Berkeley ELENG 119 - OPTICAL DETECTORS (21 pages)

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OPTICAL DETECTORS



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OPTICAL DETECTORS

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Pages:
21
School:
University of California, Berkeley
Course:
Eleng 119 - Introduction to Optical Engineering

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Chapter 4 DETECTORS Chapter 4 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 vacuum envelope D1 D7 D5 D3 A h D2 C D4 D8 D6 HV 2 KV Voltage divider chain to bias dynodes R chosen to have 100V drop per dynode 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 h 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 5 For N dynodes the total gain is then N Photocathode quantum efficiency QE probability a photon will eject one electron 0 QE 1 Typical photocathode response QE 30 20 10 nm 200 400 600 800 1000 Sensitivity 56 Jeffrey Bokor 2000 all rights reserved Chapter 4 DETECTORS For 10 dynodes 5 G 5 10 10 7 Take 2eV photons 620 nm 1 picoW 10 12W 10 12J s With QE 30 Anode current is Phototube dark current 1 random thermal excitation of electrons from photocathode 2 cosmic rays ambient radioactivity where represents the cathode work func Thermal excitation rate is proportional to tion so lower work function IR sensitivity but larger dark current For room temperature typical cathode dark current I cd is 10 4 electrons sec Anode dark current is then Dark current sets a lower limit to phototube sensitivity to low light levels To distinguish a light signal above the background dark current the photoelectric cathode current must exceed the dark current If I cd is 10 4 e sec then the sensitivity to light can be 3 10 4 photons sec assuming QE 30 3 10 4 red photons s 10 14 W Dark current can be reduced by cooling Using thermoelectric cooling T 40C is easily obtained Assume a work function of 1 5 eV 1 5 0 0225 I cd 260K e e 1 5 44 4 38 5 e 8 8 1 4 10 4 I cd 300K e 1 5 0 026 Dark



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