Tensile Strength And Its Cryogenic Sensing Characteristics Of The Metal Coated Optic Fiber

Compared with traditional sensors,FBG has a series of advantages,such as,simple structure,small size,light weight,anti-electromagnetic interference and can be used as a sensor network.Temperature monitoring of the cryogenic environments usually face the presence of strong electromagnetic interferences,traditional cryogenic sensors cannot work normally under the environments which are presence of electromagnetic interferences.Due to the unique advantage of immunity to the electromagnetic interferences,FBG is one of the best candidates for temperature monitoring in the cryogenic environments.However,the main composition of optical grating is SiO₂,bare optical grating is small,brittle and easy to be damaged.Moreover,due to the intrinsic characteristics of optical materials,the temperature sensitivity of bare fiber grating is quite low in the cryogenic environments.Therefore,fiber grating should be protected and the temperature sensitivity of fiber grating should also be increased before engineering application.In this paper,the metallization of fiber gratings is carried out by the chemical plating and electroplating method.The effects of metallization are evaluated.The sensing characteristics of the metal coated FBGs in the cryogenic environments are studied.The main contents are as follows:1)Chemical plating nickel process on the surface of fiber grating.The organic protective layers of fiber gratings were removed by mechanical stripping method and acetone immersion method respectively,and then the fiber gratings were coated with a thin conductive layer by chemical plating method.Results show that:?i?fiber optic stripper mechanical stripping method will damage the surface of fiber grating,acetone immersion method will not damage the surface of fiber grating;?ii?The chemical plating nickel layers plated on the surfaces of the fiber gratings after removing the protective layer by the acetone immersion method are smooth,compact,and continuous.There are not any obvious defects on the surface of nickel layers,the nickel layer is well bonded with the fiber grating.2)Electroplating process on the surface of fiber grating.Fiber gratings were electroplated with nickel,copper,zinc and tin after chemical plating nickel process.Results show that:?i?the surface of fiber grating obtained with nickel aminosulfonate as the main salt is smoother,brighter,and more compact than the fiber grating obtained with nickel sulfate as the main salt;?ii?There are not any obvious defects on the surface of the nickel layer,copper layer,zinc layer and tin layer,all the layers are smooth and compact;?iii?Fiber gratings,Ni pre-coating and electroplating layers are satisfactorily bonded,the fiber gratings are well protected.3)The tensile strengths of bare optical fiber and metal coated optical fibers were tested through the optical fiber tensile test setup.The effects of metallization were evaluated from the mechanical aspects.Results show that:?i?compare with the optical fibers which are removed protective layers by the mechanical stripping method,the optical fibers which are removed protective layer by the acetone immersion method have higher tensile strength;?ii?optical fibers will be damaged in the sensitization process and the activation process,the tensile strengths of optical fibers are greatly decrease after chemical plating nickel process;?iii?electroplated nickel layer,copper layer and zinc layer can protect optical fibers and increase the tensile strengths of optical fibers;?iv?tensile strength of the Ni-coated optic fibers obtained with nickel aminosulfonate as the main salt is higher than that of the Ni-coated optic fibers obtained with nickel sulfate,reaches up to about 842.46MPa,which is a little higher than the Au-coated optical fibers?718.99MPa?of FiberGuide corporation.4)Sensing characteristics of the metal coated FBGs in the cryogenic environments.The cryogenic sensing system was used to study the sensing characteristics of the metal coated FBGs in the cryogenic environments.Results show that:?i?during dynamic cooling process,the central wavelengths of the bare FBG and the metal coated FBGs both nonlinearly change with the temperature;?ii?the nonlinear characteristics of the central wavelength--temperature curve of bare FBG is not only caused by the decrease of the thermal expansion coefficient,which is also related to the decrease of thermo-optic coefficient of the optical material in the cryogenic temperature environments.?iii?nickel layer,copper layer,zinc layer and tin layer can increase the temperature sensitivity of FBG,the Zn-coated FBG and Sn-coated FBG show relatively higher temperature sensitivities,reaches up to about 20pm/°C in the range of 17°C^-100°C and about 11pm/°C in the range from-100°C to-190°C,which are about three times of that of the bare FBG;?iv?The temperature sensitivities of FBGs decrease slightly with the increases of cooling rates,the hysteresis responses of the FBGs during the rapid dynamic cooling process should be further studied in the future works.