Researchers have recently demonstrated laser-based microwave generators using built-in photonic chips developed at EPFL. Such microwave signals, in addition to their optical carriers, can potentially be utilized in satellite communications, radars, and upcoming 5G wireless networks.
In today’s information society, the production, processing, and distribution of microwave and radio signals have become prevalent in radars, telecommunications, and wireless networks.
The present inclination is to utilize the carriers in higher frequency bands, particularly with the imminent bandwidth logjams caused by demands—for example—of the “Internet of Things” and 5G wireless networks. Now, a solution could be provided by “microwave photonics,” which is a combination of optoelectronics and microwave engineering.
Optical frequency combs are the major building blocks of microwave photonics, offering an unlimited number of mutually and equidistant coherent laser lines. Optical frequency combs can also be described as ultra-short optical pulses produced with a stable repetition rate that accurately matches the comb lines’ frequency spacing. A microwave carrier is generated by the photodetection of the pulses.
In the recent past, the chip-scale frequency combs experienced a major development, especially those that were produced from nonlinear microresonators fueled by continuous-wave lasers. Such frequency combs depend on the development of dissipative Kerr solitons, which are essentially ultra-short coherent light pulses moving freely within optical microresonators. Owing to this phenomenon, these chip-scale frequency combs are often referred to as “soliton microcombs.”
Nonlinear microresonators are required to produce soliton microcombs, which can directly be constructed on-chip with the help of CMOS nanofabrication technology. The combined integration with built-in lasers and electronic circuitry may lead to miniaturization of combs and thus enable a range of application areas including communications, spectroscopy, and metrology.
A team of EPFL researchers, under the guidance of Tobias J. Kippenberg, has now uncovered the integrated soliton microcombs that have repetition rates down to 10 GHz. The study was published in the Nature Photonics journal.
The researchers achieved this feat by considerably reducing the optical losses of the built-in photonic waveguides that are built on silicon nitride. Silicon nitride is a type of material that is already utilized in CMOS micro-electronic circuits, and it has also been utilized in the last 10 years to develop photonic integrated circuits that direct the laser light on-chip.
The researchers successfully produced silicon nitride waveguides that have the lowest loss seen in any photonic integrated circuit. With the help of this technology, the coherent soliton pulses, thus produced, 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 ensuing microwave signals have phase noise characteristics that are close to or even lower than that of electronic microwave synthesizers available on the market. The successful demonstration of built-in soliton microcombs at microwave repetition rates connects the fields of microwave photonics, nonlinear optics, and integrated photonics.
The EPFL researchers were able to accomplish a level of optical loss that was significantly low and enables the light to spread almost 1 m in a waveguide measuring just 1 µm in diameter—that is, 100 times smaller than that of a single strand of human hair.
This level of loss continues to be over three orders of magnitude higher than the value found in optical fibers, and still indicates the lowest loss experienced in any closely limiting waveguide for integrated nonlinear photonics, so far. A low loss like this is the result of a novel manufacturing process—called silicon nitride photonic Damascene process—devised by EPFL researchers.
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. 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.
Junqiu Liu, Study First Author, EPFL
Liu also heads the development of silicon nitride nanophotonic chips at the Center of MicroNanoTechnology (CMi) in EPFL.
The EPFL researchers are already working with U.S. collaborators to create hybrid-integrated soliton microcomb modules that are capable of integrating chip-scale semiconductor lasers.
Such highly compact microcombs can have an impact on several applications, for example, LiDAR, transceivers in datacenters, spectroscopy, microwave photonics, optical coherence tomography, and compact optical atomic clocks.
The study was funded by the Swiss National Science Foundation (SNF) and the Defense Advanced Research Projects Agency (DARPA).