ANTONY SAVVAS DATACENTRE LASERS
OBSTACLES When going back to the full IC/laser integration question, Khan says: “It’s possible, but three issues still have to be fully addressed, and they are good product yield in manufacturing, lower cost to customers and effective power management.” Hamid Arabzadeh, CEO of Ranovus, confirms complete integration is not easy. He says, “The requirements for lasers to be used in silicon photonics subsystems are very different to today’s lasers. SiPh lasers must be designed using a simple structure that is not cost prohibitive to manufacturing.” Arabzadeh adds, “Silicon photonics also have a lot of optical loss, and therefore need higher power lasers. But higher power lasers will produce more back reflection from the SiPh, so there is a balance between these constraints when designing a laser that is suitable for SiPh subsystems. Packaging technologies are needed to bring the laser, SiPh TX/RX, driver, TIA, control IC and fibre together in a cost-effective manner.” However, Brad Booth, president of industry organisation the Consortium for On-Board Optics, says, “Integrating lasers with the co-packaged optical engine is certainly viable, as it simplifies the design. But it also places the laser in a less friendly thermal environment, which could result in a shorter lifespan. The ability to replace a laser when it fails is also extremely challenging. Therefore, it’s currently preferred to have the laser external to the optical engine to enable serviceability. As the optical networking industry will inevitably want to make sure transport systems can be as integrated as possible to meet data traffic demands, there is certainly also room for those looking to get the most out of what we already have.
successfully grew a five-channel laser array emitting at five different wavelengths, covering a 155nm-wide spectral range over silicon. “We were able to show that our ‘bonding- and-growth’ method could create a high density of high-performance components, which are needed in optical systems to maximise bandwidth,” says Besançon. By integrating the components, you also no longer need to provide separate cooling systems for them, thus lowering power consumption. The work is being done at the company’s III-V Lab in Paris-Saclay, France. IC IMPROVEMENTS Raza Khan, senior market manager for Semtech’s signal integrity products group, says it is “right and proper” for the industry to seek improvements in silicon photonics (SiPh) to better integrate the laser with the integrated circuit, but emphasises existing IC technology already exists to help do this. Khan says: “Our Tri-Edge platform takes customers to 50G lanes through driving lasers effectively, it would be counter intuitive for customers if full integration of the IC and the laser led to higher costs. We’re concentrating on using our proprietary IC technology to deliver higher- bandwidth links at lower cost to customers, through getting more wavelengths down the fibre, reducing the power needed in the lasers to do it, and delivering the same data performance despite the higher bit rates.” A rapidly evolving area for Khan in this endeavour is the wireless segment, including 5G and multi-access edge computing (MEC). He says: “Data traffic in markets like this will rapidly ramp up as users take advantage of full commercial roll-outs. IC and laser costs cannot be allowed to increase as a result of this growth, and we plan to offer 100G fibre links using 50G laser technology.”
growing more complex as we develop more sophisticated ways to transmit information over light. Our current methods of manufacturing optical devices can’t keep up with those two competing demands. We need to find new and cheaper ways of manufacturing more-efficient and higher- performance optical modules to power the fibre networks of the future.” One of the most intriguing solutions to this problem, she says, is to move away from today’s methods of building optical modules from separate components, and instead of “assembling” them, to “grow” them. Currently, optical modules consist of two distinct components: a laser source and a chip connected to that emitter via a fibre. Both components are manufactured separately, using vastly different methods, then later assembled. Nokia Bell Labs says its fully integrated module can deliver higher-performance. Placing the laser source directly on the chip suppresses lossy optical couplings that limit the overall performance of the system, meaning you get more-efficient, higher-capacity optical systems. A laser source is fabricated from alloys made from indium, gallium, aluminum, arsenic and phosphorus. These alloys are known to chemists as III-V materials due to the location of their component elements on the periodic table. “The fabrication process for these materials is expensive and they can only be produced in small volumes, in stark contrast to silicon chips. That creates a fundamental scalability roadblock. We essentially want to take an easily manufactured, cheap component and pair it with a hard-to-manufacture, costly component to build a single optical module. We’ve explored bonding III-V materials directly onto a silicon wafer, but that kind of integration is difficult. The process is complex and less cost-effective than the current methods of assembling an optical module out of separate components. But we may finally have a solution that could bring these two families of materials together on a single chip,” says Besançon. Her team at Nokia Bell Labs is developing a system to “grow” the laser source directly on the chip. Normally this would be a near impossible task because most III-V elements are incompatible with silicon. The team, however, has developed a method for layering III-V materials onto a silicon wafer. An unprocessed film of indium phosphide is bonded onto a large silicon wafer. Then, on that film, III-V alloys are grown/layered on top of each other to create the laser source. In 2020, the team
Brad Booth, president, Consortium for On-BoardOptics
Raza Khan, senior market manager, Semtech
Hamid Arabzadeh, CEO, Ranovus
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| ISSUE 28 | Q1 2022
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