A key building block of microwave photonics, optical frequency combs provide hundreds of equidistant and mutually coherent laser lines emitted as ultrashort optical pulses with a stable repetition rate, which can be turned into a microwave carrier upon photodetection.
Publishing their results in Nature Photonics under a paper titled “Photonic microwave generation in the X- and K-band using integrated soliton microcombs”, the EPFL research team led by Tobias J. Kippenberg was able to manufacture silicon nitride waveguides with the lowest loss in any photonic integrated circuit. Using this technology, the generated coherent soliton pulses have repetition rates in both the microwave K- (~20 GHz, used in 5G) and X-band (~10 GHz, used in radars).
The resulting microwave signals feature phase noise properties on par with or even lower than commercial electronic microwave synthesizers. The demonstration of integrated soliton microcombs at microwave repetition rates bridges the fields of integrated photonics, nonlinear optics and microwave photonics.
The researchers achieved a level of optical losses low enough to allow light to propagate nearly 1 meter in a waveguide that is only 1 micrometer in diameter. This loss level is still more than three orders of magnitude higher than the value in optical fibers, but represents the lowest loss in any tightly confining waveguide for integrated nonlinear photonics to date, the researchers say.