As the amount of information communication increases, optical fiber communication devices between servers and routers are densely integrated, and high-speed communication operations are required in high-temperature environments. Silicon photonics, a key next-generation device, will also require high-density integration on materials that suffer from poor heat dissipation. If such devices can operate in high-temperature environments, that situation will be advantageous in terms of data center cooling and power consumption. Expansion of the operating environment is also important from the viewpoint of reliability, especially communication in space environments, underground exploration, and in-vehicle networks. Each author described the causes of property deterioration in high-temperature environments and offered suggestions for improving the structure, materials, and properties for overcoming this problem.

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We review methods that enhance the high temperature operation of quantum dot (QD) lasers. First, we briefly review such conventional methods as p-type doping, the enhancement of carrier confinement in the ground states of QDs, and increasing the density of the QDs that contribute laser gain. We also review two recently proposed methods, n-type direct doping and a co-doping technique, both of which resolve the problem of the high threshold current of p-doped lasers at room temperature. In addition, we introduce our proposed method that uses lateral potential barrier layers (LPBLs), which enable enhanced quantum confinement and maintain carrier injection into QDs. We enhanced the temperature stability of QD lasers with LPBLs.

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Directly modulated lasers are deployed in data centers as small, cost-effective, and low-power-consumption transmitters. However, their maximum operating speed and temperature are limited mainly due to the gain reduction in the active region caused by a temperature increase under current injection. Therefore, how to reduce the thermal resistance of devices is a crucial issue. In this review, we show that a high-thermal-conductivity SiC single-crystal substrate is a suitable membrane laser platform for decreasing the thermal resistance as well as for obtaining a high optical confinement in the active region, both of which drastically improve laser operation characteristics. The membrane laser fabricated on SiC is capable of continuous-wave operation at temperatures up to 130°C. The bandwidth reaches 60 GHz thanks to the high relaxation oscillation frequency of 42 GHz. In addition, with the photon-photon resonance effect, we demonstrate uncooled 100-GBaud operation at temperatures up to 85°C.

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Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions in semiconductor quantum wells. We discuss the temperature dependences of laser performances for QCLs emitting in the mid-infrared region. The device characteristics of QCLs depend on transition strength, the lifetime of each energy level, and the coupling strength among energy levels. Temperature-insensitive QCLs have been successfully implemented by employing deep quantum wells and high barriers within the active region. In addition, QCLs based on anti-crossed dual-upper-state designs are a promising candidate for many spectroscopic applications, because of their broad bandwidth and high device performances. The lasers characterized by strong, super-linear, current-light output curves possess an exceptionally elevated characteristic temperature that ranges from T0 = 400 ~ 1085 K. The slope efficiency rises with temperature, as shown by a negative value of T1. The distinct features are due to optical-absorption quenching in the injector and reduced carrier leakage in the active region.

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A Type-II GaInAs/GaAsSb/GaInAs W-shaped quantum well applied on a GaAs substrate shows promise in achieving a temperature-stable 1.3 μm wavelength laser. In this study, we demonstrated lasing operation up to 100°C with a 1.3 μm wavelength ridge laser by improving the interface quality of the Type-II quantum well. The temperature characteristics of the laser were evaluated, and the characteristic temperature (T0) between 75°C and 100°C was found to be 92 K, which is much higher than that of InPbased Type-I quantum well lasers.

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Lasers for communications such as optical fiber communications and silicon photonics require highspeed and stable operation in high-temperature environments. Therefore, there is a need to improve the temperature characteristics of semiconductor lasers. Metamorphic growth can create virtual substrates with lattice constants between GaAs and InP substrates. Therefore, there is a high degree of freedom in material selection. Metamorphic InGaAs lasers exhibit high temperature operation in the 1.3-micron range, which is due to the large bandgap offset between the cladding and quantum well layers. So far, we have achieved laser oscillation up to 200°C and a characteristic temperature of 220 K. In this paper, we will introduce the optimization of crystal growth and the characteristics of the fabricated laser.

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