A Study Of Features Of Optical Fiber Gratings Metallized With Nickel

Optical fiber grating is one of the most important devices in optical fiber communication and optical fiber sensor. It includes fiber Bragg grating (FBG) and long-period fiber granting (LPFG). Metal coating is important for protecting the optical fiber grating and promoting its performance. With its advantages of economic, no need for high temperature and controlled thickness, the combination of electroless and electroplating is a preferred method for metallizing optical fiber gratings. In order to obtain metal coated optical fiber gratings with a property of high quality, This study, by using this method, explores the related problems in the process of metallizing optical fiber grating and features of metallized optical fiber gratings. The main contents are as follows:Firstly, on the basis of theory of guided-waves optics, it is found that the stress produced in electroless-electroplating influences the metal coated FBG (MFBG)’s resonance spectrum. Thermal stress induced by metallizing is analyzed by using finite element method on the basis of the theory of thermoelasticity and heat transfer. The result shows that it induces perturbation in FBG. Using transfer matrix method, FBG’s spectrum is simulated. The results show that under uniform strain, the resonance spectrum does not change except its center wavelength’s shifting. Under uneven strain, the FBG’s spectrum shows obvious degradation, that is multi-peak or double-peak, broadened spectrum and reduced peak value. So, homogenization in stress is the critical factor for MFBG to keep the feature of its original resonance spectrum.This study has solved the problems in the MFBG’s spectrum, that is, there commonly exist multi-peak or double-peak, broadened spectrum and reduced peak value in the process of metallization. The technology for controlling the stress is put forwards by analyzing the mechanism of producing stress, that is joining, hydrogen, foreign substances, excess energy and mismatch, and affecting mechanism of the technology on the magnitude and homogeneity of the stress. After using the technology, there is not double peak or multi-peak in the spectrum of the MFBG any more. After electroless plating, the deviation of3-dB bandwidth is less than0.005nm, the decrease of the peak loss is less than1%. After electroplating, when the thickness of the metal coating is more than150μm, the deviation of3-dB bandwidth is less than0. Olnm, the decrease of the peak loss is less than5%. When the MFBG is used as temperature sensor form room temperature to80℃, the hysteresis error is less than1.5℃. Compared with previous reports, MFBG can sustain its original feature of spectrum, and promote its performance in temperature sensor.For the first time, the study puts forwards the way to solve the problem of LPFG’s strong bending sensitivity by using a thick metal coating. However, LPFG’s spectrum faces the problem of the drastic decrease of peak loss in the process of metallization. In the previous reports, the thickest metal coating of LPFG was about2μm, and the peak loss reduced15dB due to metallization. By using electroless-electroplating method, and the new prestress device for electroplating, this study develops a metal coated LPFG (MLPFG) with thickness150μm, peak loss15.899dB,3-dB bandwidth3.942nm, and the temperature sensitivity45.7pm/℃and44.9pm/℃respectively when the MLPFB is heated and cooled. The reduced peak loss is only1.255dB due to metallization. The study has broken through the bottleneck of the thickness of metal coating of MLPFG, and provided condition for LPFG to overcome the strong bending sensitivity. It is of significance for putting LPFG into wide commercial application.The study examines the feature of MFBG’s tension response by using finite element method. Results show that there is an even change in the area marked by the core of the fiber grating when the MFBG is affected by uniform axial tension. There is a linear relation between the drift of the center wavelength of MFBG and the change of uniform axial tension. However, the present of the metal coating reduces the FBG’s uniform axial tension sensitivity. There is a descending tendency for its sensitivity with the increase of the thickness of the metal coating. The experimental results are in agreement with the theoretical analysis.The study examines the sensor feature in the temperature between100℃and300℃. The experiments reveals that, when the temperature is under100℃-300℃, the hysteresis error is more than5.5%. The research indicates that the stress is one of the main reasons that lead to the error. Heat treatment is carried out to reduce the error. In the heat treatment, the MFBG sensor is put in a drying oven at120℃for8hours, and then it is cooled with the oven. Such a heatment is repeated three times. After being heat treated, The MFBG shows that the hysteresis error is descended to3.62%, the sensitivities of rising temperature is22.84pm/℃, and the sensitivity of decreasing temperature is22.76pm/℃.