High-Spectral Purity RF Beat Note Generated by a Two-Frequency Solid-State Laser in a Dual Thermooptic and Electrooptic Phase-Locked Loop

Ultralow noise stabilization of a two-frequency solid-state laser is experimentally demonstrated using a specially designed dual phase-locked loop. Thanks to an intracavity birefringent crystal providing both thermooptic and electrooptic frequency modulation properties, the laser delivers a beat frequency that is controlled by both pump power and voltage from 0 to 1000 MHz. When phase-locked against a radio-frequency synthesizer, the beat note has a 25-mHz full-width at half-maximum. The measured phase noise is -110 dBc/Hz at 10-kHz offset, and lower than -130 dBc/Hz at 1-MHz offset and above. Improvements and applications to local oscillators in microwave-photonic systems are discussed.