Study On Tilted Fiber Bragg Grating Sensors Based On Surface Modification

Fiber Bragg grating,in which the refractive index in the fiber core is modulated periodically along the propagation axis of the fiber,has been widely used as fiber-optic sensors for measurements in temperature,stress,curvature and so on.As the power transmits in the fiber core,fiber Bragg grating cannot used for measurement in refractive index.To solve this problem,a new device called tilted fiber Bragg grating(TFBG),in which the grating's planes are written with a non-zero tilt angle relative to the propagation axis of the fiber,was proposed.With the introduction of the tilt angle,the TFBG not only possesses the advantages of fiber Bragg grating,but also has the merits of long-period fiber grating for surrounding refractive index measurement.This thesis mainly fucuses on measurements in ions and biomolecules using the TFBG with surface modifications.For the ion measurement,we proposed an ammonium sensor based on an TFBG coated with a hybrid film which was made up of polyvinylidene fluoride and bromothymol blue.The principle of the sensor is that the structure of the hybrid film is destroyed due to the reaction between the bromothymol blue and ammonium ions and the destruction is proportional to the ammonium concentration.Experimental results demonstrated that the limit of detection of our ammonium sensor was 0.02mg/L,and it had good linearity in the measured concentration range of 0.1 – 10 mg/L.Finally,we used our sensor for ammonium measurement in practical wastewater,and the results showed that our sensor could provide performance like the commercial ammonium probe.For the biomolecule detection,we proposed a glucose sensor,which was based on an TFBG coated with graphene and pyrene boric acid attached on graphene as bio-detector.Experimental results demonstrated that the graphene can not only improve the sensor sensitivity,but also simplify the attaching process.Furthermore,the results showed that our glucose sensor can measure glucose concentration of 0.5m M,which is lower than the blood glucose concentration in body,making it possible for in-situ detection of blood glucose in body in future.