Additive manufacturing (AM; 3D printing) is a fabrication process that builds an object layer-upon-layer and promotes the use of structures that would not be possible via subtractive machining. Prototype AM metal mirrors are increasingly being studied in order to exploit the advantage of the broad AM design-space to develop intricate lightweight structures that are more optimised for function than traditional open-back mirror lightweighting.
This paper describes a UK Space Agency funded project to design and manufacture a series of lightweighted AM mirrors to fit within a 3U CubeSat chassis. Six AM mirrors of identical design will be presented: two in aluminium (AlSi10Mg), two in nickel phosphorous (NiP) coated AlSi10Mg, and two in titanium (Ti64). For each material mirror pair, one is hand-polished and the other is diamond turned. Metrology data, surface form error and surface roughness, will be presented to compare and contrast the different materials and post-processing methods. To assess the presence of porosity, a frequent concern for AM materials, X-ray computed tomography measurements will be presented to highlight the location and density of pores within the mirror substrates; methods to mitigate the distribution of pores near the optical surface will be described. As a metric for success the AlSi10Mg + NiP and AlSi10Mg mirrors should be suitable for visible and infrared applications respectively.
Design for additive manufacture (AM; 3D printing) is significantly different than design for subtractive machining. Although there are some limitations on the designs that can be printed, the increase in the AM design-space removes some of the existing challenges faced by the traditional lightweight mirror designs; for example, sandwich mirrors are just as easy to fabricate as open-back mirrors via AM, and they provide an improvement in structural rigidity. However, the ability to print a sandwich mirror as a single component does come with extra considerations; such as orientation upon the build plate and access to remove any temporary support material. This paper describes the iterations in optimisation applied to the lightweighting of a small, 84mm diameter by 20mm height, spherical concave mirror intended for CubeSat applications. The initial design, which was fabricated, is discussed in terms of the internal lightweighting design and the design constraints that were imposed by printing and post-processing. Iterations on the initial design are presented; these include the use of topology optimisation to minimise the total internal strain energy during mirror polishing and the use of lattices combined with thickness variation i.e. having a thicker lattice in strategic support locations. To assess the suitability of each design, finite element analysis is presented to quantify the print-through of the lightweighting upon the optical surface for a given mass reduction.
With the recent development of new ultra fine aluminium alloys and progress in the field of directly machined freeform surfaces, diamond machined freeform gratings could play an important part in future spectrographs or integral field units, particularly at SWIR and LWIR wavelengths where the improved thermal performance of metal optics at cryogenic temperatures is well established. Freeform diamond machined gratings can offer a cost-effective, compact, and flexible alternative to gratings fabricated by other methods such as ion beam etching or complement these technologies. In this paper, both the advantages and limitations of 5 axis diamond machined freeform gratings are presented and potential applications are discussed.
Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs.
A Telescope window is a novel concept of transformation-optics consisting of an array of micro-telescopes, in our configuration, of a Galilean type. When the array is considered as one multifaceted device, it acts as a traditional Galilean telescope with distinctive and attractive properties such as compactness and modularity. Each lenslet, can in principle, be independently designed for a specific optical function. In this paper, we report on the design, manufacture and prototyping, by diamond precision machining, of 2 concepts of telescope windows, and discuss both their performances and limitations with a view to use them as potential low vision aid devices to support patients with macular degeneration.
Pixellated Optics, a class of optical devices which preserve phase front continuity only over small sub areas of the device, allow for a range of uses that would not otherwise be possible. One potential use is as Low Vision Aids (LVAs), where they are hoped to combine the function and performance of existing devices with the size and comfort of conventional eyewear. For these devices a Generalised Confocal Lenslet Array (GCLA) is designed to magnify object space, creating the effect of traditional refracting telescope within a thin, planar device. By creating a device that is appreciably thinner than existing LVA telescopes it is hoped that the comfort for the wearer will be increased. We have developed a series of prototype GLCA-based devices to examine their real-world performance, focussing on the resolution, magnification and clarity of image attainable through the devices. It is hoped that these will form the basis for a future LVA devices. This development has required novel manufacturing techniques and a phased development approach centred on maximising performance. Presented here will be an overview of the development so far, alongside the performance of the latest devices.
Plasma mirrors have become an important tool in high power laser physics due to their ability to suppress laser pre-pulses and amplified spontaneous emission allowing a cleaner and sharper rising edge pulse to be focused onto a target. A PMMA ellipsoidal plasma mirror used to increase the peak intensity of a high power laser pulses before it reaches the target is presented. The ellipse has been designed to increase by a factor 3, between input and output, the F-number of the beam, inducing in theory a factor 9 gain in peak intensity. Diamond machining, which is a technique capable of producing sub-micron accuracy on steep, freeform surfaces, is an ideal process for manufacturing these types of mirrors. In this paper, we discuss the diamond machining requirements to manufacture such near diffraction limited high numerical aperture mirrors.
