Fabrication And Sensing Characteristics Of Bamboo-like Long Period Fiber Gratings

Long period fiber gratings?LPFGs?have been extensively applicated in gain flattener and band stop filter because of their small insertion loss,wide band,low backward reflection,high response sensitivity to changes in the external environment and simple fabrication.With development of photonic devices,micro or nano fiber devices,with compactness,higher sensitivity and lower loss,are required for sensing applications in various complex environments.The traditional fabrication methods of long-period fiber gratings,including UV lithography,femtosecond laser/CO₂laser lithography,and fiber cutting and fusion,have been used for fabrication of conventional-size long-period fiber gratings.However,with scale-down fibers,there are difficulties in fabrication of gratings or poor stability.In this paper,we propose a preparation method for Bamboo-like long period fiber grating?BLPFG?:We use a femtosecond laser to produce periodic saw-teeth structure on the single-mode fiber with coating removed,and then uses the taper pulling system to pull the preform in one step.Then the sensing characteristics of this kind of grating are studied.The specific research contents and results are as follows:1.We proposed one-step taper method to fabricate a long period fiber grating.We use an800-nm infrared femtosecond laser with high-energy and high-precision light spot to mill a single-mode fiber with the coating removed.The grooves are periodically written along the fiber with widths of 5?m,depths of 5?m,and a period of 10?m.This laser milling method has no damage to the unplanned structure of the fiber preform but can precisely process it in the planned areas.We use the oxyhydrogen flame to melt cone-drawing platform to taper it.The formed grating structure has a diameter of?15?m and a period of?500?m.We have studied the structural evolution trend of the preform during the taper drawing process and the growth feature during device fabrication.2.We studied the sensing characteristics of Bamboo-like long period fiber gratings.The refractive index sensitivity measured in the experiment is?872.44nm/RIU,and the temperature sensitivity is?5.8pm/?.Because the device is made of the same material,the structure is stable,sturdy,and flexible in design,which show great potential in sensing applications.