Chemical-Assisted Femtosecond Laser Structuring of Waveguide-Embedded Wavefront-Splitting Interferometers

A comprehensive design of wavefront-splitting interferometers (WSIs) is introduced for integration into wafer light circuits and optical fiber systems. The WSI detects the coherent interference of two wavefronts after being equally split by a single buried resonator and collected into a single mode waveguide. WSIs present a desirable compact sensor as possible with Fabry Perot interferometers (FPIs), with the additional benefits of strong visibility contrast and high sensitivity to external parameters as expected with Mach-Zehnder interferometers. Theoretical models and finite-difference time-domain simulations show the WSI spectral responses to be 2χ to 120χ more sensitive to changes in refractive index, temperature, and strain over comparable Bragg grating waveguides and FPIs. Femtosecond laser irradiation with selective chemical etching provided the flexible means for 3-D geometric structuring and waveguide integration below the surface, delivering precise WSIs with a small ´12-nm rms surface roughness. Temperature and vacuum sensing were demonstrated with high sensing resolution (0.8 °C and 1.8 χ 10^-5 RIU) and sensitivity (60.6 pm/°C and 2800 nm/RIU) to match theoretically anticipated values. Such high finesse optical elements open a new realm of optical sensing and integrated optical circuit concepts without relying on tedious nanoprecision assembly methods or the use of large optical components.