**3. Characterization of ultrashort optical pulses with autocorrelations**

There are mainly two kinds of autocorrelations, i.e., intensity autocorrelations and interferometric autocorrelations [6]. Both of them have a Michelson-type interferometer setup. Intensity autocorrelation uses a translation stage scanning the split pulses to give an intensity envelope of the autocorrelation. The pulse width can be deduced from the width of the autocorrelation envelope. Intensity autocorrelation has a broad measurement range by use of the translation stage; however, it cannot provide complete information about the pulse shape. Interferometric autocorrelation uses a vibratile arm real-time scanning the split pulses to generate a distinctive autocorrelation trace. The chirp and phase of the ultrashort optical pulse are reflected in the interferometric trace; however, the vibration range is limited in several hundred micrometers, which results in a limitation of measurement range with this technique. Therefore, a multifunctional autocorrelator, which can realize both interferometric autocorrelation and intensity autocorrelation, is desirable.

Meshulach *et al* have demonstrated a third-harmonic generation (THG) autocorrelator for ultrashort optical pulses measurement [12]. They used an ordinary glass slide as the THG source, and a pair of 32:1 THG interferometric autocorrelation trace and intensity autocorrelation trace were obtained. However, THG needs the input pulse have enough high energy, and the generated signal always has a low SNR in week energy conditions. In this letter, we demonstrate a second-harmonic generation (SHG) autocorrelator. We use a two-photon detector as the source of SHG. The setup is simplified, and the sensitivity is improved.
