Mechanical Compression-Generated Long-Period Gratings in Cyclo-Olefin Polymer Optical Fibers

Polymer long-period gratings (LPGs) exhibit superior flexibility and durability compared to conventional silica-based counterparts, allowing for reliable performance in dynamic environments and broadening fabrication options. This study presents the fabrication of LPGs in single-mode polymer optical fibers (POFs) made of ZEONEX, a cyclo-olefin polymer (COP), through mechanical compression. LPGs with grating length ranging from 20 to 40 mm were produced in 70- and 110-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m cladding-diameter fibers via mechanical compression techniques. Key findings reveal that mechanically compressed LPGs require minimal force (2.2–3.4 N) to achieve significant resonance dips with attenuation up to 26.2 dB, along with strain-dependent spectral tunability of 1.64–1.74 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>. Also, permanent mechanically produced LPGs were created by heating the POF during compression, exhibiting resonance strength of 1520 dB and insertion loss of 23 dB. Spectral tunability of 1.66–1.85 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> and temperature sensitivity of 1.336–1.575 nm/°C were achieved. Furthermore, permanent LPGs in 110-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m fibers demonstrate a surrounding refractive index (SRI) sensitivity of 236.6 nm/RIU at ~1.4441 threshold, while 70-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m fiber LPGs display linear SRI response (47.3–49.6 nm/RIU) across a wide SRI range (1.34–1.45), highlighting their versatility. These results establish POFs as promising platforms for tunable and highly versatile LPG-based sensors, with mechanical compression offering a high-performance alternative to laser-based methods for diverse sensing applications.