All-in-Fiber Cladding Interferometric and Bragg Grating Components Made via Plane-by-Plane Femtosecond Laser Inscription

We introduce a method of inscribing in-fiber 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 using the same key femtosecond laser parameters, via an “inscribe and step,” plane-by-plane 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 fiber Bragg gratings; the unique sensing characteristics of the filters are maintained and provide complementary measure and information. This ensures a single inscription process, offering reliability and repetition in component manufacture, as the basic conditions to inscribe the here-demonstrated elements are common. We characterize CWG-WBG devices for axial strain, bend, and response to refractive index. The MZI-WBG is exposed to temperature and humidity excursions, confirming that 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 the development of compact, smart optical fiber sensors.