Reduced speckle shows promise for illumination and holography

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A straightforward modification of a semiconductor laser's shape can improve the quality of its beam, claim researchers at King Abdullah University of Science and Technology.

Illustration of a chaotic-cavity surface-emitting laser array. This new class of laser array combines the advantages of chaotic cavities and the surface-emission configuration to enable high-quality illumination and high-speed communication
Illustration of a chaotic-cavity surface-emitting laser array. This new class of laser array combines the advantages of chaotic cavities and the surface-emission configuration to enable high-quality illumination and high-speed communication - © 2023 KAUST; Omar Alkhazragi

According to the team, the method of altering compact semiconductor lasers will make them more practical for illumination and holography.

Semiconductor technology allows for all elements of a laser to be packed into a micrometre-scale device. This includes an optically active, light amplification region with a highly reflective mirror on each side.

One such device is the vertical-cavity surface emitting laser (VCSEL). These are built by precisely placing, or growing, alternating layers of semiconductor on a substrate to create a highly reflective stack. The active material is then grown on top, followed by a second reflective stack. Laser light can then be emitted from the top of the device.

According to KAUST, VCSELs are advantageous because hundreds can be created and used on the same substrate simultaneously. The beam, however, is prone to a speckle-like profile, which makes it unsuitable for applications including lighting, holography, projection and displays. These require uniform light in the plane perpendicular to the direction of beam propagation.

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The speckles originate from the highly ordered nature of the cavity, which allows only a small number of modes, or light-ray trajectories, to be emitted.

“VCSELs utilise an ordered cavity that allows the resonance of light in only a small number of modes with exceptionally high efficiency,” said researcher Omar Alkhazragi. “The photons in these modes interfere with each other, resulting in speckles and low illumination quality.”

Alkhazragi and colleagues have shown that speckles can be reduced in laser light from VCSELs by changing the shape of the device to break the symmetry of the cavity. This is said to introduce chaotic behaviour in the generated light and allows the emission of more modes.

Alkhazragi and the team investigated VCSELs with a D-shaped cavity and compared it with those with the standard cylindrical, or O-shaped, geometry. They observed that the D-shaped devices exhibited substantially reduced coherence and a corresponding 60 per cent increase in optical power, which is the maximum achievable.

The researchers attribute this improvement to the chaotic dynamics of the rays of light within the cavity; since light is emitted in mutually incoherent modes, the visibility of the speckles is reduced.

“Machine learning could help design cavities that further maximise the number of modes, lower the coherence and thus reduce speckle density to below human perception,” Alkhazragi said in a statement.

The team’s findings are detailed in Optica.

semiconductor lasers