VCSEL design

The novel VCSEL for next-generation datacenters and sensors. [Image: Optelligence]

From smartphone camera systems and computer mice to fingertip sensors that measure blood oxygen levels in real time, VCSELs (vertical cavity surface emitting lasers) underlie many of today’s technological marvels. Being cost-effective and reliable, they are also workhorse devices for high-speed optical interconnects in data centers and supercomputers.

Nonetheless, VCSELs’ top speeds flatten at around 30 GHz, a temporal bandwidth that limits even greater utility.

Now, a team led by U.S. researchers reports a new VCSEL design paradigm, fabricated on the same wafer structure, that it says can push speeds to 45 GHz, opening a range of potential new device applications (Nanophotonics, doi: 10.1515/nanoph-2020-0437 ).

A radical new design

It is a “radical new design that allows us to fundamentally improve the speed of this entire class of [semiconductor] laser by a factor of five and even more,” says OSA Senior Member Volker J. Sorger, an associate professor at George Washington University, USA, and an OSA division chair for optoelectronics and photonics. “We have a faster device with more output power—two of the most critical parameters for this device type,” he says.

The speed limit on current devices, says co-author Hamed Dalir, an OSA Senior Member and inventor of the technology, results from thermal effects, parasitic resistance, capacitance and especially nonlinear gain effects such as relaxation oscillations. However, earlier work suggested that temporal bandwidth could be enhanced by adding multiple transverse coupled cavities to the primary VCSEL cavity, he says.

For the new design, the researchers used six outer cavities connected to the primary cavity in a hexagonal design, thereby increasing laser aperture. The design promotes adiabatic light-energy sharing between the cavities, relative to the central cavity, they say. By creating a functional space separation between selective gain and modulation function, the researchers report simultaneously achieving high speed and high power in single-mode operation.

“Even if the feedback strength of each cavity is in a moderate range, because of its adiabatic design, the laser cavity parametrically accumulates an increased amount of the slow light portion of the light gain and makes this available for the to-be-modulated cavity,” the team writes.

Applications aplenty

This new VCSEL design concept could find even greater application, Sorger says, in high-speed data communications, advanced sensor technology, autonomous vehicles, photonic AI systems and more.

For example, instead of using fiber optic cables to communicate rack-to-rack in data centers, Dalir says, “the next step is server-to-server with light signals from these efficient and fast lasers.”

For consumers, Dalir says, it could mean smoother internet video conferencing and streaming entertainment as well as super-accurate face ID for smartphones that would unlock in a flash.

Because the devices would not only be faster but also carry more data, Sorger says, “it enables three-dimensional depth in images.” Machines that could sense depth, he says, would have an army of users for applications such as self-driving cars and self-flying drones. With this so-called sensor fusion, he says, “We can sense further and more accurately enabled by high-power lasers.”

Other applications, Sorger says, would include 6G telecommunications networks, especially given their reliance on short-hop signaling at high data speeds; cryptography for secure networks like banking systems; and quantum-computing applications that are heavily dependent on lasers.

Still another use, says Behrouz Movahhed, an entrepreneur associated with the project, would be in hashing—a cybersecurity strategy that assigns a number, or hash, to a file or message to verify that it’s uncorrupted. Many applications calculate and compare hashes automatically; for example, digital signatures use hashes within an email, and email applications automatically create and compare the hashes. But these processes require enormous computing resources that generate heat and consume gigawatts of electricity, according to Movahhed—something the research team hopes the new VCSEL technology might alleviate.

Ready manufacturing

Sorger and Dalir say that while they are boosting VCSEL speeds, they can keep device costs down and use the same manufacturing processes as conventional designs. And though the hexagonal cavity design would make for a more interesting look on a semiconductor array, the devices would not take up more space.

“We are not increasing any complexity to the system,” says Sorger, who co-founded Optelligence, a tech venture that aims to further develop and commercialize the new VCSEL technology.