Lec-14 Photodetectors contd (24-03-2011)

Lec-14 Photodetectors contd (24-03-2011) - Photodetectors...

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Photonic Network By Dr. M H Zaidi Photodetectors continued
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Photonic Network By Dr. M H Zaidi Responsivity of the photodectectors we have discussed thus far has been limited by the fact that one photon can generate only one electron when it is absorbed. However, if the generated electron is subjected to a very high electric field, it can acquire sufficient energy to knock off more electrons from the valence band to the conduction band. These secondary electron-hole pairs can generate even further electron-hole pairs when they are accelerated to sufficient levels. This process is called avalanche multiplication . Such a photodiode is called an avalanche photodiode, or simple an APD. Avalanche Photodiodes
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Photonic Network By Dr. M H Zaidi Depletion region with a reverse bias Voltage, V a . A pn-junction photodiode. Depletion region with no biased voltage applied. A reverse biased pn-junction used as a photodetector Built-in electric field on reverse bias.
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Photonic Network By Dr. M H Zaidi The number of secondary electron-hole pairs generated by the avalanche multiplication process by a single (primary) electron is random, and the mean value of this number is termed the multiplicative gain and denoted by G m . Thus there is a trade-off between the multiplicative gain and the noise factor. APDs are usually designed to have a moderate value of G m that optimized their performance. Avalanche Photodiodes
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Photonic Network By Dr. M H Zaidi Avalanche Photodiodes Characteristics Basic pn junction is highly doped Operated at high reverse bias > 50 V usually Diode operates in “avalanche” region of I-V characteristics Electrons and holes which cross depletion region gain enough energy to produce more electrons and holes Avalanche multiplication A “guard-ring” is fabricated around the active area Reduces leakage current if biased to same voltage as diode Restricts avalanche effect to middle of illuminated area Fast response Internal amplification of number of electrons Because of avalanche effect
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Photonic Network By Dr. M H Zaidi PIN and APD Sensitivity An APD typically has 10 dB better sensitivity than a PIN. -36 dBm sensitivity at 2.5 GB/s is possible.
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Photonic Network By Dr. M H Zaidi Typical Spec-sheet of a 10Gb pin
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Photonic Network By Dr. M H Zaidi
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Photonic Network By Dr. M H Zaidi Passive Devices
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Photonic Network By Dr. M H Zaidi Connectors A mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector.
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Photonic Network By Dr. M H Zaidi Connectors - contd. Type: SC, FC, ST, MU, SMA Favored with single-mode fibre Multimode fibre (50/125um) and (62.5/125um) Loss 0.15 - 0.3 dB Return loss 55 dB (SMF), 25 dB (MMF) Single fibre connector
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Photonic Network By Dr. M H Zaidi Attenuators Singlemode Variable Attenuator Repeatable, variable attenuation from 2 to 40dB <-70dB reflectance (unconnectorized) Polarization insensitive Low modal noise Long-term reliability
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Photonic Network By Dr. M H Zaidi
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This note was uploaded on 06/03/2011 for the course IT ISE808 taught by Professor Mr.tayyab during the Spring '11 term at College of E&ME, NUST.

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Lec-14 Photodetectors contd (24-03-2011) - Photodetectors...

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