Vernier Effect On A Single In-Fiber Modal Interferometer And Its Sensing Applications

In recent decades,fiber internal mode interferometer has been widely concerned in the field of optical fiber sensing due to its advantages of small size,compact structure and easy encapsulation.The sensitivity is one of the most important indexes for fiber intermodal inter ferometers.It is found that the sensitivity of the cascade interferometer can be improved by the cursor effect of the cascade interferometer.In this paper,the cursor effect based on a single fiber internal mode interferometer is proposed and realized,whi ch not only reduces the difficulty of experimental preparation,but also improves the sensitivity of a single interferometer.In this paper,the cursor effect of MZI in two different structures is analyzed,and its sensing characteristics in temperature,stress and curvature measurement are discussed.Specific research contents and achievements are as follows:(1)The principle of cursor effect based on cascade structure MZI was studied,and the cursor effect of MZI in a single fiber was theoretically analyzed and studied.(2)A method for extracting envelope information from cursor effect is introduced.The spectrum is obtained by fast Fourier transform(FFT),and the corresponding frequency component of the envelope is found.Affter that,it is filtered,and then the envelope information can be obtained by inverse fast Fourier transform(IFFT).(3)In this paper,a single malposition fusion type fiber internal MZI is fabricated,and its sensing characteristics in temperature and curvature measurement are preliminarily discussed by analyzing its vernier effect.The experimental results show that by analyzing the vernier effect of the interferometer,the response sensitivity of the interferometer to temperature is increased from 45.3pm/°C to 401.3pm/°C within the temperature range of 30-70°C,and the sensitivity is increased by about 8 times.In the curvature experiment,the curvature sensitivity of the interferometer in the curvature range of 0-1.0688m^-1 was increased from-3.62nm/m^-1 to-31.05nm/m^-1 after the cursor effect analysis,and the sensitivity was increased by about 8 times.(4)MZI with peanut structure was fabricated by single mode fiber and its response to strain and curvature was investigated by experiment.The experimental results show that the strain sensitivity of the interferometer is increased by 6 times from-1.4pm/??dielectric constant to-6.8 pm/??dielectric constant in the range of 0-1248??.However,in the curvature measurement experiment,the transmission spectrum is too messy to analyze the curvature sensitivity directly.The sensitivity of curvature in the range of0-1.0688m^-1 is-19.28nm/m^-1.