Recently, the US Defense Advanced Research Projects Agency (DARPA) “Direct On-Chip Digital Light Synthesizer” (DODOS) project has made remarkable progress again. The DODOS research team consisting of the University of California Santa Barbara (UCSB), the California Institute of Technology and the Institute of Physics (EPFL) of the Swiss Federal Institute of Technology in Lausanne developed an integrated soliton micro-nano chip. The research results were published in the journal Nature.

Optical frequency synthesizers – systems that output laser beams at precise and stable frequencies -have proven to be extremely valuable in various scientific work, including space exploration, gas sensing, quantum system control, and high-precision light detection and ranging (LIDAR).  Although they provide unprecedented performance, the use of optical frequency synthesizers is basically limited to laboratory environments due to the cost, size and power limitations of their components. In order to reduce these obstacles and achieve widespread use, DARPA launched the DODOS project in 2014. The key is to miniaturize the necessary components and integrate them into a compact module, thereby enabling the technology to be more widely deployed and unlocked at the same time.

In order to achieve the goal, the DODOS project is using the latest developments in micro resonators (the tiny structures that store light in microchips) to implement optical frequency combs in compact integrated packages. The name of the optical frequency comb comes from the conversion of a single color input laser beam into a series of uniformly arranged multiple colors, just like a comb. If there is a sufficiently wide “tooth” array of combs, innovative techniques to eliminate noise become possible, making optical frequency combs an attractive option for systems that require accurate frequency references.

In 2018, with the support of the DODOS project, the team led by UCSB and the National Institute of Standards and Technology (NIST) successfully realized the miniaturization of optical frequency synthesizer components, which laid the foundation for this research.

Manufacturing optical frequency combs from micro resonators is a complicated matter, requiring complex control schemes, special circuits, and usually requiring an expert scientist to carefully observe and fine-tune the operation. This is mainly due to the sensitive characteristics of the micro resonator, which requires the input laser to provide a perfect amount of light at a special operating frequency or color to enable the optical frequency comb to open. Even so, there is no guarantee that a coherent or stable optical frequency comb can be produced every time.

The research team has recently obtained a research result that completely changed the above situation. By closely connecting two microchips with lasers and micro resonators, these two devices can strongly affect the operation of each other. In this simplified structure, the micro resonator provides feedback, adjusts the laser to the appropriate conditions required to generate the optical frequency comb, and works in a special “soliton” mode, which has the more wide comb “tooth” array.

What is crucial to future deployment is that it’s also more convenient in operation. Due to the careful selection of the system design, as long as the laser is turned on, the optical frequency comb can be automatically started. The designed “turnkey” operation greatly simplifies systems that require optical frequency combs to achieve special functions. By removing the special electronic and optical devices that are usually used between devices, the simplified structure reduces the scale, power and cost requirements, while making the optical frequency comb have strong stability to the environment and temperature interference.

Dr. Gordon Keeler, leader of DARPA DODOS project, said: “The achievements of the UCSB team may have a broad impact on commercial and defense photonics applications-from navigation systems to optical clocks to coherent communications. This work takes us one step closer to create the next generation of light frequency control technology. This technology is not limited to the laboratory, but can be used in the real world.”