What is the Extinction Ratio?

  1. Dec 31, 2012 #1
    I am a little bit confused on what an extinction ratio is. I am looking at optical specs and it keeps on coming up. Would someone be kind enough to explain it to me and perhaps provide a mathematical background thank you!
     
  2. jcsd
  3. Dec 31, 2012 #2
    When laser diodes are used to transmit binary data (fiber optic communications), a "1" is transmitted as a higher optical power level than the "0". The extinction ratio is simply the ratio of these two power levels:

    [tex]Extinction Ratio = \frac{logic 1 power}{logic 0 power}[/tex]
     
    Last edited: Dec 31, 2012
  4. Dec 31, 2012 #3
    So a 1000:1 extinction ratio is simply that for every 1000 bits transmitted 1 bit is lost due to a loss in power?
     
  5. Dec 31, 2012 #4
    No.

    First an extinction ratio of 1000 (30dB) would not be normal. Let's take a more normal situation with a ratio of 10.

    Laser is transmitting 1mW for logic 1.
    Laser is transmitting 100uW for logic 0.
    Extinction ratio is 10.
     
  6. Dec 31, 2012 #5
    I understand that but what does that entail for the bits of information? A ratio of 1000 is a loss in signal of 30dB?
     
  7. Dec 31, 2012 #6

    berkeman

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    Staff: Mentor

    Not loss of signal.

    This is an example of AM (amplitude modulation). Do you know what a Modulation Depth is? If you look that up, the Extinction Ratio should make more sense.
     
  8. Dec 31, 2012 #7
    The bits of information are encoded as optical power level.

    Think of ordinary electrical digital logic. A TTL device encodes logic 1 as >2.2V and logic 0 as <0.8V. An ECL device encodes logic 1 as -0.9V and logic 0 as -1.7V.

    Binary digits are encoded on a fiber optic cable as logic 1 and logic 0 optical power levels. (This is a gross oversimplification, I am trying to convey the basic idea).
     
    Last edited: Dec 31, 2012
  9. Dec 31, 2012 #8

    jim hardy

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    Gold Member

    Simplification looks in order here.....??

    It's more like two people talking at some distance, say shouting down a hallway:
    IF:
    A loud shout is a "1"
    and a soft shout is a "0"
    (and heaven only knows why they're talking in binary).
    A loud shout might be 10X louder than a soft one which would be an extinction ratio of 10,
    but that says nothing about how many soft shouts were mistaken for loud ones , or were not heard at all.

    db comes from the unit BEL, after Alexander Graham.
    A bel is the logarithm of ratio between two powers, log(P2/P1).
    A decibel is a tenth of a bel, 10X log (P2/P1)
     
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