Simple modification of the shape of semiconductor laser can improve the beam quality

Scientists at King Abdullah University of Science and Technology have demonstrated a simple way to alter compact semiconductor lasers to make them more suitable for lighting and holography. The study appears in a recent issue of Optica.

Semiconductor technology allows all components of the laser to be packaged into a micron-scale device, which includes an optically active optical amplification region with a highly reflective mirror on each side.

Vertical cavity surface-emitting lasers, or VCsels, create highly reflective mirrors by precisely placing or growing alternating semiconductor layers on a substrate. The active material is then grown on the top, followed by a second mirror. A laser can then be fired from the top of the device.

Schematic diagram of a chaotic cavity surface emission laser array. This new laser array combines the advantages of chaotic cavity and surface emission configurations to enable high-quality lighting and high-speed communication.

Vcsels are advantageous because hundreds of VCsels can be created and used simultaneously on the same substrate. But beams are prone to blotchy contours, making them unsuitable for applications such as lighting, holography, projection and displays. These all require uniform light in a plane perpendicular to the direction of the beam's propagation.

The spots result from the highly ordered nature of the cavity, which allows only a small number of patterns or light trails to be emitted. "Using an ordered cavity, the VCSEL allows for light resonance with extremely high efficiency in only a few modes," explains researcher Omar Alkhazragi. . "The photons in these modes interfere with each other, resulting in spots and low lighting quality."

Alkhazragi and KAUST's colleagues, along with colleagues from China, have shown that by changing the shape of the device to break the symmetry of the cavity, it is possible to reduce the spots in the VCSEL laser. This introduces chaotic behavior in the generated light and allows more patterns to be emitted.

Alkhazragi and his team studied VCsels with D-shaped cavities and compared them to VCsels with standard cylindrical or o-shaped geometry. They observed that D-shaped devices exhibit significantly reduced coherence and a corresponding 60% increase in optical power, which is the maximum achievable.

The researchers attribute the improvement to the chaotic dynamics of the light in the cavity. Speckle visibility is reduced because the light is emitted in an unrelated pattern.

"Machine learning can help design cavities to further maximize the number of patterns and reduce coherence, thereby reducing spot density below human perception," Alkhazragi said.

Related links: https://phys.org/news/2023-03-simple-modification-semiconductor-laser-quality.html