In our information society, the synthesis, distribution, and processing of radio and microwave signals are ubiquitous in wireless networks, telecommunications, and radars. The current tendency is to use carriers in higher frequency bands, especially with looming bandwidth bottlenecks due to demands for, e.g., 5G and the Internet of Things (IoT). Microwave photonics, a combination of microwave engineering and optoelectronics, might offer a solution.
A key building block of microwave photonics is optical frequency combs, which provide hundreds of equidistant and mutually coherent laser lines. They are ultrashort optical pulses emitted with a stable repetition rate that corresponds precisely to the frequency spacing of the comb lines. The photodetection of the pulses produces a microwave carrier.
In recent years there has been significant progress in chip-scale frequency combs generated from nonlinear microresonators driven by continuouswave lasers. These frequency combs rely on the formation of dissipative Kerr solitons, which are ultrashort coherent light pulses circulating inside optical microresonators. Because of this, these frequency combs are commonly called “soliton microcombs”.
Generating soliton microcombs needs nonlinear microresonators, which can be directly built on-chip using CMOS nanofabrication technology. Co-integration with electronic circuitry and integrated lasers paves the path to comb miniaturization, allowing a host of applications in metrology, spectroscopy, and communications.
An EPFL research team has now demonstrated integrated soliton microcombs with repetition rates as low as 10 GHz. This was achieved by significantly lowering the optical losses of integrated photonic waveguides based on silicon nitride, a material already used in CMOS micro-electronic circuits, and which has also been used in the last decade to build photonic integrated circuits that guide laser light on-chip.
The scientists were able to manufacture silicon nitride waveguides with the lowest loss of 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-bands (~10 GHz, used in radar).
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 team achieved a level of optical losses low enough to allow light to propagate nearly one meter in a waveguide that is only one micrometer in diameter - 100 times smaller than a human hair. Although, this loss level is still more than three orders of magnitude higher than the value in optical fibers, it represents the lowest loss in any tightly confining waveguide for integrated nonlinear photonics to date.
Such low loss is the result of a new manufacturing process developed by the scientists - the “silicon nitride photonic Damascene process”, which is carried out using deep-ultraviolet stepper lithography. These microcombs, and their microwave signals, could be critical elements for building fully integrated low-noise microwave oscillators for future architectures of radars and information networks, according to the researchers.
The EPFL team is now working with collaborators in the US to develop hybrid-integrated soliton microcomb modules that combine with chip-scale semiconductor lasers. These highly compact microcombs can impact many applications, for example, transceivers in datacenters, LiDAR, compact optical atomic clocks, optical coherence tomography, microwave photonics, and spectroscopy.