Replicated Mach-Zehnder interferometers with focusing grating couplers for sensing applications

Abstract

A novel type of Mach-Zehnder structure which is suitable for the low-cost fabrication of miniature sensor modules is described. It consists of a nano- and micro-structured polymer substrate which is coated over its whole area with a high-index inorganic dielectric film. Ridge structures of about 3 μm height, together with focusing input and output gratings of about 20 nm depth are replicated in a single embossing step into a polymer foil, followed by waveguide deposition. The focusing grating couplers enable coupling of collimated free-space beams into and out of the stripe waveguide structures without the need for external optical elements. This leads to relaxed alignment tolerances and eases the miniaturization of the complete sensor system. In addition, fabrication based on replicated plastic substrates offers the opportunity of low-cost mass production. Waveguide and interferometer structures of this type are characterized by exciting modes in stripe waveguides via the focusing grating couplers. Successful light input and output from a 4 μm wide stripe waveguide as well as light transmission through a replicated Mach-Zehnder structure formed of 3 μm wide waveguide stripes demonstrate the feasibility of this approach. The use of high-index waveguiding films on low-index substrates makes the structure well suited for sensing applications.