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Optical Receivers
Theory and
Operation
Amitabh Shukla
Photo Detectors
- (^) 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.
Photodiodes
- (^) Photodiodes meet most the requirements, hence widely used as photo detectors.
- (^) Positive-Intrinsic-Negative ( pin ) photodiode (^) No internal gain, robust detector
- (^) Avalanche Photo Diode ( APD ) (^) Advanced version with internal gain M due to self multiplication process
- (^) Photodiodes are sufficiently reverse biased during normal operation no current flow without illumination, the intrinsic region is fully depleted of carriers
Physical Principles of
Photodiodes
- (^) As a photon flux Φ penetrates into a semiconductor, it will be absorbed as it progresses through the material.
- (^) If α s (λ) is the photon absorption coefficient at a wavelength λ, the power level at a distance x into the material is Absorbed photons trigger photocurrent I p in the external circuitry
pin energy-band diagram
Cut off wavelength depends on the band gap energy μm ( )
- 24 E E eV hc g g c λ = = Cut off wavelength:
Quantum Efficiency
- (^) The quantum efficiency η is the number of the electron–hole carrier pairs generated per incident– absorbed photon of energy hν and is given by I p is the photocurrent generated by a steady- state optical power P in incident on the photodetector.
APD Vs PIN
Responsivity ( ℜ )
Quantum Efficiency ( η) = number of e-h pairs generated / number of incident photons
APD’s have an internal gain M, hence
where, M = IM/Ip I M : Mean multiplied current 0 / / p I q P h η ν = 0 p
I q
P h
η ν
ℜ = = mA/mW
APD PIN ℜ = ℜ M M = 1 for PIN diodes
Light Absorption Coefficient
- (^) The upper cutoff wavelength is determined by the bandgap energy E g of the material.
- (^) At lower-wavelength end, the photo response diminishes due to low absorption (very large values of α s ).
Photodetector Noise
- (^) In fiber optic communication systems, the photodiode is generally required to detect very weak optical signals.
- (^) Detection of weak optical signals requires that the photodetector and its amplification circuitry be optimized to maintain a given signal-to-noise ratio.
- (^) The power signal-to-noise ratio S/N (also designated by SNR) at the output of an optical receiver is defined by SNR Can NOT be improved by amplification
Quantum (Shot Noise)
2 ( ) 2 2 i qI BM F M Q p = F(M): APD Noise Figure F(M) ~= M x (0 ≤ x ≤ 1) I p : Mean Detected Current B = Bandwidth q: Charge of an electron Quantum noise arises due optical power fluctuation because light is made up of discrete number of photons
Dark/Leakage Current Noise
2 ( ) 2 2 i qI BM F M DB D = i qI B DS L 2 2 = Bulk Dark Current Noise Surface Leakage Current Noise I D : Dark Current I L : Leakage Current There will be some (dark and leakage ) current without any incident light. This current generates two types of noise (not multiplied by M)
Signal to Noise Ratio
2 2 2 2 ( ) ( ) 2 4 / p p D L B L i M SNR q I I M F M B qI B k TB R =
Detected current = AC (i p
) + DC (I
p
Signal Power = <i p 2
M 2 Typically not all the noise terms will have equal weight. Often thermal and quantum noise are the most significant.
Noise Calculation Example