ULTRASHORT-PULSE 2ND-HARMONIC GENERATION .2. NON-TRANSFORM-LIMITED FUNDAMENTAL PULSES

Ultrashort-pulse second-harmonic generation (USP SHG) is examined for fundamental pulses that are not transform limited but still possess co herent spectra. Pulses whose spectra contain various orders of nonline ar phases are considered, and spectral phase compensation (compression ) in either the fundamental pulse (FP) or the second harmonic (SH) is investigated with regard to optimizing final SH pulse duration and ove rall conversion efficiency. For Gaussian shaped or similarly smooth sp ectra with only quadratic phase it is found that such a phase maps pro portionally over to the spectrum of the SH, and the compressed SH puls e shapes and durations are independent of whether the compensation is performed in the FP or in the SH. The optimum compensation configurati on and the conversion efficiency depend on the nonlinear interaction l ength. For a FP with multiple peaked spectra, compensation in the SH g ives poor results, accentuating the structure inherent in the pulses. FP's with higher-order spectral phases were found to give different re sults for:he two different compensation schemes. In particular, it was found that second- plus fourth-order phases. are easier to compensate than second- plus third-order. In all the cases examined, the optimum nonlinear interaction length is one to three times the pulse-width-pr eservation length. These results are useful for both understanding and experimental optimization of USP SHG with state-of-the-art systems an d pulses 10 fs or shorter in duration.