| The ocean is rich in resources and affects the change of global climate.The development of marine sensing technology will provide important and strong support for protection of marine environment,research of marine science,development of marine economics and the layout and implementation of maritime power strategy.Traditional electrical sensors are vulnerable to underwater noise,electromagnetic signal and other interference,and have high requirements for anti-pressure,anti-corrosion,anti-leakage and other special shell packaging design.Moreover,the cost for their underwater maintenance is high and service life is relatively short,which make them difficult to achieve long-distance transmission,long-term real-time detection and establish underwater stereoscopic monitoring network for marine applications.All fiber interferometric sensors which made from silica have become mainstream sensor for in-situ monitoring in marine environment,due to their characteristics of compactness,light weight,immunity to electromagnetic interference,water and corrosion resistance,high sensitivity and ease of distributed measurement.Because the optical fiber is both an optical waveguide and a sensing element,the optical fiber sensor has key advantages such as fast response speed and suitable for harsh environment.However,extrinsic optical fiber sensors usually measure seawater parameters based on the principle of chemical reaction or mechanical deformation of functional materials.Therefore,the increased sensitivity comes at the expense of increased optical loss or reduced mechanical strength of the sensing fiber.Moreover,the output spectra of optical fiber in-line interferometers,including those which based on cladding modes or other high-order modes interference,are superimposed by multiple interference signals and have different responses to multiple environmental parameters.There are some problems in their monitoring process,such as weak anti-interference ability,complicated calibration in practical application,optical loss and instability of mode transmission.By contrast,dual-mode fiber which supports only two core mode propagating in the fiber core,has key advantages in both communication and sensing fields.Therefore,our research works mainly focus on the design of two core modes interference and intrinsic all fiber interferometers.Different transmission spectrum of those sensor have been studied,such as uniform periodic interference fringes,critical wavelength(CWL)-existed spectrum,and variable period Vernier spectrum,as well the sensing characteristics of interference peaks/dips for single frequency signal or Vernier envelope which varying with several physical parameters were studied in theory and experiment.Due to the fact that the output light intensity of most conventional fiber in-line interferometers vary periodically with wavelength,and the wavelength spacing between adjacent interference peaks/dips is consistent,as well as the wavelength monitoring sensitivity of interference peaks/dips to a single measured physical parameter is basically the same,it is difficult to focus and continuously track the measured wavelength in long-term measurement and limited to use a single interferometer structure to detect multiple parameters simultaneously.In this paper,a special dual-mode fiber(DMF)which supports only LP01 and LP02 mode propagating in the fiber is used to form a fiber in-line MZI.There is a critical wavelength(CWL)existed in the transmission spectrum where the velocity of their two core mode groups are equal.The CWL is easy to identify,and measured peaks(PL1 and PR1)on the both sides closest to the CWL are easy to identify and track,which is not affected by the change of polarization state of light source.Thanks to the unique existence of CWL in the transmission spectrum,and the interference peaks/dips on the both sides of CWL varying monotonically with measured parameters,a multi-parameters sensing scheme with high sensitivity,large measurement range and suitable for marine underwater application was proposed to solve the problem that a single interference frequency signal which produced from the conventional interferometers can not be used to realize simultaneous measurement of multi-parameters.Firstly,the influence of seawater depth(pressure)on the characteristics of LP01 and LP02 mode propagating in the DMF and its sensitivity of interference peaks/dips on both sides of CWL,as well as the relationship between sensitivity of wavelength response and distance from peaks/dips to CWL were analyzed theoretically and experimentally in this paper.The wavelength sensitivities of PL1 and PR1 to a single parameter(seawater temperature or depth(pressure))shown with different values and signs,reach a maximum on both sides of CWL,respectively,therefore,simultaneous measurement of seawater temperature and depth(pressure)with high sensitivity can be realized.Secondly,as the results of theoretical and experimental analysis,the sensitivity of DMF-MZI can be further improved by using DMF with longer