Fibre cladding interferometers and Bragg gratings made via plane by plane femtosecond laser inscription

We present an extremely flexible femtosecond (fs) laser inscription method, applicable to the development of critical filtering and wave-guiding components in optical fibres. We inscribe in-fibre devices, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs) and embedded waveguide Bragg gratings (WBGs) using the same key femtosecond laser parameters, via an "inscribe and step", plane-by-plane (Pl-by-Pl) approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fibre Bragg gratings (FBGs); the unique sensing characteristics of the filters are maintained and provide complementary measurand information. The flexibility and control in waveguide/grating fabrication leads to sensing device customization, e.g. tailored bend sensing. We characterize CWG-WBG devices for their bend response, whereas the MZI-WBG is exposed to temperature and humidity excursions, confirming the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in developing compact, smart optical fibre sensors.