• 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 –


                                                                                       … (5.4.1)



is responsivity.


M is photodetector gain.

Pr is average received power.

Ip is primary photocurrent


  • Mean – square signal current at photodetector, neglecting c. term is

                                                        … (5.4.2)

  • For a photodiode detector mean noise current is sum of
  1. Mean square quantum noise
  2. Equivalent resistance thermal noise
  3. Dark noise
  4. 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.