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Photo detectors

Photo detectors. Optical Receivers. Optical receivers convert optical signal (light) to electrical signal (current/voltage) Hence referred ‘O/E Converter’ Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry

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Photo detectors

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  1. Photo detectors

  2. Optical Receivers • Optical receivers convert optical signal (light) to electrical signal (current/voltage) • Hence referred ‘O/E Converter’ • Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry • There are several photodetector types: • Photodiodes, Phototransistors, Photon multipliers, Photo-resistors etc.

  3. Photodetector Requirements • High sensitivity (responsivity) at the desired wavelength and low responsivity elsewhere • Low noise and reasonable cost • Fast response time  high bandwidth • Insensitive to temperature variations • Compatible physical dimensions • Long operating life

  4. Photodiodes • Due to above requirements, only photodiodes are used as photo detectors in optical communication systems • Positive-Intrinsic-Negative (pin) photodiode • No internal gain • Avalanche Photo Diode (APD) • An internal gain of M due to self multiplication • Photodiodes are reverse biased for normal operation

  5. Basic pin photodiode circuit • Incident photons trigger a photocurrent Ip in the external circuitry by pumping energy Photocurrent Incident Optical Power

  6. PIN Photodiode • It is a junction diode in which an undoped intrinsic ( i ) region is inserted between relatively thin p region and relatively thick n region. • Diode is reverse biased so that the entire i- region is depleted and has a strong electric field. • Light absorbed in intrinsic region produce free electron hole pairs, provided that photon energy is high enough. • These carriers which are responsible for photocurrent are swept across the region with high velocity and are collected across the reverse –biased junction. • This gives rise to a current flow in the external circuit called the photocurrent.

  7. pin energy-band diagram A particular semiconductor material can be used only over a limited wavelength range

  8. pin energy-band diagram Cut off wavelength depends on the bandgap energy Cut off wavelength for Si is about 1.06 µm and for Ge it is 1.6 µm. For longer wavelengths the photon energy is not sufficient to excite an electron from the valence band to the conduction band.

  9. Materials of construction • The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents. • Materials commonly used to produce photodiodes include:

  10. Quantum Efficiency( ) Quantum Efficiency () = number of e-h pairs generated / number of incident photons Ipaverage photocurrent produced by a steady-state average optical power Po incident on the photodetector q is the charge of the electron hvphoton energy

  11. Responsivity () Responsivity is the ratio of photocurrent to the incident optical power on the photodetector. Avalanche PD’s have an internal gain M mA/mW IM: average value of the total multiplied current M = 1 for PIN diodes

  12. Responsivity

  13. Signal to Noise Ratio For high SNR • The Photodetector must have a large quantum efficiency (large responsivity or gain) to generate large signal power • Detector and amplifier noise must be low SNR Can NOT be improved by amplification

  14. Quantum (Shot Noise) Due to optical power fluctuation because light is made up of discrete number of photons F(M): APD Noise Figure F(M) ~= Mx (0 ≤ x ≤ 1) depends on the material For PIN diodes F(M) and M are unity. Ip: Mean Detected Current B = Bandwidth

  15. Dark/Leakage Current Noise There will be some (dark and leakage ) current without any incident light. This current generates two types of noise Bulk Dark Current Noise ID: Dark Current Surface Leakage Current Noise IL: Leakage Current (not multiplied by M)

  16. Thermal Noise The photodetector load resistor RLcontributes a mean-square thermal (Johnson) noise current KB: Boltzmann’s constant = 1.38054 X 10(-23) J/K T is the absolute Temperature • Quantum and Thermal are the important noise • mechanisms in all optical receivers • RIN (Relative Intensity Noise) will also appear in analog • links

  17. Signal to Noise Ratio Detected current = AC component (ip) + DC component (Ip) Signal Power = <ip2>M2 Typically not all the noise terms will have equal weight

  18. SNR Dark current and surface leakage current noise are typically negligible, If thermal noise is also negligible For analog links, (RIN= Relative Intensity Noise)

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