Topological Cavity Widens Functionality of Surface-Emitting Lasers

Researchers at the Institute of Physics of the Chinese Academy of Sciences have incorporated a topological cavity into the design of a surface-emitting laser to create a device that addresses the challenge of simultaneously increasing output power and beam quality in semiconductor lasers — a principal bottleneck. The research team’s topological-cavity surface-emitting laser (TCSEL) achieved 10 W of peak power, subdegree beam divergence, a 60-dB side-mode suppression ratio, and 2D multiwavelength array lasing at 1550 nm — which is considered crucial for communication and is also an eye-safe wavelength.

The TCSEL was also demonstrated to operate at any other wavelength range. The laser’s specifications and capabilities give it promise for applications, including lidar and virtual reality, as well as in communications.

The laser’s design features a cavity that the same research team previously developed, called a Dirac-vortex topological cavity. The cavity offers high single-mode selection over a large area. It was proposed to overcome the bottlenecks of semiconductor lasers and simultaneously improve output power and beam quality.

Semiconductor lasers are widely used across applications due to their compact size, high efficiency, low cost, and wide spectra.

The researchers advanced the previous work and applied the cavity to a surface-emitting laser. They evaluated its performance by comparing it to existing industrial products that use single-mode semiconductor lasers — the distributed feedback (DFB) edge-emitting laser used in internet communication and the vertical-cavity surface-emitting laser (VCSEL) that enables cellphone facial recognition. These two lasers both adopt the midgap mode in their optimized 1D resonator designs.

The TCSEL realizes the 2D version of the topological midgap mode, which the researchers said is more suited for the planar process used for the manufacture of semiconductor chips.

By changing the lattice constant, the corresponding laser wavelength varies linearly from 1512 to 1616 nm. Each laser in the TCSEL 2D array works stably in a single mode with a side-mode suppression ratio greater than 50 dB. According to the researchers, the 2D multiwavelength TCSEL arrays can potentially enhance the wavelength-division multiplexing technology for high-capacity signal transmission and multispectral sensing applications. Courtesy of the Institute of Physics of the Chinese Academy of Sciences.

The far field of the TCSEL is a vector beam with radial polarizations. This narrow divergence, of less than 1° without collimating lenses, reduces the size, complexity, and cost of the laser system, which supports larger systems such as those used for 3D sensing. From microscopy images, the researchers clearly visualized the vortex structure from the Dirac-vortex cavity in the TCSEL.

Further, the laser’s wavelength flexibility gives the device the ability to achieve monolithic 2D multiwavelength arrays. In comparison, VCSELs generally lack wavelength tunability since the vertical cavity, which determines the lasing wavelength, is epitaxy-grown. Although DFB lasers can adjust wavelength, a DFB laser can only achieve a 1D multiwavelength array for edge emission. In contrast, the wavelength of TCSEL can be arbitrarily adjusted during the planar fabrication process.

Each laser in the 2D array also works stably in a single mode with a side-mode suppression ratio greater than 50 dB. The 2D multiwavelength TCSEL arrays can potentially enhance the wavelength-division multiplexing technology for high-capacity signal transmission and multispectral sensing applications.

The work was supported by the CAS, the Beijing Natural Science Foundation, the Ministry of Science and Technology of China, and the Natural Science Foundation of China.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-00972-6).