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Chapter 4: DETECTORS56 © Jeffrey Bokor, 2000, all rights reservedChapter 4OPTICAL 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 photoelectronsDynodes 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 responseSensitivity:AhD1CD2D3D4D5D6D8D7vacuum envelopeHV–~2 KVVoltage divider chain to bias dynodesR chosen to have ~100V drop per dynodeh 5NNQE probability a photon will eject one electron0 QE 160020010%20%30%QE400800 1000 (nm)Chapter 4: DETECTORS57 © Jeffrey Bokor, 2000, all rights reservedFor 10 dynodes, .Take 2eV photons (620 nm), 1 picoW = 1012W = 1012J/sWith , Anode current is Phototube dark current:1) random thermal excitation of electrons from photocathode2) cosmic rays, ambient radioactivity– Thermal excitation rate is proportional to , where represents the cathode work func-tionso lower work function  IR sensitivity, but larger dark current– For room temperature, typical cathode dark current, , is electrons/sec. Anode dark currentis then – Dark current sets a lower limit to phototube sensitivity to low light levels. To distinguish a light sig-nal above the background dark current, the photoelectric cathode current must exceed the dark cur-rent. If is , then the sensitivity to light can be photons/sec (assuming QE =30%). red photons/s 1014 W!– Dark current can be reduced by cooling. Using thermoelectric cooling is easily obtained.Assume a work function of Dark current is reduced by this amount!  down to ~1 e/sec. Minimum detectable power become< 1018 W!Photon counting: PMT is so sensitive, we are really counting photons. Often, PMT circuits arespecifically optimized to do this. 5 G 510107==QE 30%=Icd104Icd104e/sec31043104T 40C– 1.5 eV=Icd260KIcd300K-------------------------e1.5 0.0225–e1.5 0.026–---------------------------- e1.5 44.4 38.5––e8.8–1.4 104–== =Chapter 4: DETECTORS58 © Jeffrey Bokor, 2000, all rights reservedPhoton counting system:PMT output pulse– How big is the PMT output pulse from one photon? For , we get . For sec, . For , .– Discriminator eliminates electrical noise in < 1mV range. has a variation due to statistical natureof gain process. Discriminator also eliminates this.Shot noise: Photon arrival is always statistical. Generally it follows Poisson statistics. Then if thephoton arrival rate is , and we count for 1 sec, we get on average. The standard devia-tion will be found to be . This means we have noise.Shot noise is universal for light detection. Even if photons are not explicitly counted, the shotnoise is a fundamental limit. It is most significant at low light levels, though, due to depen-dence.Johnson noise: Random thermal noise in any resistor, Ramplifier/discriminatorPMTpulse counteranodeR 50=CrVppp3-5 ns=discriminatorthresholdtransit timedispersion2Vdiscriminator outputis a digital pulsettG 107107 electrons 1012–Cp108–=Iapk104–AR 50=Vp5 mVVpN ph/secNN1234567NcountsNsecNChapter 4: DETECTORS59 © Jeffrey Bokor, 2000, all rights reserved B: bandwidth (Hz)Equivalent model of noise as either current or voltage source.Channel Electron Multiplier (Channeltron)Single monolithic device functions as a PMT:1. Photon hits funnel portion2. Electrons are accelerated into the bent tube by bias field3. Secondary electron emission gives gain at each electron collision with wall4. Must be operated in vacuum5. Typical gain 6. More compact and rugged than PMTMicrochannel plate MCP – array of channeltrons– each channel is a miniature channeltron– gain ~103 IRMS4kTR---------B=~IRMSRVRMS4kTRB=R~VRMShHV–A21electron cascadeGlass tube bent around curve. One end open asa funnel shape. Coating acts as photocathodeand secondary electron emitter. Also, coatinghas high, but not infinite electrical resistance.10410-20m separationeach hole ~5-10m diameterAnodeHVhADual Plate MCP Detectorgain ~106 HV ~1-2KVChapter 4: DETECTORS60 © Jeffrey Bokor, 2000, all rights reservedMCP Image Intensifier– Electrons accelerated out of back of MCP into phosphor– Phosphor QE ~50% photons/electronsImage intensification  Night vision gogglesSemiconductor Photodetectors• Seimiconductor band structure• Optical absorption across the bandgapeye or video cameralightHVphosphor-coated plateeSingle or Dual Plate MCPEGelectronsholesECEVdopingEFEFn-type p-typeEChEVIf , an electron and hole (pair) is createdafter photon absorption.In a suitable structure, the electron and the holecan contribute to an electric current through thedevice.h EGChapter 4: DETECTORS61 © Jeffrey Bokor, 2000, all rights reserved• p-n Junction• Depletion approximation: Assumes carriers diffuse across junction and create regions that are totallydevoid of free carriersNNDNA–=zNDNA–equilibriumEFEVbiz0lp–lnzzChapter 4: DETECTORS62 © Jeffrey Bokor, 2000, all rights reserved• Reverse biasPhotodiodeReverse bias condition: electron and hole created in the depletion region follow the electric field and sep-arate.The electric field exists only inside the depletion region. So the light absorption must also occur there tocreate current.Construction• Photodiodes can be used at longer wavelength than photomultiplier – • Typically fast response time < 10 nsecEUnder reverse bias, no current flowsbecause the barrier to diffusion increases.Under forward bias, barrier to diffusion isreduced.depletion widths widerhVThese carriers are pulled apartby the field.lightly doped p-regionbackside electrodethin electrode passes lightthin, heavily doped n+ layerdepletion regionEGChapter 4: DETECTORS63 © Jeffrey Bokor, 2000, all rights reserved• Compact, inexpensivenpip-i-n photodiodeNPconstant -field in the


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Berkeley ELENG 119 - OPTICAL DETECTORS

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