**4. Conclusions**

The characterization technique of ultrashort optical pulses is introduced and demonstrated in this chapter. For accurate characterization of femtosecond optical pulses, we introduced a wavelet-transform technique for spectral phase retrieval. The wavelet-transform technique needs no filter; therefore, automatic and accurate phase retrieval and pulse reconstruction is realized because of no manual operation of selection and adjustment filter. With this technique, the ratio of autocorrelation width to pulse width can be determined, which is helpful in improving the precision of pulses width measurement with autocorrelation technique. We have demonstrated a multifunctional autocorrelator, based on a precise translation stage and a lock-in amplifier, for pulse width measurement of ultrashort optical pulses. With the autocorrelator, both interferometric autocorrelation and intensity autocorrelation can be obtained. Chirp can be clearly discerned form interferometric autocorrelation, and pulse width can be fast obtained from intensity autocorrelation. The measurement result can self-calibrate with the two correlation traces. The autocorrelator is suitable for the measurement of ultrashort optical pulses from sub-10 fs to several hundred picoseconds, and it is useful in generation and characterization of both solid ultrashort pulse lasers and fiber ultrashort pulse lasers.
