Optical Insanity: Solving the 45km Challenge

[neilgregge]

Hello,

I have been given an assignment to design an optical communication system. A 12 MHz LED (laser diode would be too easy) signal has to be sent 45km via optic fibre and read at the other end. Simple enough, you may think. however I have been told that I have to use multimodal cable, and all I have read has said that there are insane signal losses associated with multimode (one place, i read there was 2dB loss for every km, whicc was about 90% after 10km), hence its use primarily for internal networks. As repeater stations along the way are prohibited, I am at a loss (much like my signal) as to how to go about this.

Any help would be appreciated,

Neil

 

[Averagesupernova]

Treat the system as an FM radio system. Think about it. How much loss do we have between a transmitting antenna and a receiving antenna which are separated by 45 Km?

Edit: 2 db per Km is 90 db total. This has an input to output ratio (in power) of 1 billion to one.