Improving this system, making it faster and able to transmit more information is therefore crucial to global development and this report shall focus on the receiver part in this system: photodetectors, specifically avalanche photodiodes. Significance of optical fibres Photodetectors have a limited range of wavelengths they can detect and determining which wavelength to use is the first step to designing an optical communications receiver. Optical fibres make up the largest part of the system, as there are almost a million kilometres of fibre buried in the sea and all across the globe . They are made from silica glass, which has an attenuation characteristic shown Figure 1. Figure : Typical Silica attenuation characteristic The fibres exhibit minimum dispersion at ~1.3µm and minimum optical loss at 1.55 µm. These two properties define the detection wavelength at which the photodetectors must operate. Dispersion is relatively low at both wavelengths, but at distances of hundreds of kilometres, minimising optical loss becomes more important, which is why the focus in this report will be on 1.55µm wavelength photodetection. Requirements Material Considerations A wavelength of 1.55µm corresponds to an energy of ~0.79eV. For a photodiode to therefore be able to absorb photons of such a wavelength, the bandgap (the energy difference between the semiconductor valence- and conduction bands) in its absorption layer must be equal to that energy or less. Having a larger gap will cause no photons of this wavelength to be absorbed and a smaller one will increase the dark current significantly and also absorb photons that are not part of the signal, such as background radiation, which increases the noise. Wavelength and bandgap are related through equation 1.2.1,
Schottky Photodiode Schottky photodiodes are made of metal-semiconductor junctions. They have a simple structure, require only one photolithography step and have a thin active region and low capacitance to give a very high response speed. Carriers are generated when light enters the semiconductor and are collected by the electric field to produce a photocurrent. Schottky Photodiodes can be arranged with interdigited electrodes as shown in Figure 5 to form metal-semiconductor-metal (MSM) photodetectors. Figure : Schematic (left), top-view (right) diagrams and band diagram (bottom) of Schottky photodiode have pure drift photocurrent and no diffusion component but suffer from high dark currents from the back-to-back Schottky barrier junctions. Their low responsivity is caused by reflection from the metal contacts (although back-illumination is possible) and the finite carrier lifetime, since they have to cross the metal gaps before being collected. Photomultiplier Tubes Photomultiplier tubes (PMTs) are vacuum glass tubes with a photosensitive surface, which emits electrons when hit by photons. The electron is attracted to the focusing electrode and then multiplied by a process called secondary emission. It passes through a series of additional electrodes where each one is held at approximately 100 V higher than the previous electrode, as shown in Figure 6. Figure : Photomultiplier tube diagram The electrode stack allows for extremely high gains of up to 100 million at a reference bandwidth of 1 Hz. The bandwidth however is generally quite low. The device must be operated in total darkness to avoid over excitation and is operated at over one thousand Volts. Quantum efficiency (percentage of photons absorbed) is in the single percent range but can reach 20-40% for research-grade devices. PMTs are commonly used in physics laboratories, observatories and medical equipment.
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