Widely tunable femtosecond solitonic radiation in photonic crystal fiber cladding

We report on a means to generate tunable ultrashort optical pulses. We demonstrate that dispersive waves generated by solitons within the small-core features of a photonic crystal fiber cladding can be used to obtain femtosecond pulses tunable over an octave-wide spectral range. The generation process is highly efficient and occurs at the relatively low laser powers available from a simple Ti:sapphire laser oscillator. The described phenomenon is general and will play an important role in other systems where solitons are known to exist.

Figure 1

GVD as a function of wavelength for two orthogonal polarizations in a sample knot. The dispersion for polarization along the long axis (axis 1) is shown by the blue solid line, and that for the orthogonal one (axis 2) is shown by the green dashed line. The inset presents a scanning electron microscope image of the cladding.

Figure 2

Output spectra obtained by coupling the laser pulses (with center wavelength of 745 nm) into the cladding of a 3.2-cm-long PCF for the optimum polarization (along axis 1 in Fig. 1). The blue solid line and red dashed line are the spectra recorded by two different spectrometers for visible and infrared regions, respectively.

Figure 3

(a) PCF output spectrum varied by coupling the same input femtosecond pulse (with center wavelength of 800 nm) into different knots of the PCF cladding. (b) RDW tuning by varying the pump wavelength (while keeping the same pump power); blue squares correspond to blue RDWs generated in a cladding knot next to the hollow core (marked as 1 in the inset), and green circles represent those generated in an adjacent knot (marked as 2 in the inset).

Figure 4

Auto- and cross-correlation traces of the pump pulse and the 470-nm RDW. The 1-ps separation between the peaks corresponds to the delay between the two pulses. The two side peaks correspond to cross-correlation traces between the RDW pulse and the pump pulse, and it shows that the pulse duration of the 470-nm RDW is less than 160 fs (with a Gaussian pulse assumed) after PCF and other optics (with no dispersion compensation being used).

Figure 5

Results of a numerical simulation where the GNLSE [Eq. (1)] (including high-order dispersion terms and the Raman effect) is solved numerically. Parts (a) and (b) give, respectively, the temporal and the spectral domain pictures of the output field, both in logarithmic scale, for a propagation length of 3.2 cm. Part (c) shows the XFROG spectrogram of the field, showing the soliton splitting phenomenon and the emission of short RDWs in the infrared and blue regions of the spectrum. Parts (d), (e), and (f) are the same as in (a), (b), and (c), but for a slightly longer propagation distance of 5 cm. It can be seen that RDWs have rapidly increased their temporal duration. The input pump power (150 mW before coupling; coupling efficiency $15\%$) and input pulse duration (30 fs before the PCF) match the conditions of our experiments. The repetition rate is 85 MHz.

Figure 6

The output spectrum of a 10-cm-long PCF with pump laser wavelength at 1500 nm. The peak around 500 nm is the third harmonic of the pump laser, and the peak around 425 nm is the generated RDW (where here we use a 50-fs pulse obtained from an optical parametric amplifier).