Flat supercontinuum generation in cascaded fibers pumped by a continuous wave laser.

We realize the flattening and extending of a CW-pumped supercontinuum with a high spectral intensity peak at the pump region. It is achieved by cascading a long zero-dispersion wavelength high-nonlinearity fiber with the output photonic crystal fiber, in order to improve the conversion efficiency of residual pump energy to long-wavelength continuum based on the effect of cascaded stimulated Raman scattering. Compared with the non-flattened continuum of 10.3 W with 3 dB bandwidth of 62 nm and 10 dB bandwidth of 360 nm, a flat continuum of 8 W with 3 dB spectral range of 340 nm and 10 dB spectral range of 420 nm is obtained. The spectral peak at the pump region decreases more than 5 dB, below the level of long-wavelength spectral intensity. Also, the long-wavelength edge has been extended by 60 nm.