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Chapter 4:
The heart of an optical communication receiver is the opto-electronic device that is used as the photodetector. Ideally, a photodetector would detect all incident photons, respond to the fastest changes in the incoming signal that were of interest, and not introduce additional noise beyond the inherent quantum shot-noise from the received signal. In most practical applications, additional desirable characteristics can be defined. The photodetector should be small, lightweight, rugged, reliable, and cost-effective, and its characteristics should remain unaffected by age and environment. Unfortunately, realistic photodetectors have limited band widths with finite response times. They introduce unwanted noise into the detection process, and the probability of detecting an individual photon is less than 100%. Some detector technologies are fragile and environmentally sensitive; others may have finite lifetimes or may degrade unacceptably as they age. The photon-effect based photodetectors that are used in optical communication systems are those that directly generate photocurrents from interactions between photons and atoms in the detector material. When light penetrates into a photodetector material, the volume of material illuminated contains a tremendous number of atoms. The probability that a specific atom will absorb a photon and generate free carriers that form a photocurrent is quite small. However, since the number of atoms is huge, the probability that some atom will interact with an incident photon to form photo-excited free carriers can be quite high in a well designed detector. Photon-effect photodetectors can be constructed to simultaneously exhibit the high sensitivities and fast response times needed for high data-rate communications and are frequently grouped into one of four general categories; photomultipliers, photoconductors, photodiodes, and avalanche photodiodes. 4.1 Photomultipliers A photomultiplier tube (PMT) is a form of vacuum tube that utilizes the photoelectric effect and the secondary emission of electrons to provide high current gains once a photon is initially detected. A schematic illustration of a photomultiplier tube is shown in Fig. 4.1. Incident photons fall on a photo-emissive photocathode consisting of a low work-function metal or semiconductor. If the photon energy exceeds the work function of the cathode material there is a reasonable probability that an electron will be ejected from the material [1]. The electron emitted from the cathode is accelerated by an applied electric field towards a series of additional electrodes called dynodes. The dynodes are coated with a material that is prone to secondary emission of electrons and are connected in series between resistors, causing a voltage gradient to form along the dynodes. The dynodes generate additional electrons via secondary emission, with the last electrode forming an anode that collects the emitted electrons. The first dynode may generate 5–10 additional electrons, the second 25–100, etc. This current multiplication process continues for each dynode in the tube, with current gains in excess of 10 6 possible. The long-term average of the probability of detecting an incident photon is equal to the quantum yield or quantum efficiency of the photomultiplier.
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