Uniform Thin Films on Optical Fibers by Plasma-Enhanced Chemical Vapor Deposition: Fabrication, Mie Scattering Characterization, and Application to Microresonators

We demonstrate deposition of azimuthally uniform single- or multiple-layer thin films of silicon nitride and silica on fibers using plasma-enhanced chemical vapor deposition by continuously rotating the fibers during growth. Our fibers exhibit distinctive and uniform iridescence that strongly depends on coating configuration. We also report a non-invasive technique to measure refractive index and film thickness of coated fibers simultaneously based on Mie scattering. We found the films grown on fibers have very different characteristics from those grown on flat substrates. We deposit a 1-μm-thick SiN
_x
film on a spheroidal microrod resonator, which is shown numerically to push the guided fundamental mode into the silica core. We demonstrate a Q factor of 2.2 × 10
⁶
, indicating reasonably good thin film quality that could be further increased with improved process control. Our technique can be applied to coat whispering gallery mode microresonators with engineered (e.g., step, graded, or stratified) refractive index profiles, which are expected to enable many new applications.