Femtosecond Laser-Inscribed Cladding Mode Small-Period Long-Period Fiber Gratings for Temperature and Strain Sensing

In this study, we employ a femtosecond laser to inscribe a small-period long-period pattern with a period of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30~\mu $ </tex-math></inline-formula>m in a single-mode fiber (SMF). This is achieved by combining the line-by-line (LbL) scanning technique with a dry objective lens to induce the plane-by-plane (Pl-b-Pl) effect, which introduces spherical aberration (SA) for elongating the focal volume and enlarging the rectangular refractive index modulation (RIM) region. The rectangle within this region has a width of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15~\mu $ </tex-math></inline-formula>m and a length of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$32~\mu $ </tex-math></inline-formula>m. The spectral properties of the cladding mode small-period long-period fiber grating (CMSP-LPG) were systematically investigated. The reflection spectrum has a comb-like structure with alternating Bragg resonance peaks from the cladding and core modes. A weak polarization dependence was observed in the reflection spectrum. In addition, the confinement effect in the RIM region completely suppressed the resonant wavelengths of the cladding modes, resulting in refractive index insensitivity in the CMSP-LPG. In strain and temperature experiments, the fiber core and cladding exhibited detection sensitivities of 0.983 and 0.961 pm/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> for strain, respectively, and 11.37 pm/°C and 11.61 pm/°C for temperature sensing. The proposed CMSP-LPG holds great potential for applications in various fields, including communications, fiber lasers, and multiparameter sensing.