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Definition: the minimum pulse duration possible for a given optical spectrum
In ultrafast optics, the transform limit (or Fourier limit, Fourier transform limit) is usually understood as the lower limit for the pulse duration which is possible for a given optical spectrum of a pulse. A pulse at this limit is called transform-limited. The condition of being at the transform limit is essentially equivalent to the condition of a frequency-independent spectral phase (which leads to the maximum possible peak power), and basically implies that the time-bandwidth product is at its minimum and that there is no chirp.
For a given pulse duration, transform-limited pulses are those with the minimum possible spectral width. This is important e.g. in optical fiber communications: a transmitter emitting close to transform-limited pulses can minimize the effect of chromatic dispersion during propagation in the transmission fiber, and thus maximize the possible transmission distance.
Many mode-locked lasers, particularly soliton lasers, are able to generate close to transform-limited pulses. During propagation e.g. in transparent media, phenomena like chromatic dispersion and optical nonlinearities can cause chirp and thus can lead to non-transform-limited pulses. Such pulses may be brought back to the transform limit (and thus temporally compressed) by modifying their spectral phase, e.g. by applying a proper amount of chromatic dispersion. This is called dispersion compensation. For not too broad spectra, compensation of second-order dispersion is often sufficient, whereas very broad spectra may require compensation also of higher-order dispersion in order to get close to the transform limit.
See also: spectral phase, pulse duration, time-bandwidth product, dispersion, dispersion compensation
