# What is the Extinction Ratio?

by JPBenowitz
Tags: extinction, ratio
 P: 118 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!
 P: 532 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: $$Extinction Ratio = \frac{logic 1 power}{logic 0 power}$$
P: 118
 Quote by the_emi_guy 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: $$Extinction Ratio = \frac{logic 1 power}{logic 0 power}$$
So a 1000:1 extinction ratio is simply that for every 1000 bits transmitted 1 bit is lost due to a loss in power?

P: 532

## 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.
P: 118
 Quote by the_emi_guy 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.
I understand that but what does that entail for the bits of information? A ratio of 1000 is a loss in signal of 30dB?
Mentor
P: 37,615
 Quote by JPBenowitz I understand that but what does that entail for the bits of information? A ratio of 1000 is a loss in signal of 30dB?
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.
P: 532
 Quote by JPBenowitz I understand that but what does that entail for the bits of information? A ratio of 1000 is a loss in signal of 30dB?
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).
 PF Patron Sci Advisor P: 2,917 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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