Soliton Blueshift in Tapered Photonic Crystal Fibers

We show that solitons undergo a strong blueshift in fibers with a dispersion landscape that varies along the direction of propagation. The experiments are based on a small-core photonic crystal fiber, tapered to have a core diameter that varies continuously along its length, resulting in a zero-dispersion wavelength that moves from 731 nm to 640 nm over the transition. The central wavelength of a soliton translates over 400 nm towards a shorter wavelength. This is accompanied by strong emission of radiation into the UV and IR spectral regions. The experimental results are confirmed by numerical simulation.

Figure 1

(a) GVD curve of the fiber used for the fabrication of the taper transition. The open squares are the experimentally measured points, and the solid curve is a simulated dispersion profile, obtained from a silica strand model. The inset shows an SEM of the fiber. (b) Outer-fiber diameter along the transition. (c) The solid line is the simulated dispersion of the taper at its narrowest point where the two ZDWs coalesce into one at $z_0$. The solid squares are the experimentally measured GVD points of a fiber with a uniform $48\mu \mathrm{m}$ outer diameter.

Figure 2

Spectral evolution with the distance of a 130 FWHM fs pulse launched into a taper with the profile shown in Fig. 1b. The upper plots are experimental and the lower plots numerical. The peak powers are (a) 660 W and (b) 6.5 kW. The white curves show the positions of the two ZDWs, obtained from simulations (based on SEMs of the cleaved fiber tips) using the fixed-frequency plane wave method [21]. Long wavelength bend loss causes a strong increase in IR attenuation beyond the 12 cm point.

Figure 3

Evolution of the center wavelength of the first two solitons (labeled A and B) emerging after the soliton fission process. After the maximum redshift due to SSFS, a blueshift occurs in response to the blueshifting ZDW $z_2$.

Figure 4

(a) and (b) show the spectral and temporal evolution of soliton A in the 8–14 cm portion of the taper transition. In (c) and (d) the GVD profile was approximated by a frequency independent ${\beta }_2$, varying linearly from $-68.8{\mathrm{ps}}^2/\mathrm{km}$ at 8 cm to $0{\mathrm{ps}}^2/\mathrm{km}$ at 14 cm. The nonlinear coefficient was set to a constant value in (c) and (d).