Bowers "A Laser Breakthrough"

ECE Prof. John Bowers and EPFL Physics Prof. Tobias J. Kippenberg labs collaborate to develop an integrated on-chip semiconductor laser and resonator capable of producing a laser microcomb

Artist's concept illustration of electrically controlled optical frequency combs at wafer scale

Excerpt from The UCSB Current article "A Laser Breakthrough"

Fifteen years ago, UC Santa Barbara electrical and materials professor John Bowers pioneered a method for integrating a laser onto a silicon wafer. The technology has since been widely deployed in combination with other silicon photonics devices to replace the copper-wire interconnects that formerly linked servers at data centers, dramatically increasing energy efficiency — an important endeavor at a time when data traffic is growing by roughly 25% per year.

For several years, John Bowers's group has collaborated with the group of Tobias J. Kippenberg at the Swiss Federal Institute of Technology (EPFL), within the Defense Advanced Research Projects Agency (DARPA) Direct On-Chip Digital Optical Synthesizer (DODOS) program. The Kippenberg group discovered “microcombs,” a series of parallel, low-noise, highly stable laser lines. Each of the many lines of the laser comb can carry information, extensively multiplying the amount of data that can be sent by a single laser.

Recently, several teams demonstrated very compact combs by placing a semiconductor laser chip and a separate silicon nitride ring-resonator chip very close together. However, the laser and the resonator were still separate devices, made independently and then placed in close proximity to each other perfectly aligned, a costly and time-consuming process that is not scalable.

The Bowers lab has worked with the Kippenberg lab to develop an integrated on-chip semiconductor laser and resonator capable of producing a laser microcomb. A paper titled “Laser soliton microcombs heterogeneously integrated on silicon,” published in the new issue of the journal Science, describes the labs’ success in becoming the first to achieve that goal.

Soliton microcombs are optical frequency combs that emit mutually coherent laser lines — that is, lines that are in constant, unchanging phase relative to each other. The technology is applied in the areas of optical timing, metrology and sensing. Recent field demonstrations include multi-terabit-per-second optical communications, ultrafast light detection and ranging (LiDAR), neuromorphic computing, and astrophysical spectrometer calibration for planet searching, to name several. It is a powerful tool that normally requires exceptionally high power and expensive lasers and sophisticated optical coupling to function.

The working principle of a laser microcomb, explained lead author Chao Xiang, a postdoctoral researcher and newly minted Ph.D. in Bowers’s lab, is that a distributed feedback (DFB) laser produces one laser line. That line then passes through an optical phase controller and enters the micro-ring resonator, causing the power intensity to increase as the light travels around the ring. If the intensity reaches a certain threshold, non-linear optical effects occur, causing the one laser line to create two additional, identical lines on either side. Each of those two “side lines” creates others, leading to a cascade of laser-line generation. “You end up with a series of mutually coherent frequency combs,” Xiang said — and a vastly expanded ability to transmit data.

The UCSB Current – "A Laser Breakthrough" (full article)