Research On The Sensing Characteristics Of D-shaped Optical Fiber

Because of optical fiber sensors have the broad application prospects in the fields of medicine,food safety,biochemistry and environmental monitoring,in recent years,domestic and foreign scientific teams have invested a lot of energy in the research of fiber optic sensors,and have also produced rich scientific research results.Optical fiber sensors have excellent inherent conditions such as compact structure,light weight,good electrical insulation performance,high temperature resistance,and corrosion resistance,which make obvious advantages over traditional electric sensors in many application scenarios.In addition,the raw material silica for optical fiber is rich in resources and low in cost.As the manufacturing process of optical fibers becomes more and more advanced,various types of optical fibers can meet the design needs of scientific researchers for optical fiber sensors,and also allow optical fiber sensors to move towards a higher precision and a wider range of application.Due to the special structure,D-shaped optical fiber is much more sensitive to the external environment than ordinary optical fiber,so it has been widely researched in optical fiber sensing in recent years.However,almost all the D-shaped optical fibers used in research are obtained by grinding and polishing single-sided cladding of ordinary optical fibers,so those fibers have rough polishing surfaces,uneven polishing depths,and a large number of cracks on polishing surfaces.A series of problems affect the experimental results.In this thesis,the D-shaped optical fiber made by direct drawing is used as the sensor to study,avoiding the above structural defects.This thesis conducts a series of studies on the sensing characteristics of D-shaped optical fibers from both theoretical and experimental aspects,the main works as follows:1.Through theoretical analysis and using software to model and simulate the fields of the D-shaped fiber,and the mode field distribution of its core mode and main cladding modes are obtained.The representative cladding mode is selected for the simulation when the fiber is bended,and the influences of D-shaped fiber bending on the light transmission effect are obtained.A fusion method of single-mode optical fiber and non-polished D-shaped optical fiber was proposed,and the experimental samples of D-shaped optical fiber sensor with low fusion loss were successfully repeated,which have high mechanical strength.Based on the simulation,the characteristics of the D-shaped optical fiber curvature sensor are experimentally studied.The results show that the sensor has a linear relationship between the transmission light loss and the bending degree when the bending curvature is 80mm-140mm,and its loss sensitivity is 0.074dB/mm.2.Experiments on the refractive index sensing characteristics of D-shaped optical fibers are performed.Experimental results show that the average loss sensitivity of the sensor is 196.35dB/RIU in the range of refractive index from 1.38 to 1.46.A grating is fabricated on the D-shaped fiber using the contact phase mask method,and the sensing experiments for liquid refractive index are carried out on the structure.It is concluded that the reflected power of the D-type fiber Bragg grating decreases with the increase of the environmental refractive index.3.Temperature sensing characteristics of D-shaped optical fiber was studied experimentally,the results show that the sensitivity of average transmission power change reaches 0.0088dB/? in the temperature range from 35? to 75?.On this basis,a method for coating D-shaped optical fiber with polydimethylsiloxane(PDMS)colloid to enhance its temperature sensing characteristics is proposed.The experimental results show that coating the colloid can make the temperature sensing sensitivity of the D-shaped optical fiber sample reach 0.181dB/?,which is nearly 20 times that of uncoated colloid.In addition,the temperature sensing characteristic-experiment of D-shaped fiber grating was also carried out,and the sensitivity of wavelength change of the reflection peak to temperature for the D-shaped FBG is 0.01nm/?,which is equivalent to that of the ordinary single-mode fiber grating.Moreover,the reflected power also varies with the temperature,and the sensitivity to the temperature reaches 0.006dB/?.The researches in this thesis have important value and significance for exploring new fiber sensors and promoting the application of D-shaped optical fiber sensors.