Developing highly efficient and electric-field-sensitive fiber-waveguide evanescent couplers.

The slab-coupled optical fiber sensor (SCOS) is an innovative electric field detector with an ultrawide measuring range and a millimeter-level package size. The core sensing part of the SCOS is a fiber-waveguide evanescent coupler (FWEC) that directly determines the device's main performance specifications. This paper presents an investigation of the spectrum characteristics of FWECs with various structural and curing parameters. Methods for fabricating an FWEC with higher resonant depth, narrower free spectral range, and sharper spectrum slope are determined based on the experimental results. Z-cut lithium-niobate FWEC and Z-cut lithium-tantalate FWEC of about ${{1}} \times {0.5} \times {0.3}\;{\rm{mm}}^3$ are fabricated on this basis. Excellent coupling characteristics are achieved in both, according to the relevant spectra. Electric field tests indicate that the peak wavelengths shift linearly with the external AC field amplitude by 0.11 nm/(kV/cm) and 0.24 nm/(kV/cm), respectively. Three optimization methods are proposed to enhance performance: optimizing material selection, adding an antenna structure, and adjusting the calibration method. The results of this work may provide workable guidance for developing miniature, all-dielectric electric field sensors.