Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce efficient, continuous profile elements (kinoforms). Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed two accessible procedures that minimize fabrication complexity, component turnaround time, and cost. The first technique [Appl. Opt. 34, 7507-7517 (1995)] uses gray-scale masks produced by commercial slide-imager systems. Here, we report on an alternative technique for producing gray-scale masks by spatial filtering of halftone screens. Using the photoreduced gray-scale patterns as lithographic masks, we fabricated diffractiveoptic blazed gratings and lens arrays in both photoresist and quartz. First-order efficiencies as high as 70% are reported. Also, the strengths and limitations of this technique are compared with the previously reported slide-imager method as well as other fabrication methods.

Gray-scale masks for diffractive-optics fabrication: II. Spatially filtered halftone screens.

We demonstrate the fabrication of binary, multilevel, and blazed diffractive structures by a fast and flexible direct-write process by using an excimer-laser-based tabletop micromachining workstation with an integrated optical surface profiler.

Excimer laser micromachining for rapid fabrication of diffractive optical elements

Static and dynamic Fresnel zone lenses were fabricated in quartz glass by means of microstructuring techniques. Two types of on-axis and offaxis lenses with different focal lengths and of different apertures were designed to operate at wavelengths of 1·52 μm and 633 nm. The blazed profile of the onaxis and off-axis lenses was approximated by up to 16 and up to four discrete levels respectively. Dynamic, that is electrically switchable, lenses have been realized by filling the structured surface with liquid crystal. The optical properties of the lenses, such as the focal spot sizes and the diffraction efficiencies, were investigated. Further the switching behaviour of the dynamic lenses was studied. The design and fabrication of the static and dynamic, on-and off-axis Fresnel zone lenses as well as their optical and switching properties will be presented.

Static and dynamic Fresnel zone lenses for optical interconnections

Excimer-laser microetching of a variety of materials is applied to the fabrication of surface-relief optical microstructures of arbitrary morphology, with particular emphasis on computer-generated holographic structures. High-definition, high-radiation-intensity selective laser ablative etching in conjunction with step-and-repeat (period) replication or raster (pixel) scanning is used. To support such developments, the characteristic etching properties of a wide range of solid materials, from metals to semiconductors and polymers, are studied. Optical-interconnect and generic object holograms are produced by means of this alternative one-step holographic information-recording method.

Excimer laser use for microetching computer-generated holographic structures.

A method to compute diffractive elements focusing in a set of scaled lines located in different focal planes along the optical axis is considered. A phase nonlinearity is applied to the phase function of a focusator in a line. It is proved that the selection of the nonlinearity is reduced to the problem of the phase diffraction grating with preset energy distribution in the orders. Multifocal elements to focus in a set of scaled lines in a single plane are considered as well.

Multifocal diffractive elements

The use of thin-film deposition in the fabrication of antireflection-coated diffractive optical elements is discussed. The antireflection coatings for these diffractive elements are optimized on the basis of an angular spectrum approach and the method of characteristic matrices. A minimum reflectivity as low as 1 x 10 -4 is realized using in situ controlled multilayers of TiO 2 and SiO 2 . The blazed profile of the diftractive optical elements is approximated by a stepped profile with up to 32 phase levels. The highest measured diffraction efficiency for 32-level Fresnel zone lenses was 97%.

Antireflection-coated diffractive optical elements fabricated by thin-film deposition

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.

Effect of fabrication errors on multilevel Fresnel zone lenses

Planar optical waveguide structures in communication network applications require a number of indispensable features, e.g., low insertion loss, efficient fiber-to-chip coupling, polarization independent operation, and high integration density, as well as reliable and cost- efficient fabrication. The presented concept of integrated optical waveguide structures is based on the high refractive index material system of silicon-oxynitride on silicon. This system has the potential to meet all mentioned requirements. We describe the design and fabrication of such waveguide structures and demonstrate basic integrated optical components with good performance.

Optical waveguide components with high refractive index difference in silicon-oxynitride for application in integrated optoelectronics

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%.

Diffractive microlenses with antireflection coatings fabricated by thin film deposition

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.

Berndt Kuhlow

Papers

Antireflection-coated diffractive optical elements fabricated by thin-film deposition

Effect of fabrication errors on multilevel Fresnel zone lenses

Optical waveguide components with high refractive index difference in silicon-oxynitride for application in integrated optoelectronics

Diffractive microlenses with antireflection coatings fabricated by thin film deposition

Holographic elements for optical interconnects at 1.5 μm