Lecture 12

Lecture 12 - Lecture 12 Lecture 12 OPTICAL DETECTORS...

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Lecture 12 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 12 OPTICAL DETECTORS (Reference: Optical Electronics in Modern Communications , A. Yariv, Oxford, 1977, Ch. 11.) Photomultiplier Tube (PMT) Highly sensitive detector for light from near infrared ultraviolet Can detect as little as 10 19 Watt! Photocathode C:– absorbs photon ejects electron – work function is the minimum energy needed to eject an electron – the photon energy must exceed the work function to get photoelectrons Dynodes D1-D8:Secondary electron emission. Electron from cathode accelerated by ~100 eV. Impact into dynode surface causes ejection of multiple electrons, . For dynodes, the total gain is then . Photocathode quantum efficiency: Typical photocathode response Sensitivity : A h D1 C D2 D3 D4 D5 D6 D8 D7 vacuum envelope HV ~2 KV Voltage divider chain to bias dynodes R chosen to have ~100V drop per dynode h  5 N N QE probability a photon will eject one electron 0 1 600 200 10% 20% 30% QE 400 800 1000 (nm)
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Lecture 12 2 © Jeffrey Bokor, 2000, all rights reserved For 10 dynodes, . Take 2eV photons (620 nm), 1 picoW = 10 12 W = 10 12 J/s With , Anode current is Phototube dark current :1) random thermal excitation of electrons from photocathode
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Lecture 12 - Lecture 12 Lecture 12 OPTICAL DETECTORS...

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