Detection of Low-Level Optical Signals

This book is addressed to designers of photodetectors and photodetecting systems, designers of focal plane arrays, charge-coupled devices, specialists in IR technologies, designers of optoelectronic detecting, guiding and tracking systems, systems for IR direction finders, lidars, lightwave communication systems, IR imagers. All these specialists are united by one common purpose: they are all striving to catch the weakest possible optical signal. The most important characteristic of photosensitive devices is their detectivity, which determines the lowest level of optical signal they are able to detect above the noise level. These threshold characteristics define the most important tactical and technical parameters of the entire optoelectronic system, such as its range, resolution, precision. The threshold characteristics of optoelectronic system depend on many of its components; all designers agree, however, that the critically responsible part of the system is the photodetector [1]. By the end of the 1960s the physicists and the engineers were able to overcome many obstacles and to create photodetectors (at least single-element or few-element ones) which covered all the main optical bands (0. 4 . . . 2,2 . . . 3, 3 . . . 5,8 . . . 14 J. . Lm), carried out the detection almost without any loss (the quantum yield being as high as 0. 7 . . . 0. 9), and reduced the noise level to the lowest possible limit.

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This book is intended for designers of military and civil systems, such as systems for guiding and control, target acquisition, surveillance, laser range-finding, fiber-optical communications, thermal imaging and the like, as well as for designers of photodetectors for optical signal detection. The first question they face is how to detect an ultimately weak optical signal. This book gives the answer to this most important question. All the main types of photodetectors are considered, from photodiodes (including avalanche photodiodes) to focal plane arrays (FPA). Methods of matching photodetectors with preamplifiers are described. The pair photodetector plus preamplifier is treated as an integrated detection system. Much attention is paid to different types of noise and ways of maximising the signal-to-noise ratio (SNR). Foundations of theory of optimal filtering of photosignals are discussed taking due account of typical shapes of optical signals and noise spectra. Methods for tuning quasi-optimal filters to maximise the SNR are explained. The main problems associated with detection of low-level optical signals are considered: operation of avalanche photodiodes in photon count mode, filtering in the case of charge accumulation in FPA cells, and the effect of the number of pixels and geometry of FPAs on detection. Finally, using the examples of the laser range finder and IR Imager, we give guidelines for calculating the limiting parameters of optoelectronic systems to achieve the highest possible SNR. The book is based on many years' experience by the author and his colleagues in the development of photodetectors and FPAs. The book is aimed at research workers, engineers, students and postgraduates.

Contenu

1. Photodetecting Assembly as Detection System for Weak Optical Signals.- 1.1. Functions and Structure of Photodetecting Assembly, CCD, and Other Charge Accumulation Devices.- 1.2. Photodetectors.- 1.3. Amplifier.- 2. Signal and Noise in Photodetecting Assemblies.- 2.1. Pulsed Optical Signal.- 2.2. Basic Notions of Noise Process.- 2.3. Photodetector Noise.- 2.4. Noises of Photodetecting Assembly.- 2.5. Minimization of Photodetecting Assembly Noises.- 3. Foundations of Theory of Optimal Filtering as Applied to Photodetecting Assemblies.- 3.1. Signal No Signal Decision Making.- 3.2. Optimal Filter (Frequency Analysis).- 3.3. SNR at the Output of Optimal Filter (Frequency Analysis).- 3.4. Noise Equivalent Power, Detectivity, and Other Detection Invariants.- 4. Optimal Filtering of Photo Signals (Time-Domain Analysis).- 4.1. Pulse Characteristic of Optimal Filter.- 4.2. Shape of Signal at the Output of Optimal Filter.- 5. Signal Processing in Real Photodetecting Assemblies ..- 5.1. Quasi-Optimal Filtering.- 5.2. Loss in SNR in Quasi-Optimal Filter.- 5.3. Broad-Band Amplification.- 6. Detection of Weak Optical Signals with Photodetecting Assemblies Based on Avalanche Photodiodes.- 6.1. Broad-Band Amplification and Quasi-Optimal Filtering in Photodetecting Assemblies Based on Avalanche Photodiodes.- 6.2. Quantization and Fluctuation of Signal.- 6.3. Photon Counting Mode of Avalanche Photodiode.- 7. Detection and Processing of Optical Signals in Matrices.- 7.1. Integration Cell As Quasi-Optimal Filter.- 7.2. Multielement Traditional-Type Photodetecting Assemblies and Focal Plane Arrays.- 8. Detection of Weak Optical Signals in Special-Purpose Optoelectronic Systems.- 8.1. Timing of Arrival of Optical Pulse.- 8.2. IR Imager.- Conclusion.- References.