Distributed Bragg Coupler for Optical All-Dielectric Electron Accelerator

A Bragg waveguide consisting of multiple dielectric layers with alternating index of refraction provides confinement of a synchronous speed-of-light mode with extremely low loss. There are three requirements in designing input coupler for a Bragg electron accelerator: side-coupling, selective mode excitation, and high coupling efficiency. We present a side coupling scheme using a Bragg-grating-assisted input coupler to address these three requirements. Side coupling is achieved by a second order Bragg grating with a period on the order of an optical wavelength. The phase matching condition results in resonance coupling thus providing selective mode excitation capability. We demonstrate a non-uniform distributed grating structure generating an outgoing beam with a Gaussian profile, therefore, increasing the coupling efficiency.