Research On Error Detection Of Mosaic Grating Based On Interferometry

Large size diffraction grating is an important optical element in astronomical spectrum analysis and laser inertial confinement fusion.While the preparation for a single large size grating need to break too many technical problems.Therefore,it becomes a trend that splicing two or a plurality of small gratings to make a large size grating to meet the requirements.The key to whether the mosaic grating can be put into use is the mosaic accuracy.It is very important for the detection of mosaic error and the accuracy of grating attitude adjustment.In recent years,scholars at home and abroad adjust grating attitude to decrease the mosaic error using the method of far field spot or far field spot method combined with interference testing.The method of far field spot is adjusting grating and monitoring spot energy until the spot meet the requirements.However,there are great errors in the far filed spot method when the wavefront errors caused by optical system and grating are too much.For the mosaic grating can be used,the mosaic grating testing should be on the mosaic grating which is detecting the wavefront of mosaic gratings by interferometry.Therefore,the adjustment value of grating attitude can be calculated by measuring the wavefront of mosaic grating.The detection technology of mosaic gating based on interference is intensively studied.The main work is as follows:First,the model of the relationship between mosaic error with far field spot energy distribution and diffraction wavefront were established according to the Fraunhofer integral principle and interference principle.The simulation results of the two methods are compared and analyzed,and the interference method is chosen as the error detection method.Second,the mosaic grating error detection optical structure based on interferometry is designed.The wedge lens is introduced to eliminate the error of ?z to achieve the strict mosaic grating.Three steps are used to achieve the detection of five dimensional errors.The first step is to detect ??x and ??y using the light of order zero.The second step is to detect ?z using reference window produced by the light of zero order and measuring window produced by wedge lens structure.Third step is to detect ??z and ?x using the light of diffraction order.According to the adjustment process of the three steps,principle simulation results getting by Matlab and optical simulation getting by ZEMAX show good consistency.The simulation resuts shows that the three steps method can be used in grating mosaic.Third,the least squares fitting algorithm is used to calculate the wavefront error of zero order and the diffraction order according to the wavefront data measured by the interferometer.The calculation matrices are established between the wavefront of zero order with ??x,??y,??z and the wavefront of diffraction order with ?x and ?z.The numerical solution of five dimensional errors ??x,??y,??z,?x,?z can be calculated by the above calculation process.Fourth,the five dimensional mosaic error calculation program are improved using Labview according to the calculation method of mosaic error model.The program is divided into three parts: ??x and ??y calculation module,?z calculation module,??z and ?x calculation module.The function of the program is reading the measured wavefront value of interferometer system and calculating numerical value of the mosaic error ??x,??y,??z,?x,?z.In order to verify the correctness of the calculation method and the practicability of the software,ZYGO interferometer is used as the measuring system to read the wavefront of the mosaic grating.The interference fringes generated by mosaic grating are compared to the calculating results.The comparision shows that the calculation method is correct and the function of the program meet the practical needs.Fifth,the accuracy of the mosaic grating is analyzed according to the requirements of the mosaic grating.The parameters of components in the system are deduced by the accuracy analysis results,and the experimental equipment is determined according to the parameters of the components.Sixth,wavefront value is detected and mosaic error is calculated.Grating attitude is adjusted by the mosaic error using the system established before.Two small grating whose size is 35mm×35mm,groove density is 600line/mm,wavefront is 1/4 ?(632.8nm)are spliced.The wavefront of the mosaic grating is 1/3 ?,which lays the foundation for the fabrication of the large aperture plane diffraction grating in the real sense.