Research On Grin-lens Refractive Index Distribution Measurement Method Based On Wavefront Correction

A grain-lens(G-lens),also known as a gradient-index lens,is a tiny optical device whose refractive index changes gradually along the radial direction.Because of its simple structure,small volume,short optical path,light weight,and easy to mass production,easy to integrate,high coupling efficiency,it has a wide range of applications in optical communication,micro medical imaging and many other fields.Refractive index distribution is the core parameter of grin-lens,and its measurement accuracy and rapidity directly affect the production efficiency and final product quality.At present,the main measurement method is the distribution curve obtained by axial interference method,after ion exchange,the glass filament is sliced into thin slices smaller than 0.3mm and polished on both sides.Due to the need of complex secondary processing,the measurement efficiency of this method cannot meet the real-time pre-furnace detection of ion exchange.To solve this problem,radial interference method and end reflection method were proposed in the 1980s to avoid the problem of lack of real-time performance caused by secondary processing,but limited by the development of technology at that time,their efficiency and measurement accuracy could not meet the needs of the production process.In this paper,based on the radial interference method,the wavefront difference caused by the existence of matching liquid is corrected,and the fast target extraction and wavefront correction algorithm is used to effectively improve the measurement accuracy and speed.At the same time,in view of the defects of radial interference method,such as chaotic fringes and incomplete fringes,a method of measuring refractive index distribution of the end face of the grin-lens is proposed by combining the reflection spot image of the end face.The target of non-destructive,high-efficiency and high-precision detection in the whole radial area of the self-focusing lens is realized.The main research contents and results are as follows:(1)According to the principle of radial interference method,the relation between fringe offset and refractive index distribution is deduced.Based on the phase transformation principle of thin lens,the phase transformation function of self-focusing lens is derived and verified by simulation.Since the grin-lens is placed in the matching solution,in order to eliminate the wavefront difference generated by the matching solution and optical element,a wavefront correction algorithm is proposed to correct the wavefront difference in the interference system.(2)The experimental platform of radial interference method is built to collect the image of interference fringe.The fringe centerline of the interference fringe offset was extracted,the refractive index distribution of the grin-lens was calculated by combining the radial interference detection principle,and the wavefront correction algorithm was combined to eliminate the wavefront difference generated by the matching solution and optical element in the radial interference optical path,and the wavefront correction of the interference fringe image was realized.By comparing the experimental results,the measurement accuracy of refractive index distribution is improved.(3)Because the radial interference method is affected by the structure of the grin-lens,the edge interference fringe is difficult to detect.On the basis of the end-face reflection method,an image method of end-face reflected Light spots is proposed and an experimental platform is built.The grayscale uniformity of incident light spots is corrected using the Liquid Crystal Spatial Light Modulator(LC-SLM).Subsequently,the reflected light spot pattern was acquired by the CCD(Charge Coupled Device)camera,and the global reflectivity distribution was calculated according to the gray distribution reflected by the grin-lens end face,and the refractive index distribution was further derived.The experimental results show that this method improves the gray uniformity of incident light spot to a certain extent,and reduces the error of refractive index distribution calculation.On the other hand,it directly obtains the reflection spot image of the end face of the grin-lens,which effectively improves the practical problem that it is difficult to detect the entire radial refractive index distribution by radial interference method,and realizes the whole-region test of the radial refractive index distribution of the grin-lens.The combination of the two methods and their mutual correction is more conducive to the rapid real-time and high precision monitoring in front of the furnace in the process of grin-lens manufacturing.