Transform limited spectrum

From RP photonics:

In ultrafast optics, the transform limit (or Fourier limit, Fourier transform limit) is usually understood as the minimum pulse duration which is possible for a given optical spectrum of a pulse. A pulse at this limit is called transform-limited. This condition of being at the transform limit is close to (although not exactly identical with) 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.

Many mode-locked lasers, particularly soliton lasers, are able to generate close to transform-limited pulses. Processes such as dispersion or optical nonlinearities can cause chirp and thus can lead away from the transform limit. Non-transform-limited pulses may be brought to the transform limit (and thus temporally compressed) by modifying their spectral phase, e.g. by applying a proper amount of dispersion or a pulse shaper. This is called dispersion compensation. For not too broad spectra, compensation of second-order dispersion is often sufficient, while very broad spectra may require compensation also of higher-order dispersion in order to get close to the transform limit.