1. 1 Stephen SchultzFiber Optics Fall 2005 Semiconductor Optical Detectors

2. 2 Stephen SchultzFiber Optics Fall 2005 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 –Electrical power is proportional to optical power squared –Called square law device Important characteristics –Modulation bandwidth (response speed) –Optical conversion efficiency –Noise –Area

3. 3 Stephen SchultzFiber Optics Fall 2005 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

4. 4 Stephen SchultzFiber Optics Fall 2005 p-n Diode

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

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

7. 7 Stephen SchultzFiber Optics Fall 2005 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 (  requires the photon energy to be smaller than the material band gap

8. 8 Stephen SchultzFiber Optics Fall 2005 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

9. 9 Stephen SchultzFiber Optics Fall 2005 Detector Responsivity Each absorbed photon generates an electron hole pair I ph = (Number of absorbed photons) * (charge of electron) Rate of incident photons depends on –Incident optical power P inc –Energy of the photon E photon = hf Generated current Detector responsivity –Current generated per unit optical power  in units of  m

10. 10 Stephen SchultzFiber Optics Fall 2005 Responsivity Depends on quantum efficiency , and photon energy

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