RP Photonics logo
RP Photonics
RP Consulting RP Software RP Encyclopedia RP Buyer's Guide
… combined with a great Buyer's Guide!
VLib part of the


<<<  |  >>>  |  Feedback

Buyer's Guide

Use the RP Photonics Buyer's Guide to find suppliers for photonics products! You will hardly find a more convenient resource.

Definition: a logarithmic measure for power ratios, applied e.g. to optical powers or to noise powers

German: Dezibel

Category: optical metrology

How to cite the article; suggest additional literature

The decibel (dB) is often used for quantifying the gain of an amplifier or the loss of some optical element, such as an optical fiber or an optical attenuator. The number of decibels is 10 times the logarithm (to base 10) of the power amplification factor or loss factor, or alternatively 20 times the logarithm of the amplitude ratio of the electric field strengths.

Such a logarithmic quantity is useful because e.g. the decibel gain values of several amplifiers in a sequence can simply be added to obtain the total gain of the amplifier chain.

decibel scale

Figure 1: Scale for converting decibels to power amplification factors and vice versa.

Decibels in the Context of Optical Signals

The decibel is also often used in the context of transmitted signals (e.g., for optical filters) and of noise e.g. of lasers or amplifiers. Great care is recommended in the context of optical signals, where one is dealing with two different kinds of power:

For example, if the peak-to-peak modulation of an optical power is increased by a factor of 10 (e.g. by amplification in a 10-dB fiber amplifier), this increases the signal power by 20 dB, not 10 dB.

Frequently Used Specifications

Some frequently used related specifications are:

See also: gain, amplifiers, optical attenuators

How do you rate this article?

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you like our website, you may also want to get our newsletters!

If you like this article, share it with your friends and colleagues, e.g. via social media:


A Quiz Question

taken from the Photonics Quiz:

The zero-point fluctuations (vacuum fluctuations) of the electromagnetic field

(a) are associated with a certain small density of photons (one photon per mode), which cannot be further reduced because quantum mechanics predicts some balance of absorption and emission processes for any absorber material

(b) are occurring when no photons at all are present

After selecting your answer(s) and pressing this button, find the explanations on the left side.

– Show all banners –

– Get your own banner! –