定义：
对于一个给定光谱能达到的最小脉宽

在超快光学中，变换极限脉宽（也称傅立叶极限脉宽或者傅立叶变换极限脉宽）通常指的是对于脉冲的光谱所能得到的最短的脉宽。此时的脉冲被成为变换极限脉冲。要得到变换极限脉宽需要脉冲具有一个与频率无关的谱相位（此时脉冲具有最大的峰值功率），这意味着时间带宽积达到了最小且脉冲不具有啁啾。时间带宽积的最小值取决于脉冲形状，对于高斯型脉冲，其最小值约为0.44；对于双曲正割型脉冲，其最小值约为0.315。（以上脉宽和谱宽的取值均为其半高全宽值。） 
对于给定的脉宽，变换极限脉冲是那些具有最小谱宽的脉冲。这一点对于光纤通信是很重要的：如果发射机发射一个近变换极限脉冲时，可以尽量减少传输过程中色散的影响，从而最大化可传输的距离。 
许多锁模激光器，尤其是孤子激光器，可以产生近变换极限脉冲。当脉冲在透明介质中传输时，色散和非线性效应会引起啁啾从而导致脉冲不是变换极限脉冲。这样的脉冲可以通过对其施加合适的色散从而修改其谱相位，使得其变回变换极限脉冲（同时在时域上被压缩），这就是所谓的色散补偿。对于不是很宽的光谱，进行二阶色散补偿通常就足够了，而对于很宽的光谱，则可能需要补偿较高阶的色散，从而获得近变换极限。

Definition: a limit for the time–bandwidth–product of an optical pulse

Alternative term: Fourier transform limit

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. The minimum time–bandwidth product depends on the pulse shape, and is e.g. ≈ 0.315 for bandwidth-limited sech²-shaped pulses and ≈ 0.44 for Gaussian-shaped pulses. (These values hold when a full-width-at-half-maximum criterion is used for the temporal and spectral width.)

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 mode-locked lasers, are able to generate close to transform-limited pulses. During propagation e.g. in transparent media, phenomena such as 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 approach the transform limit.