Michael T. Gale

Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists

A laser writing system for the fabrication of continuous-relief micro-optical elements in photoresist is described. The technology enables a wide range of planar micro-optical elements to be fabricated and replicated into polymer film using Ni shims electroformed from the photo-resist originals. The advantages and limitations of laser writing technology for micro-optics fabrication are discussed. Examples of fabricated micro-optical elements include Fresnel microlenses and microlens arrays, kinoforms, and other continuous-relief phase elements.

Replication technology for diffractive optical elements

Replication technologies such as embossing, molding and casting in polymer materials are highly attractive for the fabrication of surface-relief diffractive optical elements (DOEs) and are expected to become key technologies for their production in the future. They have very high resolution, typically in the nanometer range, and allow the fabrication of large area, complex microstructure by low-cost, high volume industrial production processes. This paper gives a summary the current main replication technologies for surface microstructure and discusses their extension to a wider range of DOE microstructures, involving developments such as the replication of deeper and higher aspect ration microstructure and the maintenance of an optically precise form for critical surfaces such as imaging DOEs. Examples and recent results for DOE fabrication by replication technology are presented. New possibilities resulting from the combination of replication technology with other processes such as dry etching and thin film coating are discussed.

Optimized kinoform structures for highly efficient fan-out elements

We discuss the realization of highly efficient fan-out elements. Laser-beam writing lithography is available now for fabricating smooth surface relief microstructures. We develop several methods for optimizing microstructure profiles. Only a small number of parameters in the object plane are necessary for determining the kinoform. This simplifies the calculation of M x N arrays also for large M and N. Experimental results for a 9-beam fan-out element are presented.

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of mu > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-mum period to a value of 79%.

Continuous-relief diffractive optical elements for two-dimensional array generation

Continuous surface-relief diffractive optical elements for two-dimensional array generation (fan-out) are designed and fabricated. Separable and nonseparable solutions for the two-dimensional element design are compared. The phase-grating microstructures are generated by laser-beam writing lithography in a single exposure step and converted to nickel shims by electroplating, enabling low-cost replicas to be produced by using laboratory and commercial replication processes. Results are presented for a 9 x 9 fan-out diffractive optical element with a measured efficiency of 94% and an overall uniformity within +/-8%; replicas in epoxy have the same efficiency and a uniformity of +/-15%.

Zero-order diffractive microstructures for security applications

The optical characteristics of diffractive microstructures designed for zero-order read-out are discussed. Such devices offer new optical behaviour which can be exploited for applications in the field of visual and machine readable security features. They can be fabricated by low-cost embossing and evaporation techniques.

Replicated chirped waveguide gratings for optical sensing applications

A novel type of integrated optical sensor chip based on chirped grating couplers is presented. It can be used as a powerful basic building block for realizing miniature sensor modules for measuring and monitoring the values of a wide variety of chemical and physical quantities. The feasibility of fabricating these chips by means of very low-cost processes such as replication and thin film deposition is demonstrated. The most important theoretical fundamentals for calculating the performance of these chips are given. First experimental results, reported for replicated polycarbonate chips used in a flow cell to determine the refractive index of liquids, show their suitability for practical applications.

Grid polarizer for the visible spectral region

We report on the fabrication of a grid polarizer for the visible spectral region, based on metallic grids with periods below 100 nm. Theoretical calculations of the degree of polarization predict useful values for the visible region, if the period is in the range of 100 nm. The properties of the polarizer are shown to be strongly dependent on the optical constants of the metal and the substrate. For the fabrication of the grids, direct e-beam writing in combination with a dry etching process has been used. Metallic grids with 50 nm lines and a period of 100 nm were fabricated on a glass substrate. Measurements of the degree polarization were made at a wavelength of 670 nm. The results are compared to theoretical calculations made in the framework of a rigorous diffraction theory.

The Fabrication Of Fine Lens Arrays By Laser Beam Writing

Lenticular arrays with near diffraction-limited performance have been generated by laser beam writing. A photoresist film is exposed in a line raster mode by scanning under an intensity-modulated focused laser beam; subsequent development of the resist film produces the desired lenticular relief profile. The writing of exposure patterns by raster scanning is analyzed, and it is shown that very precise positioning of each raster line is required to avoid unwanted modulation terms. A low cost xy positioning table with better than 20 nm dynamic positioning accuracy was constructed to satisfy this requirement, and has been used to generate high quality fine lenticular arrays with lenslets of widths down to 20 μm and relief depths up to about 4 μm.

High-efficiency continuous surface-relief gratings for two-dimensional array generation

Continuous surface-relief phase gratings for two-dimensional (2-D) array generation have been realized by laser-beam writing lithography. For a 9 × 9 fan-out element, a diffraction efficiency of 94% and a uniformity of better than ±8% have been achieved. These are, to our knowledge, the best published results for 2-D surface-relief fan-out elements. Separable and nonseparable solutions for the design of 2-D fan-out elements are discussed.