Diffraction-arrested soliton self-frequency shift of few-cycle laser pulses in a photonic-crystal fiber

The balance between diffraction and index-step guiding in photonic-crystal fibers is controlled by modifying the fiber structure, leading to different wavelength dependences of the effective mode area $S_{\mathrm{eff}}\left(\lambda \right)$ and providing a mechanism to control nonlinear-optical phenomena. In optical fibers with a steep $S_{\mathrm{eff}}\left(\lambda \right)$ profile, the guided mode of the light field tends to become much less compact with an increase in radiation wavelength, slowing down the Raman-induced soliton self-frequency shift of an ultrashort laser pulse. A $100\text{−}\mathrm{nm}$ reduction of the soliton self-frequency shift is demonstrated for $6\text{−}\mathrm{fs}$ input laser pulses.

Figure 1

(Color online) The effective mode area (1, 2) and the group-velocity dispersion (3, 4) for the PCFs of the first (1, 3) and second (2, 4) types. The inset shows a scanning electron microscopy image of the first-type PCF with a core diameter of $2.5\mu \mathrm{m}$.

Figure 2

(Color online) Output spectra measured (circles) and calculated (solid line) for a $2\text{−}\mathrm{nJ}$ few-cycle laser pulse (the input spectrum is shown in the inset) transmitted through the first-type PCF with a variable length—from top to bottom 4.5, 10, 15, and $19\mathrm{cm}$.

Figure 3

(Color online) The spectral intensity of the redshifted output of the second-type PCF: (1, 2) GNSE solution with a frequency-independent effective mode area (1) and with the $S_{\mathrm{eff}}\left(\lambda \right)$ profile shown by curve 2 in Fig. 1 (2); (3) results of experiments performed with the PCF of the second type. The spectrum of the input pulse is shown in the inset to Fig. 2. The input pulse energy is 0.45 (a) and $0.30\mathrm{nJ}$ (b).