**2. Characterization of femtosecond pulses with wavelet-transform for spectral phase retrieval**

The spectrum of a pulse can easily be measured with a spectrometer. The pulse would be completely known, if we could, in addition, determine the phase across the spectrum [9]. SPIDER is a technique for measurement spectral phase of femtosecond optical pulses. In SPIDER setup, two replicas of the input pulse to be characterized are generated with a fixed time delay between them. These two replica pulses are then upconverted by sum-frequency mixing with a strongly chirped pulse derived from the same original input pulse. Because the two replica pulses are separated in time domain, they interact with different parts of the chirped pulse and are therefore upconverted to different frequencies. From the interferogram of these spectral shearing pulses, it is possible to extract the amplitude and phase of the initial pulse by using an algebraic inversion algorithm [2].
