  Efficient supercontinuum generation (SCG) requires excitation of solitons at
the pump laser wavelength. Quadratic nonlinear waveguides may support an
effective self-defocusing nonlinearity so solitons can directly be generated at
common ultrafast laser wavelengths without any waveguide dispersion
engineering. We here experimentally demonstrate efficient SCG in a standard
lithium niobate (LN) waveguide without using quasi-phase matching (QPM). By
using femtosecond pumps with wavelengths in the $1.25-1.5 \mu\rm m$ range,
where LN has normal dispersion and thus supports self-defocusing solitons,
octave-spanning SCG is observed. An optimized mid-IR waveguide design is
expected to support even broader spectra. The QPM-free design reduces
production complexity, allows longer waveguides, limits undesired spectral
resonances and effectively allows using nonlinear crystals where QPM is
inefficient or impossible. This result is important for mid-IR SCG, where
QPM-free self-defocusing waveguides in common mid-IR nonlinear crystals can
support solitons directly at mid-IR ultrafast laser wavelengths, where these
waveguides have normal dispersion.
