Generation of tightly compressed solitons with a tunable frequency shift in Raman-free fibers

Optimization of the compression of input N-solitons into robust ultra-narrow fundamental solitons, with a tunable up- or downshifted frequency, is proposed in photonic crystal fibers (PCF) free of the Raman effect. Due to the absence of the Raman self-frequency shift, these fundamental solitons continue propagation maintaining the acquired frequency, once separated from the input N-soliton's temporal slot. A universal optimal value of the relative strength of the third-order-dispersion (TOD) is found, providing the strongest compression of the fundamental soliton, is found. It depends only the order of the injected N-soliton. The largest compression degree significantly exceeds the analytical prediction supplied by the Satsuma-Yajima (SY) formula. The mechanism behind this effect, which remains valid in the presence of the self-steepening, is explained.