Experimental research of dynamic long-period fiber gratings based on ultrasound modulation

Long-Period Fiber Grating (LPFG)-based biosensors operate on the fundamental principle that changes in the surrounding refractive index (SRI) can be translated into measurements of resonance wavelength shifts or variations in transmission loss. Through continued exploration of writing method for LPFGs, various gratings with special structures and novel functions have been designed and developed garnering increasing attention in the realm of novel optic devices. Currently, the fabrication of LPFGs primarily involves altering the effective refractive index of the fiber core using methods such as the mask method and holography, which result in a permanent change in the refractive index of the fiber core. However, this paper explores a method of fabricating dynamic LPFGs based on the principle of acousto-optic effect, whose applications could be more convenient in certain cases. The propagation of ultrasonic waves within the fiber optic induces periodic elastic strain of the fiber, leading to corresponding changes in the fiber's refractive index over time and space. By modulating the characteristics of the ultrasound, various types of LPFGs can be generated. The feasibility of this method is verified through experiments, and two special gratings, namely dynamic phase-shifted long-period fiber grating and dynamic chirped long-period fiber grating, are fabricated based on the variations in ultrasonic characteristics. This development presents a new technique for addressing fiber optic sensing requirements and opens avenues for future applications such as the detection of immunoglobulin, bacteria, DNA, blood antibody analysis and other targets.