On-Chip Frequency Combs for Microwaves

These silicon nitride photonic chips are used for frequency comb and photonic microwave generation. (Source: J. Liu & J. He,EPFL)

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”. 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 comb lines. The photo­detection of the pulses produces a microwave carrier. In recent years there has been significant progress on chip-scale frequency combs generated from nonlinear micro­resonators driven by continuous-wave 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 called soliton microcombs.

Generating soliton microcombs needs nonlinear micro­resonators, and these can be directly built on-chip using CMOS nano­fabrication technology. The co-inte­gration with electronic circuitry and integrated lasers paves the path to comb minia­turization, allowing a host of appli­cations in metrology, spectroscopy and communi­cations. Now, an EPFL research team led by Tobias J. Kippenberg has 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 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 team 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. Such low loss is the result of a new manu­facturing process – the “silicon nitride photonic Damascene process”. “This process, when carried out using deep-ultraviolet stepper litho­graphy, gives truly specta­cular performance in terms of low loss, which is not attainable using conven­tional nano­fabrication techniques,” says Junqiu Liu, who lead the fabrication of silicon nitride nano­photonic chips at EPFL’s Center of MicroNano­Technology (CMi). “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.”

The team is already working with colla­borators in US to develop hybrid-inte­grated soliton microcomb modules that combine chip-scale semi­conductor lasers. These highly compact microcombs can impact many appli­cations, e.g. transceivers in data­centers, LiDAR, compact optical atomic clocks, optical coherence tomo­graphy, microwave photonics, and spectro­scopy. (Source: EPFL)

Reference: J. Liu et al.: Photonic microwave generation in the X- and K-band using integrated soliton microcombs, Nat. Phot., online 20 April 2020; DOI: 10.1038/s41566-020-0617-x

Link: Laboratory of Photonics and Quantum Measurements, Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland

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