Thermoforming Structure Modified Plastic Optical Fiber SPR Refractive Index Sensor Study

Refractive index is an important physical property of a substance and is often tested to identify the composition,concentration,purity and quality of a substance.New refractive index sensors are emerging in this industrial environment,and the emergence of optical fiber has brought great innovative advantages to this field in terms of cost,safety,miniaturization and intelligence,as well as promoting the continuous cross-fertilization of physics,chemistry,biology and other disciplines.GOFs sensors are the most common in daily life,but GOFs require extremely high fiber coupling accuracy and expensive supporting devices,which greatly increases the cost of preparing GOFs sensors and limits their application in the field of fiber optic sensing.In contrast to GOFs,plastic optical fiber has become an ideal choice for optical fiber refractive index sensors due to its huge cost advantage,and the soft texture and high plasticity of plastic optical fiber provide the possibility of various structures for plastic optical fiber refractive index sensors.Surface plasma resonance（SPR）technology is a highly sensitive,small size,immune to electromagnetic interference and large dynamic test range sensing technology that enables the sensing of small refractive index changes in the external world.Compared with GOFs,the excellent flexibility of plastic fiber makes it easier to enhance the SPR effect by structural modification.To this end,this paper proposes a flat structure and a screw-shaped structure of plastic optical fiber SPR refractive index sensor by thermoforming structure modification.The effects of the structural parameters of the sensing part of these two sensors on the overall performance are investigated from the sensor structure,and the main results obtained are as follows.A flat plastic optical fiber SPR refractive index sensor was prepared by pressing the plastic optical fiber into a flat shape using mechanical hot pressing technique and combining the surface plasmon resonance technique with the magnetron sputtering coating technique.The results of the study on the refractive index sensing characteristics showed that the SPR sensor prepared from a flat plastic optical fiber with a sensing zone thickness of 500μm has better refractive index sensing characteristics.In addition,the sensing characteristics were optimized by varying the sensing zone length,single/dual side coating,and changing the coating type.The results showed that when the flat thickness was 500μm,the sensing zone length was20 mm,and both sides of the flat zone were coated with 50nm thick metal Ag film,the highest sensitivity was obtained in the measurement range of refractive index from 1.335 to 1.400 at 2568.4nm/RIU with a resolution of 2.73×10^-4RIU.The plastic optical fiber was prepared into a screw-shaped shape by mechanical hot pressing and twisting,and the same combination of surface plasmon resonance a screw-shaped plastic optical fiber SPR refractive index sensor was prepared and its refractive index sensing characteristics were investigated by combining surface plasmon resonance and magnetron sputtering coating techniques.After structural optimization,the refractive index measurement sensitivity was 2168.9nm/RIU when the length of the screw-shaped plastic fiber was 10 mm and the number of threads was 4.