physical length to make PL1 and PR1 in spectrum as close to CWL as possible.Finally,experimental results shows that the CWL and the interference peaks/dips on both sides of CWL are stable with variation of refractive index of solution around the surface of DMF,which shows superiority of DMF-MZI sensor for simultaneous measurement of underwater depth(pressure)and temperature in practical marine environment.The fabrication of all fiber seawater salinity sensors are usually based on the open cavity or ’evanescent structures’,so that energy of guide modes in those structures are partially or completely exposed to the measurand medium,and then change the effective refractive index difference of interference modes and cause the shift of interference peaks/dips in the transmission spectrum.Compared to the open cavity sensing structures with the relatively large loss of light transmission in general,microor nano-fiber show more promising optical properties of low transmission loss,a large evanescent field and tight optical confinement,beneficial for designing a high RI-sensitive fiber sensor.However,when the fiber required was tapered,by using a heating and pulling approach,the diameter of the fiber core will also gradually decrease,which increase the number of the high-order modes propagating in the sensing fiber and also create a "cutoff" of the core mode,which was then transformed to several cladding modes,and lead to the problems such as instablility in mode transmission or a long taper area,etc.By contrast,reducing the cladding thickness by chemical etching is an ideal way to maintain the stability of interference modes propagating in the fiber core.However,it is difficult to obtain a large evanescent field by reducing fiber cladding where designed with air holes or eccentric cores,which limit the improvement of refractive index sensitivity of those microstructure fiber sensors.The multi-cladding fiber which supports only LP01 and LP02 modes propagating in the fiber,for example,the interference mode(LP02)tends to cut off as the decrease of fiber diameter.Therefore,based on the characteristics of both microfiber and dual-mode interferometer,a special 40μm dual-mode fiber(TDF)with only LP01 and LP11 modes propagating in the fiber core was used to fabricate an all fiber MZI seawater salinity(refractive index)sensor,and TDF-MZI sensor with TDF diameter of 9μm was used to obtain a high sensitivity of～21292.2nm/RIU(1.4000～1.4025).As the results of theoretical analysis and experimental study,influence of environmental refractive index and sensing fiber diameter on the transmission characteristics and sensitivity of LP01 and LP11 interference modes in TDF-MZI uniform periodic transmission spectrum was studied,and its tunability of sensitivity on monitoring the shift of resonant wavelength,including the tunability of direction of wavelength shift in the transmission spectrum,were verified both in liquid and gas media.Generally,functioned as a sensing fiber in the fiber interferometer,core-doped fiber has difficult to detect the temperature over～800℃,and the temperature sensitivity of pure silica fiber is extremely low.In this paper,a fiber in-line TDF-MZI based on a special core-doped 40μm sensing fiber which supports only LP01 and LP11 mode propagating in the fiber,was used to realize the temperature measurement in the range of 100～1000℃,and solve common problems in the schemes of using core-doped fibers,such as interference fringe of dual-mode interferometer(e.g.,LP01 and LP02 interference)disappear or its monotonicity of resonance wavelength which response to the temperature changes suddenly when ambient temperature up to 800℃.Using pure silica photonic crystal fiber,Vernier effect was first applied in monitoring of high temperature up to 1200℃ in this work.The effect of multiple amplification of sensitivity by tracing peaks/dips of Vernier envelope was studied in theory and experiment.Small optical path difference of two intrinsic Fabry-Perot interferometer was easily controlled by arc discharge.The transmission spectrum of single interferometer was modulated by variable period Vernier envelope and influence of variation of optical path difference between two interferometers produced on Vernier envelope and its magnification was analyzed theoretically and experimentally.The shift of interference peaks/dips in Vernier envelope was detected in high temperature experiment,and its sensitivity was successfully improved by 1～2 orders of magnitude in a high temperature measurement range.The two compact schemes mentioned above based on core-doped sensing fiber and pure silica sensing fiber,respectively,have key advantages of fast response,high sensitivity,large measurement range,electromagnetic isolation and chemical stability,and thus provide an important high temperature measurement technology for marine engineering applications,such as marine oil and gas engineering,marine electric power engineering or development of marine mineral resources. |
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