Silicon Wafer based Laser
John Bowers, a professor of electrical and materials at UC Santa Barbara, pioneered a way to integrate a laser onto a silicon wafer fifteen years ago. This technology is now widely used in conjunction with other silicon photonics devices to replace copper-wire interconnects which once linked servers at data centers. It dramatically increases energy efficiency, which is important at a time where data traffic is increasing by approximately 25% annually.
The Bowers group has been working with Tobias J. Kippenberg, at the Swiss Federal Institute of Technology (EPFL), for several years. This collaboration is part of the Defense Advanced Research Projects Agency's (DARPA), Direct On-Chip Digital Optical Synthesizers (DODOS). The "microcombs" were discovered by the Kippenberg group. They are a series low-noise and highly stable laser lines. Each line of the laser comb can contain information, increasing the number of data that can easily be sent using a single laser.
Recent demonstrations showed that a number of teams were able to create compact combs by placing both silicon nitride-ring-resonator and semiconductor laser chips very close together. The laser and resonator were separate devices that were made separately and placed close to each other. This was a time-consuming and costly process that is not easily scalable.
The Bowers laboratory has collaborated with the Kippenberg laboratory to create an integrated on-chip semiconductor resonator and laser capable of producing a microcomb. A paper titled "Laser soliton microcombs heterogeneously integrated on silicon(link is external)," published in the new issue of the journal Science describes the labs' success in becoming the first to achieve that goal.
Soliton microcombs, which emit optical frequency lines in mutually coherent phases, are optical frequency combs with the ability to produce laser lines that are constant and unchanging relative to one another. This technology can be used in 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. This powerful tool requires extremely high-power lasers, expensive optical coupling, and exceptional precision in order to work.
Chao Xiang (postdoctoral researcher) explained that a laser microcomb works on the principle of a distributed feedback laser producing one laser line. The line passes through an optical phase control and enters the microring resonator. This causes the power intensity of the light to increase as it travels around the ring. Non-linear optical effects can occur when the intensity exceeds a certain threshold. This causes the laser line to produce two identical lines on each side. Each of these "sidelines" creates another, resulting in a cascade generation of laser-line generators. "You end up having a series mutually coherent frequency combs," Xiang stated -- and a greatly expanded capability to transmit data.
This research allows semiconductor lasers to seamlessly integrate with low-loss optical micro-resonators. "Low-loss" is because light can travel through the waveguide without losing any of its intensity over time. The device can be controlled entirely by electricity and no optical coupling is necessary. The new technology is able to be commercially scaled because it can make thousands of devices from a single wafer by using industry-standard complementary metal oxide semiconductors (CMOS-compatible) techniques. Researchers stated that their approach "paves the way to large-volume, low cost manufacturing of chip-based frequency combiners for next-generation high capacity transceivers and datacenters, as well as mobile platforms"
The main challenge when making the device was that both the semiconductor laser (which generates the comb) and the resonator (which creates it), had to be constructed on different materials platforms. Lasers cannot be made with materials other than those listed in the Periodic Table's III and V groups. The best combs are made from silicon nitride. "So, we had the challenge of putting them all together on one wafer," Xiang said.
The researchers used UCSB's heterogeneous process for making high-performance lasers on a silicon substrate, and their EPFL collaborators' ability to create record-setting high-Q silicon-nitride microresonators using their "photonic damascene" process. They worked sequentially on the same wafer. This wafer-scale process, which is different from making individual devices and then combing them one-by-one, allows thousands of devices to come out of a single wafer measuring 100 mm in diameter. It also gives the ability to scale up production levels beyond that of the 200mm or 300-mm industry-standard substrates.
The device must function properly if the laser, resonator, and optical phase between them are controlled in order to create a coupled system that is based on "self-injection locking". Xiang explained how the laser output is partially reflected by the microresonator. The laser is locked to the micro-resonator when a certain phase is reached between the laser's light and the back-reflected light of the resonator.
Back-reflected light is not good for laser performance but it is essential to generate the microcomb. The laser light locked triggers soliton formation within the resonator. It also reduces frequency instability or laser light noise. This transforms something bad into something positive. The team was able not only to create the first integrated laser soliton microcomb on a single chip but also the first narrow linewidth laser sources that have multiple channels on one chip.
"Optical comb generation is a very exciting field that is moving at a rapid pace. It has applications in optical clocks and high-capacity optical networks, as well as many spectroscopic uses," Bowers, who is the Fred Kavli Chair for Nanotechnology and director of the College of Engineering’s Institute for Energy Efficiency, said. "The missing element was a self-contained chip which includes both the pump laser as well as the optical resonator. This key element was demonstrated and should allow for rapid adoption.
Xiang said, "I believe this work will become very large." He said that the potential of this technology reminds him of how putting lasers onto silicon 15 years ago helped both research and commercialization of silicon-photonics. He said that Intel has shipped millions of transceiver units per year because this transformative technology was commercialized. Future silicon photonics that use co-packaged optics are likely to be a powerful driver for transceivers with higher capacities and a wide range of optical channels.
Xiang stated that the current comb produces approximately twenty to thirty usable comb line and that the goal is to increase that number. "Hopefully, one hundred combined lines will be possible from each laser-resonator with low power consumption," he said.
Based on the soliton's low energy use and ability to provide a large amount of high-purity optical comb line lines for data communications, said Xiang: "We believe our achievement could be the backbone of efforts in optical frequency comb technology in many areas, including efforts in keeping up with fast-growing data traffic, and hopefully slowing down the growth in energy consumption in mega-scaled datacenters."