Microwave and radio signals play a pivotal role in radar technology and telecommunications including wireless networks. Moving away from the current tendency of using high-frequency bands for such applications, EPFL scientists have demonstrated laser-based microwave generators using built-in photonic chips developed at EPFL. This is an important breakthrough because high-frequency bands are prone to logjams because of high demand. On the other hand, microwave photonics offers high bandwidth, low transmission loss, and immunity to electromagnetic interference.

Microwave photonics, a combination of optoelectronics and microwave engineering, is built using optical frequency combs. Recently, a major advancement in this field was the development of chip-scale frequency combs from nonlinear microresonators fueled by continuous-wave lasers. These chip-scale frequency combs are often referred to as “soliton microcombs” because they depend on the development of ultra-short coherent light pulses called solitons.

In their study published in the Nature Photonics journal, EPFL researchers led by Tobias J. Kippenberg present integrated soliton microcombs that have repetition rates down to 10 GHz. They achieved this by significantly reducing the optical losses of integrated photonic waveguides based on silicon nitride, which is already being used in CMOS micro-electronic circuits. The silicon nitride waveguides produced by the researchers have the lowest loss recorded in any photonic integrated circuit. The resulting coherent soliton pulses have repetition rates in the microwave X-band (~10 GHz, utilized in radars) and the microwave K-band (~20 GHz, utilized in 5G network).

The microwave signals through this technology have phase noise characteristics that are on par or lower than that of electronic microwave synthesizers available on the market. By successfully demonstrating built-in soliton microcombs at microwave repetition rates, the research integrates the fields of microwave photonics, nonlinear optics, and integrated photonics.

The low optical losses achieved by the EPFL researchers allow light to spread nearly 1 meter in a waveguide that is only 1 micrometer in diameter, i.e., 100 times smaller than a human hair. The level of loss is the lowest seen in any closely limiting waveguide for integrated nonlinear photonics. The low loss is due to the innovative manufacturing technique called silicon nitride photonic Damascene process, devised by EPFL researchers.

The EPFL team is currently working with U.S. collaborators to create hybrid-integrated soliton microcomb modules. Such highly compact microcombs can be used in LiDAR, transceivers in datacenters, spectroscopy, microwave photonics, optical coherence tomography, and compact optical atomic clocks.

The research was funded by the Swiss National Science Foundation (SNF) and the Defense Advanced Research Projects Agency (DARPA).