Photonic waveguide mode to free-space Gaussian beam extreme mode converter

• Published in: arXiv.org
• Main Authors: Kim, Sangsik, Westly, Daron A, Roxworthy, Brian J, Li, Qing, Yulaev, Alexander, Srinivasan, Kartik, Aksyuk, Vladimir A
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 06.03.2018
• Subjects: Beams (radiation); Converters; Coupling; Gaussian beams (optics); Light diffraction; Optical components; Organic chemistry; Photonics; Waveguides
• ISSN: 2331-8422

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 (\(\approx10^6\) 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 \(160~\mu\)m. Using two identical converters, we demonstrate a grating-to-grating 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.