Efficient Coupling of Tapered Optical Fibers to Silicon Nanophotonic Waveguides on Rare-Earth Doped Crystals

Citation

Huan, Yan Qi (2019) Efficient Coupling of Tapered Optical Fibers to Silicon Nanophotonic Waveguides on Rare-Earth Doped Crystals. Senior thesis (Major), California Institute of Technology. doi:10.7907/QXEA-0C75. https://resolver.caltech.edu/CaltechTHESIS:06032019-143215628

Abstract

Quantum networks are a rapidly developing field of quantum information processing that have the potential to enable long-range entanglement between future quantum computers as well as the implementation of secure communication through quantum key distribution. Two key components of such networks are quantum memories to store entangled photon pairs for use in the quantum repeater protocol and quantum light-matter interfaces to efficiently interconvert between stationary qubits and flying qubits in the form of photons. Rare-earth ion-doped crystals are a promising solid-state platform that show promise for both of these applications due to their long optical and spin coherence times. Due to their relatively weak optical transitions, rare-earth ions have been coupled with nanophotonic resonators to enhance their transition strengths, with past work in the Faraon group utilizing focused ion beam milled photonic crystal resonators with 45-degree angled couplers to couple light in and out. Such resonators have the disadvantage of requiring manual alignment to fabricate, and the couplers are also relatively inefficient which limits the performance of such devices. It is therefore desirable to move towards silicon photonics, where mature techniques such as electron-beam lithography can allow for scalable fabrication of nanophotonic cavities together with high coupling efficiencies.

In this thesis, we demonstrate significant progress towards the usage of acid-etched tapered optical fibers as an efficient interface for coupling light into tapered silicon nanophotonic waveguides. We show comprehensive simulations of the taper geometries required to achieve adiabatic coupling with theoretical efficiencies of more than 99%, and design a silicon photonic crystal mirror to be used in the measurement of the fiber-waveguide coupling efficiency. We then optimize the hydrofluoric acid fiber etching process and demonstrate the ability to make tapered fibers which are 200 microns long with a taper half-angle of 2 degrees and a tip diameter of 50 nm. Using these tapered fibers to couple light into tapered silicon waveguides fabricated using electron-beam lithography shows a moderately high coupling efficiency of 11.4% with the potential for improvement. This method of tapered fiber coupling shows promise to be integrated into silicon nanophotonic resonators on rare-earth ion doped crystals and allow for highly efficient quantum memories and quantum light-matter interfaces in the solid-state.

Thesis Files