Study Of Circular-Core Few Mode Fiber Sensor

Few mode fiber(FMF)is a special type of fiber which can propagate the fundamental mode and a few high order modes simultaneously.FMF have less nonlinearity like multimode fiber(MMF),at the same time they have the same performance as single mode fiber(SMF)in terms of low mode dispersion and additional loss.The essence of FMF sensing is inter-mode interference.The sensitivity of the different modes in the FMF to the external physical quantity is different,so the FMF can realize the multi-parameter sensing and also the cross-sensitive problem.In this paper,the interference between two circularly symmetric modes in a circular-core FMF is studied in detail.By simulating the strain and temperature phase sensitivity of the inter-mode interference,a temperature-insensitive FMF is designed.The experimental system is established to verify the sensing characteristics of the fiber,and effectively solve the problem of strain and temperature cross sensitivity which is widely existed in the traditional fiber sensor.The spectral sensing characteristics of FMF are studied,and a method of realizing dual parameter sensing based on small mode fiber is proposed.In this paper,the following research work has been done based on the temperature-insensitive sensing characteristics of circular-core FMF:(1)Theoretical analysis of the inter-mode interference principle of the small-mode fiber,the strain and temperature sensing characteristics of the small-mode fiber are studied,and the phase sensitivity formula of strain and temperature is deduced.(2)According to the theory of phase sensitivity and simulation results,a temperature-insensitive FMF is designed,which can solve the cross-sensing problem in fiber sensing.The power distribution of the circularly symmetric mode in the small mode fiber is simulated and analyzed,which provides the theoretical basis for the mode excitation in the experiment.(3)The peak wavelength shift and the image shift demodulation method of the strain and temperature sensing spectra of the small-mode fiber are studied.The image frequency demodulation method based on the spatial frequency has the advantages of both the wavelength domain and the frequency domain.(4)Design the experiment system to complete the strain and temperature sensing test of the FMF.According to the experimental results,we can draw the following conclusions: 1)The strain and temperature independent tests of four kinds of FMF verify the correctness and universality of our simulation algorithm;2)Design the strain and temperature crossings of the FMF.The results verify the design of the temperature insensitivity of the low modulus fiber,which is of great significance to guide the design of the fiber parameters.