Combining physical micro-machining and direct UV writing to fabricate novel planar tilted Bragg grating refractometers

Tilted Bragg gratings (TBG) offer discrimination between different environmental perturbations, such as localised strain (including thermal expansion) and external refractive index [1]. Discrimination can usually be made in this way as the external refractive index is not in direct contact with the core mode. Such technology has been primarily exploited in optical fibre [1–3], with some recent progress demonstrating multiplexed multi-point sensing networks [2], and the employment of enhancement layers to increase external refractive index resolution [3]. Despite significant developments made in fibre, relatively few reports have been made for such components in a planar geometry, which have the added advantage of compact integration and offers ready integration with microfluidics. A recent patent application for a refractometer device [4], has proposed the use of planar tilted Bragg gratings (PTBG) but it only describes geometries in which the measurand is in direct contact with the core of the waveguide. This work details a novel PTBG structure whose core mode is not in contact with the external refractive index.