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

Area of Science:

• Photonics and Optical Engineering
• Materials Science
• Resonator Technology

Background:

• High-aspect-ratio silicon nitride (Si₃N₄) waveguide ring resonators are crucial components in integrated photonics.
• Accurate measurement of their free spectral range (FSR) and thermal properties is essential for device characterization and system integration.
• Existing methods may lack the real-time capability or precision required for advanced applications.

Purpose of the Study:

• To demonstrate a novel real-time scheme for measuring the FSR of Si₃N₄ waveguide ring resonators.
• To achieve high-precision FSR measurements using a fiber-based hybrid unbalanced Mach-Zehnder modulator (MZM) and optical single-sideband (OSS) technique.
• To determine the FSR coefficient of thermal expansion for Si₃N₄ resonators.

Main Methods:

• Utilized a fiber-based hybrid unbalanced Mach-Zehnder modulator (MZM) combined with an optical single-sideband (OSS) technique.
• Implemented resonance-tracking loops employing the Pound-Drever-Hall (PDH) technique for precise locking of resonance modes.
• Applied the developed scheme to a 35-mm Si₃N₄ waveguide ring resonator.

Main Results:

• Achieved a relative precision of 3.25 × 10^-6 in FSR measurements.
• Measured the FSR of the 35-mm Si₃N₄ resonator as 1,844,628 kHz with a quality factor (Q) of 3.211 × 10⁶.
• Determined the FSR coefficient of thermal expansion for the Si₃N₄ resonator to be -16.735±0.002 kHz/°C.

Conclusions:

• The demonstrated real-time measurement scheme offers a flexible and precise method for characterizing Si₃N₄ waveguide ring resonators.
• The high precision achieved and the measurement of thermal expansion are vital for the development of stable and advanced photonic systems.
• This technique provides a valuable photonic interface for realizing complex integrated optical devices and systems.