Mid-IR laser plays a significant role in ultrafast spectroscopy and strong-field experiments, and has attracted great attentions, both scientifically and technologically. Therefore, the accurate characterization of ultrashort mid-IR pulses is very essential and necessary in order to facilitate the potential applications of these long wavelength laser sources.

Recently, the researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) proposed a new initial guess in PCGPA. The study, published in Optics and Laser Technology, was based on second-harmonic-generation frequency-resolved optical gating (SHG-FROG) technique, and the SHG-FROG device can be used for accurately characterizing ultrashort pulse in the mid-IR.

In the experiment, a mid-infrared pulse at 3580 nm from the home-built optical parametric amplifier (OPA) system was measured with a good agreement between the retrieved and measured spectrums, and different initial guesses were studied. The spectrum retrieved by the proposed method was closer to the measured spectrum in spectrum shape and bandwidth.

Moreover, a further verification was made to compare the measured spectral phase difference with the calculated one by inserting a dispersion plate. The excellent phase agreement verified the accuracy of the measurement and high reliability of the device.

This initial guess with constraint both in time and frequency domain outperforms other guesses for the data errors analysis in these SHG-FROG apparatus. Therefore, it is capable of providing powerful support on dispersion compensation in the mid-IR.

The spectra retrieved by PCGPA. (Image by SIOM)