Simultaneous SHG of orthogonally polarized fundamentals in single QPM crystals

Fabrication of quasi-phase-matching (QPM) gratings suitable for cascading of two second-order parametric nonlinear processes in a single lithium niobate crystal is being undertaken using a new technique - electric field poling assisted by laser micro-machined topographical electrodes. To date, single period poled gratings with 45.75, and 45.8 μm periods have been fabricated in order to demonstrate second harmonic generation of 1064nm laser light with 1^st order type-I and 7^th order type-0 QPM simultaneously. The two frequency doubling processes share a common Z polarized second-harmonic wave which allows exchange of energy between the two orthogonally polarized fundamental waves and several second order cascading interactions can be realized. The use of the higher QPM orders (3rd, 5th or7th) for the type-0 second harmonic generation process leads to comparable efficiencies of the two processes, as the respective nonlinear coefficients are d_zzz ∼27 pm/V and d_yyz ∼4.7 pm/V in lithium niobate crystals. Possible applications include; polarization switching, parametric amplification and polarization mode dispersion monitoring, and polarization insensitive second harmonic generation.