  We report experimental determination of the phase-matching function for
type-II three-wave mixing in a periodically poled KTiOPO_4 waveguide in the
792-815 nm spectral region. The measurement was performed by sum-frequency
generation of spectrally tuned fundamental components. Strong dependence of the
observed signal on the excited spatial modes in the waveguide has been observed
and fully interpreted. These results indicate a route to employ the waveguide
for spontaneous parametric down-conversion producing photon pairs in
well-defined spatial modes.
