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.

Understanding the origin of noise is important because it gives hints on how to reduce its effects even from the electronic point of view. This chapter analyzes the physics background of some sources of random processes that are limiting sensing systems referred to as “thermal,” “shot,” and “flicker” noises. It also shows how thermal and shot noises are at the base of other observed electronic effects such as “kTC,” “phase,” and “current” noises. The discussion uses analogies between mechanical and electronic effects of thermal agitation. This is important not only for understanding the process but also to unify the model of noise in microelectromechanical sensor systems so as to use the same analysis framework.