Margit Ferstl

Effect of fabrication errors on multilevel Fresnel zone lenses

Blazed Fresnel zone lenses for the 1.5-μm wavelength were fabricated in quartz glass by means of microstructuring technology. The blazed profile in each zone of the lenses was approximated by two, four, and eight discrete levels. The effects of fabrication errors, such as depth and alignment errors, on the diffraction efficiency of the different Fresnel zone lenses were investigated. Further the location and intensity of the parasitic foci appearing due to the discrete level approximation are calculated. Theoretical results along with experimental measurements are presented.

Diffractive microlenses with antireflection coatings fabricated by thin film deposition

Two-dimensional arrays of Fresnel zone microlenses were fabricated and coated with antireflection layers by an ion beam sputter deposition technique. The reflection of these lenses was analyzed on the basis of an angular spectrum approach for different substrate materials. A minimum reflectivity as low as 2 x 10-4 was realized by means of in situ controlled multilayers of TiO2 and SiO2. The lenses have a circular aperture of 2 mm and different focal lengths for the wavelengths of 1.52 and 0.63 μm, respectively. The kinoform profile in each zone of the Fresnel zone lenses was approximated by an eight-level profile. Such stepped profiles were realized with several masks written with an electron beam and transferred by photolithographic technology. Our measurements reveal that the spot sizes of the fabricated microlenses are close to the diffraction-limited values, and the highest measured diffraction efficiencies for the eight-level structures are greater than 80%.

Integrated optical pickup system for axial dual focus

We propose an optical pickup that acquires data from both layers of a dual-layer digital versatile disk simultaneously. An adaptive optical element that uses liquid crystals creates two axial foci separated by a spacing of 55 mum, which is the distance between the two layers. The spacing between the foci can be varied by the adaptive element. The separation of the reflected light into TE and TM polarized light, corresponding to each of the layers, is made by dielectric gratings that are characterized by high aspect ratios. Electron-beam lithography and reactive ion etching techniques were used to produce the submicrometer structures. All fabricated elements were assembled in a pickup system, whose properties were measured.

High-frequency gratings as polarization elements

Binary gratings with feature sizes smaller than the illumination wavelength were fabricated in quartz glass by means of microstructuring techniques. Using rigorous coupled wave analysis polarization elements like polarizing beam splitters and phase retardation plates were designed for operation in transmission at the wavelength of 650 nm. High frequency polarization gratings with feature sizes down to 140 nm and aspect ratios up to 7 were realized. For the polarization selective beam splitting elements we measured diffraction efficiencies of about 80% in the -1st order for TE polarization, and 90% in the 0th order for TM polarization. The values are in good agreement with the theoretical values. Furthermore we realized phase retarding elements e.g. (lambda) /8-plates which showed a phase difference of (Phi) equals 44.8 degree(s) ((Phi) theor. equals 45 degree(s)) between TE and TM polarized light. The design and the fabrication process as well as the optical properties of our high frequency binary phase gratings will be presented. Experimental results will be compared with theoretical values.

Holographic elements for optical interconnects at 1.5 μm

Holographic optical elements for interconnecting electronic switching stages with light of 1.5 micrometers wavelength are presented. These elements include deflection holograms recorded in dichromated gelatin for deflecting the light and diffractive spherical gratings fabricated by microstructuring for focusing and collimating the light. The diffraction efficiency of these elements can reach 90% and focused spot sizes can be within the diffraction limit.

High-efficiency computer-generated holograms

Various diffractive optical elements have been fabricated for the visible wavelength region, mainly for laser beam splitting purposes, but also for the generation of arbitrary but predetermined intensity patterns (e.g. spirals, logos etc.). To obtain high efficiencies the computer-generated holograms were realized as transmissive diffractive phase elements (DPE). More detailed we report on a beam splitter which was intended to distribute an incoming laser beam into 40 partial beams of equal intensities arranged equidistantly on a circle. These circular beam splitters--designated to be used in a measuring system--were realized as binary phase elements. In addition DPEs, that generate a given arbitrary intensity pattern, were produced in 2- and 8-level approximation. The computer-generated phase elements with feature sizes down to the sub-micrometer range were fabricated in quartz-glass by means of microstructuring techniques. Due to our precise and well developed processes we realized binary- and multilevel microstructures of high optical quality. For the binary 1:40 beam splitters we reached diffraction efficiencies of about 60% and uniform spot intensities of better than +/- 2.5% of the average intensity value. The measured efficiency of the eight-level pattern generators was higher than 80%. The optical characterization of our components showed a good agreement with the results expected from simulations. Using simple embossing techniques we were able to replicate first test samples in organic polymers which showed good optical performances.

Blazed Fresnel zone lenses approximated by discrete step profiles: effects of fabrication errors

Blazed Fresnel zone lenses for 1.5 micrometers wavelength were fabricated in quartz glass by means of microstructuring technology. The blazed profile in each zone of the lenses was approximated by 2, 4, and 8 discrete levels. The effects of fabrication errors, such as depth and alignment errors, on the diffraction efficiency of the different Fresnel zone lenses were investigated. Further the location and intensity of the parasitic foci appearing due to the discrete level approximation are calculated. Theoretical results along with experimental measurements are presented.

Integrated optical pickup for optical disks

We report on a new integrated optical pickup for double layer DVDs. The optics is almost integrated by means of diffractive optical elements. Dual focus as well as focal control is done by a liquid crystal cell.

Multilevel Fresnel zone lenses capable of being fabricated with only one binary mask

Multi-level approximated Fresnel zone lenses with reduced level numbers in the outer zones are investigated and compared with Fresnel zone lenses of unique level numbers over the whole lens. Calculations of the fabrication error effects on diffraction efficiency for both lens types are performed. Measurements of focussing efficiency show that. especially for Gaussian beam illumination, segmented Fresnel zone lenses can reach nearly as high focussing efficiencies as normal Fresnel zone lenses. Arrays of segmented lenses can be fabricated using only one binary Fresnel zone lens mask with the aid of a modified optical stepper with fixed scaling down factors in the ratio of 1: 1/(root)2 from one to the next step of pattern transfer in the lithographic process. The fabrication of such lenses could be advantageous because the generation of precise e-beam written masks with a large number of binary ring zones is time consuming and expensive.

Antireflection-coated arrays of binary lenses for interconnection networks at 1.5 μm

Arrays of two different sizes, each consisting of 16 by 16 blazed Fresnel zone lenses, were fabricated in quartz glass by means of microstructuring techniques. The lenses were designed for free space optical interconnection networks operating in the IR. In order to achieve high diffraction efficiencies the kinoform profile of the microlenses was approximated by a staircase-like profile. The fabrication of the lenses involved multiple steps of repeated pattern transfer by photolithography and successive reactive ion etching. We fabricated Fresnel zone lenses with 16 levels per zone and with different focal lengths. The individual elements of the two arrays have circular and square apertures with a diameter of 2 mm and 200 micrometers respectively and were designed to operate at a wavelength of 1.52 micrometers .