Photoreceivers

Abstract

AbstractPhysics, functionality, design concept and electronic circuits of photoreceivers are explained in eight sections. The chapter opens with photodiodes—the most reliable, compact, and inexpensive sensing component of photoreceivers. Physics of a pn-junction and carrier separation mechanism of light detection are carefully explained. The basic knowledge of electrodynamics is necessary to understand this subject. Electrical field distribution inside the pn-junction, open and short circuit characteristics of a photodiode are explained in detail. Advantages and disadvantages of photovoltaic and reverse bias connections are clearly identified. Current–voltage curves determine linearity and sensitivity of response. Functionality and field distribution inside the PIN structure—widely used type of photodiodes—is explained. Antireflection coating increases quantum efficiency up to 90 %. Simple theoretical formulas help to determine quantum efficiency from experimentally measured spectral sensitivity curves. Phototransistors, although being highly sensitive elements, are highly non-linear as well, therefore they are mostly suitable for switching applications. Avalanche photodiodes possess inner amplification but suffer from instability and noise. Multi-element sensors are typically used in beam stabilization circuits, like the one described in Chap. 10. Electronic read-out circuit is a very important component of a photoreceiver, and several practical recommendations and detailed schemes are presented in the fifth section of the chapter. Moving from the simplest switching solutions to highly linear circuits with operational amplifiers, this section introduces the concepts of negative feedback, reverse and zero bias schemes, cut-off frequency, gain-bandwidth product, and transient response. Some practical tricks are suggested to improve frequency characteristic of the photodiode receiver. Practical schemes for connecting multi-element sensors are also examined. Operational amplifiers, although being very efficient instrument for building photodiode receivers, are not the only solution, and several high-speed circuits on discrete transistors are presented. Helping to avoid bitter mistakes, this section also presents typical examples of wrong circuits that do not work. Photomultipliers (PMTs)—the most sensitive photodetectors based on inner amplification—are explained in detail in the sixth section. Some mathematics and knowledge of statistics are needed to explain why inner amplification increases signal-to-noise ratio. Special features, like the total internal reflection prism, non-linearity on saturation, and inner design of compact PMT modules, are presented. Microchannel plates (MCPs) are the subject of the seventh section. This information helps to understand performance of the gated intensified spectrometers in Chap. 9. The chapter ends with PMT receivers—complete devices, incorporating both the PMT sensor and subsequent electronics. This section recommends the simplest and most reliable transimpedance solution based on operational amplifiers. Very essential rules must be observed in order to avoid common mistakes that make the entire system not working.KeywordsOutput VoltageQuantum EfficiencyOperational AmplifierInput ResistanceDepletion RegionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.