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

Abstract

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-dispersion-scanned (d-scan) second-harmonic generation (SHG) spectra together with the fundamental spectrum as inputs. In order to ensure the technique is robust when pulseto-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 the phaseretrieval is accurate even with high levels of laser fluctuation. The trade-off is that pulses which have abruptly changing or highly modulated phase profiles are not retrievable. The d-scan uses the compressor gratings intrinsic to the fiber chirped-pulse amplification (CPA) systems so the method is low cost. For experimental verification, we used a 470 fs ytterbium-fiber CPA system, so the method is applicable to measurements on fiber based fs laser systems which generally have an order of magnitude less bandwidth than the Ti:sapphire lasers used to test earlier variants of the d-scan approach.