Abstract
Crystal nonlinearity is a widely employed technique in measurements and analyses of femtosecond pulse lasers. To solve the Group Delay Dispersion (GDD) problem and enable the proper usage of thick crystal, which possess excessively high conversion efficiency, in pulse measurements, we thoroughly examine the Second-Harmonic-Generation Frequency-resolved Optical Gating (SHG FROG) traces strongly distorted by excessive GDD. The SHG FROG traces from fundamental pulse with short pulse duration (25fs) in thick Aperiodically Poled Lithium Niobate (APPLN) crystal (~15mm) are considerably different from the standard SHG FROG traces, indicating that GDD severely influences the FROG traces and the standard retrieving method of phases of fundamental fields in SHG FROG is no longer applicable. Here we introduce a new retrieving method making use of the GDD-distorted FROG traces and applying the iterative optimization algorithm: Particle Swarm Optimization (PSO), which allows us to successfully retrieve the spectral phase of the fundamental pulse. In the experiment, we apply a 15mm APPLN as the nonlinear crystal to demonstrate serious GDD effect and meanwhile obtain a PM spectrum that is sufficient for 25-fs Ti:S laser as fundamental pulse. A shaper-assisted interferometer with collinear output beam duplicates is set to scan the GDD-distorted FROG traces with APPLN. The experimental results prove the strength and usefulness of this retrieving algorithm and unprecedentedly demonstrate pulse retrieval using thick crystal and under severe GDD.