Enhanced intermodal four-wave mixing for visible and near-infrared wavelength generation in a photonic crystal fiber

We demonstrate experimentally an enhanced intermodal four-wave mixing (FWM) process through coupling positively chirped femtosecond pulses into the deeply normal dispersion region of the fundamental mode of an in-house fabricated photonic crystal fiber (PCF). In the intermodal phase-matching scheme, the energy of the pump waves at 800 nm in the fundamental mode is efficiently converted into the anti-Stokes waves around 553 nm and the Stokes waves within the wavelength range of 1445-1586 nm in the second-order mode. The maximum conversion efficiency of η_as and η_s of anti-Stokes and Stokes waves can be up to 21% and 16%, respectively. The Stokes frequency shift Ω is 5580 cm^-1. The fiber bending and intermodal walk-off effect of pulses do not have significant influence on the nonlinear optical process.