Research On Sensing Characteristics Of The Fiber Laser Based On MMI Fiber Structure

Due to many kinds of requirement of automotive industry, aerospace and military,oil exploration, environmental monitoring and testing of building, scholars andexperts in many countries have paid much attention to the technology of sensor.Optical fiber sensing technology was developed in the late1970’s, which was newand different from the traditional sensor technology. Optical fiber sensor technologyhas many advantages, such as light weight, small volume, low loss on power, beingisolated to electromagnetic interference and being easily controlled far away. So theoptical fiber sensing technology has been widely used for monitoring the temperature,stress, humidity, displacement, vibration, acoustics, electricity, magnetism, andbiochemistry. The multimode interference (MMI) effect of single-mode andmultimode-mode and single-mode (SMS) fiber structure can be applied to filter,sensor and signal processing. The MMI structure can be easily made, packed andconnected to other optical fiber structure, so it has been widely applied to the study ofsensor. Because of high resolution and sensitivity of the laser, fiber lasers can improvethe effect of sensor based on a MMI fiber structure. So, this paper compared MMIfiber structure and optical fiber laser, and analyzed its sensing properties throuththeory, simulation and experiment.In this paper, the main research is as follows:（1）The research background and significance of this topic have beendescribed. The research statuses at home and abroad about fiber sensor structure basedon MMI and fiber laser sensor have been introduced as well.（2）The MMI effect in SMS fiber structure has been theoretically described,and the resonant wavelength expression of the SMS fiber structure has been deduced,according to the dispersion equation. Starting from the wavelength expression, theanalysis of filtering characteristics of the SMS fiber structure have been made, as wellas the sensing mechanism of the SMS fiber structure on temperature, external strainand environmental refractive index.（3）Based on the beam propagation method, the distribution of the internaloptical field in a SMS fiber structure and the filtering characteristics have beensimulated by using the Rsoft software. At the same time, the influence of MMF’slength, core diameter and the cladding refractive index on the output characteristics ofSMS fiber structure have been simulated. It has been theoretically demonstrated thatSMS fiber structure can be applied to sensor and monitor of temperature, strain andrefractive index.（4）The output characteristics of the fiber laser based on a SMS fiber structurehave been theoretically analyzed. The influence of erbium-doped fiber’s (EDF) length,Erbium ion’s density, coupling ratio and the interference center wavelength of SMS fiber structure on the output characteristic of laser has been simulated by theMATLAB software, which provided theoretical basis for design, production anddebugging of the fiber laser.（5）The fiber ring laser sensor based on SMS fiber structure has been designedand produced, and the experimental devices of temperature, strain and refractiveindex have been made. The sensing experiments research on temperature, strain,refractive index and bending have been carried out separately. The sensingcharacteristics of the fiber laser based on SMS fiber structure have been discussed,and difference, advantages and disadvantages of different experimental results havebeen analyzed, respectively. Part of the results of the sensing experiments have beenshown as follows: the sensitivity of temperature sensor was13.24pm/oC, thesensitivity of strain sensor was1.29pm/με, and the sensitivity of refractive indexsensor was180.21nm/RIU. Compared to traditional fiber sensors, resolution has beenimproved to some extent, individual result is slightly low than that of fiber lasersensor based on fiber Bragg grating structure. Although, the practical potential of thefiber laser sensor based on SMS fiber structure on temperature sensing, strain sensingand bending sensing has been confirmed once more. The sensor trend of micro-bendsensor experiment is consistent with the theoretical analysis, but the regularity is notstrong, the testing device used in our experiments needs to be further improved.