VCSELs Employ Slow Light to Boost Speed

George Washington University (GWU) researchers have combined multiple transverse coupled cavities to develop a new vertical-cavity surface-emitting laser (VCSEL). The cavities in the laser system enhance the laser’s optical feedback capabilities, helping the laser demonstrate record-fast temporal bandwidth, the researchers reported.

VCSELs, a class of semiconductor laser diodes accompanying a monolithic resonator that emits light in a direction perpendicular to the surface of a chip, are comparatively energy-efficient and high-speed optical interconnects in data centers and supercomputers. VSCELs are compact and deliver high optoelectronic performance. As miniaturized lasers, VSCELs function as an optical source in high-speed, short-wavelength communications and optical data networks. They also support a combination of dense traffic and high-speed transmission in data communications, as well as in other applications, including those in automotive.

Traditionally, however, thermal effects, parasitic resistance, capacitance, and influence from nonlinear gain all limit the 3-dB bandwidth, regarded as the VSCEL “speed limit.” Nonlinear optical amplification effects, or gain relaxation oscillations, prevent direct VSCEL modulation from exceeding 30 GHz.

The researchers debuting the new design relied on a multifeedback approach, combining multiple, coupled cavities to fortify the system feedback known as “slow-light.” The use and design of the cavities extended the temporal laser bandwidth, or laser speed, beyond the known limit of the relaxation oscillation frequency.

Fast, powerful, compact lasers: a novel VCSEL for next-generation data centers and sensors. Courtesy of Volker Sorger/GWU.

To achieve functionality, the direct feedback from each cavity needed only to be of a moderate strength; further, users are able to precisely control and manipulate the feedback by way of the coupled cavities, delivering an extended range of design freedom. The research team said it anticipates achieving a modulation bandwidth in the 100-GHz range.

“This invention is timely since demand for data services is growing rapidly and moving toward next-generation communication networks such as 6G, but also in automotive as proximity sensor of smartphone’s face ID,” said Hamed Dalir, co-author of the paper describing the laser and inventor of the technology. “Furthermore, the coupled cavity system paves a way for emerging applications in quantum information processors such as coherent Iasing machines.”

The research was published in Nanophotonics (www.doi.org/10.1515/nanoph-2020-0437).