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Real-time free spectral range measurement based on optical single-sideband technique.

We demonstrate a real-time scheme for measuring the free spectral range (FSR) of a high-aspect-ratio Si3N4 waveguide ring resonator with a fiber-based hybrid unbalanced Mach-Zehnder modulator (MZM) using an optical single-sideband technique. Resonance-tracking loops were established with the Pound-Drever-Hall technique for locking resonance modes. A relative precision of 3.25 × 10-6 was achieved for a 35-mm waveguide ring resonator with FSR = 1,844,628 kHz and Q = 3.211 × 106. Furthermore, the Si3N4 resonator FSR coefficient of thermal expansion was measured as -16.735±0.002 kHz/°C. This method will provide a flexible photonic interface for realizing advanced photonic systems.