Coherence engineering of VCSELs via eccentric annular cavity design for speckle-free imaging and ultrafast random number generation

Coherence engineering of vertical-cavity surface-emitting lasers (VCSELs) is a powerful means to explore advanced light–matter interactions and manipulate optical dynamics, such as speckle-free imaging and random number generation (RNG) using broadband chaotic emission. While low-coherence VCSELs have been used to study geometry-dependent laser dynamics, achieving systematic coherence control through cavity design remains challenging. Here, we address this challenge by studying and simulating a sequence of graduated off-centered positioning of an eccentric annular cavity design, enabling spatial distribution tuning of the transverse modes to engineer coherence and modal diversity. Spatial coherence is evaluated through speckle contrast in imaging, and temporal fluctuations resulting from chaos are characterized via autocorrelation functions and radio frequency spectra. Among all designs, the A-20 VCSEL, featuring an annulus displaced by 20 μm from the center, achieves the best imaging performance and an RNG rate of 250 Gbit/s, successfully passing the NIST SP 800-22 randomness test. These results demonstrate a straightforward annulus-positioning strategy for lateral refractive index engineering, offering a scalable pathway toward coherence-engineered VCSELs that integrate high-speed RNG and speckle-free illumination within a single compact platform.