Designing And Packaging Techniques Of Ultrahigh Resolution FBG Strain Sensors For Three-Dimension Strain Sensing Measurement

The crustal deformation is a strain tensor in three-dimension space.Traditional high precision borehole strain meters are the dominant sensors in the measurement of crustal deformation,which are based on the principle of volumetric strain measurement of liquid with electronic technology.Due to the large size of the sensing unit,the high precision borehole strain meters are only suitable for body strain(body strain meter)and crustal deformation in a horizontal plane(multi-component strain meter).They cannot measure the strain in the vertical direction.The lack of a dimension of the strain information severely limits the understanding of the crustal strain state.In addition,the borehole strain meters are based on electronic instrument,suffering from the lightning damage problem.The optical fiber sensors are well known for their small size,low cost,ultrahigh strain resolution,robust against harsh environment,immune to electronic-magnetic interference,and capability of remote sensing.They are also very attractive in the crustal strain field measurement with the potential of measuring crustal deformation in three-dimension space.This paper focuses on the design and package technologies of optical fiber Bragg grating(FBG)sensor for the strain sensing of the crustal deformation in the three-dimension space.One FBG sensor can provide the strain information in one-dimension.By measuring multiple FBG sensors in different directions,the strain tensor in three-dimension can be obtained.In this work the relationship between the one-dimension grating strain in different directions and the three-dimension strain tensor is established.Firstly,it is shown that the two-dimension strain tensor can be obtained by three FBG sensors arranged in certain directions.The self-consistency checking method of four FBG sensors is also derived.Then,the relationship between the FBG sensors and the strain tensor is extended to the three-dimension space,which proves that the three-dimension strain tensor can be analyzed by the method similar to the two-dimension strain tensor.Based on the above theoretical analysis,the design of crustal strain tensor of the three-dimension space are achieved.The measurement of crustal deformation requires high stability over a long period up to years.FBG sensors are usually packaged with glue.It is very simple,but suffers from poor long time stability due to slip and creep of glue,so it is not suitable for crustal resolution deformation measurement with ultrahigh resolution.Recently,welding the FBG to the measurand after surface metallization has been widely adopted for industrial applications.This method has good long time stability,but the welding of the fiber to the measurand is very complex and difficult to be performed in field experiments.We developed a new package technique for the FBG sensor with both high stability and simple field assembly.The fiber is pretreated,magnetron sputtered,and plated with copper in sequence.The fiber is metallized with diameter around 200 ?m after the above treatment.Then the metallized fiber is fixed to the baseboard with V-shaped groove with a cover,and the pressure can be measured.According to the data analysis for several tests over a long period,it is found that the proposed scheme is more stable and reliable compared with the glue based packaging method.At the end of the thesis,the total work is reviewed.