Roland Bitterli

Fabrication and characterization of linear diffusers based on concave micro lens arrays

We present a new approach of beam homogenizing elements based on a statistical array of concave cylindrical microlens arrays. Those elements are used to diffuse light in only one direction and can be employed together with flys eye condensers to generate a uniform flat top line for high power coherent light sources. Conception, fabrication and characterization for such 1D diffusers are presented in this paper.

Refraction limit of miniaturized optical systems: a ball-lens example

We study experimentally and theoretically the electromagnetic field in amplitude and phase behind ball-lenses across a wide range of diameters, ranging from a millimeter scale down to a micrometer. Based on the observation, we study the transition between the refraction and diffraction regime. The former regime is dominated by observables for which it is sufficient to use a ray-optical picture for an explanation, e.g., a cusp catastrophe and caustics. A wave-optical picture, i.e. Mie theory, is required to explain the features, e.g., photonic nanojets, in the latter regime. The vanishing of the cusp catastrophe and the emergence of the photonic nanojet is here understood as the refraction limit. Three different criteria are used to identify the limit: focal length, spot size, and amount of cross-polarization generated in the scattering process. We identify at a wavelength of 642 nm and while considering ordinary glass as the ball-lens material, a diameter of approximately 10 µm as the refraction limit. With our study, we shed new light on the means necessary to describe micro-optical system. This is useful when designing optical devices for imaging or illumination.

Multifunctional Micro-Optical Elements for Laser Beam Homogenizing and Beam Shaping

Refractive, diffractive and reflective micro-optical elements for laser beam shaping and homogenizing have been manufactured and tested. The presented multifunctional optical elements are used for shaping arbitrary laser beam profiles into a variety of geometries like, a homogeneous spot array or line pattern, a laser light sheet or flat-top intensity profiles. The resulting profiles are strongly influenced by the beam properties of the laser and by diffraction and interference effects at the micro-optical elements. We present general design rules for beam shaping and homogenizing. We demonstrate the application of such multifunctional micro-optical elements for a variety of applications from micro-laser machining to laser diagnostic systems.

Refractive statistical concave 1D diffusers for laser beam shaping

Certain high power laser applications require thin homogeneous laser lines. A possible concept to generate the necessary flat-top profile uses multi-aperture elements followed by a lens to recombine separated beamlets. Advantages of this concept are the independence from entrance intensity profile and achromaticity. However, the periodic structure and the overlapping of beamlets produce interference effects especially when highly coherent light is used. Random optical elements that diffuse only in one direction can reduce the contrast of the interference pattern. Losses due to undesired diffusion in large angles have to be minimized to maintain a good quality and high efficiency of beam shaping. We have fabricated diffusers made of fused silica for a wide range of wavelengths that diffuse only in one direction. Structures are based on an array of concave cylindrical microlenses with locally varying size and position following a well defined statistical distribution. The scattering angle can be influenced by process parameters and is typically between 1° and 60°. To predict the influence of process parameters on the optical properties, a simplified model for the fabrication process and geometrical optics have been used. Characterization of the fabricated devices was done by stylus measurements for the surface shapes, microinterferometry to measure phase profiles and high resolution goniometry to obtain far field distribution of light. The simulated data compare very well to measured optical properties. Based on our simulation tool we discuss limits of our fabrication method and optimal fabrication parameters.

Miniaturized concentrator arrays as compact angle transformers for light collection and distribution

Efficient light management is one of the key issues in modern energy conversion systems, might it be to collect optical power or to redistribute light generated by high power light emitting diodes. One problem remains: How can one realize small size elements with high quality of light management. We propose a novel scheme by using miniaturized angle transformers or concentrators that have size of several millimeters. none In this size range diffraction effects play rarely a role and the design can be based on classical ray tracing. Dimensions are chosen to allow effective solution for high power light emitting diodes as well as solar cells. In most solar cell designs, the photocurrent is extracted through a conducting window layer in combination with a silver grid at the front of the device. The trade-off between series resistance and shadowing requires either buried contacts or screen printing of narrow lines with high aspect ratio. We propose an alternate approach where an array of parabolic concentrators directs the incoming light into the cell. The front metallization can thus be extended over the area between the paraboloids without shadowing loss. High power light emitting diodes are source with certain far field distribution and composed often out of several chips. Applying the concentrator array technology not on the whole source but locally on each chip promises small and effective solutions. We demonstrate realization of linear and hexagonal arrays of micro-concentration systems, discuss details of application and results of simulation of their optical properties in applications.

