All-in-fibre cladding interferometric and Bragg grating components made via plane by plane femtosecond laser inscription
We introduce a method of inscribing in-fibre devices using a femtosecond laser that is applicable to crucial components, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs), embedded waveguide Bragg gratings (WBGs), and waveguide Fabry-Perot cavities (WFPs) 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 FBGs. We show that the unique sensing characteristics of the filters are maintained and can be used to provide complementary measurand information, such as temperature and humidity. This approach ensures a single inscription process for each device, offering reliability and repetition in component manufacture, as the basic conditions used to inscribe the here-demonstrated CWGs, MZIs, WBGs and WFPs are common to all devices. We characterize CWG-WBG devices for axial strain, extract the WBG location through bend measurements and measure the response to surrounding refractive index (RI). The MZI-WBG device is exposed to temperature and humidity excursions, which confirm that the unique sensor responses are maintained for this compound and compact 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 the development of compact, smart optical fibre sensors.
Published in IEEE Journal of Lightwave Technology 2019 – https://ieeexplore.ieee.org/document/8746574