Optical Insanity: Solving the 45km Challenge

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SUMMARY

The discussion centers on designing an optical communication system using a 12 MHz LED signal transmitted over 45 km of multimode fiber optic cable. The primary challenge is the significant signal loss associated with multimode fiber, estimated at 2 dB per kilometer, resulting in a total loss of 90 dB over the distance. The user, Neil, highlights the prohibition of repeater stations, complicating the transmission further. The analogy to FM radio systems is suggested to conceptualize the loss between antennas over long distances.

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neilgregge
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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
 
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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.
 
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