We theoretically investigate a phase-matching (PM) between the HE11 and HE13 modes at wavelengths 1596 and 532 nm, respectively, of a real germania-silica fiber waveguide, whose preform was made by MCVD technology. For several measured refractive index profiles of the fiber preform, the corresponding waveguide diameters, providing homogeneous PM, both with modal dispersion and power characteristics, are calculated. The PM parameters obtained for the real fiber are compared to that calculated for a standard step-index fiber model.

We analyze the origins of the luminescence in germania-silica fibers with high germanium concentration (about 30 mol.% GeO2) in the region 1-2 µm with a laser pump at the wavelength 532 nm. We show that such fibers demonstrate the high level of luminescence which unlikely allows the observation of photon triplets, generated in a third-order spontaneous parametric down-conversion process in such fibers. The only efficient approach to the luminescence reduction is the hydrogen saturation of fiber samples, however, even in this case the level of residual luminescence is still too high for three-photon registration.

We prepare qudits based on angular multimode biphoton states by modulating the pump angular spectrum. The modes are prepared in the Schmidt basis and their intensity distributions do not overlap in space. This allows one to get rid of filtering losses while addressing single modes and to realize a single-shot qudit readout.

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