Study Of Modal Interferences In Mach-Zehnder Fiber Sensors

Mach-Zehnder fiber sensors based on mode interferences have beenintensively studied and several applications based on the Mach-Zehnder structurehave been proposed in this thesis. The main researching works are listed as thefollowing.1. The principle of modal interferences is introduced, providing an importanttheoretical basis for the following studies of the fiber sensors. Then the two kinds ofmodal interferences fiber sensors are demonstrated. The beam propagation methodsimulating the distribution of mode fields is introduced.2. A fiber in-line Mach–Zehnder interferometer using thin-core fibers havebeen proposed and demonstrated. This in-line interferometer is composed of a shortsection of thin-core fiber inserted between two single mode fibers （SMF）, anddemonstrated as a strain and temperature sensor in this study. It is well known thatthe interference spectrum is sensitive to the environmental parameters, such as strainand temperature. The multi-mode interference pattern will shift when thesurrounding environment changes. By demodulating the wavelength shifts ofmulti-mode interferences, the surrounding parameters’ changes can be obtained. Astrain sensitivity of1.83pm/με with a measurement range of0-2000με, and thetemperature sensitivity of72.89pm/oC with a temperature variation of50oC areachieved. We also discussed that the influence of strain and temperature change onthe relative power ratios among the excited cladding modes in thin-core fibers. Withspectrum differential integration method, intensity-modulated to strain andtemperature is studied. The strain sensitivity is27.956nw/με, and the temperaturesensitivity is844.56nw/oC.3. A fiber curvature sensor based on the structure of singlemode-multimode-single mode fiber is proposed. The principle of fiber curvaturesensor is based on the change of propagation mode in no core fiber. Simulations onlight distribution propagating along the fiber are carried out when the fiber is straight or bent. The spatial Fourier transform of the interference spectrum is plotted todetermine the interference spectrum formed with multimode overlapped. Asensitivity of-22.84nm/m^-1is achieved when the applied curvature increases from3.62m^-1to4.84m^-1. With comparison to other curvature sensors with the similarstructure, our proposed one has a much higher sensitivity and can be easilyfabricated.