Patent ID: 6369948
Filing Date: 2002-04-09
Classification: G02B

Abstract:
A method of producing a multilevel diffractive optical element having a phase function &phgr;=&phgr;(x, y) and phase zones of different local modulation depth d =d(x,y) defined by different numbers of phase levels &xgr;=&xgr;(x, y) per phase zone, each phase zone consisting of phase levels having identical width and, in at least one phase zone, one additional level that is narrower than the other levels in that zone, the phase levels of all phase zones being spaced, in the direction along the modulation depth, by a substantially identical distance h, said method comprising:generating M binary amplitude masks having fringes configured to provide, in each phase zone, its local number of said phase levels Î¾&af;(x,y)=d&af;(x,y)h,â€ƒthe number M of masks being chosen so as to enable the production of a number of phase levels N0, which is at least not less than a maximal number of phase levels per phase zone over the optical element; and utilizing the masks serially for serial etching of said phase levels into said phase zones of the optical element; wherein at least some of said fringes have a varying width in accordance with the transmittance TM of a binary amplitude mask, which varies, in each mask, in X and Y directions in accordance with the variation of &phgr;=&phgr;(x, y) and d=d(x,y), and is calculated in accordance with the mathematical function TM&af;(x,y)=T&it;{sin&af;(P&it;â€ƒ&it;Mod[Ï†&af;(x,y)2&it;â€ƒ&it;Ï€]&it;d&af;(x,y)d0)}where T(x)=1 for x>0,t(x)=0 for x&lE;0, P is a parameter which is defined by a serial number of the mask, and which determines a number of the fringes provided in said mask for each phase zone, and d0 is a maximal achievable modulation depth: d0=N0Â·h.