Photopatterned surface polymer-stabilized alignment (SPSA) for tunable liquid crystal long-period fiber gratings in hollow core fibers: a novel fabrication strategy inspired by SPSA

Abstract A novel and straightforward method for fabricating liquid crystal (LC)-based long-period fiber gratings was developed by employing a surface polymer-stabilized alignment technique in conjunction with a binary grating photomask. In this approach, polymer networks were formed along the inner surface of a hollow-core fiber (HCF), enabling stable alignment of LC molecules. Simultaneously, the binary photomask induced a periodic modulation of LC orientation, facilitating the formation of well-defined gratings. The effects of UV exposure time and curing voltage on the pretilt angle and operating voltage of the proposed LC-LPFGs were systematically investigated. To qualitatively analyze LC alignment within the HCF, polarized optical microscopy was employed, and the resulting textures were compared with the Michel-Lévy interference chart to interpret structural characteristics. Experimental results revealed that increasing the UV exposure time and curing voltage led to a larger pretilt angle, thereby reducing both the threshold voltage required to initiate LC reorientation and the voltage needed to reach saturation. The proposed technique enabled precise, reproducible, and defect-free control of LC alignment in HCFs, yielding uniform and stable long-period gratings. This innovative fabrication method offers a robust platform for the development of reconfigurable photonic components with potential applications in optical sensing, filtering, and communication systems.