# What is the Extinction Ratio?

1. ### JPBenowitz

141
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. ### the_emi_guy

629
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:

$$Extinction Ratio = \frac{logic 1 power}{logic 0 power}$$

Last edited: Dec 31, 2012
3. ### JPBenowitz

141
So a 1000:1 extinction ratio is simply that for every 1000 bits transmitted 1 bit is lost due to a loss in power?

4. ### the_emi_guy

629
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. ### JPBenowitz

141
I understand that but what does that entail for the bits of information? A ratio of 1000 is a loss in signal of 30dB?

### 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.

7. ### the_emi_guy

629
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
8. ### jim hardy

5,131
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)