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,
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,
CORRECTED%20Max%20Rowe%20-%20MPhil%20Thesis.docx