The Studies On Biconical-tapered Tiber-optic Interferometers And Its Sensing Applications

Compared with the traditional sensing equipment,optical fiber sensors have the advantages of compact structure,high sensitivity,strong anti-electromagnetic interference ability,and simple structure.They are widely used for the measurements of many physical measurands,such as displacement,temperature,pressure,refractive index,and many others.At present,most optical fiber sensors require complicated preparation processes such as fusion splicing and chemical etching,and the mass production cost is relatively high.In addition,high-precision displacement sensing has important applications for the precision micro-machining and semiconductor manufacturing processes.However,most of the existing optical fiber sensors are difficult to achieve the nanoscale displacement sensing.The main innovation of this paper is described as follows: The single-mode fiber is pulled away to introduce nonadiabatic waveguiding and thus fractional amount of the optical power in core mode will be converted into cladding modes.On the other hand,at the other side of the end-cleaved single-mode fiber after the abrupt taper,the conical shape was produced using mechanical polisher so as to reflect the different cladding modes toward the different propagation paths when the desired polished angle is determined.This fiber interferometer,composed of an abrupt taper and a polished cone taper,is featured with several advantages including small foot-print size,compact structure,easy fabrication,low cost,and strong anti-EMI ability when a suitable polished angle was achieved.It can be used to realize the measurement of the physical measurands such as linear and angular displacement.Obviously,this is crucial for the latest developments for the advanced precision industrial fabrications.The main research contents can be categorized into three parts as follows:(1)Analyzing the working principles of fiber cladding mode excitation and coupling technology,Fabry-Perot interferometer and vernier effect to generate new thinking ways and the theoretical guidance.(2)The single-mode fiber is abrupt-tapered and then polished taper to prepare a small-size biconical fiber interferometer.The influence of the tapering and polishing on the interference effect in the biconical fiber interferometer is studied.The experimental results show that the different taper angles of single-mode fiber have significantly different control effects on the cladding mode.At the same time,the polished shape,the polished length and the abrupt-taper diameter all have an impact on the interference effect.(3)Using the Vernier effect in the biconical optical fiber interferometer,the linear displacement sensors and the angular displacement sensors using cantilever were designed and investigated.From the experimental results,it is shown that the linear displacement sensitivity is 0.023nm/nm when the displacement is ranging from 0 nm to 120 nm.On the other hand,the angular displacement sensitivity is 0.17417nm/degree over the angle range of 0°-8.76°.In this thesis,we have largely improved the accuracy of the most of optical fiber interferometric sensors,especially in terms of the linear and angular displacement.This biconical fiber interferometer is promising for achieving high accuracy sensing for the scientific and industrial applications.