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Decibel

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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

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

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Bragg mirror

Evolution of the reflectivity spectrum of a Bragg mirror during fabrication.

This diagram has been made with the RP Coating software.

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