Optical Multiplexer / Demultiplexer

You could try DWDM. These devices are small, compact, and VERY expensive!

Not a terribly difficult or even interesting problem, has been done many times before. You need a stable clock source (Crystal controlled thank you), a 2 bit counter, or larger just ignore the higher order bits, a 4 line data selector, and the appropriate IC to interface your fiber. (depends on which fiber connector you plan to use)...

Hi. Try www.osd.com.au. They have a number of different types of modems you are refering to...analogue, digital, IP. I have bought from them to do exactly what you request. I think they have anything from 1 channel to 16 channel types with/without data (ie to control PTZ of camera), audio, Ethernet.

The info you will need to provide though before contacting them (or any other manufacturer) is the type of fiber you are using or planning to use - multimode (cheaper but less distance) or singlemode (dearer but further distance is achievable).

Your benefit (if you are from the USA - they are from Australia) is the exchange rate - you will get about 30% savings without even starting to haggle!