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Optical Communications
Chapter 4: Photodiode
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Content
Working principles
Optical characteristics
Modulation characteristic
PIN and APD
Noise
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PN Junction
Reverse biased:increases depletionregion, preventscarriers from
moving across thejunction
Forward biased:
decreases depletion
region, diffusescarriers across thejunction
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Working principles
Working principles
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Working principles
Working principles
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Optical characteristics
Optical characteristics
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Optical characteristics
Optical characteristics
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Optical characteristics
Optical characteristics
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Modulation characteristics
Modulation characteristics
Shunt Resistance, RSHthe slope of the I-V curve at V=0 Series Resistance, RSarises from the resistance of the contacts and
the resistance of the undepleted silicon
Junction Capacitance, CJthe boundaries of the depletion region act asthe plates of a parallel plate capacitor
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Modulation characteristics
Modulation characteristics
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Modulation characteristics tDRIFT, the charge collection time of the carriers in the depleted
region of the photodiode.
tDIFFUSED, the charge collection time of the carriers in the undepletedregion of the photodiode.
tRC, the RC time constant of the diode-circuit combination.
Response time
Modulationbandwidth
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PIN
PIN photodiode
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APD
APD photodiode
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PIN vs APD
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PIN vs APD High reverse voltage (~10 to 100V), sometimes just below breakdown. M depends strongly on the reverse voltage.
The excess noise factor increases with M. Therefore, the reverse voltage isoften chosen such that the multiplication noise approximately equals thenoise of the electronic amplifier
The detection bandwidth can be very high, because operation with a smallershunt resistor is acceptable.
InGaAs APDs are significantly more expensive than Ge APD, but exhibit
superior noise performance and a higher detection bandwidth.
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Noise
Noise sources
Quantum (shot noise)
Bulk dark current
Surface dark current
Thermal noise
2 2 2
2 2 2
2 2
2 2
2 ( )
2 ( )
2
4
Q Q p
DB DB D
DS DS L
BT T
L
i qI BM F M
i qI BM F M
i qI B
k TBi
R
< >= =
< >= =
< >= =
< >= =
B: bandwidth
M: APD gainF(M): exceed noise factor
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Noise
Signal to Noise ratio
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Conclusion
Photodiodes use reverse-bias PN junction to
convert photons to electrons Due to the semiconductor material, the optical
bandwidth of photodiode is limited
Due to the working principles and structure of aphotodiode, the modulation bandwidth is limited
PIN photodiode is cheaper but has a lower gain
APD photodiode has better gain but increasesnoise