Markus Rossi

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.

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%.

Refractive and diffractive properties of planar micro-optical elements

The refractive and the diffractive properties of planar micro-optical elements are investigated. The transition between purely refractive and purely diffractive planar microlenses is numerically simulated for the example of differently designed phase-matched Fresnel elements. Results obtained from numerical simulations and experiments show that the refractive and diffractive types exhibit a distinctly different behavior in the presence of small fabrication errors or wavelength deviations. Based on these results, design rules for various applications, including low- and high-numerical-aperture lenses and hybrid refractive-diffractive elements, are derived. For a high-numerical-aperture (ƒ /# = 1.0) lens the experimental characterization of the irradiance distribution in the image space is presented and shown to agree well with theoretical predictions.

Single-point diamond turning and replication of visible and near-infrared diffractive optical elements.

High-fidelity diffractive surfaces have been generated with single-point diamond-turning techniques. A key to the success of this technique is the ability to shape the diamond tool tip to provide the optimum phase-relief profile, given manufacturing constraints. Replication technology is used to transfer the phase-relief surface into a thin epoxy or photopolymer layer on a glass substrate. Diffraction efficiency results for a wide range of zone widths are presented to provide the reader with a baseline of expected performance for replicated visible and near-infrared diffractive optical elements. In addition, a new method for analyzing diffractive surface structures is presented. The ray-trace algorithm quickly provides accurate results of predicted diffraction efficiency for arbitrary zone profiles, which is extremely valuable in predicting manufacturing errors.

Efficient beam shaping of linear, high-power diode lasers by use of micro-optics

We have designed, fabricated, and characterized a micro-optical beam-shaping device that is intended to optimize the coupling of an incoherent, linearly extended high-power diode laser into a multimode fiber. The device uses two aligned diffractive optical elements (DOEs) in combination with conventional optics. With a first prototype, we achieved an overall efficiency of 28%. Straightforward improvements, such as antireflective coatings and the use of gray-tone elements, are expected to lead to an efficiency of approximately 50%. The device is compact, and its fabrication is suited for mass production at low cost. This micro-optical device, used in a range-finder measurement system, will extend the measurement range. In addition to the direct laser writing technique, which was used for fabrication of the DOEs of the prototype, we applied two other technologies for the fabrication of the micro-optical elements and compared their performance. The technologies were multiple-projection photolithography in combination with reactive-ion etching in fused silica and high-energy beam-sensitive glass gray-tone lithography in photoresist. We found that refractive-type elements (gray tone) yield better efficiency for large deflection angles, whereas diffractive elements (multilevel or laser written) give intrinsically accurate deflection angles.

Arrays of anamorphic phase-matched Fresnel elements for diode-to-fiber coupling

A method for designing microlens arrays that inherently takes into account application requirements and fabrication constraints is presented. Elements with numerical apertures of up to 0.5 have been designed and fabricated by laser beam writing in photoresist and replication in plastic material. In a laser-diode-to-fiber array coupling experiment, an overall optical throughput of 60% was achieved. By means of anamorphic microlens arrays, correction of the laser-diode longitudinal astigmatism and circularization of the image-plane irradiance distribution are demonstrated.

Wafer scale micro-optics replication technology

For many high-volume applications of micro-optical elements and systems the most cost-effective fabrication technology is replication in polymer materials with techniques such as UV embossing, hot embossing, and injection molding. Replication significantly reduces the cost in volume production in comparison to silicon-based etched components. However, the temperature and humidity stability of most commercial polymers is not suitable for the application of replicated elements in areas such as telecom or datacom. A process based on UV-replication in chemically durable polymers has been developed. Technologies for all fabrication steps from mastering over tooling to replication on wafer-scale, post-processing and characterization are described. We present results of various projects with double-sided micro-optics for telecom/datacom and various sensor applications.

Design and fabrication technologies for ultraviolet replicated micro-optics

A wafer scale UV replication process is described that is suitable for mass production of micro-optical components. The fabrication process is divided into two phases. In phase 1, the design and mastering is done and a wafer-sized embossing tool is fabricated. In phase 2 of the process chain, the batch fabrication process is performed. From a single embossing tool, several identical wafer-sized replicas are fabricated that are then diced into the final components. Single-sided as well as double-sided replicas can be made. The economical flexibility of the UV replication chain is demonstrated through two application examples: High-quality components that pass strict environmental tests, which compete with etched silica structures, and low-cost micro-optical components, which compete with microinjection molded parts. All UV-replicated components have in common that they exhibit excellent optical quality and pass an IR reflow process.

Replicated micro-optics for automotive applications

Replicated micro-optics is playing an increasingly important role in illumination and sensing systems in automobiles. The introduction of new design methods and improvements in materials and production technology has led to components which can offer superior performance, size and weight compared with classical optical elements. Diffractive Optical Elements (DOEs) for applications such as beam shaping can achieve optical performance which is not possible with conventional optics. Beam forming elements for use with red and white LEDs play a major role in automotive optics. Customised DOEs can offer significantly more design flexibility and functionality over Fresnel lenses for the complex optical system based on a single or multiple LED source with reflector and wavelength converting resin. Thinner modules and improved efficiency are achieved. With CMOS imager sensors, micro-optical lenslet arrays can improve the effective sensitivity by many factors. UV-embossing and injection moulding are used to produce components in high volumes at low production costs. Replicated mounting and alignment features reduce assembly costs. New materials and processes have been developed to enable wafer-scale production by UV-embossing, producing glass-like components with excellent humidity and elevated temperature stability as well as IR-reflow process compatibility.