Kim Sangsik, Westly Daron A, Roxworthy Brian J, Li Qing, Yulaev Alexander, Srinivasan Kartik, Aksyuk Vladimir A
1Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA.
2Maryland Nanocenter, University of Maryland, College Park, MD 20742 USA.
Light Sci Appl. 2018 Oct 10;7:72. doi: 10.1038/s41377-018-0073-2. eCollection 2018.
Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment. Here, we develop an extreme mode converter, which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 µm waist, which corresponds to an increase in the modal area by a factor of >10. The beam quality is thoroughly characterized, and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated. To achieve low phase error over a beam area that is >100× larger than that of a typical grating coupler, our approach separates the two-dimensional mode expansion into two sequential separately optimized stages, which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space. Developed at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication, or other wavelengths. The technique can be expanded to more arbitrary phase and intensity control of both large-diameter, free-space optical beams and wide photonic slab modes.
将光子芯片与毫米级原子、微机械、化学和生物系统集成，可推动科学发展，并实现新型小型化混合设备和技术。通过小消逝体积的光学相互作用限制了诸如气体光谱学等应用中的性能，因此需要具备在大自由空间体积中以光子方式访问光场的通用能力。然而，传统的反向锥形器和光栅耦合器无法直接扩展以产生宽的、高质量的准直光束，以便在自由空间中进行数毫米的低损耗无衍射传播，这就需要额外的笨重准直光学器件和昂贵的对准操作。在此，我们开发了一种极端模式转换器，它是一种紧凑的平面光子结构，能够将300 nm×250 nm的氮化硅高折射率单模波导高效地耦合到具有约160 µm束腰的准直近表面法线高斯光束，这相当于模式面积增大了10倍以上。对光束质量进行了全面表征，并展示了其在自由空间中4 mm的传播以及通过单独的相同模式转换器以低损耗耦合回单模光子波导的过程。为了在比典型光栅耦合器的光束面积大100倍以上的区域实现低相位误差，我们的方法将二维模式扩展分为两个连续的、分别优化的阶段，在将其外耦合到自由空间之前，先创建一个完全扩展且准直的高斯平板模式。我们的设计在波长780 nm下开发，用于与芯片级原子蒸汽池腔集成，可适用于可见光、电信或其他波长。该技术可扩展到对大直径自由空间光束和宽光子平板模式进行更任意的相位和强度控制。
