Published papers

A Micro-lens Array (MLA) is an optical element in which multiple lens segments are integrated onto a single substrate. Traditional MLAs consist of segments with the same geometric shape arranged at regular intervals atop a planar surface. They are used for applications that include beam homogenization, to equalize light intensity distribution, and light field imaging to change post-exposure the optical configuration such as viewpoint and focal distance of the captured image data. However, the limitations imposed by the identical segment geometry and the regular arrangement pattern constrain the range of possible applications and their associated performance. In this paper, novel MLA geometries and design methods are introduced that enable new generation products. With this type of MLA, each segment can have a different free planar shape and freeform surface shape in height direction. The segments of MLA are designed with topology optimization method based on internal stresses, that allows unsupervised design of optical segments. The designed freeform segments can maximize the use of light the lens, according to the target illumination pattern and intensity defined by each segment.

Industrial robots have been in increasing demand in recent years, although their lower positioning accuracy and velocity controllability (compared to CNC machine tools) makes it difficult to adopt them in machining processes such as milling. However, precision polishing using compliant tools, fluid jets, etc., belongs to the class of time-dependent processes where the amount of material removed depends on the surface feed rate of tool influence on the workpiece. Therefore, there is a possibility to maintain high accuracy even when using industrial robots, so long as the velocity profile is well predicted. In this paper, the attitude and motion of an industrial robot tool center point is estimated using a stereoscopic high-speed camera setup. 3 light markers are mounted above a fluid jet polishing nozzle attached to the robot end-effector. Their positional relationship to the tool center point is ascertained by collecting posture data while maintaining a precision ball at a fixed location with an R-test device. Next, raster paths are generated and the robot motion is recorded with the cameras. This data is used to predict the surface profile to be generated when polishing a BK7 optical window. Actual polishing confirms a good correlation, within 10 nanometers, between measured and predicted material removal.

Because of their many applications, including automotive, aeronautics, optics, and so forth, demand for grinding tools sees unabated growth. But the energy and resource intensive nature of their manufacture leads to questions of sustainability. This paper details how recent developments in basic tool technology (structure, binder, abrasives) is increasing tool and abrasive resilience while reducing production inputs. The advent of smart grinding tools involving aspects of surface engineering, process hybridization, and process monitoring is also discussed. The paper concludes with an outlook of further energy savings, tool recycling and uses of additive manufacturing with the aim of achieving long-term sustainability.

© 2017 Fluid jet polishing is a versatile process used for super-fine finishing of small and complex optical and prosthetic surfaces. Advantages of this process include highly controllable sub-millimetre polishing footprints and absence of tool wear, though the main drawback is very low material removal rate. To address this issue, a novel system was developed in which ultrasonic cavitation causes micro-bubble generation directly upstream of the nozzle outlet. Experimental data shows that these micro-bubbles boost removal rate by up-to 380%, without causing any degradation of the surface finish. This paper reports on the modelling, implementation, and testing of this new polishing system.

© 2019 The demand for miniaturized products and functionalized parts is increasing, and many such products are made of hard and brittle materials machined by abrasive processes. The first part of this keynote paper is dedicated to a discussion of micro parts and micro structures machined – at least in part – by abrasive processes, followed by a discussion of the associated abrasive processes. The strengths and limitations of these processes, among which include dicing, micro grinding, micro abrasive blasting, and vibration and magnet field assisted finishing are discussed. The paper concludes with a discussion of future trends in the field.

In this paper we address two interrelated issues important to primary mirror segments for extremely large telescopes - edge-control, and the detailed topography over the segment surface. Both affect the intensity and distribution of stray light and infrared emissivity. CNC polishing processes typically deploy spiral or raster tool-paths, tending to leave repetitive features. We compare and contrast two novel families of pseudo-random tool-paths for Precessions CNC polishing. We then show how CNC control of the three-dimensional tool-path can optimize edge-profiles. Finally, we demonstrate fluid-jet polishing used to clean up residual edge defects.

Free-form surfaces look set to revolutionise optical design. We report on recent development of the Precessions process for polishing free-form surfaces from the precision-ground state, and correcting form-errors. On-going experimental results are presented.

© 2020 Elsevier Ltd Time-dependent sub-aperture surface processing is widely used in industry for finishing of optical surfaces. It can often be viewed as the convolution process between a tool influence function (TIF, also known as footprint) and the equivalent dwell time derived from the velocity at discrete locations across the tool path. While the direct convolution problem has been extensively studied through numerical computation, this approach leads to limited and inaccurate prediction of the processed surface. Therefore, this paper proposes a simple and universal analytical model that uncovers the intrinsic relationship between process parameters and processed surface, and which has potential for a wide range of manufacturing processes. Furthermore, sensitivity of the process to TIF fluctuations is considered, which leads to highly accurate stochastic predictions of the processed surface waviness variation by Monte Carlo simulation. Experimental results in fluid jet polishing confirm correctness of the proposed analytical solution and effectiveness of the waviness prediction model. The presented solution provides a better understanding and insight into time-dependent sub-aperture surface processing and opens the door to efficiently tackling the related direct and inverse problem.

