| The distributed fiber optic sensing(DFOS)technology is gaining increasing interest in geological and geotechnical monitoring.In DFOS,for some fiber sensing technologies,such as Brillouin scattering based technologies(BOTDR,BOTDA,BOFD A,etc.),the fiber Bragg grating(FBG)sensing technology,etc.,temperature compensation is often needed in engineering strain monitoring.Therefore,it is important to understand the temperature effects on the fiber optic cables(sensors)and the monitoring results.Based on previous studies,in this paper the author tests and analyzes the temperature effects on a host of cables of various structures under different conditions.The main findings are summarized as follows:(1)A new type of temperature control device is designed.The sensing optical cable is heated in a well-insulated insulating tube with a water bath,so that the temperature of the sensing optical cable can be changed without the influence of stress.Through the test of eight kinds of sensing optical cables commonly used in engineering,the important conclusion that there is a good linear relationship between their temperature change and Brillouin frequency shift is obtained,that is,there is a unique temperature coefficient in the test temperature range.(2)In the test temperature range,there is an inflection point in the fitting curves of temperature coefficients of the three kinds of optical cables of 0.9mm Hytrel tightly sheathed strain sensing cable,0.9mm Hytrel tightly sheathed strain sensing cable and 2mm polyurethane tightly sheathed strain sensing cable.That is to say,at about 45 C,the sheaths of the three kinds of optical cables begin to soften,which reduces the shear strength between them and the fibers;slippage occurs,which reduces the temperature effect of sheath.Tests on the temperature coefficients of two different coated optical cables show that the coated layer has little effect on the temperature coefficient of bare fibers,which can be neglected.Without the influence of thermal expansion of the substrate,the temperature coefficients of non-sheathed sensing optical cables and FBG are equal in free state and stress state,which indicates that the stress of the optical fibers has little effect on the temperature coefficient(3)Through the tests,the temperature coefficients of four fully distributed sensing optical cables attached to the surface of seven kinds of substrates(before 45 ? of PVC material),the temperature coefficients of the FBG attached to five kinds of substrates,and the temperature coefficients of two kinds of tightly sheathed optical cables embedded in reinforced concrete and plain concrete are obtained.(4)The tests show that the fully distributed sensing optical cables attached to the surface of the substrate can also limit the deformation of some non-metallic materials,which has a certain effect on the temperature coefficient of the optical fiber.When the attached substrate is a non-metallic material with small stiffness,the change of Brillouin optical frequency shift or wavelength in the optical fi'ber is affected by the bare fiber,sheath and substrate.When the attached substrate is a metal material and a non-metallic material with large stiffness;when the nonmetallic material is large,the change of Brillouin optical frequency shift or wavelength in the optical fiber is affected by both the bare fiber and the substrate(5)On the basis of tests,it is obtained that the temperature coefficients of the four distributed optical cables are almo st the same when the attached substrates are the same.Therefore,the respective temperature coefficient of the four distributed optical cables can be used in applications,or one temperature coefficient can be selected.The temperature coefficient of each grating point of the FBG is basically the same,which can be expressed by a temperature coefficient.As far as the sensing optical cable is concerned,the point-type attaching method has little effect on the temperature coefficient when it is attached to the substrate.(6)The temperature coefficient of the sensing optical cable attached to the substrate is determined by the temperature coefficient of the bare fiber and the thermal expansion coefficient of the attached substrate,and is not affected by other factors.The fitting formulas of the sensing optical cable(fiber)and the fiber grating based on the thermal expansion coefficient of the substrate are obtained.According to these formulas,the temperature coefficient of the optical fiber attached to the substrate can be obtained by querying the thermal expansion coefficient of the substrate.(7)In the distributed optical fiber geostress meter test,it is found that the stainless steel material is not suitable for the substrate of the stress meter,and PPR,PVC and plexiglass can be used as the substrate of the geostress meter,and the strain and temperature responses of the three are both good,the data of the geostress meter of the PPR substrate is relatively poor,and its thermal expansion is large,which easily affects the results of stress monitoring.Therefore,in the ground stress monitoring,PVC and plexiglass should be preferred as the substrate of the distributed optical fiber geostress meter.(8)By testing the temperature effect of FBG static level,it can be seen that there is no obvious linear relationship between the strain wavelength of sensor and the wavelength of thermometer in bellows static level and thin film static level when the temperature changes significantly,so it is not suitable for accurate temperature compensation.When testing the temperature effect of the buoy static level,it can be seen that the temperature coefficient fitting of the sensor made of stainless steel buoy material is better than that made of quartz glass and aluminium alloy buoy material.When the temperature changes significantly,the strain wavelength of the buoy static leveler has the same trend as the temperature wavelength,and has a good linear relationship,that is,there is a relatively fixed proportionality coefficient.Therefore,the buoy static level can effectively conduct temperature compensation itself. |
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