Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 135

Quantum cascade lasers (QCLs), developed in 1994 (Faist et al. 1994), are another option for gas monitoring systems. Operating at room temperature, they emit light in the mid‐infrared region of the spectrum, from 2.5 to over 20 μm, a region where many pollutant molecules strongly absorb. In contrast to diode lasers, where the light emitted depends upon the bandgap of the semiconductor material constituting the laser, quantum cascade lasers incorporate dozens of alternating semiconductor layers. The electric potential varies over the length of the device, where the semiconductor layers form potential “wells.” In QCLs, the output light wavelength is dependent upon the layer structure constructed by design, whereas in tunable diode lasers, it is a function of the material.

The QCL relies on transitions between excited states in the conduction band (intersubband transitions) for photon generation. In operation, electrons tunnel through the “quantum wells,” generating photons as they cascade through different energy levels. One electron emits a photon in each intersubband transition within the quantum well and then tunnels into the next quantum well to emit another photon, cascading down the quantum wells of the structure to emit multiple photons.

The flexibility in manufacturing, their ability to emit light in the mid infrared, their high optical power output, and their ability to operate at room temperature have made QCLs increasingly attractive for gas monitoring applications (Kosterev et al. 2008).