Abstract

Supercontinuum generation (SCG) in optical fibers has found a wide range of applications. SCG is especially efficient when solitons can be excited at the pump laser wavelength, which requires pumping the fiber above the zero-dispersion wavelength (ZDW) where material positive Kerr nonlinearity is balanced with the negative anomalous dispersion. Therefore the fiber is expected to have large capabilities of dispersion engineering so that the ZDW can be shifted below the pumping wavelength. In contrast, for a negative Kerr nonlinearity, soliton formation requires positive normal dispersion, i.e. pumping below the ZDW. This would remove the waveguide dispersion constraints. Uniquely, the negative nonlinearity is possible when using a quadratic nonlinear crystal, in which cascaded (phase-mismatched) second-harmonic generation (SHG) could produce an equivalent Kerr nonlinearity but is negative-signed. In bulk, such Kerr-like nonlinearities have found numerous applications [1]. Meanwhile, SCGs in quadratic nonlinear lithium niobate (LN) waveguides have also been observed [2], by pumping below the ZDW. However, quasi-phase-matching (QPM) is always employed in such waveguides, with the purpose to strengthen the cascaded nonlinearity. But QPM is detrimental to produce a broadband cascaded nonlinearity that is necessary to the soliton formation and to the SCG. Using QPM will cause light radiations from the soliton due to the sideband phase matching or high-order QPM effects, together with a reduction on the nonlinear susceptibility.

© 2015 IEEE