Vertical-cavity surface-emitting laser (VCSEL) light sources are used in optical storage, laser printing, 3D sensing, and other critical applications. Conventional, oxide-confined VCSELs typically operate in single-transverse mode, with an oxide aperture smaller than 4 μm. This can lead to increased series resistances and low output power in the VCSEL. To overcome these limitations, researchers at the Changchun Institute of Optics, Fine Mechanics, and Physics of the Chinese Academy of Sciences developed a mode filtering technique that uses metal apertures to flexibly regulate transverse modes in VCSELs. The new metal-dielectric film mode filter structure demonstrates the potential of metal apertures to enhance optical mode control and improve the performance of single-mode VCSELs. The researchers began by creating a finite element simulation model of a metal mode-filtered VCSEL (MMF-VCSEL). They simulated variations in the position of the metal layer, the metal apertures, and oxide apertures on the fundamental mode optical spot of the MMF-VCSEL, and the effect of the variations on modal control. They systematically calculated the modal loss and optical confinement factor variations with different metal apertures and metal layer positioning. The structure of a metal mode-filtered vertical-cavity surface-emitting laser (MMF-VCSEL). Courtesy of Sensors (2024). DOI: 10.3390/s24144700. The researchers then defined the optical gain, which made it possible for them to describe the modal discrimination and provide alternative computational schemes and parameter ranges to achieve optimal device performance according to specific requirements. The simulation results showed that the modal control performance of the MMF-VCSEL was significantly influenced by the number of P-distributed Bragg reflector (P-DBR) pairs, the metal aperture size, and the oxide aperture size. The transverse optical field was strongly confined within the metal aperture when the number of P-DBR pairs was low, but the confinement weakened as the number of P-DBR pairs increased. The researchers found that when the metal aperture was smaller than the oxide aperture, the optical scattering effect intensified as the distance between the two apertures shrank. The mode discrimination and modal loss of the VCSEL increased, improving single-mode stability. The team observed that when the metal aperture exceeded the size of the oxide aperture, the optical mode in the VCSEL was controlled primarily by the oxide aperture. This finding shows how the complex relationship between the metal and oxide apertures can affect optical field confinement and mode discrimination in the VCSEL. By selecting the appropriate metal aperture and oxidation aperture values, the VCSEL can maintain good single-mode stability while retaining low threshold characteristics. By defining optical gain, the researchers introduced a new parameter to characterize the change in the threshold gain of the different transverse modes that is caused by optical scattering. By balancing the difference in optical gain between modes and the optical gain of the fundamental mode, they could identify the optimal structural parameters needed to enhance both single-mode stability and slope efficiency of the MMF-VCSEL. In summary, the researchers showed that number, volume, and stability of transverse modes inside the VCSEL can be adjusted according to three key parameters — the oxide aperture, the metal aperture, and the distance between the oxide aperture and the metal aperture — to form a flexible window. A new parameter — optical gain — can be defined to describe the mode identification. The researchers demonstrated that high-power, single-mode VCSELs with improved performance characteristics can be achieved by flexibly modulating the transverse mode. The study underscores the critical role of metal apertures in enhancing mode control in VCSELs, and could lead to new approaches to developing high-performance VCSELs for broad use. The research was published in Sensors (www.doi.org/10.3390/s24144700).