- Fiber optic transmission also supports analog links i.e. voice channels. The performance of analog receiver is measured interms of S/N ratio (ratio of mean square signal current to mean square noise current).
- The current generated at optical receiver by analog optical signal is given as –
M is photodetector gain.
Pr is average received power.
Ip is primary photocurrent
- Mean – square signal current at photodetector, neglecting c. term is
- For a photodiode detector mean noise current is sum of
- Mean square quantum noise
- Equivalent resistance thermal noise
- Dark noise
- Surface leakage noise .
Ip is primary photocurrent. ID is primary dark current.
IL is surface leakage current.
F (M) is photodiode noise factor. B is effective noise BW.
Req is equivalent resistance of photodetector and amplifier.
Ft is noise figure of baseband amplifier.
- Signal – to – noise ratio (S/N ratio) is given as –
This limits the sensitivity of analog receiver.
S/N ratio for PIN and APD receiver as a function of received optical power is shown in Fig. 5.4.1.