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Bandwidth-limited Pulses

Definition: pulses with a duration as short as possible with their optical spectrum

Alternative term: transform-limited pulses

German: bandbreitebegrenzte Pulse

Category: light pulses

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URL: https://www.rp-photonics.com/bandwidth_limited_pulses.html

A bandwidth-limited light pulse (or transform-limited pulse) is a pulse which is as short as its spectral bandwidth permits. In other words, its time–bandwidth product is as small as possible for a given temporal or spectral shape For example, the minimum time–bandwidth product of sech2-shaped pulses is 0.315, which implies that bandwidth-limited sech2 pulses with a duration of 100 fs must have a bandwidth of 3.15 THz.

Note that the considered spectral bandwidth is related to the Fourier transform of the electric field. That bandwidth is necessarily large when the pulses are very short; that is due to the basic mathematical properties of Fourier transforms [1], i.e., not related to any physical processes e.g. of the used pulse generation mechanism.

A slightly different definition for a bandwidth-limited pulse is that its peak power is as high as the optical bandwidth allows (for a given pulse energy). This is equivalent to the pulse having a flat spectral phase, but does not always precisely lead to the shortest possible pulse duration in terms of full width at half maximum.

optical bandwidth of bandwidth-limited pulses
Figure 1: Optical bandwidth of sech2-shaped bandwidth-limited pulses. Different curves apply for different center wavelengths due to the conversion from frequency to wavelength intervals.

If an initially transform-limited pulse propagates through a medium, its time–bandwidth product can increase due to the influences of chromatic dispersion or nonlinearities. For example, the influence of dispersion can temporally stretch the pulse while leaving its spectral width constant. This is associated with a chirp. This chirp can later be removed by dispersive pulse compression, restoring the original pulse duration.

Many mode-locked lasers, particularly soliton mode-locked solid-state lasers, but also some mode-locked diode lasers, can generate nearly bandwidth-limited pulses. This feature is very desirable e.g. in optical fiber communications, as it minimizes dispersive temporal broadening.

Questions and Comments from Users

2020-11-10

What happens when you have a very narrow linewidth laser beam and you shut on/off (e.g. by a mechanical shutter) this beam to get below the time–bandwidth product?

Answer from the author:

You cannot get below the time–bandwidth limit with optical switching, since the switching is a kind of modulation which increases the width of the optical spectrum – even if the instantaneous frequency of the optical field stays constant.

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Bibliography

[1]R. Paschotta, Spotlight article “Understanding Fourier Spectra

(Suggest additional literature!)

See also: time–bandwidth product, spectral phase, light pulses, chirp, pulse compression, The Photonics Spotlight 2008-06-13
and other articles in the category light pulses

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