Shaping Light with MOEMS

Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for shaping light in the space and time domain.

Liquid as template for next generation micro devices

Liquids have fascinated generations of scientists and engineers. Since ancient Greece, the perfect natural shape of liquids has been used to create optical systems. Nowadays, the natural shape of liquid is used in the fabrication of microlens arrays that rely on the melting of glass or photoresist to generate high quality lenses. However shrinkage normally associated to the liquid to solid phase transition will affect the initial shape and quality of the liquid structure. In this contribution, a novel fabrication technique that enables the encapsulation and replication of liquid templates without affecting their natural shape is presented. The SOLID (SOlid on LIquid Deposition) process [1] allows for a transparent solid film to be deposited and grown onto a liquid template (droplet, film, line) in a way that the liquid shapes the overgrowing solid layer. The resulting configuration of the SOLID devices is chemically and mechanically stable and is the base of a huge variety of new micro-nano systems in the field of microfluidics, biomedical devices and micro-optics among others. The SOLID process enables in a one step process the encapsulation of liquid microlenses, fluidics channels, drug reservoir or any naturally driven liquid structure. The phenomenon and solid-liquid interface resulting from the SOLID process is new and still unexploited. The solid layer used for the SOLID process chosen in this paper is poly-para-xylylene called Parylene, a transparent biocompatible polymer with excellent mechanical and chemical properties. Moreover, as the solid layer is growing over a liquid template, atomically smooth surfaces channels can be obtained [2]. The polymerization of Parylene does not exert stress and does not change the shape of the liquid; this latter aspect is particularly interesting for manufacturing naturally driven liquid structures. In this paper the authors explore the limits of this new method by testing different designs of SOLID encapsulated structures and their potential to deliver next generation micro devices.

Dynamically deformable reflective membrane for laser beam shaping and smoothing

We show a laser beam shaping device made of a deformable continuous reflective membrane fabricated over a scanning stage. The combination of two actuator schemes enables shaping and smoothing of a laser beam with a unique compact device. It is designed to shape an input laser beam into a flat top or Gaussian intensity profile, to support high optical load and to potentially reduce speckle contrast. One single electrode is needed to deform the whole membrane into multiple sub-reflecting concave elements. The scanning stage is used simultaneously to smooth out the remaining interference patterns. The fabrication process is based on SOI wafer and parylene refilling to enable the fabrication of a 100 % fill factor 5 by 5 mm2 deformable membrane. Applications for such device are laser machining and laser display.

Deformable silicon membrane for dynamic linear laser beam diffuser

We present a dynamic laser beam shaper based on MEMS technology. We show a prototype of a dynamic diffuser made of single crystal silicon. A linearly deformable silicon micromembrane is used to diffuse a laser beam in one dimension. Resonance frequencies of the membrane can range from 1 kHz to 20 kHz. Mode shapes of the deformable mirror are excited using magnetic actuation. Diffusing angle can be tuned by adjusting the driving current in the membrane. We measured a diffusing angle of 1° for an applied current of 40 mA. The aluminum coated mirror can handle 140 W/cm2 of visible to infrared optical power. Application to smooth out interference pattern generated by a static diffuser is shown.

Dynamic array spot shaping for laser micro machining

Laser manufacturing of microstructures using a single focus is a well known technology. Multi-spot optics are applied for process parallelizing if the demand on throughput in mass production rises or large areas of material have to be processed. Diffractive optical elements (DOEs) are used for parallel laser processing of a repetitive structure. These elements split the beam into a periodic spot pattern, where each spot shows the same shape and energy. This allows simultaneous manufacturing of several equal shaped structures at the same time. For patterning a surface this is state of the art and the appropriate technique to reduce processing time while maintaining a high lateral resolution as well as a good relative positioning of the structure due to the DOE. We investigate the usage of microlens arrays as multifunctional elements for forming an arbitrary shaped laser beam into a spot-, a ring-spot- or a line-array pattern. It can be shown that the intensity distribution of each spot is equal to the intensity distribution of all other spots in the whole pattern. The shape of the spots is defined by the angular distribution of the incident beam. We demonstrate that besides other optical properties the output beam profile strongly depends on the Fresnel-Number and is influenced by diffraction and interference effects. We present important design rules which consider geometrical and physical optics. The properties of the spot arrays, like spot diameter, Rayleigh length and beam divergence in dependency of beam and system properties are investigated. Finally we will show some laser micro structuring and micro drilling results in different materials.