Biophotonics_Concepts_to_Applications

maintained for proper signal interpretation. The powersignal-to-noise ratio(des-
ignated by SNR) at the output of an optical receiver is defined by

SNR¼

signal power from photocurrent

photodetector noise power + amplifier noise power

¼

mean square signal current

P

mean square noise currents

ð 5 : 11 Þ

The noise currents in the receiver arise from various photodetector noises and the
thermal noise associated with the combined resistance of the photodiode and the
amplifier circuitry.
To see the interrelationship of the different types of noises affecting the
signal-to-noise ratio, consider the circuit of a simple receiver model shown in
Fig.5.6. The photodiode has a small series resistance Rs, a total capacitance Cd
consisting of junction and packaging capacitances, and a bias (or load) resistor RL.
The amplifier following the photodiode has an input capacitance Caand a resistance
Ra. For practical purposes, Rstypically is much smaller than the load resistance RL
and can be neglected.

5.3.2 Noise Sources

The mean square value of the signal current isis given by

i^2 s

¼ i^2 pðtÞ

DE

M^2 ð 5 : 12 Þ

where ip(t) is the primary time varying current resulting from a time varying optical
power Pin(t) falling on the photodetector and M is the avalanche gain (M = 1 for a
pinphotodiode).
The principal noise sources associated with photodetectors are shot noise (also
calledquantum noise) and dark-current noise generated in the photodiode material.
Theshot noisearises from the statistical nature of the production and collection of
photoelectrons when an optical signal is incident on a photodetector. These

Load

resistor

Photodiode circuit Amplifier circuit

Fig. 5.6 Simple circuit
model of a photodetector
receiver

5.3 Photodiode Noises 129