Frequency combs as microwave spectral filters

Low-noise microwave signals are of high importance in numerous applications including high-speed telecommunication, ultrafast data processing, low-concentration detection, and testing fundamental physics constants. Conventionally, such signals are generated with bulky, complex, and delicate microwave oscillators that are not easily accessible to most laboratories.

As an alternative, low-noise microwave generation based on optical microresonators has been intensively explored in the past decades. Relying on the high optical frequency and spectral purity of lasers, optical microresonators provide the possibility of supplying low-noise microwaves in a simple, compact, and energy-efficient manner. However, the generated microwaves suffer from large frequency drifts and limited frequency range due to the vulnerability of the optical resonators to environmental instabilities and the intrinsic properties of the materials that the resonators are made of.

Published in Physical Review Letters, researchers at Tobias Kippenberg’s lab at EPFL and the Russian Quantum Centre have now developed a novel and effective method for generating low-noise microwaves using optical microresonators. The approach uses a laser to generate equally spaced Kerr solitons – self-organized dissipative temporal pulses – from a crystalline microresonator to form frequency combs – also known as “microcombs”.

The researchers input microwave signals from an off-the-shelf oscillator to the laser to trap the pulses in the microresonator. As a result, the timing of the pulses could be regulated by the input signals. Detecting the disciplined pulses with a photodetector, the team was surprised to find that they could generate microwave signals with the same frequency but much lower phase noise.

Disciplined by an input microwave signal, laser pulses that are generated in a microresonator can produce microwaves with significantly purified phase noises (credit: Weng Wenle, EPFL)

This phenomenon is caused by the competition between the self-organization of the Kerr solitons and the forces exerted by the optical traps. “In the long-time scale, the solitons are captured by the traps, so the generated microwave frequency is following that of the input signal”, explains Wenle Weng, the paper’s first author. “But in very short-time scales, the solitons can keep their extremely orderly rhythm without being disturbed by the traps.”

Consequently, the phase noise of the generated microwave signals maintains the original level of the Kerr solitons, which is significantly lower than those of most commercially available microwave oscillators.

Compared to other optical microresonator technologies, the microwaves produced with this method show excellent frequency stabilities. And, in principle, the frequency of the microwaves can be adjusted from a few GHz up to the THz scale, simply by using microresonators with different sizes.

The scientists are now working on implementing the technology with micro-fabricated chip-scale ring-resonators instead of crystalline ones. Micro-fabricated resonators can be integrated with a variety of photonic and microelectronic components, and can also be mass-produced. The superior performance of such “microwave purifiers” based on integrated photonics could revolutionize the emerging market for low-noise microwave sources.