Pixellated optical components, for example generalised confocal lenslet arrays (GCLAs), enable the design of optical devices which cannot be realised without introducing pixellation or a similar compromise. A key concern is the degradation of imaging quality due to the combined effects of diffraction, worst for smaller pixels, and the visibility of the pixels. Here we examine the effects of these two factors on image quality through use of our custom raytracer, Dr TIM. We also outline future work in developing these ideas more rigorously and applying the conclusions to more complicated devices.
CHOUGH is a small, fast project to provide an experimental on-sky high-order SCAO capability to the 4.2m WHT telescope. The basic goal has r0-sized sub- apertures with the aim of achieving high-Strehl ratios (> 0:5) in the visible (> 650 nm). It achieves this by including itself into the CANARY experiment: CHOUGH is mounted as a breadboard and intercepts the beam within CANARY via a periscope. In doing so, it takes advantage of the mature CANARY infrastructure, but add new AO capabilities. The key instruments that CHOUGH brings to CANARY are: an atmospheric dispersion compensator; a 32 × 32 (1000 actuator) MEMS deformable mirror; 31 × 31 wavefront sensor; and a complementary (narrow-field) imager. CANARY provides a 241-actuator DM, tip/tilt mirror, and comprehensive off-sky alignment facility together with a RTC. In this work, we describe the CHOUGH sub-systems: backbone, ADC, MEMS-DM, HOWFS, CAWS, and NFSI.
Concave blazed gratings greatly simplify the architecture of spectrographs by reducing the number of optical components. The production of these gratings using diamond-machining offers practically no limits in the design of the grating substrate shape, with the possibility of making large sag freeform surfaces unlike the alternative and traditional method of holography and ion etching. In this paper, we report on the technological challenges and progress in the making of these curved blazed gratings using an ultra-high precision 5 axes Moore-Nanotech machine. We describe their implementation in an integral field unit prototype called IGIS (Integrated Grating Imaging Spectrograph) where freeform curved gratings are used as pupil mirrors. The goal is to develop the technologies for the production of the next generation of low-cost, compact, high performance integral field unit spectrometers.
Adaptive optics (AO) can potentially allow high resolution imaging deep inside living tissue, mitigating against the loss of resolution due to aberrations caused by overlying tissue. Closed-loop AO correction is particularly attractive for moving tissue and spatially varying aberrations, but this requires direct wavefront sensing, which in turn requires suitable "guide stars" for use as wavefront references. We present a novel method for generating an orthogonally illuminated guide star suitable for direct wavefront sensing in a wide range of fluorescent biological structures, along with results demonstrating its use for measuring time-varying aberrations, in vivo.
Free form surfaces are now commonly used components in optics applications and can be widely found in fields such as ophthalmics, car illumination and head-up display systems and laser optics. The machining of free form optics on a 3-axis diamond turning machines is made possible with the use of tool servo machining which synchronises either or both the axial and radial motions of the tool and surface positions (X and Z axes) to the angular position of the spindle (C axis).
However, the machining of surfaces with non-zero gradient at the surface centre is particularly troublesome because the tool is still subject to a relatively large amplitude motion when reaching the central area of the surface. As a result, a small tool offset in either X (radial) or Y (height) creates a particular central signature that can be readily identified, measured and subsequently corrected by the machine operator.
In this paper, we report on a method to optimise the tool offset (X axis) in the particular case of non-zero central gradient and illustrate our discussion with simulation and measurement results.
We report on the incorporation of adaptive optics (AO) into the imaging arm of a selective plane illumination microscope (SPIM). SPIM has recently emerged as an important tool for life science research due to its ability to deliver high-speed, optically sectioned, time-lapse microscope images from deep within in vivo selected samples. SPIM provides a very interesting system for the incorporation of AO as the illumination and imaging paths are decoupled and AO may be useful in both paths. In this paper, we will report the use of AO applied to the imaging path of a SPIM, demonstrating significant improvement in image quality of a live GFP-labeled transgenic zebrafish embryo heart using a modal, wavefront sensorless approach and a heart synchronization method. These experimental results are linked to a computational model showing that significant aberrations are produced by the tube holding the sample in addition to the aberration from the biological sample itself.
We report on the development of a widefield microscope that achieves adaptive optics correction through the use of a
wavefront sensor observing an artificial laser guide star induced within the sample. By generating this guide star at
arbitrary positions and depths within the sample we allow the delivery of high-resolution images. This approach delivers
much faster AO correction than image optimization techniques, and allows the use of AO with fluorescent imaging
modalities without generating excessive photo-toxic damage in the sample, or inducing significant photo-bleaching in
the flurophore molecules.
We report on recent developments in the use of adaptive optics (AO) in wide-field microscopy to remove both system
and sample induced aberrations. We describe progress on using both a full AO system and image optimization
techniques (wavefront sensorless AO). In the latter system the determination of the best mirror shape is found via two
routes. In the first an optimization algorithm using a Simplex search pattern is used with an initial random set of mirror
shapes. We then explore the use of specific Zernike terms as our starting basis set. In both cases the final optimization
performance is not affected by the choice of optimization metric. We then describe an open loop AO system in which the
equivalent of a laser guide star is used as the light source for the wavefront sensor.