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±kHz/°C. This method will provide a flexible photonic interface for realizing advanced photonic systems.