6/4/2021
A group of researchers from HMNTL are working to improve VCSEL technology through single-mode polarization, without compromising the device in newly-funded research.
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In the last six years, the demand for vertical-cavity surface-emitting lasers, or VCSELs, has exploded. Prior to 2017, they were mainly used in data centers, laser printers and optical mice. The easy-to-manufacture, low-cost, energy-efficient lasers had a major glow up in the mid-2010s after Apple used them for facial recognition on the iPhone X, which has sparked interest in using these devices for artificial reality (AR) and virtual reality (VR) headsets and automotive LiDAR.
For VCSEL researchers at HMNTL, the VCSELs rise in popularity isn’t surprising.
“It’s exciting to see many big names using these devices to get newer technologies into people’s hands in a very low-cost way,” said Patrick Su, HMNTL and electrical and computer engineering (ECE) graduate student leading the project. “There is a lot of excitement around the resurgence of VCSELs that has helped enable newer technologies like LIDAR, AR, VR, and it has people working on these devices.”
Traditional applications for VCSELs, such as fiber optic communications, required researchers to improve the energy efficiency or speed of the devices. Newer applications such as AR/VR headsets need improved optical beam power and improved beam quality. A previous project by the group, also funded by the II-IV Foundation, looked to improve beam quality by decreasing the noise in the communication process. This research looks to take beam improvement a step further.
“Previously we were focused on high single-mode power. This year we’re focused on trying to apply those same techniques for single-mode, single-polarization operation,” said John Dallesasse, HMNTL faculty member and ECE professor. “That’s important because every iPhone 10 and beyond has a VCSEL array and they’re also used for things like 3D photography. Even in traditional applications, having devices that have single polarization is important.”
In a traditional VCSEL, there is no preferred polarized state, so the team had to break down the inner workings to improve it to a single mode of polarization. Previous researchers have succeeded in doing this, but their techniques required etching into the VCSEL. This means material is taken away, compromising the flow of electricity and negatively impacting the device. The team’s technique involves depositing an additional layer of silicon as an anti-phase coating onto the device or selectively intermixing the top layers via impurity-induced layer disordering rather than taking material away. This results in a preferred polarizing effect without negatively affecting the reflectivity or electrical flow.
The team’s first steps will be designing and simulating the desired size and shape of the apertures, which they will be doing in two different ways. Su is using the impurity-induced layer disordering technique mentioned above, and graduate student Kevin Pikul will be using an antiphase coating, trying to reach the same result.
“We’re basically doing the same thing except Patrick is modifying the VCSEL emission aperture through selectively intermixing the layers by diffusing zinc, and I am modifying the emission
aperture using an antiphase coating, both in the shape of an ellipse,” said Pikul, also an ECE student. “Right now, the technology being used is not well suited for emerging applications. With our work in polarizing VCSELs we can accomplish and satisfy the needs of these applications better.”
The elliptical-shaped aperture being designed by Pikul and Su is crucial to developing singular polarization emission because it breaks the symmetry of the previously circular device. Since the laser threshold modal gain is greater in regions outside of the aperture, those optical modes and polarization states are suppressed. This limits the VCSEL to a single mode and single polarization to lase in, which would improve the emission quality of VCSEL devices.
This project is funded by the II-IV Foundation, whose mission is “to encourage and enable students to pursue a career in engineering, science and mathematics while maintaining a standard of excellence in that pursuit.” This is the fourth time Dallesasse’s research group has been awarded a grant from the foundation.
“We’re grateful for the II-VI Foundation for the opportunity to keep working on this,” said Dallesasse. “They are extremely supportive of our students and their development.”