Abstract
We report the generation of square temporal-shape pulses with no loss of spectral bandwidth, using an analytic expression for the spectral phase modulation dependent only on the input spectrum and stretching factor. We demonstrate numerically and experimentally conversion of 40fs pulses into 150 times longer flat top pulses with sharp on and off fronts. Applications in pulse amplification and free electron lasers are considered.
Highlights
Energy scaling of ultrafast lasers is limited by the self-action of the electromagnetic field propagating through optical media, manifested as self-phase modulation (SPM) or selffocusing, limits the peak intensity that can be transmitted
We present an analytic solution to the generation of square pulses in the time domain starting from essentially any arbitrary spectrum
Of particular importance is that the approach presented causes no spectral bandwidth loss and can be reversed to obtain the original input pulse by introducing the complementary phase to the one used to stretch the pulse (Figs. 7 and 8)
Summary
Energy scaling of ultrafast lasers is limited by the self-action of the electromagnetic field propagating through optical media, manifested as self-phase modulation (SPM) or selffocusing, limits the peak intensity that can be transmitted. Flat top 200fs FWHM pulses were created using a phase mask that was found using both a genetic algorithm and a Gerchberg-Saxton algorithm [9] This approach works; for every input laser spectrum and final square pulse duration the phase needs to be determined by the optimization algorithm. There is an intriguing approach that is based on the concept of stacking pulse replica one after the other in order to create a flattop pulse with pulse duration equal to the number of pulses stacked in time [11] The advantage of this method is its simplicity. The disadvantage is that spectral interference among the replicas results in the sync-function spectrum of the output square pulse, which is much narrower than the input pulse spectrum
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