Segmented waveguides and their applications for biosensing

The periodically segmented waveguide (PSW) has been used for a variety of applications, including frequency conversion, filtering, mode expansion and evanescent wave sensing. The PSW functionalities include index averaging and linear/nonlinear index grating, with the advantage of an inherent single-step fabrication. The paper briefly reviews some aspects of the PSW, and then focuses on a new type of segmented waveguides, and on applications of the PSW for biosensing. The inherent problem of the PSW is radiation loss, particularly in cases of high index contrast, such as in rib waveguides. Partial solution is to choose the segmentation period to obtain minimal loss. Another would be to replace (or combine) the longitudinal segmentation with transverse segmentation. In that context, we present results for the transversely segmented waveguide (TSW). Due to the low-cost potential, an attractive application of the PSW is in evanescent wave sensing. The PSW has been used to realize evanescent wave sensors, in a Mach-Zehnder configuration. The waveguides have been fabricated by ion-exchange in glass. To further increase the sensitivity, the chip was covered by a thin (40-60 nm), high index TiO₂ film. The resulting refractive index sensitivity is currently approximately 2×10^-5, and biofilm thickness sensitivity of approximately 0.2 angstrom have been achieved so far.