DEVELOPMENT AND APPLICATION OF ON-MACHINE SURFACE MEASUREMENT FOR ULTRA-PRECISION TURNING PROCESS

Optical freeform components, featured with high functional performance, are of enormous demand in advanced imaging and illumination applications. However, the geometrical complexity and high accuracy demand impose considerable challenges on the existing ultra-precision freeform machining technologies. Surface measurement and characterisation become the key to further improving machining performance. In order to further increase the metrology availability and efficiency, a shift in the approach of surface metrology from offline lab-based solutions towards the use of metrology upon manufacturing platforms is needed. On-machine surface measurement (OMSM) will not only allow the assessment of manufactured surfaces just-in-time without transportation and repositioning, but also provide feedback for process optimization and post-process correction with consistent coordinate frame. In the thesis, a single point robust interferometer is integrated onto a diamond turning lathe to establish the metrology-embedded ultra-precision manufacturing platform. To extract a priori information for the subsequent OMSM, a theoretical and experimental study of surface generation was carried out for ultra-precision turning of optical freeform surfaces. With the proposed machining methodology and surface generation simulation, two freeform surfaces (sinusoidal grid and micro-lens arrays) were successfully fabricated using the slow tool servo technique. The machined topography of freeform surfaces was uniformly distributed and in agreement with simulated results. Since it operates in the manufacturing environment, the machine tool effects on the OMSM were comprehensively evaluated, including on-machine vibration test, machine kinematic error mapping and linearity error calibration. A systematic calibration methodology for single point OMSM was proposed. Both theoretical and experimental investigation have been conducted to prove the validity of the proposed calibration methodology and the effectiveness of OMSM. With the aid of OMSM, potential applications were explored to exploit the integration benefits to further enhance the ultra-precision machining performance. OMSM integration will increase the automation level of the manufacturing. As OMSM preserves the coordinate system between the machining and measurement, the process investigation can be carried out in a more deterministic manner. The effect of process parameters on the surface form errors was investigated for ultra-precision cylindrical turning process. An empirical model based on response surface methodology has been established and validated with the experimental results. Moreover, a corrective machining methodology was proposed to further improve the accuracy of diamond turned surfaces with OMSM. According to different correction tasks, corresponding OMSM data processing methods were presented. Profile and surface correction experiments were performed to validate the proposed corrective machining methodology and 40% improvement of surface accuracy was achieved

Femtosecond Laser Patterned Templates and Imprinted Polymer Structures

Femtosecond laser machining is a direct-write lithography technique by which user-defined patterns are efficiently and rapidly generated at the surface or within the bulk of transparent materials. When femtosecond laser machining is performed with tightly focused amplified pulses in single-pulse mode, transparent substrates like fused silica can be surface patterned with high aspect ratio (\u3e10:1) and deep (\u3e10 μm) nanoholes. The main objective behind this dissertation is to develop single-pulse amplified femtosecond laser machining into a novel technique for the production of fused silica templates with user-defined patterns made of high aspect ratio nanoholes. The size of the nanoholes, both lateral and vertical, is controlled to a certain degree by controlling laser machining parameters or by chemical etching in a post-machining treatment. Fused silica templates produced by this new technique, both as-machined and chemically etched, are shown to be useful for imprinting polymer structures by a simple replication procedure using polymer thin films or solutions. In particular, a solution-based replication procedure, termed solution casting, is developed to imprint polymer structures from fused silica templates. Polymer structures in the form of nanowires, nanocones, and micropillars are successfully imprinted from various polymer types. Imprinted polymer structures are easily functionalized by subsequent surface treatment processes like cryogenic sputter coating and vapor deposition. A novel low-temperature chemical vapor deposition process is developed to coat polymer nanowires with silica to produce silica nanoneedles. Silica nanoneedles thus produced are shown to be useful as synthetic cell culture substrates to study the behavior of NIH 3T3 fibroblasts. In the final part of this dissertation, a report is given on more in-depth collaborative experiments to study the role of optical aberrations as part of the mechanism for producing high aspect ratio nanoholes by single-pulse amplified femtosecond laser machining. The results indicate that (i) precise optical alignment of the focusing lens is needed to avoid coma, which significantly deteriorates the ability to produce nanoholes, and (ii) 10-micron deep nanoholes can be produced by focusing a beam without spherical aberration but even deeper nanoholes are formed when the beam is focused with undercorrected spherical aberration

Aspheric geodesic lenses for an integrated optical spectrum analyser

Abstract available p. xiii-xi

Development of a measurement of upper MID frequency errors on arbitrary grazing incidence optics

Two state-of-the-art subsystems (detection and servo) were developed and performance generally more than an order of magnitude better than comparable commercial subsystems was achieved. Most importantly, though, the breadboard instrument measured more than an order of magnitude more accurately than the original 1 Angstrom goal within the primary spatial period range. Extensive measurements were taken, and a useful set of Phase 3 modifications were defined

Electrostatic discharge and roughness modelling in diamond turning of contact lenses

