Clean optical solitons generation at the 1700 nm window in a short photonic-crystal rod

Tunable optical soliton generation from optical waveguides is especially suitable for multiphoton microscopy (MPM). Among the different waveguides, photonic-crystal (PC) rod has been mainly used for high-energy soliton generation at the 1700 nm window, which is promising for deep-tissue MPM. In order to obtain high-energy soliton, short PC rod is commonly used. This leads to dramatic overlap between the soliton spectrum and the residual pump or a second soliton spectrum and strong modulation of the soliton spectrum, when the pump pulse is relatively long, typical of energetic pump lasers for the PC rod. This spectral overlap and modulation make the spectral filtering of the solitons quite ambiguous and difficult. In this paper, we propose an effective method to generate clean solitons at the 1700 nm window, in a 44 cm short PC rod, by compressing the pump pulse width at 1550 nm. We demonstrate comparison experiments with both 500 and 175 fs pump pulse at 1550 nm. Our results clearly show that a shorter excitation pulse enables cleaner soliton generation at the 1700 nm window, in good agreement with theoretical expectations, which facilitates subsequent spectral filtering. We also demonstrate detailed comparison experiments and simulations of the filtered 1660 nm high-energy solitons. Compared with solitons generated by 500 fs pump pulse, the clean solitons generated by the 175 fs pump pulse also show clean temporal profiles manifested by both measured interferometric autocorrelation trace and simulated temporal intensity profile. We expect this technique will be beneficial for deep-tissue MPM excited at this window. (paper)