Laser feedback interferometry, based on the self-mixing (SM) effect in quantum cascade lasers (QCLs), is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and SM is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. The SM phenomenon in QCLs has been exploited for a wide range of applications, including the measurement of internal laser characteristics, two- and three-dimensional imaging, materials analysis and near-field imaging. This chapter provides an overview of the SM effect in QCLs, and reviews the state of the art in sensing using this technique.

This chapter begins with a brief review of electronic circuitry and terminology because optical detection and signal processing are in the realm of electrical engineering. A detailed discussion of analog detection follows, with circuitry including transimpedance amplifiers and equivalent circuits for analyzing noise and bandwidth. Two electronic noise sources are introduced, Johnson noise and amplifier noise, and their effects on SNR are modeled. Photon counting is then discussed in terms of its instrumentation, advantages, and limitations. The basic principles of coherent detection are elucidated through a mathematical derivation, and the advantages of coherent detection are shown: high SNR, optical background discrimination, and the measurement of Doppler shifts to sense winds. The main types of detectors used in lidar systems are then discussed, including intrinsic and PIN photodiodes, photomultipliers, avalanche photodiodes, and single-photon avalanche diodes. The advantages of internal detector gain for optimizing SNR are quantified.