John M. Tamkin

Effects of structured mid-spatial frequency surface errors on image performance

Optical designers are encouraged to adopt aspheric and free-form surfaces into an increasing number of design spaces because of their improved performance. However, residual tooling marks from advanced aspheric fabrication techniques are difficult to remove. These marks, typically in the mid-spatial frequency (MSF) regime, give rise to structured image artifacts. Using a theory developed in previous publications, this paper applies the fundamentals of MSF modeling to demonstrate how MSF errors are evaluated and toleranced in an optical system. Examples of as-built components with MSF errors are analyzed using commercial optical design software.

Theory of point-spread function artifacts due to structured mid-spatial frequency surface errors

Optical design and tolerancing of aspheric or free-form surfaces require attention to surface form, structured surface errors, and nonstructured errors. We describe structured surface error profiles and effects on the image point-spread function using harmonic (Fourier) decomposition. Surface errors over the beam footprint map onto the pupil, where multiple structured surface frequencies mix to create sum and difference diffraction orders in the image plane at each field point. Difference frequencies widen the central lobe of the point-spread function and summation frequencies create ghost images.

Photolithographic processing of hybrid glasses for microoptics

Hybrid glass materials are used in the photolithographic fabrication of optical and optomechanical structures. Two different photolithographic hybrid glass processing methods are described. The first one is referred as photolithographic patterning and the second as direct photolithographic deforming of hybrid glass materials. No etch transfer of the photoimaged structures is needed. In the latter method even the chemical development step can be excluded from the fabrication. Fabrication of lens-arrays, gratings and other binary structures is presented. The synthesized hybrid glass materials feature minimum optical transmission of 97% at wavelengths ranging from 450 nm to 1600 nm and refractive index of, e.g., 1.53 at 632.8 nm. The photolithographic patterning resulted in structure heights in excess of 180 /spl mu/m with rms surface roughness values ranging from 10 to 45 nm. The direct photolithographic deforming resulted in structure heights in excess of 27 /spl mu/m with rms surface roughness values ranging from 1 and 15 nm.

High-speed gray-scale laser direct-write technology for micro-optic fabrication

Binary Laser Direct-Write (LDW) raster-scan technology for UV exposure of photosensitive materials has been used for single or multiple-pass exposure applications. Gray-scale LDW can be applied to manufacture of 3-D optical structures, but the system requirements are substantially different, since edge slopes and surface departures must be controlled to within fractions of a wavelength.. In this paper, we explore the differences between binary and gray-scale raster imaging applied to micro-optic fabrication, and compare the system model with a prototype high-speed, gray-scale LDW tool that was developed from a Laser Direct Imaging tool originally designed for binary applications.

Analysis and tolerancing of structured mid-spatial frequency errors in imaging systems

Structured mid-spatial frequency surface errors on aspheric optics can create ghost images and reduced contrast. This reduction in performance is shown to be non-linear with surface height using Fourier methods without small signal or statistical approximations. Tolerancing MSF errors can use traditional MTF metrics, and derives peak-to-valley limits on MSF surface height components.

Evolution of catadioptric scan optics for wide-field high-resolution applications

Most high resolution scanning applications use a rotationally symmetric scan lens and precision motor polygon mirror assemblies or active facet error correction. The realm of passive motion compensation and low-cost motor polygon assemblies tend to be limited to lower-performance systems. The reason for this lies in the symmetry that is broken by tangential beam input to the rotating polygon mirror. An alternative design method is presented, along with examples of application for UV maskless lithography.