What is Photo Detectors?Optical receivers convert optical signal (light) to electrical signal (current/voltage)Hence referred O/E Converter Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitryThere are several photodetector types:Photodiodes, Phototransistors, Photon multipliers, Photo-resistors etc.
Photodetector RequirementsGood sensitivity (Responsivity) at the desired wavelength and poor Responsivity elsewhere wavelength selectivity Fast response time high bandwidthCompatible physical dimensionsLow noiseInsensitive to temperature variations Long operating life and reasonable cost
PhotodiodesDue to above requirements, only photodiodes are used as photo detectors in optical communication systemsPositive-Intrinsic-Negative (pin) photodiodeNo internal gainAvalanche Photo Diode (APD)An internal gain of M due to self multiplication
Principles of PIN PhotodiodesAs a photon flux penetrates into a semiconductor, it will be absorbed as it progresses through the material.If s() is the photon absorption coefficient at a wavelength , the power level at a distance x into the material is
Absorbed photons trigger photocurrent Ip in the external circuitryPhotocurrent Incident Light Power
PIN Energy-Band BiagramCut off wavelength depends on the band gap energy
Avalanche Photodiode (APD)APD has an internal gain obtained by having a high electric field that energizes photo-generated electrons and holesThese electrons and holes ionize bound electrons in the valence band upon colliding with themThis mechanism is known as impact ionizationThe newly generated electrons and holes are also accelerated by the high electric field and they gain enough energy to cause further impact ionizationThis phenomena is called the avalanche effect.
StructureAvalanche Photodiode (APD)
Quantum EfficiencyThe quantum efficiency is the number of the electronhole carrier pairs generated per incidentabsorbed photon of energy h and is given by
Ip is the photocurrent generated by a steady-state optical power Pin incident on the photodetector.
APD vs PINAPD has high gain due to self multiplying mechanism, used in high end systemsThe tradeoff is the excess noise due to random nature of the self multiplying process. APDs need high reverse bias voltage (Ex: 40 V)Therefore costly and need additional circuitry
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