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Semiconductor Optical Detectors

Semiconductor Optical Detectors. Semiconductor Optical Detectors. Inverse device with semiconductor lasers Source: convert electric current to optical power Detector: convert optical power to electrical current Use pin structures similar to lasers Electrical power is proportional to i 2

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Semiconductor Optical Detectors

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  1. Semiconductor Optical Detectors Fiber Optics Fall 2005

  2. Semiconductor Optical Detectors • Inverse device with semiconductor lasers • Source: convert electric current to optical power • Detector: convert optical power to electrical current • Use pin structures similar to lasers • Electrical power is proportional to i2 • Electrical power is proportional to optical power squared • Called square law device • Important characteristics • Modulation bandwidth (response speed) • Optical conversion efficiency • Noise • Area Fiber Optics Fall 2005

  3. p-n Diode • p-n junction has a space charge region at the interface of the two material types • This region is depleted of most carriers • A photon generates an electron-hole pair in this region that moves rapidly at the drift velocity by the electric field • An electron-hole pair generated outside the depletion region they move by diffusion at a much slower rate • Junction is typically reversed biased to increase the width of the depletion region Fiber Optics Fall 2005

  4. p-n Diode Fiber Optics Fall 2005

  5. Semiconductor pin Detector • Intrinsic layer is introduced • Increase the space charge region • Minimize the diffusion current Fiber Optics Fall 2005

  6. I-V Characteristic of Reversed Biased pin • Photocurrent increases with incident optical power • Dark current, Id: current with no incident optical power Fiber Optics Fall 2005

  7. Light Absorption • Dominant interaction • Photon absorbed • Electron is excited to CB • Hole left in the VB • Depends on the energy band gap (similar to lasers) • Absorption (a) requires the photon energy to be smaller than the material band gap Fiber Optics Fall 2005

  8. Quantum Efficiency • Probability that photon generates an electron-hole pair • Absorption requires • Photon gets into the depletion region • Be absorbed • Reflection off of the surface • Photon absorbed before it gets to the depletion region • Photon gets absorbed in the depletion region • Fraction of incident photons that are absorbed Fiber Optics Fall 2005

  9. Detector Responsivity • Each absorbed photon generates an electron hole pair Iph = (Number of absorbed photons) * (charge of electron) • Rate of incident photons depends on • Incident optical power Pinc • Energy of the photon Ephoton= hf • Generated current • Detector responsivity • Current generated per unit optical power l in units of mm Fiber Optics Fall 2005

  10. Responsivity • Depends on quantum efficiency h, and photon energy Fiber Optics Fall 2005

  11. Minimum Detectable Power • Important detector Specifications • Responsivity • Noise Equivalent noise power in or noise equivalent power NEP • Often grouped into minimum detectable power Pmin at a specific data rate • Pmin scales with data rate • Common InGaAs pin photodetector • Pmin=-22 dBm @B=2.5 Gbps, BER=10-10 • Common InGaAs APD • Pmin=-32 dBm @B=2.5 Gbps, BER=10-10 • Limited to around B=2.5 Gbps Fiber Optics Fall 2005

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