Dennis W. Prather

Diffractive Lens Design

This chapter describes the uses of diffractive optics in lens design. In some applications an optical component may require a diffractive surface combined with a classic lens element. In other cases the requirements can be satisfied with just a diffractive element. Both types are described and analyzed here. After describing the concepts and approaches used to design a classic lens, we demonstrate how they can be applied to a single diffractive lens. The use of diffractive surfaces in hybrid lenses to correct chromatic aberration is also described. In Chapter 10, the use of diffractive optics in lens design is extended to multielement systems and to their use in thermal compensation of lenses. The lens design process is depicted in Fig. 4.1. After determining the specifications of the optical system, a starting lens design is evaluated by entering its description in a lens design program. The specifications of the performance of the component as a linear set of weighted target values, called a merit function, are set. Then some of the parameters of the optical system are varied and the merit function is evaluated. As the actual parameters approach the desired values, the merit function will approach zero. After a number of iterations, the design has improved (optimized) in comparison with the starting point. It is up to the designer to determine if the performance is good enough for the desired application or if some design changes need to be made, with additional optimization steps.

Application of Diffractive Optics to Lens Design

In Chapter 4 the diffractive lens, including a simple hybrid achromat, was discussed in some detail. In this chapter, we extend our discussion of lens design by introducing the wavefront aberration polynomial and the Seidel coefficients. Although it is not our intention to sneak a lens design text into this book, we do have to provide sufficient information on the expression of aberrations and their correction so that you can gain an appreciation of the power of diffractive surfaces in lens design and the additional degrees of freedom that they provide. The limitations of these approaches are also discussed. The chapter begins with an introduction to the aberration polynomial and its expression for a thin lens. Then pure diffractive designs, including superzone versions, are discussed. This is followed by a number of examples of hybrid lenses. Diffractive surfaces can also be used in the design of systems with reduced thermal sensitivity. The chapter concludes with a short discussion of applications that use microlenses.