Investigation Of The Sensing Characteristics Of A Specially Designed Few-mode Fibre Based Interferometer And Their Applications In Fibre Optic Sensing

Fibre optic in-line Mach-Zehnder interferometer(MZI)structures have merits of small and compact size,high sensitivity,immunity to electromagnetic interference,and durability in harsh and corrosive envirmonents,which have became an important kind of sensing structure in the fields of fibre optic sensing.However,problems arise in that the transmission spectrum of a typical reported in-line MZI shows periodic interference fringes,where the interference peaks/dips exhibit the same behaviour,i.e.shift in the same direction,with the same sensitivity,when monitoring environmental effects,for example.Therefore,when such a sensor system is being used to detect any particular parameter,through monitoring the wavelength variations of a particular interference peak/dip at a particular wavelength,a number of important issues will limit the veracity of the sensor performance,for a range of applications including:overlapping of adjacent peaks/dips induced by periodically-changing interference fringes,and limitations in using this configuration in multi-parameter sensing,caused by the uniformity of the interference fringes and their wavelength sensitivities.Besides,the Vernier-effect,which can increase the wavelength sensitivity of the interference peak/dip up to an order of magnitude,has been employed to a typical in-line MZI by cascasing two fibre optic interferometers with nearly the same free spectrum range.However,similar as the interference fringes in the transmission spectrum of a single in-line MZI,the envelope of the superimposed spectrum has periodically changed interference peaks/dips,which may cause problems of difficults in wavelength identification,limit in measurement scales and restrictions in multi-parameter sensing.Thus,alternative approaches to sensor designs are needed.The equalization wavelength,which is also called the critical wavelength(CWL)in the field of fibre optic sensing,is the wavelength where the group verocities of the two interference modes equals to each other or the group optical phase difference of the two interference modes equals to zero.In the transmission spectrum of a fibre optic in-line Mazh-Zehnder interferometer based on modal interference,at the CWL,the wavelength response period is infinite and the wavelength spacing between adjacent interference peaks reaches maximum.Therefore,the critical wavelength,which is entirely different from the interference peaks/dips in a transmission spectrum of a fibre optic interferometer,is exclusive and easy to identify from the transmission spectrum.This thesis,based on theoretically studying the transmission and sensing characteristics of the CWL and the interference fringes on both sides of the CWL in the transmission spectrum of a fibre optic modal interferometer,proposed the wavelength sensitivity theory of the transmission spectrum of fibre optic interferometers based on two modes interference.Based on the current problems arised in the typical single and cascaded fibre optic interferometers,the sensing characteristics and wavelength sensitivities of the CWL and the interference peaks/dips on each side of the CWL under a wide range of measurands,such as temperature,strain,surrounding refractive index,and static pressure,have been theoretically analyzed and experimentally studied,and innovative sensing approaches have been proposed in this thesis with competitive advantages of high sensitivity,large measurement sensing range,and co-located multi-parameter sensing.Meanwhile,a specially designed few-mode fibre(FMF),which only the fundamental core mode LP01 and the first symmetric high order core mode LP02 are excited in the it under the axial to axial splicing condition,and there is a CWL exists in the transmission spectrum of the SMF-FMF-SMF(SFS)structure at the operational wavelength,has been studyed in this thesis.The FMF is specially designed so that only the fundamental core mode LP01 and the first symmetric high order core mode LP02 are excited in the few-mode fibre under the axial to axial splicing condition,and there is a critical wavelength exists in the transmission spectrum of the SFS structure at the operational wavelength.By applying the proposed sensing approaches employing the CWL and the interference peaks/dips on each side of the CWL in the transmission spectrum of the SFS structure,and theoretically analyzing the transmission characteristics of the FMF under a wide range of sensing paramters,a variety of sensors have been constructed and practically demonstrated:temperature,strain,surrounding refractive index,static pressure sensing in large measurement scales;temperature,strain,static pressure sensing with high sensitivities;simultaneous measurement of temperature and strain,static pressure and temperature with co-located fibre sensors.Comparation between two different dominating approaches of fibre structure designs have been investigated,and the optimizated FMF structure design is presented,with temperature and strain wavelength sensitivity simulated to demonstrate its feasibility.Finally,a cascaded SFS structure is proposed in this thesis,which the envelope of the superimposed transmission spectrum also has a critical wavelength,and the wavelength spacing between the adjacent interference peaks/dips of the envelop reseches maximum at the envelope critical wavelength.Theoretical ayalysis and experimental results shows that the Vernier-effect can increase the sensitivity of both the envelope interference peaks/dips and the envelope critical wavelength.All fibre static pressure sensors and temperature sensors with ultra-high sensitivities,large measurement scales,and potenitals in multi-parameter sensing are proposed and demonstrated.