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Using a photodiode in optical communications

  1. Mar 16, 2017 #1
    1. The problem statement, all variables and given/known data

    As part of my coursework in a sensor systems module at university I was given the question:

    State the most important reason why the photovoltaic (as opposed to photoconductive) mode would be the most suitable choice for a long distance fibre optic transmission system.

    2. Relevant equations

    From my lecturers notes, I could draw a two main answers to that although the question asks for the most important reason.

    3. The attempt at a solution

    My reasons would be:
    There isn't a dark current present in the photovoltaic mode (i.e. zero bias).
    There is no chance of an avalanche effect being experienced.
    Any help would be greatly appreciated!

    Many thanks,
    Henry
     
  2. jcsd
  3. Mar 16, 2017 #2

    berkeman

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    Welcome to the PF. :smile:

    So by "photovoltaic" you mean a photodiode, and by "photoconductive" you mean a Light Dependent Resistor?

    Photodiodes do have a small dark current (leakage), so I don't think that's an answer. What characteristics are important for fiber-optic communication?
     
  4. Mar 16, 2017 #3
    Thank you!

    No sorry, by photovoltaic I mean a no bias setup with a photodiode and by photoconductive I mean a reverse bias setup like in the attachment :). That's a slide from my lecture notes.

    Capture.PNG
     
  5. Mar 16, 2017 #4
  6. Mar 16, 2017 #5

    berkeman

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    Hmm. So you are supposed to say why a receiver with no reverse bias on the photodiode is best for long-distance FO communication? That's counterintuitive to me, so I guess I should let others chime in. I know why all of my FO receivers use reverse biased photodiodes (can you think of that reason?)...
     
  7. Mar 16, 2017 #6

    berkeman

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    Yeah, that higher speed of the reverse biased photodiode is what I was looking for. But I guess they are saying that the "no dark current" with no reverse bias is more important in FO detectors for very weak signals. I hadn't heard that before, but I suppose it's possible. Still, the bandwidth of an unbiased photodiode is so much worse...
     
  8. Mar 16, 2017 #7
    Yeah I was a little confused when looking around because I saw a lot of people saying the same (i.e. that reverse bias is better). I'm a little confused to be honest, I might go and talk to my lecturer and see what he says, do you think he was implying weak signals by saying "long distance"?.
     
  9. Mar 16, 2017 #8

    berkeman

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    That would be my guess.

    It might be worth looking around some to see if you can find some typical FO receiver circuits used in the trans-Atlantic FO cables or other long-haul FO systems. I would think that high BW is so important that they would just deal with the dark current in their S/N calculations. After all, the dark current is mostly DC, so filtering it out should be possible.
     
  10. Mar 16, 2017 #9
    Okay, well thank you very much for your help :smile:
     
  11. Mar 16, 2017 #10

    berkeman

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  12. Mar 16, 2017 #11

    berkeman

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    I meant to post the link to the top of the article, but that link is a good place to start...
     
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