Integrated optical sensor based on refractive index gratings in photorefractive LiNbO_3:Ti:Fe channel waveguides

摘要

We present the results of simulations of an integrated optical sensor that uses two narrow-bandwidth interference filters in a single-mode LiNbO_3:Ti:Fe channel waveguide operating in the near infrared wavelength region around λ = 1.55 μm. The interference or Bragg filters consist of thermally fixed gratings recorded with green light (λ = 532 nm) in transmission geometry. With these Bragg gratings a Fabry-Perotresonator is formed. The evanescent field of the guided mode senses changes in the refractive index of the layer covering the waveguide. The resulting changes of the effective refractive index of the mode alter the resonance condition of the resonator. These phase changes can be compensated by using the electro-optic effect and applying an electric field to the waveguide, thus this electric field acts as a sensor signal. Covering the waveguide with a thin layer of a material with high refractive index (e.g. TiO_2) leads to an increase of the evanescent field and therefore to a higher sensitivity. In combination with an additional functional layer that is sensitive only to, e.g., specific molecules, a highly efficient sensor with a resolution in the ppb range may be realized.