SIGNAL-TO-NOISE-RATIO (SNR)
Signal shot noise
Dark current shot noise
arising from discrete nature of incident nature of photodiode dark current, Idark
quantum noise arising from discrete nature of incident signal
Thermal noise
fluctuation due to random motion of electrons
photon absorption is non-continuous process
Number of absorption fluctuates statistically
Photocurrent fluctuation conforms to Poisson statistic
Means square noise current:
current with no incident light
Dark current - buld dark current + surface leakage current
Mean square dark current shot noise:
arise from random thermal motion of carriers in resistive circuit elements
Means square thermal current:
SNR & detection sensitivity
ratio of signal urrent squared to total mean square noise current
total mean square noise current:
SNR: 
the dominant noise component will determine the minimum detectable power, Pmin at the receiver/ the detection sensitivity of the detector
SNR for an APD
UMMI SHOBIRAH BINTI ALI