With the increased application of ultra-high precision machining of polymers and the limited research in single point diamond turning (SPDT) of contact lens polymers, it became imperative to gather understanding on the production of contact lenses using the above-mentioned technology. A limiting factor in SPDT of polymers is wear of the diamond tool, resulting into poor surface finish due to unintended charges generated as a result of the contact/rubbing action between the cutting tool and the cut material. Central Composite Design (CCD) Face Centred experimental design was developed and applied to the SPDT of ONSI-56 and Polymethly methacrylate (PMMA) contact lens buttons. An electrostatic sensor coupled to a computer monitored the electrostatic discharge generated and a profilometer measured the surface roughness. The Response Surface Method (RSM) was utilised during the development of predictive models for both the surface roughness and the electrostatic discharge generated, to deduce the effects of cutting parameters during machining. The cutting speed and the feed rate deemed as the influential parameters on the surface roughness and electrostatic discharge, for both materials. The depth of cut induced more charge generation for PMMA. Predictive models were successfully developed and they were aimed at creating a database a guide to the SPDT of contact lens polymers

Microfabrication of photonic devices for mid-infrared optical applications

This thesis details research into the microfabrication of photonic devices for mid-infrared optical applications using the technique of ultrafast laser inscription. A diverse range of devices and materials is explored, including the first fabrication and development of an ultrafast laser inscribed mid-infrared waveguide laser source in thulium-doped sesquioxide ceramic gain media. The source produced 81 mW of output power at 1942 nm with a maximum slope efficiency of 9.5% demonstrating progress towards compact, low-threshold and efficient ultrafast laser written waveguide laser sources near 2 μm with the potential for high pulse repetition rate and ultrashort pulse operation. Also shown is the first demonstration of ultrafast laser inscription enabled selective chemical etching of chalcogenide glass. Investigations into the etching of modified regions in gallium lanthanum sulphide glass showed they could be etched at a rate ~13.3 times greater than the un-modified bulk. This result was explored further as a potential route to the production of optofluidic sensors for gas, liquid chemical or biomedical samples. The first demonstration and characterisation of ultrafast laser written waveguides in the chalcogenide glass GASIR-1 is also described. The waveguides were employed for chip scale supercontinuum generation producing the broadest and deepest Infrared supercontinuum from an ultrafast laser inscribed waveguide to-date, spanning ~4 μm from 2.5 to 6.5 μm, which has applications in remote sensing. Finally, the design, build and commissioning of an advanced laser processing setup suitable for ultrafast laser inscription is also detailed

Ultra-high precision machining of contact lens polymers

Contact lens manufacture requires a high level of accuracy and surface integrity in the range of a few nanometres. Amidst numerous optical manufacturing techniques, single-point diamond turning is widely employed in the making of contact lenses due to its capability of producing optical surfaces of complex shapes and nanometric accuracy. For process optimisation, it is ideal to assess the effects of various conditions and also establish their relationships with the surface finish. Presently, there is little information available on the performance of single point diamond turning when machining contact lens polymers. Therefore, the research work undertaken herewith is aimed at testing known facts in contact lens diamond turning and investigating the performance of ultra-high precision manufacturing of contact lens polymers. Experimental tests were conducted on Roflufocon E, which is a commercially available contact lens polymer and on Precitech Nanoform Ultra-grind 250 precision machining. Tests were performed at varying cutting feeds, speed and depth of cut. Initial experimental tests investigated the influence of process factors affecting surface finish in the UHPM of lenses. The acquired data were statistically analysed using Response Surface Method (RSM) to create a model of the process. Subsequently, a model which uses Runge-Kutta’s fourth order non-linear finite series scheme was developed and adapted to deduce the force occurring at the tool tip. These forces were also statistically analysed and modelled to also predict the effects process factors have on cutting force. Further experimental tests were aimed at establishing the presence of the triboelectric wear phenomena occurring during polymer machining and identifying the most influential process factors. Results indicate that feed rate is a significant factor in the generation of high optical surface quality. In addition, the depth of cut was identified as a significant factor in the generation of low surface roughness in lenses. The influence some of these process factors had was notably linked to triboelectric effects. This tribological effect was generated from the continuous rubbing action of magnetised chips on the cutting tool. This further stresses the presence of high static charging during cutting. Moderately humid cutting conditions presented an adequate means for static charge control and displayed improved surface finishes. In all experimental tests, the feed rate was identified as the most significant factor within the range of cutting parameters employed. Hence, the results validated the fact that feed rate had a high influence in polymer machining. The work also established the relationship on how surface roughness of an optical lens responded to monitoring signals and parameters such as force, feed, speed and depth of cut during machining and it generated models for prediction of surface finishes and appropriate selection of parameters. Furthermore, the study provides a molecular simulation analysis for validating observed conditions occurring at the nanometric scale in polymer machining. This is novel in molecular polymer modelling. The outcome of this research has contributed significantly to the body of knowledge and has provided basic information in the area of precision manufacturing of optical components of high surface integrity such as contact lenses. The application of the research findings presented here cuts across various fields such as medicine, semi-conductors, aerospace, defence, telecom, lasers, instrumentation and life sciences