Ultra-precision diamond turning can deliver very accurate form, often less than 100nm P-V. A possible manufacturing method for thin Wolter type-1 mirrors in hard X-ray space telescopes thus involves generating electroless nickel plated mandrels by diamond turning, before coating them with a reflective film and substrate. However, the surface texture after turning falls far short from the requirements of X-ray and EUV applications. The machining marks need to be removed, with hand polishing still widely employed. There is thus a compelling need for automated finishing of turned dies. A two step finishing method is presented that combines fluid jet and precessed bonnet polishing on a common 7- axis CNC platform. This method is capable of finishing diamond turned electroless nickel plated dies down to 0.28nm rms roughness, while deterministically improving form error down to 30nm P-V. The fluid jet polishing process, which consists of pressurizing water and abrasive particles for delivery through a nozzle, has been specially optimized with a newly designed slurry delivery unit and computer simulations, to remove diamond turning marks without introducing another waviness signature. The precessed bonnet polishing method, which consists of an inflated membrane rotated at an angle from the local normal to the surface and controlled by geometrical position relative to the work-piece, is subsequently employed with a novel control algorithm to deliver scratch-free surface roughness down to 0.28 nm rms. The combination of these two deterministic processes to finish aspheric and freeform dies promises to unlock new frontiers in X-ray and EUV optics fabrication. © 2012 SPIE.

In recent years, transparent resin organ models created by 3D printing have found application in the medical fields. The inside of the organ model contains diseased parts, blood vessels and so on, but they cannot be seen due to the layered appearance of printed surfaces. To address this issue, an automated system was developed using an industrial robot to replace the manual polishing currently needed. The automated system consists of three aspects: fixturing of the organ model, polishing tool path generation and polishing process conditions. From polishing experiments on a relatively simple shape, it was found that the system has the potential to deliver transparent surfaces on complex resin organ models.

The requirements of space and defence optical systems and ground-based astronomy (especially extremely large telescopes) are providing optical fabricators with new challenges. These challenges particularly concern process speed, determinism and automation, and tighter tolerances on surface form and texture. Moreover, there is a growing demand for complex off-axis and 'freeform' surfaces and for larger components of the ~lm scale. With this in view, we first report on form-correction on a smaller analogue of the LRP1200 - an IRP400 in service in industry. We then report on the design, commissioning and preliminary process-development results from the first of the scaled-up 1.2m capacity CNC polishing machine from Zeeko, Ltd. This machine delivers the 'Classic' bonnet-based process, together with two new processes - fluid-jet polishing and the hybrid soft-grinding/polishing process called 'Zeeko-Grolish.' We indicate how this trio of processes running on the same machine platform with unified software can provide an unprecedented dynamic range in both volumetric removal rate and removal spot-size. This leads into a discussion of how these processes may be brought to bear on optimal control of texture and form. Preliminary performance of the 1.2m machine is illustrated with results on both axially-symmetric and more complex removal regimes. The paper concludes with an overview of the relevance of the technology to efficient production of instrumentation-optics, space optics and segmented telescope mirrors.

© OSA 2016. Shape Adaptive Grinding (SAG) is novel process for rapid finishing of ceramic freeform optics. In this paper, we report results from an investigation into the brittle-ductile transition observable on ceramic materials such as silicon carbide.

Finishing of complex surfaces needs simultaneous movement of multiple axes and precessing the tool on the surface in a desired tool path. For finishing narrow internal surfaces, the conventional spherical or flat end tools are difficult to precess inside the geometry. Therefore, this work presents a new tooling system for grinding narrow channels on tungsten carbide surface, to be used in optical applications. A thin wheel type finishing wheel and related jig is designed to use on an ultra-precision finishing machine. Finishing using the newly developed wheel type tool with different grade shape adaptive grinding pads could generate excellent surface finish within Ra = 3 nm. Form correction capability of the developed tooling system was also verified where the form error of the channel could be reduced to ∼7 lm P-V from an initial error of ∼42 lm P-V, in 5 h of processing time.

Thin films are widely adopted to improve resistance to chemicals, high temperature, etc. However, their hard and brittle nature imposes great challenges to achieve satisfactory surface integrity control in finishing process. To address this, an active bending methodology via piezoelectric actuator is proposed to investigate the thin film response on and beneath the surface, under different stress states. An analytical model is established and verified by experiments, which demonstrate distinct nano scratching regimes on a Si3N4 film. Based on this active stress control approach, macroscale bonnet polishing was conducted and showed great improvement in surface roughness and material removal.