It was shown in Chaps. I and 6 that the aberration properties of DLs differ significantly from those of their refractive analogies, refracting surfaces. This is because the aberration expansion of a flat axial DL has better convergence. In addition, the technology of making DL structures (by means of optical or electron-beam lithography, for example) is free of limitations peculiar to the holographic recording method and allows effective control over the lens spherical aberration without affecting field aberrations. Finally, the Petzval condition (6.64) determining the possibility of forming a flat stigmatic image is automatically fulfilled in DL-based optical systems independently of the optical power and spherical aberration of the components.
In view of the above features, a number of high-resolution diffractive objective designs, much simpler than analogous refractive systems, can be offered. This chapter deals with the design and analysis of such DL-based objectives. Note that all essential characteristics of optical systems considered below can be achieved after computing only two first orders of the aberration expansion. The further optimization by ray tracing, of course, improves these characteristics but does not significantly change the objective design parameters obtained earlier. The fact that methods of aberration theory lead to good results before using optimization makes the design of DL-based systems quite different from the design of refractive analogs.
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