
#1
Dec3112, 12:38 AM

P: 121

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
Dec3112, 01:21 AM

P: 580

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] 



#3
Dec3112, 01:39 PM

P: 121





#4
Dec3112, 02:57 PM

P: 580

What is the Extinction Ratio?
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. 



#5
Dec3112, 04:12 PM

P: 121





#6
Dec3112, 04:20 PM

Mentor
P: 39,571

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. 



#7
Dec3112, 04:44 PM

P: 580

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). 



#8
Dec3112, 05:19 PM

Sci Advisor
P: 3,131

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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