Adaptive optics for laser processing

The overall aim of the work presented in this thesis is to develop an adaptive optics (AO) technique for application to laser-based manufacturing processes. The Gaussian beam shape typically coming from a laser is not always ideal for laser machining. Wavefront modulators, such as deformable mirrors (DM) and liquid crystal spatial light modulators (SLM), enable the generation of a variety of beam shapes and furthermore offer the ability to alter the beam shape during the actual process. The benefits of modifying the Gaussian beam shape by means of a deformable mirror towards a square flat top profile for nanosecond laser marking and towards a ring shape intensity distribution for millisecond laser drilling are presented. Limitations of the beam shaping capabilities of DM are discussed. The application of a spatial light modulator to nanosecond laser micromachining is demonstrated for the first time. Heat sinking is introduced to increase the power handling capabilities. Controllable complex beam shapes can be generated with sufficient intensity for direct laser marking. Conventional SLM devices suffer from flickering and hence a process synchronisation is introduced to compensate for its impact on the laser machining result. For alternative SLM devices this novel technique can be beneficial when fast changes of the beam shape during the laser machining are required. The dynamic nature of SLMs is utilised to improve the marking quality by reducing the inherent speckle distribution of the generated beam shape. In addition, adaptive feedback on the intensity distribution can further improve the quality of the laser machining. In general, beam shaping by means of AO devices enables an increased flexibility and an improved process control, and thus has a significant potential to be used in laser materials processing

Optical and single element transducers for the generation of arbitrary acoustic fields

Precise control over the temporal and spatial properties of acoustic fields in 2 or 3-D is essential for nearly all modern, biomedical applications of ultrasound. At present, piezoelectric arrays dominate, however, despite their ubiquity they have a number of drawbacks that compromise the fidelity with which the output field can be manipulated, particularly at high frequencies and in three dimensions. The development of new novel alternatives for manipulating acoustic fields in 3-D is therefore essential. This thesis presents several new techniques through which this can be achieved using both the optical generation of ultrasound and single element piezoelectric transducers. First, the use of multiple Q-switch laser sources in combination with binary amplitude holograms is investigated for the generation of single and multi-focal acoustic fields. The conditions required for the generation of a focus are established numerically and the method is validated experimentally. Next, two approaches are developed for the generation of arbitrary spatial distributions of pressure using a single optical pulse. The first employs multi-layer optical absorbers: structures composed of several absorbing layers each individually patterned such that the field constructively interferes at a set of target points. The second uses tailored optically absorbing surface profiles: arbitrary surface shapes, fabricated through 3-D printing, designed to geometrically focus over a continuous pattern. Finally, the last chapter of the thesis investigates the use of multi-frequency kinoforms for mapping the field of single element piezoelectric transducers onto multiple complex target distributions. The properties of these kinoforms are explored in depth numerically and experimentally it is shown that multiple complex distributions can be generated in a target plane using this approach

AN ITERATIVE INTERLACING APPROACH FOR SYNTHESIS OF COMPUTER-GENERATED HOLOGRAMS

A new approach to optimizing computer-generated holograms (CGH\u27s) is discussed. The approach can be summarized most generally as hierarchically designing a number of holograms to add up coherently to a single desired reconstruction. In the case of binary holograms, this approach results in the interlacing (IT) and the iterative interlacing (IIT) techniques. In the IT technique, a number of subholograms are designed and interlaced together to generate the total binary hologram. The first sttbhologram is designed to reconstruct the desired image. The succeeding subholograms are designed to correct the remaining error image. In the IIT technique, the remaining error image after the last subhologram is circulated back to the first subhologram, and the process is continued a number of sweeps until convergence. The IT and the IIT techniques can be used together with most CGH synthesis algorithms, and result in substantial reduction in reconstruction error as well as increased speed of convergence in the case of iterative algorithms

