Large Aperture Diffractive Optical System For Lidar Receiver

The development of space-borne Lidar applications proposed the requirement of developing large aperture receiver optical system,so as to enlarge detection range and improve detection precision and sensitivity.Traditional refractive optical systems and reflective optical systems are difficult to develop large aperture applications,because of factors such as system mass and surface shape figure tolerance.While diffractive optical systems are competitive to form Lidar receiver optical systems,on account of the superiority of light weight and perfect surface shape figure tolerance.Characters of a diffractive lens are discussed,aiming at Lidar applications.The diffraction efficiency is related to factors like surface pattern depth,material refractive index,lens f-number,number of quantized steps.It is also influenced by manufacturing errors like etch depth error and lateral alignment error.Diffractive lenses have different monochromatic aberration,chromatic aberration and thermal dispersion from traditional lenses.The former provides excellent imaging quality of on-axis monochromatic object point.Aberrations are mainly from off-axis object point and broad spectrum.When focusing light,diffractive lenses introduces serious negative color dispersion.The coefficient of thermal expansion(CTE)is only determined by coefficient of linear expansion(CLE),and is independent of thermal coefficient of refractive index(TCRI).Technologies such as manufacturing,folding and deploying are also introduced.The chromatic aberration character of diffractive lenses inherently demands special achromatic design for diffractive optical systems.In a practical diffractive optical system,color dispersion caused by lens material and diffraction forms the chromatic aberration of a diffractive lens together.The two sources have different sign,while the latter's ability to introduce dispersion is one order of magnitude stronger than the former's in most cases.Achromatic structure based on high dispersion materials is able to compensate dispersion,but the scope of its applicability is narrow.Achromatic structure based on Schupmann's theory,however,is able to correct chromatic aberration thoroughly.The problem is that classic achromatic structure based on Schupmann's theory appears to be complex for Lidarapplications.The simplified achromatic structure,consisting of three parts,the primary lens,the correcting lens and the focusing part,is proposed then.The structure is demonstrated to correct chromatic aberration perfectly,and more widely adaptable than high-dispersion-material-based structure.A Lidar receiver optical system design is provided,with 1-meter aperture,3.333-meter system focal length and 52?rad max FOV.The image of the designed optical system attains diffraction limit.And it remains perfect in the given tolerance level and thermal environment.Further,a system with 1-meter aperture,8-meter system focal length and 1mrad max FOV is demonstrated,which also attains diffraction limit in the given tolerance condition and thermal environment.It also proves the application potential of the simplified achromatic structure.