What is the Extinction Ratio?

  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. 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. 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. 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. 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. berkeman

    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. 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. jim hardy

    jim hardy 4,367
    Science Advisor
    Gold Member
    2014 Award

    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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