Robust Phase Retrieval Using Group-Delay- Dispersion-Scanned Second-Harmonic Generation Demonstrated in a Femtosecond Fiber Chirped-Pulse Amplification System

We propose and demonstrate a phase retrieval method using a novel variant of the dispersion scan (d-scan) technique via both simulations and experimental measurements on a femtosecond fiber laser. The method combines a map of group-delay d-scan second-harmonic generation (SHG) spectra together with the fundamental spectrum as inputs. In order to ensure that the technique is robust when pulse-to-pulse fluctuations are present, we use only the wavelengths of the resulting SHG peaks, and avoid the areas with low signal in the borders of the data trace. Simulations confirmed that phase-retrieval is accurate even with high levels of laser fluctuation. The tradeoff is that pulses that have abruptly changing or highly modulated phase profiles, are not retrievable. The d-scan uses the compressor gratings intrinsic to fiber chirped-pulse amplification (CPA) systems, so the method is low cost. For experimental verification, we used a 470-fs ytterbium-fiber CPA system. However, the method is applicable to measurements on fiber-based femtosecond laser systems, which generally have an order of magnitude less bandwidth compared with the Ti:sapphire lasers used to test earlier variants of the d-scan approach.