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 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 is found, providing the strongest compression of the fundamental soliton is found. It depends only on the order of the injected 𝑁 -soliton. The largest compression degree significantly exceeds the analytical prediction supplied by the Satsuma–Yajima formula. The mechanism behind this effect, which remains valid in the presence of the self-steepening, is explained. 38 18 3623-3626 3623-3626