Using Open Data as a Material for Introductory Programming Assignments

This case study explores why and how open data can be used as a material with which to produce engaging challenges for students as they are introduced to programming. Through describing the process of producing the assignments, and learner responses to them, we suggest that open data is a powerful material for designing learning activities because of its qualities of ease of access and authenticity. We conclude by outlining steps to take in devising and implementing open data-based assignments

MRF Applications: On the Road to Making Large-Aperture Ultraviolet Laser Resistant Continuous Phase Plates for High-Power Lasers

Over the past two years we have developed MRF tools and procedures to manufacture large-aperture (430 X 430 mm) continuous phase plates (CPPs) that are capable of operating in the infrared portion (1053 nm) of high-power laser systems. This is accomplished by polishing prescribed patterns of continuously varying topographical features onto finished plano optics using MRF imprinting techniques. We have been successful in making, testing, and using large-aperture CPPs whose topography possesses spatial periods as low as 4 mm and surface peak-to-valleys as high as 8.6 {micro}m. Combining this application of MRF technology with advanced MRF finishing techniques that focus on ultraviolet laser damage resistance makes it potentially feasible to manufacture large-aperture CPPs that can operate in the ultraviolet (351 nm) without sustaining laser-induced damage. In this paper, we will discuss the CPP manufacturing process and the results of 351-nm/3-nsec equivalent laser performance experiments conducted on large-aperture CPPs manufactured using advanced MRF protocols

Scratch Forensics

Scratches on optical components which are formed during fabrication, cleaning, handling and end-use, are widespread and almost always detrimental. The impact of scratches on the end-use of the optic includes increased optical scatter, reduced system performance, and reduced strength. In the case of optics used in high intensity laser applications, prevention of scratches is paramount because they are closely associated with laser damage. Evaluation of the characteristics (dimensions, location on optic, shape, and orientation) of a scratch can serve a powerful tool to identify the cause of the scratch and lead to mitigations to prevent their reoccurrence. It is likely that opticians have used such techniques for hundreds of years. In recent years, by applying techniques of fracture mechanics and tribology, several new semi-quantitative rules-of-thumb have been developed allowing one to estimate the size and shape of the scratch inducing asperity or rogue particle, the load on the particle, the depth of the fractures in the scratch, and properties of material housing the rogue particle. The following discussion reviews some these techniques, which as a whole, we refer to as 'Scratch Forsenics'

Laser damage initiation and growth of antireflection coated S-FAP crystal surfaces prepared by pitch lap and magnetorheological finishing

Antireflection (AR) coatings typically damage at the interface between the substrate and coating. Therefore the substrate finishing technology can have an impact on the laser resistance of the coating. For this study, AR coatings were deposited on Yb:S-FAP [Yb{sup 3+}:Sr{sub 5}(PO{sub 4}){sub 3}F] crystals that received a final polish by both conventional pitch lap finishing as well as magnetorheological finishing (MRF). SEM images of the damage morphology reveals laser damage originates at scratches and at substrate coating interfacial absorbing defects. Previous damage stability tests on multilayer mirror coatings and bare surfaces revealed damage growth can occur at fluences below the initiation fluence. The results from this study suggest the opposite trend for AR coatings. Investigation of unstable HR and uncoated surface damage morphologies reveals significant radial cracking that is not apparent with AR damage due to AR delamination from the coated surface with few apparent cracks at the damage boundary. Damage stability tests show that coated Yb:S-FAP crystals can operate at 1057 nm at fluences around 20 J/cm{sup 2} at 10 ns; almost twice the initiation damage threshold

Imprinting continuously varying topographical structure onto large-aperture optical surfaces using magnetorheological finishing

Over the past four years we have advanced Magnetorheological Finishing (MRF) techniques and tools to imprint complex continuously varying topographical structures onto large-aperture (430 x 430 mm) optical surfaces. These optics, known as continuous phase plates (CPPs), are important for high-power laser applications requiring precise manipulation and control of beam-shape, energy distribution, and wavefront profile. MRF's unique deterministic-sub-aperture polishing characteristics make it possible to imprint complex topographical information onto optical surfaces at spatial scale-lengths approaching 1 mm and surface peak-to-valleys as high as 22 {micro}m. During this discussion, we will present the evolution of the MRF imprinting technology and the MRF tools designed to manufacture large-aperture 430 x 430 mm CPPs. Our results will show how the MRF removal function impacts and limits imprint fidelity and what must be done to arrive at a high-quality surface. We also present several examples of this imprinting technology for fabrication of phase correction plates and CPPs for use in high-power laser applications

