We report detailed experimental studies in a single wavelength-band system of correlated photon-pair generation in a 1.5 μm telecommunication wavelength-band using cascaded processes, second-harmonic generation, and the following spontaneous parametric down conversion (c-SHG/SPDC), in a periodically poled (PPLN) ridge-waveguide device. By using a PPLN module with of the SHG efficiency, we achieved a coincidence-to-accidental ratio (CAR) of 2380 at of the mean number of the signal photon per pulse. Detailed investigation on the origin of uncorrelated noise photons was also discussed in this paper. We revealed that the noise photons mainly originated from spontaneous Raman scattering induced in pigtail optical fibers and also that the PPLN device itself had poor contribution to the noise photons. This feature of the c-SHG/SPDC process is promising for the realization of a noise-photon-free, high-purity quantum entangled photon-pair source.
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