Refractive Index Detection Based On Beat-Frequency Of Cladding-Carved DBR Fiber Lasers

Fiber sensor technology has been extensively used in building,oil drilling,aerospace and biochemistry detection areas due to its advantages of compact,multiplex available,anti-corrosion and electromagnetic immunity.With the development of fiber sensing,distributed Bragg Reflective?DBR?fiber lasers,acting as one of the excellent candidates of fiber sensors,have drawn great research attention.They have been applied in measurement of physical quantities such as temperature,stress,magnetic force,ultrasound,electric current and has become a mature and widely used sensor because of its high signal to noise ratio,narrow line width,multiplexibility,and so on.We have reviewed DBR fiber lasers that are developed in recent years.Based on this,we proposed a method to machine the laser cavity by use of CO₂ laser milling technique.We use the beat signal to detect external refractive index.Compared with the traditional refractive index sensor such as gratings,interferometers,microfibers,our device combines the advantages of robustness,high sensitivity,and low cost.This study opens a new possibility for RI sensing with beat-frequency fiber lasers.The main contents of this article are summarized as follows:1.We have analyzed the literature about fiber refractive index sensing,DBR fiber lasers,and fiber micromachining techniques,which are strongly related to our experiments and theory in this research.Both the fiber Bragg gratings and DBR fiber lasers are theoretically analyzed.The sensing principle of cladding-carved DBR fiber lasers are demonstrated as well.2.The working principle of DBR fiber laser is demonstrated.The influence of CO₂ laser on the spectral and spectral characteristics of the laser is described.The effect of changing CO₂ laser parameters on the output characteristics of DBR structure are discussed.Compared with femtosecond laser irradiation technique,the thermal effect of CO₂ laser can make a smoother surface on the fiber structure and thus a low transmission loss of light.We have given a detailed investigation on the center wavelength and beat frequency of the DBR fiber laser.It is shown that the CO₂-laser milled structure can have potential of applications for optical sensing and filtering.3.We have described the sensing characteristics utilizing the milled DBR fiber lasers.We have investigated the long-term stability of the carved DBR fiber after.The influences of different carve depths and widths on the refractive index sensitivity are studied and discussed in detail s.With the increase of slot depth and width,the beat frequency drifts more rapidly with the change of external refractive index.The obtained maximum refractive index sensitivity is-271.76 MHz/RI-unit.The temperature response is investigated as well.