Nonlinear-optical spectral transformation of few-cycle laser pulses in photonic-crystal fibers

Photonic-crystal fibers with special dispersion profiles are shown to provide a high efficiency of spectral transformation of chirped sub-$6\text{‐}\mathrm{fs}$ Ti:sapphire laser pulses. With the wavelength of zero group-velocity dispersion of the fiber lying within the broad spectrum of the input few-cycle pulse, the output spectra feature well-resolved spectral peaks, indicative of soliton self-frequency shift, four-wave mixing, and Cherenkov emission of dispersive waves. We demonstrate that up to 3% of radiation energy at the output of the fiber can be confined within a spectrally isolated soliton peak centered at $1060\mathrm{nm}$, which is ideally suited as a seed for Nd:YAG- and ytterbium-based laser devices.

Figure 1

(Color online) (a) Group-velocity dispersion as a function of the wavelength for the fundamental mode of (curve 1) the fused silica photonic-crystal fiber with a core diameter of $1.6\mu \mathrm{m}$ (shown in the inset) and (curve 2) NL-PM-750 PCF from Crystal Fibre. Curve 3 shows the intensity spectrum of the input laser pulse (Ti:sapphire oscillator output). (b) Temporal envelope and chirp of the Ti:sapphire oscillator output reconstructed from SPIDER data.

Figure 2

(Color online) Temporal (a) and spectral (b) evolution of a laser pulse with an initial energy of $0.25\mathrm{nJ}$ and an input temporal envelope and chirp shown in Fig. 1b propagating through the PCF with the dispersion profile as shown by curve 2 in Fig. 1a.

Figure 3

(Color online) Spectral evolution of a laser pulse with an initial energy of $0.3\mathrm{nJ}$ and an input temporal envelope and chirp shown in Fig. 1b propagating through the PCF with the dispersion profile as shown by curve 1 in Fig. 1a (a) with and (b) without the Raman term in the generalized nonlinear Schrödinger equation. The insets show the time-domain structure of the radiation field in the PCF for $z=25\mathrm{cm}$ (a) with and (b) without the Raman effect.

Figure 4

(Color online) Filled circles connected with a solid line show experimental spectra measured at the output of (a) a $20\text{‐}\mathrm{cm}$ NL-PM-750 fiber and (b) a $23\text{‐}\mathrm{cm}$ PCF with a cross-section structure shown in the inset to Fig. 1a. The dashed line in (a) presents the input spectrum, provided by the Ti:sapphire oscillator. The inset in (b) shows the close-up of the $1064\text{‐}\mathrm{nm}$ soliton peak in the output of the PCF output spectrum.

Figure 5

(Color online) Spectrum of radiation intensity at the output of a $20\text{‐}\mathrm{cm}$ NL-PM-750 fiber with a modified spectrum of the input ultrashort laser pulse (shown in the inset).