Review of iterative Fourier-transform algorithms for beam shaping applications

We present a comparison of some of the most used iterative Fourier transform algorithms (IFTA) for the design of continuous and multilevel diffractive optical elements (DOE). Our aim is to provide optical engineers with advice for choosing the most suited algorithm with respect to the task. We tackle mainly the beam-shaping and the beam-splitting problems, where the desired light distributions are almost binary. We compare four recent algorithms, together with the historical error-reduction and input-output methods. We conclude that three of these algorithms are interesting for continuous-phase kinoforms, and two, namely the three-step method proposed by Wyrowski and the over-compensation of Prongué, still perform well with multilevel- and binary-phase DOE. © 2004 Society of Photo-Optical Instrumentation Engineers

Point Spread Function and Modulation Transfer Function Engineering

A novel computational imaging approach to sensor protection based on point spread function (PSF) engineering is designed to suppress harmful laser irradiance without significant loss of image fidelity of a background scene. PSF engineering is accomplished by modifying a traditional imaging system with a lossless linear phase mask at the pupil which diffracts laser light over a large area of the imaging sensor. The approach provides the additional advantage of an instantaneous response time across a broad region of the electromagnetic spectrum. As the mask does not discriminate between the laser and desired scene, a post-processing image reconstruction step is required, which may be accomplished in real time, that both removes the laser spot and improves the image fidelity. This thesis includes significant experimental and numerical advancements in the determination and demonstration of optimized phase masks. Analytic studies of PSF engineering systems and their fundamental limits were conducted. An experimental test-bed was designed using a spatial light modulator to create digitally-controlled phase masks to image a target in the presence of a laser source. Experimental results using already known phase masks: axicon, vortex and cubic are reported. New methods for designing phase masks are also reported including (1) a numeric differential evolution algorithm, (2) a “PSF reverse engineering” method, and (3) a hardware based simulated annealing experiment. Broadband performance of optimized phase masks were also evaluated in simulation. Optimized phase masks were shown to provide three orders of magnitude laser suppression while simultaneously providing high fidelity imaging a background scene

Fundamentals of phase-only liquid crystal on silicon (LCOS) devices

This paper describes the fundamentals of phase-only liquid crystal on silicon (LCOS) technology, which have not been previously discussed in detail. This technology is widely utilized in high efficiency applications for real-time holography and diffractive optics. The paper begins with a brief introduction on the developmental trajectory of phase-only LCOS technology, followed by the correct selection of liquid crystal (LC) materials and corresponding electro-optic effects in such devices. Attention is focused on the essential requirements of the physical aspects of the LC layer as well as the indispensable parameters for the response time of the device. Furthermore, the basic functionalities embedded in the complementary metal oxide semiconductor (CMOS) silicon backplane for phase-only LCOS devices are illustrated, including two typical addressing schemes. Finally, the application of phase-only LCOS devices in real-time holography will be introduced in association with the use of cutting-edge computer-generated holograms.This is the final version. It has been published by NPG in Light: Science & Applications here: http://www.nature.com/lsa/journal/v3/n10/full/lsa201494a.html

Photochemisch strukturierte computergenerierte Hologramme in Bakteriorhodopsin-Schichten

Ziel dieser Dissertation ist die Untersuchung des photochemischen Aufzeichungsprozesses von computergenerierte Hologrammen (CGH's) innerhalb des Biomaterials Bakteriorhodopsin (BR). Dabei werden folgende Schwerpunkte detailliert analysiert: - Implementierung und Untersuchung verschiedene Algorithmen zur Berechnung von CGH's. Die Algorithmen sind insbesondere hinsichtlich der Verwendung des Biomaterials BR als Hologrammedien zu beurteilen. - Untersuchung des Aufzeichungsprozesses von CGH's mittels eines direkten Laserschreibsystems. Bei den CGH-Herstellungsverfahren werden die physikalischen und biochemischen Eigenschaften von BR gezielt ausgenutzt, um die für die CGH-Aufzeichnung optimale Oberflächenprofilierung und/oder Brechungsindexmodulation zu erreichen