Research On Accurate Measurement Method Of Fiber Grating Reflectivity Based On Scale Grating

Large-mode-area double-clad fiber gratings are important components of highpower fiber laser systems.For example,fiber Bragg grating pairs are the core components of fiber oscillators,and chirped tilted fiber gratings are effective means to suppress stimulated Raman scattering.Reflectance is an important characteristic parameter of fiber gratings,which has an important influence on the output characteristics of fiber laser oscillators and the suppression effect of stimulated Raman scattering.Therefore,it is very important to accurately measure the reflectance of fiber gratings.In this paper,aiming at the current difficulty in accurately measuring the reflectance of double-clad fiber gratings with low reflectivity and large mode field,innovatively proposed the concept of "scale grating",and carried out detailed research on reflectance measurement theory and experiment based on scale grating.The main contents are as follows:(1)A new method of measuring the reflectivity of large-mode-area fiber gratings using scale grating is proposed.The basic concepts and reflectance measurement principles of scale grating are introduced,and the feasibility is verified.In order to further improve the accuracy of reflectance measurement,the concept of fiber Bragg grating Fabry-Perot cavity(FBG-FP)scale grating is proposed,and its spectral characteristics are theoretically analyzed and simulated,and the pairs of structural parameters in FBG-FP are explored.The influence of the characteristics of the scale grating provides theoretical guidance for the preparation of the FBG-FP scale grating.(2)Preparation and application of fiber scale grating based on ultraviolet laser phase mask.The FBG-FP structure is introduced to prepare the FBG-FP scale grating with higher measurement accuracy.A test system for measuring the reflectivity of large-modearea fiber gratings was built,and low-reflectivity experiments for measuring large-modearea low-reflection fiber gratings and chirped tilted gratings(CTFBG)were carried out.The experimental results of measuring the reflectivity of large mode area low reflection FBG show that compared with the 5.26% calculated by traditional analysis of transmission spectrum,the measured results of grating reflectivity measured by scale grating and data fitting are 1.20% and 1.32% respectively,the accuracy of measuring the reflectivity of the grating with the scale grating are greatly improved.(3)Preparation and application of fiber scale grating based on femtosecond laser line-by-line writing.Using the advantages of femtosecond lasers that can theoretically inscribed fiber gratings in any wavelength,a flexible and adjustable FBG-FP scale grating with a flexible and adjustable working wavelength is made,which gets rid of the limitation of the fixed period of the phase mask and expands the use band of the FBG-FP scale grating.The experiment of calibrating the reflectance of the scale grating was carried out to verify the feasibility of the measurement method.At the same time,the reflectivity of the large-mode-area and low-reflection fiber grating was measured.