Optical frequency division using a mode-locked- laser, frequency comb has revolutionized time keeping and the generation of stable microwave signals. A different approach for optical frequency division and microwave generation using a comb of frequencies is demonstrated. A tunable electrical oscillator (as opposed to an optical resonator) sets the line spacing of the comb by direct phase modulation of two optical signals whose difference frequency is very stable. Phase-locked control of the electrical oscillator by op- tical division of the higher, difference frequency produces stable microwaves. Functionally, the approach transposes in the frequency domain the oscillator and frequency reference functions of a conventional microwave frequency synthesizer. A significant benefit is magnification of the phase noise stability of the electrical oscillator as opposed to its reduction relative to the frequency reference. Besides simplicity, the approach is tunable and scalable to higher division ratios, potentially approaching that of a conventional mode- locked laser frequency comb.
Citation: Science Magazine
Pub Type: Journals
frequency comb, frequency division, microwave synthesis