More on detectors Apr. 5, 2018

+ Avalanche photodiode (APD): photodiode with internal gain, i.e. [equation]; [equation] current gain through impact ionization by energetic electrons under high electric field, i.e. high reverse biasing voltage.
As temp increases, [equation] decreases.
modulation bandwidth [equation], [equation] is the effective transit time.

+ Noise in detection: the generation of each electron by photon is a random process [equation] signal plus noise
amplifier introduces additional noise measured by noise figure F = SNR at input/ SNR at output; [equation].
Shot noise of APD [equation].

+ Signal to noise ratio in analog system: SNR = average signal power / noise power [equation]
Note: dc [equation] [equation] [equation]; ac sinusoidal modulated power [equation] [equation] [equation] & [equation]; with APD, [equation] [equation] [equation] & [equation] [equation] [equation]
[equation]
[equation]

+ Thermal noise limited detection [equation]

+ Shot noise limited (quantum-limited): [equation]
fundamental limit under strong signal.


Last Modified: March 30, 2018
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