Study On Active Fiber Laser Refractive Index Sensor

Fiber laser is a kind of active components,which not only has the traditional passive components' good features but also has the features like narrow line width,high signal-to-noise ratio.Fiber laser can achieve high precision sensor measurements.In recent years,some new sensing technology based on the fiber laser has been implemented and achieve the application.Fiber laser active sensing technology is based on the passive fiber Bragg grating sensing technology,using fiber laser as a sensor instead of fiber Bragg grating constitute the sensing system to achieve the experimental study.Its significant characteristic is to improve the sensing wavelength's signal-to-noise ratio and the power.Comparing to the traditional broadband light source,its signal-to-noise ratio of the sensing signals has been greatly improved,which can be easily detected by the sensor.This paper is based on the National Natural Science Fund projects,researching “the refractive index sensing technology based on active fiber laser”.The specific work content is as follows:Firstly,the historical development and current research status of fiber laser are discussed in detail.Meanwhile,the types of fiber grating and the sealing method are also described.Secondly,the theory of fiber gratings written through the refractive index modulation are described based on the modal properties of the optical fibers and coupled mode theory of fiber gratings.The mode couplings of long period fiber gratings written through the structural change are discussed with the coupled local-mode theory,and the simulations of transmission spectra are achieved.Based on the above methods,the transmission characteristics and mode couplings of various fiber gratings can be explained.Thirdly,a simplified model of equivalent refractive index is established to study the refractive index sensing properties of fiber gratings.The relationships between the resonance wavelength of fiber Bragg grating and the refractive index are obtained.The refractive index sensitivity of long period grating is achieved by numerical simulation,and the relationships between refractive index sensitivity and different fiber grating parameters are obtained.In addition,the refractive index sensitivity of the structural change long period grating is studied.Finally,using the fiber Bragg grating and long period fiber grating inscribed in optical fiber with the other active devices,the dual mode laser output is realized through the continuous optimization of each device parameter,then the feasibility of the refractive index sensitivity system is theoretically proved.