Broadband quasi-phase-matched second-harmonic generation in MgO-doped periodically poled LiNbO_3 at the communications band

A novel scheme for broadband second-harmonic generation was developed with type I quasi phase matching by use of an off-digital nonlinear optical coefficient $d_{31}$. This was achieved by matching of the group velocities of the second harmonic and the fundamental, while the phase velocities were quasi phase matched. The desired group-velocity dispersion could be obtained with ${\mathrm{LiNbO}}_3$ doped with 5 mol. % of MgO. Efficient second-harmonic generation with a bandwidth of 52 nm was obtained for the fundamental wavelength centered at 1566 nm in MgO-doped periodically poled ${\mathrm{LiNbO}}_3$, compared with a bandwidth of 1.3 nm for the conventional scheme using $d_{33}$. Efficient frequency doubling of an ultrafast pulse is expected without group-velocity walk-off or pulse distortion.