Iterative calculation, manufacture, and investigation of DOE forming unimodal complex amplitude distributions

We present results of iterative calculation, manufacturing and experimental as well as theoretical investigations of a novel diffractive optical element (DOE) which transforms a Gaussian TEM00 input beam into a unimodal Gauss-Hermite (1,0) complex amplitude distribution. The iterative calculation procedure is based on the application of the method of generalized projections. The projection operator onto a set of modal functions is implemented through partition of the focal plane into a 'useful' and an 'auxiliary' domain. To improve the error reduction during the iterative calculation procedure, a stochastic predistortion in the auxiliary domain is chosen. This calculation results in a 2-D phase distribution which has to be transferred into an optical element. This element has been manufactured as a 16 level surface profile by (variable dose) electron-beam direct- writing into a PMMA resist film and a subsequent development procedure of the resist. Each of the generated 15 steps of the resist profile corresponds to a certain electron dose, comparable to a usual 'isobathic process' or a 'monotone etching method.' The final element consists of a fused silica substrate coated with the structured PMMA film. Both computational and experimental results are presented and demonstrate a good conformity with each other. Energy efficiency has been measured in the focal plane as 37.7%, compared with the calculated value of 45.5%. The achieved results show good prospects of such an approach for the formation of unimodal distributions.