Study On The Fabrication Of Echelle Grating With 90° Apex Angle By Femtosecond Laser And Its Formation Mechanism

Grating,as an important diffraction element,has important applications in many fields.There are many kinds of grating processing methods,but the disadvantages are obvious.Ultrafast laser has the characteristics of ultrafast pulse,ultra-high energy density and nonlinear machining,which can realize ultrafast 3D micro-machining.Compared with other processing methods,ultrafast laser processing has the advantages of high efficiency,environment-friendly,simple process,low cost,high quality and accuracy,and high repeatability,so it is necessary to use ultrafast laser to prepare grating.In this dissertation,the femtosecond laser was used to fabricate Echelle grating with 90°apex angle on silicon substrate based on the combination of simulation and experiment.The experiment focused on two processing methods,namely,direct writing and multi-line writing,to prepare the Echelle grating wih 90 ° apex angle by femtosecond laser.The simulation focused on the analysis of the diffraction efficiency of gratings under different incident wavelengths and different incident angles.The main research contents and results are as follows:(1)By controlling the change of single factor,the influence of femtosecond laser processing parameters on the groove structure(groove width,groove depth,etc.)was studied.Under the optimal parameters,femtosecond laser was used to directly carve out the echelle grating with a period of 30 μm,90°apex angle and V-groove structure close to the theoretical direction.It was shown that the structure of the direct-writing echelle grating was clear and the lines were even.Compared with the theory,the errors of the actual processing grating mainly included the deviation between the groove depth and the groove width,the existence of a platform at the top,the uneven bottom,the asymmetry of both sides of the grating groove and the existence of a certain arc at the bevel edge.(2)Based on the Finite-Difference-Time-Domain method,the simulation was established to analyze the theoretical period of 30 μm,90°apex angle and V-groove structure.It was shown that TE and TM were basically coincident in the wavelength domain.At the wavelength of 800 nm,most of the reflection and flash occurred in the-53 order;when the incident angle changed,the reflected energy of-53 order was less and less,and more energy was concentrated at other diffraction orders;in addition,at 45 ° incident,the diffraction efficiency of transverse electric wave(TE)reached the maximum.Considering the deviation of width and depth in the actual machining process,the deviation of ±2 μm and ±1 μm were simulated,analyzed respectively and compared with the data of theoretical groove structure.The results showed that with the increase of the actual machining groove width,the longer the wavelength corresponding to the higher-53 diffraction rate,the higher the theoretical peak diffraction efficiency was than the actual machining peak diffraction efficiency.(3)Aiming at the echelle grating of large period with 90 °apex angle,a method,using multiple lines,was introduced to etch a single groove structure in this dissertation,and by using this method,three kinds of large period grating with 90°apex angle were successfully fabricated:the grooves of echelle grating with period 80 μm and 120 μm were isosceles right triangle,but the groove shape of 104 μm grating was a non isosceles right triangle.It was shown that the prepared grating had uniform lines and clear structure,which proved the feasibility of this method in the preparation of large period grating.In addition,compared with symmetrical groove,asymmetric groove was more difficult to operate and control in etching preparation,with lower similarity to theoretical groove,worse straightness of groove boundary;in general appearance,asymmetric groove grating was seriously missing in groove top angle and groove bottom angle,while symmetrical groove grating had better quality.