The University of Illinois combines the light emitted by multiple VCSEL into a single coherent mode

Today, VCSELs (vertical cavity surface-emitting lasers) are used in everything from computer mice to face-scanning hardware in smart phones. They are renowned for their ability to integrate seamlessly into semiconductor chips, VCSELs are still considered to be an active field of research, and many researchers believe there are still important applications waiting to be discovered.
The laboratory of Kent Choquette, a professor of electrical and computer engineering in Grainger College of Engineering at the University of Illinois Urbana-Champaign, has developed a new design in which light from multiple VCSELs is combined to form a single coherent pattern called a “supermode”.

As the researchers report in IEEE Photonics Journal, the result is a controllable pattern brighter than what is possible with an array of independent devices.

940 nm dual-cavity photonic crystal VCSEL array

‘Challenging VCSELs’

“VCSELs are more challenging to work with than other kinds of lasers because they naturally tend to emit light in many special patterns, or modes, so the central problem has been figuring out how to get the light to stay in the mode you want,” Choquette said.

“The design we explore in this study is noteworthy because it shows how to extend mode control across more than one VCSEL and use an array of them in tandem to get a single desired mode. With this level of cooperation across arrays of VCSELs, we’re confident that new uses for these devices will emerge.”

Ordinarily, VCSELs are individually controlled with electrical signals, making the problem of coordinating a coherent beam across laser cavities difficult. The researchers proposed a design that makes use of a photonic crystal connecting adjacent VCSELs. So, although they are electrically independent, they act in tandem optically. This makes it possible to control both cavities in a way that produces one of two pre-determined collective patterns, or supermodes.

The details of the design, including the use of a special “anti-guided” crystal to achieve the optical coupling, were studied by Dan Pflug, an Illinois Grainger Engineering graduate student in Choquette’s laboratory and the study’s lead author.

The Illinois team then turned the design over to the company Dallas Quantum Devices, where a working device was fabricated in a foundry-level process, demonstrating that the design can be practically realized.

“Our collaboration with Dallas Quantum Devices originates in a call from the National Science Foundation for Small Business Innovation Research proposals in high-speed VCSELs,” Choquette said. “I have known some of these people for over 20 years. It’s a case where what started out as informal exchanges has led to a long-term relationship.”

For Choquette, this work is a product of discovery and innovation for its own sake. He observed that this is often where some of the most important end uses for new technologies originate. “When I started working with VCSELs 30 years ago, the interest in them was purely academic,” he said. “But one day, I got a call from Microsoft, and laser computer mice entered the market. Now, everyone uses VCSELs every day. This is the reason we do research like this: applications aren’t always obvious, and the only way to know is to try it out.”

Source: optics.org