agree with above;
1.for post amplifiers (used after transmitter)you have to keep NON-linear effects (NLE) in mind. NLE is hiher at higher power levels.
2.pre-amplifers are required to compensate losses in optical line.THere is also dipersion,NLE in optical fiber ,which contribute to noise at RECEIVER END(RX).this amplifiers also amplifies noise at same time .there choice of gain of this amplifier depend on threshold level and type of RX