Lecture8 - 1 Optical Detectors; p-n Photodiodes; p-i-n...

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Unformatted text preview: 1 Optical Detectors; p-n Photodiodes; p-i-n Photodiodes; Avalanche Photodiodes EE4035 Optical Communications Semester A 2010-11 Lecture 8 2 Intended Learning Outcomes (ILOs) • Describe the considerations in choosing a photodetector. • Explain the operation principles of p-n photodiodes, PIN photodiodes, and APDs. • Explain and evaluate the characteristics of photodetectors. • Explain the advantages and drawbacks of different types of photodetectors. 3 Criteria when choosing an optical detector: 1. High sensitivity at the operation wavelength First generation 0.8 – 0.9 μ m; At present, 1.3 μ m and 1.55 μ m where attenuation and material dispersion can be minimized. 2. High fidelity For analogue transmission, the response of an optical detector must be linear with respect to the optical source over a wide dynamic range. 3. Large electrical response to the received signal waveform Maximum conversion efficiency 4. Short response time to obtain a suitable bandwidth At present 10 GHz and above Optical Detectors (Photodetectors) An optical detector is a device to convert the received optical signal into an electrical signal, which is then amplified before further processing. Improvement of detector characteristics and performance allows the installation of fewer repeaters and lowers both the capital investment and maintenance costs. 4 5. Minimum noise Low dark current, leakage current and shunt conductance Gain mechanism must be of low noise. 6. Stability of performance characteristics Because characteristics (i.e. sensitivity, noise, internal gain) vary with temperature, temperature compensation is necessary. 7. Small size Efficient coupling to fibre and easy packaging 8. Low bias voltage No excessive bias voltage and current 9. High reliability Continuous stable operation at room temperature for many years 10. Low cost Materials Semiconductors: Silicon; Germanium; III-V Alloys 5 Operation of the p-n photodiode: (a) photo-generation of an electron-hole pair in an intrinsic semiconductor; (b) the structure of the reverse biased p-n junction illustrating carrier drift in the depletion region; (c) the energy band diagram of the reverse biased p-n junction showing photo- generation and the subsequent separation of an electron-hole pair. A photon incident in or near the depletion region that has energy larger than or equal to E g will excite an electron from the valence band into the conduction band. This process leaves an empty hole in the valance band and is known as photo-generation of an electron-hole (carrier) pair. Carrier pairs are separated and swept under the influence of the electric field to produce a displacement by the current in the external circuit in excess of any reverse leakage current....
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This note was uploaded on 04/17/2011 for the course EE 4035 taught by Professor Prof.chiang during the Spring '10 term at City University of Hong Kong.

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Lecture8 - 1 Optical Detectors; p-n Photodiodes; p-i-n...

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