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Lighting up the machines: A half century of laser advances at ISLC

Di_Liang

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By Di Liang, Hewlett Packard Labs Research Scientist

We are living in an era when nearly every bit of human society is constantly being transformed by new technological advances and scientific discoveries. Many of the fruits we are harvesting right now were actually planted in a short span of a few decades in the past century.

Arguably the most important scientific invention in the 20th century, also the fundamental building block in computer chips, the transistor, was built in 1947. The first laser with a comparable significance in optics was built in 1960, followed by the first visible light-emitting diode (LED) and semiconductor laser demonstration in 1962.

My participation in the 50th anniversary of International Semiconductor Laser Conference (ISLC) in Kobe, Japan, granted me a rare opportunity to learn from the people who actually made this history.

Hewlett, Packard, and the light

Widely recognized as the symbolic founding company of Silicon Valley, Hewlett-Packard also contributed tremendously to the technological and commercial development of LEDs and semiconductor lasers. The erstwhile HP Labs developed the first commercially-available LED in 1966, the laser interferometer five years later, and the laser printer in 1980. Those innovations and product development have been and are still changing the world in the areas of lighting, display, industrial measurement, printing and so on.

Fast forwarding to about 11 years ago, the Large-Scale Integrated Photonics (LSIP) Group, started by Ray Beausoleil, began to look into an optical interconnect solution to carry exponentially-increasing digital data loads from one place to another place within a computer chip, between chips and racks, within a datacenter, and between datacenters and customers.

This represented a major step in opening up the R&D work at Labs to the semiconductor laser's biggest application: The optical engine in an optical communication system.

Hewlett-Packard-Labs-researcher-Geza-Kurczveil.gifHewlett Packard Labs researcher Geza Kurczveil

The rainbow connection

A semiconductor laser exists to transform external energy, e.g., electrical power, into optical energy with extremely pure color(s), i.e., wavelength or frequency, depending on the specific material property and structure design. With the light being generated in the laser, electrical "1 0 1 0 ..." can be converted to optical "1 0 1 0 ..." This can be done by directly brightening or dimming the laser output. This is known as direct modulation. You can also open or close a "curtain" (optical modulator) in front of a steady laser output, a process known as external modulation.

Both modulation schemes lead to a stream of light with differentiable brightness. How fast this modulation occurs determines how much data can be carried in this stream of light per second. The light travels through a waveguide structure for short distance reach (centimeter scale) or through optical fiber for much longer distance reach (kilometer scale). When the optical data reaches its destination, the photodetector which functions in an opposite manner to the laser, converts the optical data back into electrical current stream with differentiable amplitude resembling the initial electrical "1 0 1 0 ..."

Multiple colors of streams of light, like a rainbow, with each carrying independent electrical data, can transmit using a single waveguide or optical fiber, drastically enhancing the data transmission capacity in a unique fashion others cannot match. This is a technology we have been relying on for trans-continental data transmission for the past four decades. Now a global effort is on to develop a similar optical communication system but at a higher volume, in a smaller size, with better tolerance for the harsh environments of datacenters and high-performance computer applications, and at much lower cost.

Significant progress has been achieved in deploying optical interconnects with HPE's own IP into future products. At last Hewlett Packard Enterprise Discover 2016 in Las Vegas, a special type of laser called vertical-cavity surface-emitting lasers (VCSELs) developed by Mike Tan's team at Labs and HPE’s Silicon Design Lab showcased the power to directly modulate VCSELs and transmit tens of Gigabits of data per second optically within a fiber.

The Large-Scale Integrated Photonics (LSIP) group's long-term effort aims to fully take advantage of the unique multi-color data transmission scheme by either using an array of power efficient and directly modulated small lasers with different colors or a single master laser with the capability to generate a steady "rainbow" light with hundreds of colors and code the electrical information by external modulation.

More importantly, they are building them directly on the silicon, a basic semiconductor material whose intrinsic material properties determined its dominating role in microelectronics shortly after the invention of transistor, and its fate of being widely abandoned for making light-emitting devices even before the birth of the first semiconductor laser.

Recent technical breakthroughs in other silicon-based photonic building blocks (e.g., low-loss passive waveguide, modulator, Ge and SiGe photodetectors) have triggered the blooming Si photonics market. There, three key driving factors – cost, size and volume – can be satisfied simultaneously in a conventional microelectronics foundry. Putting the light-emitting materials on silicon in a smart way not only allows you to integrate on-chip light sources with all other Si photonics building blocks for huge technical and cost advantages but also enables novel laser structure and enormous energy savings.

My colleague Geza Kurczveil is using more advanced quantum-dot light emitting material on silicon to further boost laser performance. His presentation in this ISLC emphasized the significance of choosing the best laser material for specific applications.

HPE is leading tremendous efforts to continue advanced research and accelerate commercialization of robust semiconductor lasers on silicon and the full silicon photonics package with minimal energy consumption and volume manufacturability. We are combining the new device concept and best materials, – a message many renowned experts echo and applaud – a message even more unforgettable than the famous Kobe beef served in the conference banquet.

There is an old Chinese saying: At fifty, I knew the decrees of Heaven. But after over 50 years of technology advances, people still go on the journey to extend the semiconductor laser application territory and surprise the world with its new usefulness. HPE has been a passionate adventurer on this journey. People at HPE inherited the mindset of "thinking outside the box" and will continue leaving recognizable footprints down the road.

Whether I will see the celebration of the 100th anniversary of ISLC is unknown, but our dedication to innovate new technologies will continue resonating for another 50 years and more for sure.

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About the Author

Di_Liang

Di Liang is a research scientist at Hewlett Packard Labs, and leading the advanced development of hybrid silicon photonics for optical interconnect system in The Machine and other applications. He has authored and coauthored over 130 journal and conference papers, five book chapters, and was granted by nine US/international patents with another 30+ pending. He is a senior member of IEEE, and member of OSA.