  We demonstrate a compact photon pair source based on a periodically poled
lithium niobate nonlinear crystal in a cavity. The cavity parameters are chosen
such that the emitted photon pair modes can be matched in the region of telecom
ultra dense wavelength division multiplexing (U-DWDM) channel spacings. This
approach provides efficient, low-loss, mode selection that is compatible with
standard telecommunication networks. Photons with a coherence time of 8.6 ns
(116 MHz) are produced and their purity is demonstrated. A source brightness of
134 pairs(s.mW.MHz)$^{-1}$ is reported. The high level of purity and
compatibility with standard telecom networks is of great importance for complex
quantum communication networks.
