Microwave signal generation with high spectral purity using two-frequency solid-state lasers

Two-frequency solid-state lasers are shown to provide beat notes at microwave frequencies with continuous tunability, high spectral purity and 100% modulation depth. Owing to an intra-cavity birefringent crystal providing both electro- and thermo-optic properties, the laser delivers a beat frequency that is both voltage- and temperature-controlled. First, a dual phase-locked loop is experimentally demonstrated to stabilize a two-frequency solid-state laser against a RF synthesizer, resulting in a linewidth of 25 mHz. Second, we show that pump-power modulation can lead to a new two-frequency pulsed operation with pulse-to-pulse RF coherence. Third, a specially designed two-frequency composite Er-Yb:glass-LiTaO₃ microchip laser is shown to provide a beat note up to 100 GHz with continuous tunability. Applications of these lasers to local oscillators in microwave-photonic systems and to lidar-radars are discussed.