First Quantum Dot Mode-Locked Laser Grown on Silicon

Illustration of a high-channel-count, 20-GHz, passively mode-locked quantum dot laser directly grown on silicon. (Source: B. Long Illustr. / UCSB)

Ten years into the future. That’s about how far UC Santa Barbara elec­trical and computer engi­neering professor John Bowers and his research team are reaching with the recent develop­ment of their mode-locked quantum dot lasers on silicon. It’s tech­nology that not only can massively increase the data trans­mission capacity of data centers, telecommunications companies and network hardware products to come, but do so with high stability, low noise and the energy effi­ciency of silicon photonics.

“The level of data traffic in the world is going up very, very fast,” said Bowers. Generally speaking, he explained, the trans­mission and data capacity of state-of-the-art tele­communications infra­structure must double roughly every two years to sustain high levels of per­formance. That means that even now, technology companies such as Intel and Cisco have to set their sights on the hardware of 2024 and beyond to stay compe­titive. Enter the Bowers Group’s high-channel-count, 20 gigahertz, passively mode-locked quantum dot laser, directly grown – for the first time, to the group’s knowledge – on a silicon substrate. With a proven 4.1 terabit-per-second trans­mission capacity, it leaps an estimated full decade ahead from today’s best commercial standard for data transmission, which is currently reaching for 400 gigabits per second on Ethernet.

The tech­nology is the latest high-perfor­mance candidate in wave­length-division-multi­plexing (WDM), which transmits numerous parallel signals over a single optical fiber using different wavelengths. It has made possible the streaming and rapid data transfer we have come to rely on for our communi­cations, enter­tainment and commerce. The new tech­nology takes advantage of several advances in telecommu­nications, photonics and materials with its quantum dot laser – a tiny, micron-sized light source – that can emit a broad range of light wave­lengths over which data can be trans­mitted.

“We want more coherent wave­lengths generated in one cheap light source,” said Songtao Liu, a post­doctoral researcher in the Bowers Group. “Quantum dots can offer you wide gain spectrum, and that’s why we can achieve a lot of channels.” Their quantum dot laser produces 64 channels, spaced at 20 GHz, and can be utilized as a trans­mitter to boost the system capacity. The laser is passively ‘mode-locked’ – a technique that generates coherent optical combs with fixed-channel spacing – to prevent noise from wave­length compe­tition in the laser cavity and stabilize data trans­mission.

This technology represents a signi­ficant advance in the field of silicon electronic and photonic integrated circuits, in which the primary goal is to create components that use light and wave­guides alongside and even instead of electrons and wires. Silicon is a good material for the quality of light it can guide and preserve, and for the ease and low cost of its large-scale manu­facture. However, it’s not so good for generating light. “If you want to generate light effi­ciently, you want a direct band-gap semi­conductor,” said Liu, referring to the ideal electronic structural property for light-emitting solids. “Silicon is an indirect band-gap semi­conductor.” The new quantum dot laser, grown on silicon molecule-by-molecule, is a structure that takes advan­tage of the electronic properties of several semi­conductor materials for per­formance and function, in addition to silicon’s own well-known optical and manu­facturing benefits.

This quantum dot laser, and components like it, are expected to become the norm in telecommu­nications and data processing, as technology companies seek ways to improve their data capacity and trans­mission speeds. “Data centers are now buying large amounts of silicon photonic trans­ceivers,” Bowers pointed out. “And it went from nothing two years ago.” Since Bowers a decade ago demon­strated the world’s first hybrid silicon laser, the silicon photonics world has continued to create higher effi­ciency, higher performance tech­nology while main­taining as small a footprint as possible, with an eye on mass pro­duction. The quantum dot laser on silicon, Bowers and Liu say, is state-of-the-art tech­nology that delivers the superior perfor­mance that will be sought for future devices. “We’re shooting far out there,” said Bowers, “which is what univer­sity research should be doing.” (Source: UCSB)

Reference: S. Liu et al.: High-channel-count 20  GHz passively mode-locked quantum dot laser directly grown on Si with 4.1  Tbit/s transmission capacity, Optica 6, 128 (2019); DOI: 10.1364/OPTICA.6.000128

Link: Optoelectronics Group, University of California, Santa Barbara, USA

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