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# FAQ-Optical power, what is the difference between dBm and dB unit?

Publication Date:  2012-12-13  |   Views:  2  |   Downloads:  0  |   Document ID:  EKB1000021515

Contents

## Issue Description

Q：
Optical power, what is the difference between dBm and dB unit?

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## Handling Process

A：
1. dBm
dBm is a value of the power absolute value, formula is: 10lgP (power value / 1mw).
[Example 1] if the transmitter power P is 1mw, converted to dBm is 0dBm.
[Example 2] for 40W power value, translated in dBm unit, value is as follows:
10lg（40W/1mw)=10lg（40000）=10lg4+10lg10000=46dBm。
2. dB
dB is the value of a characterization relative value, when consider A power compared to B power is more or less hoe many dB, calculated by the following formula: 10lg (A power / B power)
【Example 3 】A power is twice times than B power, then 10lg (A power / B power) = 10lg2 = 3dB. That is to say, the A power is 3 dB greater than B power.
[Example 4] If A power is 46dBm, B power is 40dBm, it can be said, A is larger than B.

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## Suggestions

Optical power is generally related to the dBm and dB two units.
dBm means 1mw relative value, can be said to be a absolute value.
dB used to measure the relative values of random two power.