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

Abstract

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.