Optoelectronics New VCSEL Technology Cuts Production Times in Half

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Researchers at TU Berlin (Germany) have revolutionized VCSEL production by replacing Bragg mirrors with nano-precise surface gratings. The production time is reduced by 50 percent, while at the same time different laser wavelengths can be realized in parallel on one wafer.

Researchers at TU Berlin have developed an innovative approach for the production of VCSEL laser diodes that should make the manufacturing industry sit up and take notice. The team led by Prof. Dr. Stephan Reitzenstein from the "Optoelectronics and Quantum Devices" department, in collaboration with the University of Łódź (Poland), has developed a method that reduces production times by 50 percent by replacing the complex upper Bragg mirrors with precisely etched optical gratings, while at the same time realizing different laser wavelengths in a single production step.

Proven Technology With New Possibilities

VCSELs (Vertical Cavity Surface Emitting Lasers) have been in use for more than two decades and have established themselves as key components. These laser diodes, which emit their light perpendicular to the semiconductor chip, can couple the laser beam into optical fibers very well, unlike laser diodes with lateral light emission. The reason for this is the round geometry with a small design as well as good beam quality and focusability. VCSELs are therefore used in large data centers for the efficient transmission of information between servers. They are also used in cell phones for facial recognition.

Grids Instead of Elaborate Mirror Layers

A VCSEL essentially consists of an active layer in which the light particles are created, as well as layers of so-called Bragg mirrors above and below it. They repeatedly reflect the light back into the active layer so that it stimulates the creation of further light particles there—the typical laser effect. The upper mirror layers reflect the laser light slightly less, allowing part of the laser beam to escape and be used.

The new technology promises considerable cost savings in industrial production. While the conventional production of the complex mirror layers takes around twelve hours, the Berlin researchers can save around six hours with their grid-based solution. "The big difference is that we no longer have to apply so many semiconductor layers one after the other. This is a lengthy and expensive process," explains first author Niels Heermeier.

Precision in the Nanometer Range Enables Flexibility

"The even more important advantage of the line grating is that laser diodes with different output wavelengths can be manufactured simultaneously on a semiconductor wafer in a single production step," explains Heermeier. For this purpose, several geometric parameters of the line grating are varied during the etching process depending on the diode: the thickness of the grating bars, their distance from each other and the depth of the etched grooves between them. "We don't have this flexibility when applying the mirror layers, as these have to grow on the entire wafer and therefore define the properties for all laser diodes in a uniform way."

Extremely high accuracy is required in order to be able to precisely manufacture the new microlasers with their individual properties: The sizes may deviate from their target value by less than five nanometers. Compared to the distance between the earth and the moon of almost 249,000 miles, this would correspond to a maximum permissible deviation of 7 feet. This achievement was only possible thanks to an electron beam lithography system available at the department and procured with funding from the German Research Foundation (DFG) and TU Berlin, which has to be adapted to its task by the researchers in a complex process.

Broad Range of Applications in Growth Markets

The potential applications are diverse and target current growth markets. In addition to the proven application in data centers for efficient signal coupling in fiber optic cables, the microlasers could also be used for the lidar process (light detection and ranging), which plays a major role in autonomous driving. Here, arrangements with laser diodes of different wavelengths, such as those that can be easily produced with the new process, deliver significantly better resolution.

The new VCSEL diodes could also become an important building block for the hardware of so-called neuromorphic computing with optical processors, which Stephan Reitzenstein's team is developing together with colleagues at the Universities of Berkeley and MIT in Boston. These computers are designed in the same way as the human brain and work with optical components and light as information carriers instead of electrical circuits.

"It is important that the wavelength of the light from the laser diodes is exactly the same. However, even the smallest deviations in production can lead to different wavelengths," explains Reitzenstein. With the new method, it would be possible to measure the wavelengths after production of the optical chip and then match them exactly by means of subsequently etched grating structures.

Quantum Physics As A Research Bonus

The new microlasers are also interesting for basic research, as the researchers from the collaborating working group in Poland were able to realize the exotic quantum state of a "Bose-Einstein condensate" with light particles in conventional VCSEL diodes for the first time by chance. This phenomenon can now be investigated in more detail with the new, more easily adjustable laser diodes.

The team is already planning the next development step: the complete replacement of the lower mirror layers with grating structures. "However, this task is much more complex because the substrate material of the diode must then be removed on the underside," Reitzenstein points out. From his point of view, the new Center for Integrated Photonics Research (CIPHOR) is therefore of particular importance as a central component of the new experimental physics building on the East Campus of TU Berlin. This will be built from 2028 and will open up new production possibilities with excellently equipped clean rooms. (heh)

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