Performance improvement of CGHs for optical testing

Abstract

The expansion of the field of diffractive optics applications is accompanied by toughening performance requirements for CGHs. Optical testing sets especially high requirements, concerning wavefront accuracy and diffraction efficiency. The key point in fabrication technology is the writing system creating the photomask or the profiled pattern. The diffractive optics fabrication facility at ITO (University of Stuttgart) is based on the circular laser writing system CLWS-300. This flexible and high-accurate tool was originally designed for binary diffractive optics fabrication. This paper presents novel enhancements of this system allowing direct laser writing of a wide range of binary and continuous-relief CGHs on photoresist layers, chromium films and LDW-glass. Main topics of the enhancements were the scanning accuracy and exposure control.Many types of CGHs (binary precision holograms for optical testing, Shack-Hartmann arrays, microlens discs for confocal microscopy, diffractive interferometer objectives, doughnut generators etc.) have been manufactured using the developed algorithms and hardware.

Highly Efficient and Broadband Wide-Angle Holography Using Patch-Dipole Nanoantenna Reflectarrays

“…The efficiencies we present here exceed 40% over an operational bandwidth of at least 180 nm, which is substantially broader than previously reported results. The results show significantly higher efficiencies than that of previous reports on holograms at wide angles − , and approach the efficiency values of the simple waveforms projected to high angles by volume holograms …”

“…These methods facilitate the realization of holographic elements that radiate complex waveforms efficiently to small angles or simple grating lobes to large angles through specific conditions of resonant gratings. − Wide-angle projection of computer generated holograms requires a steep phase gradient between adjacent pixels. Attaining such gradient necessitates a small number of pixels in each period and renders the projected hologram inefficient. − Thus, efficient projection of complex waveforms to large angles still remains a challenge. , …”

Diffractive/refractive (hybrid) UV-imaging system for minimally invasive metrology: design, performance, and application experiments

“…They were fabricated at the ITO by laser direct writing with optimized writing strategies (principle described in [34,35]) to obtain steep backward slopes of the blaze structures and thereby high diffraction efficiencies. For the minimal structure sizes of about 10 μm, a diffraction efficiency of more than 85% at the design wavelength was achieved.…”

“…The database for the efficiencies can be generated by a fabrication model or by direct measurements of test elements. For the simulation of the diffractive elements in the hybrid imaging system, we took into account the orders −10 to 10 and used a simple model for the fabrication by direct laser writing of diffractive gray scale structures in photo resist with a Gaussian writing beam using edge enhancement algorithms [34,35]. To give an impression of the fabrication quality, for a period size of about 10 μm and writing parameters of our fabrication system (405 nm wavelength, NA 0.6), resulting efficiencies of the image forming order of more than 85% are calculated for the design wavelength and the neighboring orders (0th and 2nd) can be suppressed down to about 0.5% each, which was also experimentally confirmed.…”

Increasing the spatial resolution of direct laser writing by using a non-Gaussian intensity distribution in the writing laser spot

“…The technology of direct laser writing has proven itself well for the producing of diffractive optical elements and computer-generated holograms (CGH) [1][2][3][4][5]. One of the ways to develop direct laser writing technology is to increase the spatial resolution in the manufacture of microstructured surfaces.…”