Characteristic Studies Of Sensing And Demodulation Based On Mmf Modal Interference

Modal interference sensor not only has advantages of the traditional optical fiber sensor such as corrosion resistance,anti-electromagnetic interference and low cost,but also has the advantages of easily fabrication and high sensitivity.Due to these merits,Modal interference sensor has attracted more and more researchers' attentions and developed rapidly in recent years.The modal interference sensor belongs to the phase modulation optical fiber sensor.This paper introduced the interference principle of the common phase modulation optical fiber sensor and expatiated the principle of all-fiber Mach-Zehnder interference.The expression of the wavelength under the condition of interference minimum was deduced by the multimode interference theory.The parameters influencing the trough position were obtained.The theoretical model of wavelength drift and temperature was established by theoretical analysis in modal interference sensor.In this paper,three types of Mach-Zehnder interference sensors were designed by modal interference theory,which were core mismatch-offset full-fiber Mach-Zehnder interference sensor,bitaper-offset all-fiber Mach-Zehnder interference sensor and core mismatch-offset-core mismatch full-fiber Mach-Zehnder interference sensor.The finite-difference beam propagation method was used to numerically analyze the mode and the distribution of the internal modal power in the sensor.Structure parameters such as the length of multimode fiber,single mode fiber and bitaper region,the offset of fiber and the waist diameter of bitaper region were optimized to obtain the interference transmission spectrum with high contrast.The parameters of Mach-Zehnder interference structure were optimized and the high linearity of the transmission spectrum was obtained for sideband filtering.The core mismatch-offset all-fiber Mach-Zehnder interference sensor,bitaper-offset all-fiber Mach-Zehnder interference sensor and the modal interference sideband filter were fabricated by the fiber fusion splicer.The experimental systemof monitoring temperature was established to study the temperature characteristics of these two sensors.The characteristic of sideband filtering demodulation,which used all-fiber modal interference structure,was verified by established sideband filtering demodulation system.In addition,the method of calculating the transmitted optical power through the sideband filter by spectral data,was proposed to improve the power sensitivity of sideband filtering.