Design And Application Research Of Coupling Fiber-Optic Vibration Sensor Based On Local Mode Coupling Theory

Vibration is a very universal problem in the filed of modern engineering technology and most of vibration phenomenon would bring damage to the vibration object.How to control the vibration effectively and eliminate destory in maximum are becoming an important research topic of technicians.From the beginning of 20 centuries,vibration examination technique has gotten a huge development.It has experienced mechanical,electricial,and optical measurement technology stages.Currently,the mechanical method has already been washed out because of its low accuracy,easily be wear away and signals can't be delivered.The electricial vibration seneor is widely used because of its mature technique and high senstivity,but it need insulating process and easily be affected by electromagnetism interference conduced it hardly attain the safe production purpose,on account of these disadvantages of electricial vibration seneors,people turn to optical measurement technology these years.The fiber-optic sensing technology has many advantage such as:high senstivity, small size,light weight,essential safety,anti-electromagnetism interference and corrosion resistance etc,representing the development direction of new optical sensors.The fiber-optic sensor based on intensity,frequency,wavelength,and polarization modulation get a rapid development in recent years.The biconical fiber-optic coupler is one of the most widely used fiber-optic components.The lightwaves transmitted in its specail fused structure coupling and separate(combinate) to each other,so it can be used as filter,splitter,and polarimeter etc.The manufacture process is putting the two single mode optical fibers whose claddings be wiped off closed to each other,heating and meltdown it with high temperature and draging it towards two sides in the meantime,then a biconical stucture comes into being at last.When the light transmits in the coupling zone of this biconical stucture,the power reassign will be occurred,one part of light power continues transmits in the "direct arm", another part transmitte in the "coupling arm".The coupling output ratio of the coupler is senstive to the length and refractive index variety of the coupling zone. Making use of this characteristic of the fiber-optic coupler,we can make coupling fiber-optic sensor to achieve the exmanation of strain/vibration signals via maesure the changes of coupling output ratio.Beside having the merites of common fiber-optic sensor,the coupling fiber-optic sensor also has the advantage such as easy manufacture,low cost,and modulation simple etc.Based on the local coupling mode theory of gradual changing fiber-optic wave guide,The relation between coupling output and vibration was deduced,and a new type of coupling fiber-optic sensor was presented.The following several topics are Mainly studied:1.Summarize the development of vibration measurement technology and fiber-optic sensing,eapecially the advantage of coupling fiber-optic sensor such as:simple structure,low cost,high senstivity etc.Giving an overview of research course and trends home and abroad.The theoretical significance and application value of the fiber-optic coupler used for vibration measurement is discribed at the same time.2.According to the transverse coupling mechnism of two parallel light waveguide,the mathematical differential equation of its coupling output is deduced.The biconical fused fiber-optic coupler which was produced by two same singal-mode fibers be devided into some fragments in which their section is equal to each other.The local mode coupling theory is proposed to study the rule of the lightwave transmit in the two coupling optical fiber.The coupling rules in the fragments of the coupler was discussed with the mode coupling theory of two parallel optical fiber,and the accurate resolution function was obtained at last.The influence of the vibration to coupling output be divided as low frequency and high frequency case.The coupling output variation induced by low frequency vibration was studied with a second-order single-freedom forced vibration system.The influence of high frequency vibration was studied through analysing the refractive index changes conduce by the elastic-optic effect and then affect the coupling output.The relation formula of coupling output and vibration was built.And the new idea of using single-mode coupler as a senser to realize the measurement of vibration was put forward.3.The accurate calculation formula between coupling output and the strain of coupling section was obtained.The characteristic of the fiber-optic coupler was studied with spiral micrometer to avoid the disturbance of cantilever self-weight and self-vibration,enhance the measurement precision effectively.The fiber-optic coupler was sticked to the cantilever and the coupler's response to shock and periodic vibration be obtained.Compared with piezoelectrical sensor shows that the experiment results were the same with theory analysis.The result indicates that the fused biconical taper coupler's output not only sensitive to the strain,but also changes monotone and linear.At the same time it is temperature stabilization and anti-disturber transversely.The singal-mode fiber-optic coupler can be used for strain/vibration measurement.4.Two kinds of coupling-region length dependent vibration sensors were designed.In the first kind,the coupler was sticked to the cantilever beam.Its vibration will arouse the coupling region's length change and its mathematical model was established.Bring the cantilever beam's strain value into the coupling output calculating formula,the precision calculation formula between coupling output and vibration was obtained and simulated with MATLAB.This kind of coupling low-frequency fiber-optic vibration sensor realize exact measurement to viration frequency below 10Hz and suitable for structural health monitoring. Another kind of vibration meaurement method was realized through fixed the coupling fiber-optic sensor on a hollow aluminum mass with V groovy.According to the theories of the material mechanics analysis,the strain of coupler focus on the thinest section and it is inversely proportional to cross section's square diameter.The strain changes with vibration remarkably when its diameter less than 20μm,the coupling section acts as an vibration converter.The changing curver and the precision calculation formula between coupling output and vibration was obtained.This kind of coupling universal fiber-optic vibration sensor realizes exact measurement to vibration frequency below 300Hz.5.A new kind of coupling region dependent vibration sensor was designed. According to the theory of elastic mechanics and fiber optics,the intermolecular relative displacement conduced by its interaction energy will bring with media density fluctuate when the high-vibration transmits in the coupler.Thus cause the refractive index changes periodically and form a phase grating in the coupler, arouse the changes of dielectric constant and coupling coefficient,the coupling output ratio will changes consequently.The precision calculation formula between coupling output and high frequency vibration was educed.In the light of this character a new kind of coupling vibration sensor used for Composite and intelligent structuresperformance evaluation was designed,the object vibration less than 8KHz was measured and ideal results was acquired.The character of the coupling fiber-optic sensor to different intensity and different direction signals is also dissussed simultaneously.6.An integrated coupling fiber-optic vibration system was designed.Because of the coupling output power is very small(commonly is nW level),the photoelectricity conversion effciency is discussed and the PIN photoelectric diode is chose.The photoelectricity conversion and 2-grade amplified circuits were designed,the filter circuits was designed according to different request at the same time.The vibration parameter acquisition and analysis were implemented with properdata acquire card(DAQ-2005) and application software LabVIEW.The signals were processed with hard and soft threshold wavelet de-noising method to increase the signal-to-noise ratio.Making use of the conclusion educed from previous chapter and experimental results,the coupling fiber-optic sensor was used in the vechile dynamic weighting field,experiments indicate that its weighting error is less than 5%at 20KM/h.It can meet the requests of dynamic weighting. the coupling fiber-optic sensor was also used for large generator stator's vibration online monitor.A new kind of online monitor system of generator stator based on coupling fibre-optic vibration sensor was also designed simultaneously.7.The conclusion for the theorical and experimental work of coupling fiber-optic sensor was made;the impending problems to be resolved and new research direction were presented.