stripes and 3–5 mm cavity lengths. All samples were tested under 150 ns pulsed pumping with a 12 kHz repetition rate. Our experiments show that QCLs based on both structures with InGaAs contact layer with uniform and gradient cladding doping (Types II and III) demonstrate better efficiency while the lasers based on Type I design with InP contact layer and uniformly doped upper cladding feature improved power characteristics resulting in the record-high power value $> 16\ \mathrm{W}(> 8\mathrm{W}/\text{facet})$ . We claim the latter is directly related to the better thermal conductivity of InP contact layer comparing to InGaAs counterpart. This was confirmed by the chirp measurements demonstrating the lower heating rate of the active region in structure with InP contact layer (Type I), see Fig. 1a. At the same time, in our experiments InP contact layer had lower electrical conductivity, that finally affected the laser efficiency as shown in Fig 1.b.

A comparison is made of conventional pin photodetectors and modern uni-traveling-carrier photodetectors based on InGaAs/InP heterostructures. The differences in the designs of heterostructures of these photodetectors are considered and the resulting features of their functioning are analyzed. It is shown that with comparable photosensitivity, the saturation photocurrent of uni-traveling-carrier photodetectors is approximately four times greater than that in conventional pin photodetectors. Devices with a photosensitive area 40 μm in diameter reach the saturation current at an input optical power of 10–12 and 60–70 mW for pin and uni-traveling-carrier photodetectors, respectively (reverse bias voltage of 5 V).

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

The effective design of (In)G aAsP/AlG aInP/G aAs semiconductor heterostructures grown by MOVPE, which allows to fabricate cw laser bars emitting in the $770-880 \mathrm{~nm}$ spectral range, were developed. It was demonstrated that reduced threshold current density and increased external differential efficiency could be achieved.

We analyze the influence of optical coatings on the electro-optical characteristics of quantum cascade lasers. We compare light-current characteristics of devices without and with different combinations of optical coatings. The highest output power is achieved with combination of anti- and high-reflection coatings, while the lowest threshold with partial-high- and highreflection coatings.

We study quantum-cascade lasers with active region designs based on strained and lattice-matched heterostructures. Lasers based on strained well/barrier pairs demonstrate improved efficiency, temperature stability and record-high optical power.

Measurements of the output optical power, laser-oscillation spectra, optical-pulse duration, and switching-on delays of semiconductor lasers–thyristors with a strip width of 200 μm and a length of 980 μm were performed in the operating temperature range from 20 to 70°C at a nominal value of the discharge capacitor of 22 nF and a control-current amplitude of 10.4 mA. It is shown that lasers–thyristors have high temperature stability. When the devices were heated from 20 to 70°C, the level of reduction of the peak output power did not exceed 15

We present a study of quantum cascade laser dynamical properties accounting for the Joule heating released in the active region. In particular, we study the QCL emitting at 8 mu m in the pulsed pumping mode and present experimental measurements, as well as a theoretical description of the QCL build-up time, showing the features appearing due to the Joule heating released inside the active region.

The main characteristics of 1550 nm few-mode laser diodes with aperture width $20 \mu \mathrm{m}$ and different cavity lengths (power, electrical, spectral and spatial characteristics) were measured. It is shown that there are two ranges of pump currents in which the characteristics of lasers behave differently: the first current range corresponds to operation in a low-mode mode, in which a scatter in the output characteristics of lasers is observed; in the second range the laser operates predictably and stably.

Quantum-cascade lasers (QCLs) based on InGaAs/AlInAs/InP with reflective and antireflective coatings are fabricated and studied. The manufactured lasers emit in the spectral range from 4 to 5 μm. The effect of variation in the front mirror reflectance on the output power of a QCL with a highly reflective rear mirror is investigated. It is shown that the use of an antireflective coating on the front face leads to a simultaneous increase in both the threshold current of the QCL and the slope of the light–current characteristic. This allows a higher output power to be achieved at high pump currents. In contrast, the use of a partially reflective coating on the front face not only reduces the threshold current of the QCL, but also decreases the slope of the light–current characteristic. Such QCLs may have an advantage over other emitters at low pump currents.

We report the fabrication of 1.5–1.6 μm light-emitting modules based on an AlGaInAs/InP heterostructure with strain-compensated quantum wells in new, smaller, thermally stabilized housings. Their performance is tested under extreme operating conditions. The reliability of such modules and stability under the influence of climatic factors is studied.

We report the results of a comparison of metal–dielectric mirror coatings for mid-IR quantum-cascade lasers (QCLs). Samples of QCLs with Al2O3/Ti/Au and SiO2/Ti/Au optical coatings are fabricated and their characteristics are studied. It is shown that the use of metal–dielectric mirror coatings allows the output optical power of devices to be increased up to 93

We analyze the characteristics of semiconductor emitters based on separate-confinement double heterostructures with quantum wells with different configurations of waveguide layers. Lasers with narrow and broad waveguides are considered as applied to the problem of increasing the output power. Semiconductor emitters with undoped and doped waveguide layers are compared. We consider lasers with ultranarrow and broad strongly asymmetric waveguides. It is shown that the reduction of series and thermal resistance reduces the self-heating of lasers and increases the output power and efficiency. The prospects of using the epitaxial integration for designing lasers with several tunnel-coupled emitting sections for increasing the output power and luminosity are considered. The possibility of constructing monolith-integrated thyristor lasers combining the emitting section and an electron switch in a single crystal is demonstrated.

Current switches based on low-voltage InP heterothyristors with a maximum blocking voltage of 20 V were developed and studied. In current pulse generation mode, the efficient operation of InP heterothyristors with a low-resistance load in the form of a capacitor was demonstrated. It has been shown that the minimum turn-on delay time is about 6 ns at a control current amplitude of 60 mA. The possibility of generating current pulses with a duration of 53–154 ns and amplitudes of 38–130 A was demonstrated when the capacitor values were changed in the range of 56–1000 nF.

We have demonstrated a quantum cascade laser (QCL) with a generation frequency of about 3.8 THz, grown by metal-organic vapor phase epitaxy. The multilayer heterostructure for QCLs consists of 185 repetitions of an active module containing four GaAs/Al0.15Ga0.85As quantum wells. The threshold current and threshold voltage of the fabricated QCL were 2.25 kA/cm2 and 19.7 V, respectively. The QCL oscillations were carried out in the multimode regime, and the detection of terahertz radiation continued with an increase in the laser temperature up to 60 K.

We report theoretical and experimental results of comparison of high-power laser diode arrays made of (Al)GaAs/AlGaAs and GaAsP/GaInP heterostructures for the spectral range from 800 to 810 nm. The best results are demonstrated for arrays based on GaAsP/GaInP heterostructures. The maximum values of the output optical power of 1-cm-long laser diode arrays in the quasi-cw pump regime reach 370–380 W. The possible reason for the difference in the output powers of the arrays based on the material systems in question is discussed and methods for further increasing the radiation power are presented.

Current switches based on low-voltage InP heterothyristors with a maximum blocking voltage of 20V were developed and studied. In current pulse generation mode, the efficient operation of InP heterothyristors with a low-resistance load in the form of a capacitor was demonstrated. It has been shown that the minimum turn-on delay time is about 6 ns at a control current amplitude of 60 mA. The possibility of generating current pulses with a duration of 53–154 ns and amplitudes of 38–130 A was demonstrated when the capacitor values were changed in the range of 56–1000nF.