Fate of a Soliton in a High Order Spatial Mode of a Multimode Fiber

We show numerically that under certain conditions noise-induced soliton self-mode conversion dominates over soliton self-frequency shift for a soliton in a high order spatial mode of a multimode optical fiber. The input soliton has to be group index matched to a lower order mode for a frequency separation for which the Raman gain is non-negligible, and this condition determines the wavelength of the pulse growing from noise. The phenomenon has no known analogs in single-mode or graded-index fibers. The results demonstrate that it is possible for a noise-induced physical process to dominate over a seeded one even for noise levels at the fundamental limit.

Figure 1

The spectral evolution of a soliton with an input duration of (a) 80 and (b) 40 fs in the multimode step-index fiber. The spatial modes have been indicated by the dashed lines that follow the respective spectral maxima. Note the different length scales in (a) and (b).

Figure 2

The spectral evolution of a soliton with an input duration of 15 fs.

Figure 3

The frequency separation needed to group-velocity match ${\mathrm{LP}}_{0,19}$ to a lower frequency in ${\mathrm{LP}}_{0,18}$ as a function of the ${\mathrm{LP}}_{0,19}$ soliton wavelength (solid black line). The blue crosses correspond to the stages of energy transfer in the simulations where 1% of the total energy has been transferred from ${\mathrm{LP}}_{0,19}$ to ${\mathrm{LP}}_{0,18}$. The red circles correspond to 50% energy transfer for the same simulations. The normalized Raman gain is shown on the right.

Figure 4

(a) The distance of 50% energy transfer (solid black curve) and the interaction length (dashed blue curve) as a function of the input soliton FWHM duration. (b) The percentage of energy in the ${\mathrm{LP}}_{0,19}$ mode (color coded) as a function of input soliton duration and propagation distance.