Hollow Eccentric Fiber Bragg Gratings And Its Sensing Application Study

The optical fiber sensors,with its small size,light weight,immunity to electromagnetic interference,low loss,lost cost,ease of multiplex,and embedded ability,are widely applied in the field of civil engineering,aerospace,energy surveys,biomedicine and so on.As one kind of the optical fiber sensor which was the most widely and frequently used,fiber Bragg grating?FBG?has received a great deal of interest.Basing on the study of fabrication and sensing principle of the hollow eccentric-core fiber?HEF?FBG,we propose a bending sensor,which can distinguish bending direction and eliminate the cross-sensitization of temperature and axial strain.The main works are as follows:The fabrication of the single-core and two-core hollow eccentric fiber FBG was accomplished by using the 248nm excimer laser and phase mask technique,the FBGs are fabricated at different exposure directions,and the influence of the exposure direction on the characteristic parameters of the FBGs is analyzed.The results can be summarized as:the fiber core which close to the exposure direction the most,can received more modulate in refractive index,and the resonance wavelength of grating will longer.The temperature,axial strain,external and internal refractive index of the single-core HEF-FBG were measured.The experimental results indicate that:the single-core HEF-FBGs are not sensitive to the change of refractive index,and the temperature sensitivity?5.2pm/??and axial strain sensitivity?0.6pm/???are a bit smaller than that of conventional SMF-FBGs.To enhance the sensing characteristics,we had successfully fabricated tilted grating in single-core HEF,the resonance wavelength depends on temperature and strain were measured.For two-core HEF,we choosed the sample which exposure direction was perpendicular to the line that connects two core,and measured the temperature and axial strain of the grating in each core,respectively.The data shows that:the gratings in each core have the same temperature and strain characteristics,and similar to that of single-core HEF-FBG.Furthermore,the bending characteristics of the two-core HEF-FBG on different bending direction were studied.The experimental results indicate that the FBG fabricated in the two-core HEF is suitable for distinguishing bending direction due to the strong direction-dependent bending sensitivities.In addition,through a subtraction between the resonance wavelength of the two gratings,the two-core HEF-FBG can not only immune to temperature and axial strain,but also increase the bending sensitivities,the largest bending sensitivity after a subtraction is-50.2pm/m^-1.The two fiber core in two-core HEF have a large distance,and a big air hole lie between them.So it can avoid the signal crosstalk the most,and have a obvious advantage in enlarging the signal capacity of fiber and application to the two-channel filters.