Engineerable compression of ultrashort pulses by use of second-harmonic generation in chirped-period-poled lithium niobate

We demonstrate the use of an aperiodic quasi-phase-matching (QPM) grating to generate second-harmonic pulses that are stretched or compressed relative to input pulses at the fundamental frequency. We frequency doubled an externally chirped erbium-doped fiber laser generating 17-ps (FWHM) pulses at 1560 nm to produce near-transform-limited 110-fs (FWHM) pulses at 780 nm by use of a 5-cm-long lithium niobate crystal poled with a QPM grating chirped from an 18.2- to a 19.8-$\mathit{\mu }\mathrm{m}$ period.