High-Visibility Franson Interference Enabled by Passive Photonic Integrated Interferometers at Telecom Wavelengths

High-visibility Franson interference at telecom C-band wavelengths is achieved using a cascaded periodically poled lithium niobate (PPLN) waveguide photon-pair source combined with fully passive, path-imbalanced Mach-Zehnder interferometers implemented on photonic integrated circuits (PICs). The interferometers require neither on-chip phase shifters nor active stabilization; instead, the phase is scanned via thermal tuning of the chip. By employing a narrow-linewidth continuous-wave (CW) pump and dense wavelength-division multiplexing (DWDM) filtering, energy-time entangled photon pairs with high spectral indistinguishability are generated. We achieve a 4.8% heralding efficiency and a two-photon interference visibility of 97.1% from sinusoidal fringe fitting (raw visibility 95.2% and background-corrected visibility 95.6%), alongside a coincidence-to-accidental ratio (CAR) exceeding 1000 at only 1.7 mW of pump power. These results represent one of the highest Franson-interference visibilities reported for a PIC-based analyzer within a compact, fiber-integrated platform.