Such advantages as compact size, high reliability, and ease of application make conventional quantum cascade lasers ideal in telecommunications and consumer electronics in applications such as optical fiber communications and compact disk players. To be more precise, the quantum cascade lasers are considered to be a specific type of semiconductor laser, generally emitting mid-infrared light.
The operation of the quantum cascade laser is based on laser transitions not between various electronic bands but on intersubband transitions of a semiconductor structure. The thing is that an electron “undergoes the first transition between two sublevels of a quantum well (which is the laser transition on which stimulated emission occurs), then a non-radiative transition (red arrow) to the lowest sublevel, before tunneling into the upper level of the next quantum well”.
Herewith, the use of several tens of quantum wells in a quantum cascade laser allows obtaining a higher optical fiber gain and numerous photons per electron. Moreover, the transition energy of quantum cascade lasers is determined not by specific material features but rather by parameters of quantum lasers, therefore, their operating wavelengths vary from a few microns to well above 10 μm, or even in the terahertz region.
It should be noted that the quantum well structure is installed in a waveguide of a quantum cascade laser, herein, the resonator is mostly regarded as of DBR or DFB type. Additionally, there are external-cavity lasers, where a wavelength tuning element, for example, fiber Bragg grating is embedded right in the quantum laser resonator.
The quantum cascade lasers produce the power conversion efficiency of a few tens of percent. Nevertheless, there are quantum lasers with efficiencies around 50% but only for cryogenic operation conditions. Although most quantum cascade lasers produce mid-infrared laser beam light, they also can generate terahertz waves. Such quantum laser systems are considered to be highly compact and simple sources of terahertz radiation.
The most potential applications for quantum cascade lasers include laser absorption spectroscopy of trace gases, for example, for tracing tiny concentrations of pollutants in the air. Quantum lasers offer a suitable wavelength range, a relatively narrow linewidth, and good wavelength tunability, making them ideal for such applications.
Nowadays quantum cascade lasers are commonly used in modern technology due to their compactness, high power, and peculiar frequency range. In spite of the progress, however, quantum laser systems face several challenges that are pursued by the scientific community.
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