  We demonstrate photon-pair generation in a reverse proton exchanged waveguide
fabricated on a periodically poled magnesium doped stoichiometric lithium
tantalate substrate. Detected pairs are generated via a cascaded second order
nonlinear process where a pump laser at wavelength of 1.55 $\mu$m is first
doubled in frequency by second harmonic generation and subsequently
downconverted around the same spectral region. Pairs are detected at a rate of
42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair
generation process is similar to four-wave-mixing where two pump photons
annihilate and create a correlated photon pair.
