Optical Frequency Combs on Wafers

A Laser Breakthrough: First Commercially Scalable Built-in Laser and Microcomb on a Single Chip

Optical Frequency Combs on Wafers

Artist’s idea illustration of electrically managed optical frequency combs at wafer scale. Credit score: Brian Lengthy

Fifteen years in the past, UC Santa Barbara electrical and supplies professor John Bowers pioneered a way for integrating a laser onto a silicon wafer. The know-how has since been broadly deployed together with different silicon photonics gadgets to interchange the copper-wire interconnects that previously linked servers at knowledge facilities, dramatically rising vitality effectivity — an vital endeavor at a time when knowledge visitors is rising by roughly 25% per 12 months.

For a number of years, the Bowers group has collaborated with the group of Tobias J. Kippenberg on the Swiss Federal Institute of Expertise (EPFL), inside the Protection Superior Analysis Initiatives Company (DARPA) Direct On-Chip Digital Optical Synthesizer (DODOS) program. The Kippenberg group found “microcombs,” a sequence of parallel, low-noise, extremely steady laser traces. Every of the numerous traces of the laser comb can carry info, extensively multiplying the quantity of information that may be despatched by a single laser.

Not too long ago, a number of groups demonstrated very compact combs by inserting a semiconductor laser chip and a separate silicon nitride ring-resonator chip very shut collectively. Nevertheless, the laser and the resonator have been nonetheless separate gadgets, made independently after which positioned in shut proximity to one another completely aligned, a pricey and time-consuming course of that’s not scalable.

The Bowers lab has labored with the Kippenberg lab to develop an built-in on-chip semiconductor laser and resonator able to producing a laser microcomb. A paper titled “Laser soliton microcombs heterogeneously built-in on silicon,” revealed within the new subject of the journal Science, describes the labs’ success in changing into the primary to realize that objective.

Soliton microcombs are optical frequency combs that emit mutually coherent laser traces — that’s, traces which are in fixed, unchanging section relative to one another. The know-how is utilized within the areas of optical timing, metrology and sensing. Latest subject demonstrations embody multi-terabit-per-second optical communications, ultrafast gentle detection and ranging (LiDAR), neuromorphic computing, and astrophysical spectrometer calibration for planet looking, to call a number of. It’s a highly effective device that usually requires exceptionally excessive energy and costly lasers and complicated optical coupling to perform.

The working precept of a laser microcomb, defined lead creator Chao Xiang, a postdoctoral researcher and newly minted Ph.D. in Bowers’s lab, is {that a} distributed suggestions (DFB) laser produces one laser line. That line then passes via an optical section controller and enters the micro-ring resonator, inflicting the ability depth to extend as the sunshine travels across the ring. If the depth reaches a sure threshold, non-linear optical results happen, inflicting the one laser line to create two further, equivalent traces on both facet. Every of these two “facet traces” creates others, resulting in a cascade of laser-line era. “You find yourself with a sequence of mutually coherent frequency combs,” Xiang stated — and a vastly expanded capability to transmit knowledge.

This analysis permits semiconductor lasers to be seamlessly built-in with low-loss nonlinear optical micro-resonators — “low-loss” as a result of the sunshine can journey within the waveguide with out dropping a big quantity of its depth over distance. No optical coupling is required, and the machine is completely electrically managed. Importantly, the brand new know-how lends itself to commercial-scale manufacturing, as a result of hundreds of gadgets might be created from a single wafer utilizing trade normal complementary metallic oxide semiconductor (CMOS)-compatible strategies. “Our strategy paves the best way for large-volume, low-cost manufacturing of chip-based frequency combs for next-generation high-capacity transceivers, datacenters, house, and cellular platforms,” the researchers said.

The important thing problem in making the machine was that the semiconductor laser and the resonator, which generates the comb, needed to be constructed on totally different materials platforms. The lasers might be made solely with supplies from the III and V teams on the Periodic Desk, corresponding to indium phosphide, and the most effective combs might be made solely from silicon nitride. “So, we needed to discover a strategy to put them collectively on a single wafer,” Xiang defined.

Working sequentially on the identical wafer, the researchers leveraged UCSB’s heterogeneous integration course of for making high-performance lasers on silicon substrate and the power of their EPFL collaborators to make document ultra-low-loss high-Q silicon nitride micro-resonators utilizing the “photonic damascene course of” they developed. The wafer-scale course of — in distinction to creating particular person gadgets after which combining them one after the other — permits hundreds of gadgets to be created from a single 100-mm-diameter wafer, a manufacturing degree that may be scaled up farther from the trade normal 200-mm- or 300-mm-diameter substrate.

For the machine to perform correctly, the laser, the resonator and the optical section between them have to be managed to create a coupled system primarily based on the “self-injection locking” phenomenon. Xiang defined that the laser output is partially back-reflected by the micro-resonator. When a sure section situation is achieved between the sunshine from the laser and the back-reflected gentle from the resonator, the laser is claimed to be locked to the resonator.

Usually, back-reflected gentle harms laser efficiency, however right here it’s essential for producing the microcomb. The locked laser gentle triggers soliton formation within the resonator and reduces the laser gentle noise, or frequency instability, on the identical time. Thus, one thing dangerous is remodeled right into a profit. Because of this, the staff was in a position to create not solely the primary laser soliton microcomb built-in on a single chip, but additionally the primary narrow-linewidth laser sources with a number of obtainable channels on one chip.

“The sphere of optical comb era could be very thrilling and transferring very quick. It’s discovering functions in optical clocks, high-capacity optical networks and plenty of spectroscopic functions,” stated Bowers, the Fred Kavli Chair in Nanotechnology and the director of the Faculty of Engineering’s Institute for Power Effectivity. “The lacking factor has been a self-contained chip that features each the pump laser and the optical resonator. We demonstrated that key factor, which ought to open up fast adoption of this know-how.”

“I believe this work goes to grow to be very large,” stated Xiang. The potential of this new know-how, he added, reminds him of the best way placing lasers on silicon 15 years in the past superior each analysis and industrial commercialization of silicon photonics. “That transformative know-how has been commercialized, and Intel ships tens of millions of transceiver merchandise per 12 months,” he stated. “Future silicon photonics utilizing co-packaged optics will probably be a powerful driver for higher-capacity transceivers utilizing numerous optical channels.”

Xiang defined that the present comb produces about twenty to thirty usable comb traces and that the objective going ahead shall be to extend that quantity, “hopefully to get 100 mixed traces from every laser-resonator, with low energy consumption.”

Primarily based on the soliton microcombs’ low vitality use and their capability to offer numerous high-purity optical comb traces for knowledge communications, stated Xiang, “We consider that our achievement might grow to be the spine of efforts to use optical frequency comb applied sciences in lots of areas, together with efforts to maintain up with fast-growing knowledge visitors and, hopefully, gradual the expansion of vitality consumption in mega-scale datacenters.”

Reference: 1 July 2021, Science.

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