Vertical-Cavity Surface-Emitting Laser Linewidth Narrowing Enabled by Internal-Cavity Engineering

Vertical-cavity surface-emitting lasers (VCSELs), featuring the advantages of low energy consumption, miniaturization, and high-beam quality, show potential for various applications from atomic clock to light detection and ranging (LiDAR). A high-performance atomic clock system requires laser linewidths below 10 MHz to ensure compatibility with the natural atomic linewidth (e.g., 5 MHz for cesium). However, the current prevalent method for reducing VCSEL linewidths relies on external cavities, which adds complexity and cost to the devices and hampers seamless integration into atomic clock systems. While narrow-linewidth VCSELs have been successfully demonstrated using extended cavities, there remains a need for a comprehensive and systematic study on the underlying design principles and optimization strategies. Here, we propose a VCSEL linewidth narrowing strategy enabled by internal-cavity engineering for cesium atomic clock applications. We investigate strategies to narrow the cold cavity linewidth without introducing additional optical round-trip loss. We provide a general approach to constructing the extended cavity (EC) and showcase the ability of manipulating the phase of light. To optimize the electrical properties, we explore variations in the extended layer thickness based on a monolithic VCSEL structure. We proposed an EC-VCSEL configuration with a theoretical laser spectral linewidth of approximately 1.7 MHz and a calculated output power of about 3 mW. Through exploiting gain-cavity offset, the EC-VCSEL exhibits a stable emission (894.6 nm) and a high gain of cavity mode ( $\sim $ 4000 cm $^{-1}$ ) at high-temperature (e.g., 360 K). This work may serve as a reference for the realization of narrow-linewidth VCSELs, offering potential benefits in reducing device complexity and facilitating the system integration.