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Researchers at EPFL’s Laboratory of Photonics and Quantum Measurements (K-Lab), Trinity College Dublin (TCD), and Dublin City University (DCU) have teamed up to develop a new technique for generating variable low-noise microwaves with a single optical microresonator. The paper was recently published in Science Advances.

Optical frequency combs (OFCs) based on femtosecond pulse lasers have the potential to revolutionize the fields of optical metrology and spectroscopy. Development of the frequency division technique has allowed the use of photodetection of pulse trains to synthesize microwaves with lowest phase noise levels. However, the use of mode-locked laser-based OFCs has been limited to the laboratory due to their unwieldy size, high power consumption, and delicate structure. Although some approaches have been proposed to make OFCs field-deployable, they have limitations that prevent wider application.

The new research proposes a frequency division scheme in which two compact frequency combs, (a soliton microcomb and a semiconductor gain-switched comb) are combined to demonstrate low-noise microwave generation. Using the technique, the team successfully generated new microwaves that showed much lower phase-noise levels than those of a microresonator frequency comb oscillator and off-the-shelf microwave oscillators.

The technique presented by the authors enables spectral purity transfer between different microwave signals. Lead author Wenle Weng explains:

“Traditionally, executing perfect microwave frequency division in a variable fashion has not been easy. Thanks to the fast-modulated semiconductor laser developed by our colleagues at TCD and DCU, now we can achieve this using a low-cost photodetector and a moderate control system.”

While the traditional optical injection locking method uses a continuous-wave (CW) laser as the master, the new scheme locks a semiconductor laser to the entire microcomb, transferring both the carrier phase coherence and the soliton repetition rate spectral purity to the gain-switched laser (GSL). Consequently, the GSL can generate additional comb teeth that are fully coherent and equally spaced, facilitating the application of high-repetition rate microcombs in metrology and spectroscopy.

With the ability to be portable and mass-produced, the variable microwave oscillator and frequency comb generator developed by the team can revolutionize the market for portable low-noise microwave and frequency comb sources.

The research was funded by Swiss National Science Foundation; Defense Advanced Research Projects Agency, Defense Sciences Office (US); Science Foundation Ireland (SFI); and SFI/European Regional Development Fund.