An electro-optic sensor capable of detecting electric fields with a high degree of sensitivity and linearity is fabricated using optical D-fiber. The slab coupled optical sensor utilizes weak coupling and long evanescent interaction with a lithium niobate waveguide. Transmission dips from mode resonances have a linewidth of $0.12\mathrm{nm}$ and a Q factor of $\sim \mathrm{13,000}$ . These sharp resonances improve device sensitivity and are achieved due to the unique fabrication process possible with D-shaped fibers. The sensor deviates $<0.1\%$ from linearity while monitoring fields between $200\mathrm{V}/\mathrm{m}$ and $20\mathrm{kV}/\mathrm{m}$ and promises high sensitivity to fields well beyond that range.