Measurement And Control Research Of Effective Refractive Index Of Femtosecond Laser Direct-written Glass Waveguides

Femtosecond laser direct writing technology is an important processing technique for integrated optics,which has been widely researched due to its advantages of the wide range of processed materials,cold processing,and true 3D processing,and has been applied to many fields such as communication,topological photonics,and astronomy.As a basic element of integrated optics,the study of the fundamental properties of optical waveguides is very important to improve the performance and functionalization of optical waveguides.Among them,the accurate measurement of the effective refractive index of waveguides is significant for applications such as waveguide mode coupling and phase delay.The work in this paper focuses on the measurement theory and simulation verification of waveguide effective refractive index,experimental measurement of waveguide effective refractive index,and the study of the control of waveguide effective refractive index by adjusting the laser processing parameters.The main research contents are as follows.1.A method for measuring the effective refractive index of optical waveguides based on the waveguide structure of a broken-arm Mach-Zehnder interferometer(MZI)is proposed.The principle of the method for measuring the effective refractive index of waveguides is explained in detail in theory,and the simulation of waveguide devices using Rsoft software is used to demonstrate the feasibility of the method.Specifically,the research includes the loss and phase change caused by Gaussian beam transmission in a broken waveguide,and the working principle of measuring the effective refractive index of a waveguide using the broken-arm MZI.2.Accurate measurement of the effective refractive index of the waveguide is achieved experimentally using the broken-arm MZI structure.Specifically,the broken waveguide with different break lengths was prepared experimentally using a femtosecond laser direct write at 1030 nm,pulse width 290 fs,and repetition frequency1 MHz,and its loss was tested and fitted using a continuous laser at 808 nm to verify the practicality of the broken waveguide loss theory.The results were tested and fitted to obtain the effective refractive index of the optical waveguide of 1.504+7.7×10-4.Meanwhile,this work discussed the range of data selection for experimentally prepared MZI devices with disconnection lengths and the reliability of the test results.For experiments,we prepared MZIs with interferometric arm breaking lengths of 20 μm-1970 μm,and selected different fitting lengths,and obtained the best-fit data in the range of 20 μm-970 μm after comparing the effective refractive index differences of the waveguides obtained from their fits.To verify the stability of the femtosecond laser processing system,the MZI were fabricated under the same experimental conditions To investigate the reliability of the effective refractive index test results of waveguides,four sets of broken-arm MZI were prepared and their effective refractive indices were compared to obtain their standard deviations of 9.54×10-6.3.The quantitative control of the effective refractive index of the femtosecond laser direct-written glass waveguide was investigated,and the effective refractive index of the prepared optical waveguide was changed by adjusting the processing parameters of the femtosecond laser.Specifically,the control range of the effective refractive index difference of the fabricated waveguides reached 5.01×10-4-7.46×10-4 and 2.65×10-4-7.89×10-4,respectively,by controlling the number of scans and the scanning speed.