Photonic waveguide to free-space Gaussian beam extreme mode converter
Sangsik Kim, Daron A. Westly, Brian J. Roxworthy, Qing Li, Alexander Yulaev, Kartik A. Srinivasan, Vladimir A. Aksyuk
Integration of photonic chips with atomic, micromechanical, chemical and biological systems can advance science and open many possibilities in chip-scale devices and technology. Compact photonic structures for direct coupling of light between high-index single-mode waveguides and arbitrary free-space modes spanning hundreds of waves in cross-section would eliminate bulky optical components and enable integration of photonics into many new applications requiring wide beams, structured light and centimeter-scale propagation distances with low diffraction- limited losses. Conventional fiber-coupling approaches do not scale well for accurate, low-loss coupling across the extremely large mode scale mismatch (approximately 10^6 times in modal area). Here we present an extreme mode converter that can transform the photonic waveguide mode to the diffraction-limited, free-space Gaussian beam, with a beam waist of about 160 um. Using two identical converters, we demonstrate a mirror-back coupling that couples the radiating beam back to the chip through a mirror reflection in free-space. Operating at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication or other wavelengths. Furthermore, other types of beams can be implemented by using the 2-stage expansion approach presented in this paper.