Removal of Lattice Imperfections that Impact the Optical Quality of Ti:Sapphire using Advanced Magnetorheological Finishing Techniques

Advanced magnetorheological finishing (MRF) techniques have been applied to Ti:sapphire crystals to compensate for sub-millimeter lattice distortions that occur during the crystal growing process. Precise optical corrections are made by imprinting topographical structure onto the crystal surfaces to cancel out the effects of the lattice distortion in the transmitted wavefront. This novel technique significantly improves the optical quality for crystals of this type and sets the stage for increasing the availability of high-quality large-aperture sapphire and Ti:sapphire optics in critical applications

MRF Applications: Measurement of Process-dependent Subsurface Damage in Optical Materials using the MRF Wedge Technique

Understanding the behavior of fractures and subsurface damage in the processes used during optic fabrication plays a key role in determining the final quality of the optical surface finish. During the early stages of surface preparation, brittle grinding processes induce fractures at or near an optical surface whose range can extend from depths of a few mm to hundreds of mm depending upon the process and tooling being employed. Controlling the occurrence, structure, and propagation of these sites during subsequent grinding and polishing operations is highly desirable if one wishes to obtain high-quality surfaces that are free of such artifacts. Over the past year, our team has made significant strides in developing a diagnostic technique that combines magnetorheological finishing (MRF) and scanning optical microscopy to measure and characterize subsurface damage in optical materials. The technique takes advantage of the unique nature of MRF to polish a prescribed large-area wedge into the optical surface without propagating existing damage or introducing new damage. The polished wedge is then analyzed to quantify subsurface damage as a function of depth from the original surface. Large-area measurement using scanning optical microscopy provides for improved accuracy and reliability over methods such as the COM ball-dimple technique. Examples of the technique's use will be presented that illustrate the behavior of subsurface damage in fused silica that arises during a variety of intermediate optical fabrication process steps

Utilization of Magnetorheological Finishing as a Diagnostic Tool for Investigating the Three-Dimensional Structure of Fractures in Fused Silica

We have developed an experimental technique that combines magnetorheological finishing (MRF) and microscopy to examine fractures and/or artifacts in optical materials. The technique can be readily used to provide access to, and interrogation of, a selected segment of a fracture or object that extends beneath the surface. Depth slicing, or cross-sectioning at selected intervals, further allows the observation and measurement of the three-dimensional nature of the sites and the generation of volumetric representations that can be used to quantify shape and depth, and to understand how they were created, how they interact with surrounding material, and how they may be eliminated or mitigated

Strength of Protection for Geographical Indications: Promotion Incentives and Welfare Effects

We address the question of how the strength of protection for geographical indications (GIs) affects the GI industry\u27s promotion incentives, equilibrium market outcomes, and the distribution of welfare. Geographical indication producers engage in informative advertising by associating their true quality premium (relative to a substitute product) with a specific label emphasizing the GI\u27s geographic origin. The extent to which the names/words of the GI label can be used and/or imitated by competing products—which depends on the strength of GI protection—determines how informative the GI promotion messages can be. Consumers’ heterogeneous preferences (vis-à-vis the GI quality premium) are modeled in a vertically differentiated framework. Both the GI industry and the substitute product industry are assumed to be competitive (with free entry). The model is calibrated and solved for alternative parameter values. Results show that producers of the GI and of the lower-quality substitute good have divergent interests: GI producers are better off with full protection, whereas the substitute good\u27s producers prefer intermediate levels of protection (but they never prefer zero protection because they benefit indirectly if the GI producers’ incentives to promote are preserved). For consumers and aggregate welfare, the preferred level of protection depends on the model\u27s parameters, with an intermediate level of protection being optimal in many circumstances

The distribution of subsurface damage in fused silica

Managing subsurface damage during the shaping process and removing subsurface damage during the polishing process is essential in the production of low damage density optical components, such as those required for use on high peak power lasers. Removal of subsurface damage, during the polishing process, requires polishing to a depth which is greater than the depth of the residual cracks present following the shaping process. To successfully manage, and ultimately remove subsurface damage, understanding the distribution and character of fractures in the subsurface region introduced during fabrication process is important. We have characterized the depth and morphology of subsurface fractures present following fixed abrasive and loose abrasive grinding processes. At shallow depths lateral cracks and an overlapping series of trailing indentation fractures were found to be present. At greater depths, subsurface damage consists of a series of trailing indentation fractures. The area density of trailing fractures changes as a function of depth, however the length and shape of individual cracks remain nearly constant for a given grinding process. We have developed and applied a model to interpret the depth and crack length distributions of subsurface surface damage in terms of key variables including abrasive size and load