Characterisation of the spatial purity of photon pairs generated in a multimode non-linear waveguide

We have demonstrated recently the feasibility of controlling the spatial characteristics of photon pairs produced in a multimode waveguide through intermodal dispersion. Here we present experimental methods to characterise the spatial purity of the generated photons. Our sample was a 1 mm long periodically poled potassium titanyl phosphate (PPKTP) multimode waveguide suitable for type-II down-conversion producing photons in the 800 nm wavelength region. The phase matching function for these waveguides was characterized using the sum-frequency generation process, and the nonlinear process was carefully arranged to generate photons in fundamental spatial modes.