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Nonlinear cascaded femtosecond third harmonic generation was experimentally investigated pumped by 100 fs pulses at optical communication band 1550 nm using a multi-grating 5 mol. % MgO-doped periodically poled lithium niobate crystal. The optimized efficiency of 10.8% was achieved with the simultaneous phase-matching of the second harmonic and sum frequency process. And the third harmonic spectrum reached as broad as 8.7 nm because of the choosing of a small group velocity mismatching between the fundamental and second harmonic pulses. Nonlinear cascaded method will provide a reference for the efficient frequency conversion in the high intensity range.

The authors report the filamentary waveguide formation and the significant spectral broadening based on periodically poled lithium niobate substrate. The modified morphology contributes to the combined effects of optical diffraction and self-focusing with the dependence on pulse intensity. Up to 4 times broadening of the FF wave and about 47 nm spanning of the SH wave with the pump power of 19.5 mW are achievable under 1550 nm excitation. Spectral evolution by cubic nonlinearity inside the waveguide has been obtained numerically, and provides a reasonable agreement with the experimental results.

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