Investigation of wave breaking in the normal dispersion region of nano-structured photonic crystal fibers

The propagation of femtosecond laser pulses with wavelengths of 1550 nm, 1064 nm, 800 nm and 700 nm, respectively, which are in the normal dispersion region of the nano-structured photonic crystal fiber (N-PCF) with interesting broadband normal dispersion and highly nonlinear properties, is studied. For the effect of chirp variation mainly induced by group velocity dispersion (GVD) and self-phase modulation (SPM), after propagation over a short length, the wave breaking occurs. Namely, oscillatory structures are presented near pulse edges and sidelobes appear in the pulse spectrum. In the case of 800 nm, after the propagation of 20 mm, a super flat spectrum is obtained. The bandwidth of the super flat spectrum is associated with the dispersion length and the nonlinear length. By choosing N-PCF and laser pulse with appropriate parameters, a broadband super flat spectrum in a short length can be achieved.