Research On The Temperature And Strain Characteristics Of Optical Delay And Its Applications

With the rapid development of fiber-optic transmission technology,fiber-optic transmission-based optical fiber delay techniques have been widely applied in various engineering environments.Due to the negative effects of environmental temperature and stress,fiber-optic channel may be affected by the delay fluctuations,thereby worsening the performance of time-frequency transmission system.Therefore,the effects of temperature and strain on fiber-optic delay were thoroughly researched in this dissertation,and the applications of fiber-optic delay in the field of optical fiber sensor were further researched as well.In addition,fiber-optic delay based temperature and strain sensors were designed,the prototype experimental setup was built,and the large range and high precision measurements of temperature and strain were achieved,which show great scientific significance and application value.Firstly,starting from the Lorentz-Lorenz equation,the effect of environmental temperature on the refractive index of fiber was researched,which showed the temperature characteristic of the refractive index was related with the volume and polarization of the fiber affected by temperature.Furthermore,on basis of the proposed electric-dipole theoretical model,the expression of the thermo-optical coefficient was obtained,which indicated the linear relationship between the refractive index of fiber and temperature.Besides,by utilizing the classical elastic mechanics theory,combined with the practical application environment of fiber,the expression of the elasto-optical coefficient was derived as well.Relying on the optical fiber transmission theory,the thermal equation and elastic equation of fiber-optic delay were deduced,thereby demonstrating that fiber-optic delay was linearly related with temperature and strain.Secondly,an experimental setup for high precision measurement of the refractive index of fiber was built,thereby realizing the measurements of thermo-optical coefficient and elasto-optical coefficient.The experimental results showed that the refractive index of fiber linearly increased with the increment of environmental temperature,and the measured value of thermo-optical coefficient was1.3?10^-5/?;the refractive index of fiber linearly decreased with the increment of strain,and the measured value of elasto-optical coefficient was5.48?10^-7/??.Besides,a high precision fiber-optic delay measurement system was built to detect the thermal coefficient?K?and the elastic coefficient?J?with a measurement precision of better than 0.1 ps.The experimental results showed that fiber-optic delay was linear with temperature,and the thermal coefficient was 39.5 ps/km?within the range of-60?-750?;fiber-optic delay was linear with strain,and the elastic coefficient was 3.82ps/km??.Here,the experimental results are consistent with the theoretical ones.Thirdly,a fiber-optic delay based temperature sensor was proposed,which no longer relies on the precision measurement of spectrum but realizes the measurement of temperature by detecting the temperature-induced delay.Based on its sensing principle,two fiber-optic sensor system structures were designed,which are dual-wavelength structure and cascade-able structure,respectively.Combined with the temperature sensing experiments,the two system structures were tested in the aspects of repeatability,stability,measurement precision,etc.,and the testing result showed that this sensor method was reliable for high precision temperature measurement.In addition,a quasi-distributed fiber-optic temperature sensor,with 8 nodes distributing along a32.61 km long fiber,was presented,which could realize a temperature precision of better than 0.1?within the range of-30?-80?.Finally,this dissertation presents a high-precision temperature insensitive fiber-optic strain sensor system.By utilizing a high precision fiber-optic cutting platform to control the length difference between sening path and reference path,this sensor system can overcome the negative effects of temperature fluctuation through self temperature compensation.Besides,this strain sensor can achieve a strain sensitivity of4.75??within the range of 350??.Furthermore,to overcome the limitation of phase-discriminator module,an improved fiber-optic strain sensor based on the autocorrelation of optical pulse was proposed as well.This sensor uses pulsed laser as light source and realizes the measurement of strain by detecting the time-interval between the main peak and the secondary peak of the autocorrelation peak.In addition,this sensor can transform the measurement of delay into the measurement of length,thereby improving the strain sensitivity by one order of magnitudea under the condition of same sensing fiber length,and it has the advatages of polarization-and temperature-insensitivity.