Frequency-Bin Photon-Pair Comb Generation via Cavity-Enhanced SPDC in a Grating-Based Waveguide

We numerically investigate spontaneous parametric down-conversion (SPDC) in a thin-film lithium niobate waveguide cavity formed by wavelength-selective Bragg gratings (VSBGs). Using a 5 mm-long periodically poled lithium niobate (PPLN) waveguide, the cavity supports frequency-multiplexed photon-pair generation with a comb-like joint spectral structure. More than 8 resolvable frequency bins are obtained across the phase-matching bandwidth, with a cavity-defined spacing of approximately 12.8 GHz and individual joint spectral linewidths as narrow as 34.09 MHz. The cavity discretizes the spectrum without external filtering, while fabrication-tolerant grating mirrors enable robust and scalable implementation. This compact integrated platform provides a practical route toward dense frequency-bin quantum light sources for quantum communication and networking applications.