A Revolutionary Development in Silicon Photonics: The World’s Smallest Comb Generator

June 21, 2020 by Gary Elinoff

Photonics leads the way today, and a new development now revolutionizes the potential of photonic data systems.

What worked in the past won’t cut it anymore! Today, the links between servers are longer, and the amount of heat generated by those links is becoming ever more troublesome. Modern systems also require greater bandwidth capacity, and can’t tolerate the latencies inherent to copper links. That’s where photonics comes in.

Photonics to the Rescue

Information carried by photons travels at the speed of light, and photons generate no heat. The technology for using photons to send data was given a massive boost by a demonstration from UC Santa Barbara (UCSB) and Intel of silicon laser technology. And now, fifteen years later, Intel is providing data centers millions of silicon-based photonic transceivers each year.

However, practical utilization of silicon laser technology, at present, requires the employment of complex, cumbersome optical systems. A group of researchers from UCSB, Caltech, and the Swiss Federal Institute of Technology Lausanne (EPFL) has developed a way to fabricate systems onto a silicon photonic chip. This breakthrough allows for lowering production costs, and for integration within established silicon chip manufacturing processes.


Multicolored Lasers

Multiple radio stations can deliver numerous programs to the world over the same airspace because each station broadcasts on a different frequency. Similarly, lasers of different colors can transmit separate data streams from the same data link without interference. The problem is that each silicon laser can transmit only on one frequency.

As described by UCSB’s John Bowers, who led the research group, “You might literally need 50 or more lasers in that chip for that purpose.” Among the multiple disadvantages therein is that the laser frequencies can drift into each other, just like radio stations used to create interference.


The internal structure of the microcomb chip.

The internal structure of the microcomb chip, next to a quarter to demonstrate it's size. Image credited to Lin Chang

Optical Frequency Combs

The problem was relieved with the development of optical frequency combs, an amalgamation of equally spaced channels of laser lights. A graphical plot of frequencies vs. intensity reveals peaks and voids that look like the outline of hair comb, hence the name. Up till now, creating that frequency comb was a challenging endeavor.


The Smallest Comb Generator in the World

Bowers’ team’s solution was remarkably simple, composed of a silicon nitride photonic chip and a feedback laser. According to Bowers, “What we have is a source that generates all these colors out of one laser and one chip. That’s what’s significant about this.”

The new solution represents savings in power, cost, and board real estate. It also makes the production of a stable comb, called a soliton, far more manageable. According to co-author Kerry Vahala of Caltech, “The new approach makes the process as easy as switching on a room light.” 

Vahala is seconded by EPFL’s Tobias J. Kippenberg, the provider of the silicon nitride photonics chips, a technology previously commercialized by LIGENTEC. Kippenberg states that “What is remarkable about the result is the reproducibility with which frequency combs can be generated on demand."

As described by the announcement from UCSB, “The magic behind all these improvements lies in an interesting physical phenomenon. When the pump laser and resonator are integrated, the interaction between them forms a highly coupled system that is self-injection locking. It simultaneously generates “solitons,” pulses that circulate indefinitely inside the resonator and give rise to optical frequency combs.” 


Further Implications of this New Technology

Optical clocks are the most accurate clocks available today but are so large and heavy that, Bowers says, few exist in the world. Optical clocks can be made small and cheap enough to fit on a wristwatch with this new technology.

While this might be just a bit of overkill, it has implications in GPS and a host of other applications. According to Bowers, “It is the key step to transfer the frequency comb technology from the laboratory to the real world.”