A novel Vision ray metrology technique is reported that estimates the geometric wavefront of a measurement sample using the sample-induced deflection in the vision rays. Vision ray techniques are known in the vision community to provide image formation models even when conventional camera calibration techniques fail. This work extends the use of vision rays to the area of optical metrology. In contrast to phase measuring deflectometry, this work relies on differential measurements, and hence, the absolute position and orientation between target and camera do not need to be known. This optical configuration significantly reduces the complexity of the reconstruction algorithms. The proposed vision ray metrology system does not require mathematical optimization algorithms for calibration and reconstruction – the vision rays are obtained using a simple 3D fitting of a line.
Precision glass molding is a viable process for the cost-effective volume production of freeform optics. Process development is complex, requiring iterative trials of mold manufacture and metrology, molding prototyping, optic metrology, and functional testing. This paper describes usage of chalcogenide glass molding as a lower cost, faster turnaround mold prototyping process for an Alvarez lens intended for use in visible light. The target optic has a clear aperture of 42 mm, aspect ratio of 16:1, error specifications of 20 nm – 25 nm RMS across a spatial wavelength band from 1 mm to the clear aperture (e.g. form and mid-spatial frequencies) and microroughness of 2 nm Sq. The evaluation process uses diamond turning of trial molds in metal, molding of chalcogenide glass prototypes, and comparative metrology of molds and optics to close the manufacturing and metrology loops and more rapidly converge on a viable process starting point for visible glass.
This manuscript is the second of three submissions to describe the concurrent engineering of a 250 mm aperture class three mirror anastigmatic (TMA) visible spectrum imager. The system is an off axis, F/3, 3 degree FOV imager with all freeform mirrors. The major drivers for the mechanical design were low mass, stiffness, robustness, thermal stability, and scalablility. The system is designed using silicon carbide (SiC) for the optics and housing to meet these requirements. The scope of this manuscript is limited to aluminum prototype optics, which are identical to the SiC components. Here, we describe the mechanical design process to light weight the optics while maintaining adequate stiffness. Also described is the pairing of the optic with an optical cell for both the manufacture and system integration using kinematic mounts. The mount design pairs with a matched kinematic mount for off machine figure metrology. This allows for an iterative figure convergence process. A design for manufacture and metrology approach was used to ensure the system elements can be both manufactured and measured by providing feedback to the optical designers.
Precision glass molding is a viable process for the cost-effective volume production of freeform optics. Process development is complex, requiring iterative trials of mold manufacture and metrology, glass mold prototyping, metrology and functional testing. This paper describes the first iteration in the development of a process for an Alvarez lens for visible light. The challenges of this optic are extremely tight band-RMS tolerances on a freeform shape over a maximum clear aperture of 45 mm, a 16:1 aspect ratio and a freeform departure of 329 micrometers. A freeform glass mold for an Alvarez lens was manufactured by coordinated-axis diamond turning in a mold substrate using a custom tool error correction method. The results of prototype precision glass molding are also reported. Mold surfaces and molded optical surfaces are analyzed with scanning white light interferometry. A surface roughness of approximately 3 nm RMS is obtained for both the mold substrate and the glass optic with high-fidelity reproduction of micro-surface structure in the glass. These measurements also identify challenging areas, particularly the presence of mid-spatial frequency errors on the optic originating from the machine thermal control system. The form of the molds was also measured with a profilometer; however, the mold surface does not agree with the expected prescription with an overall deviation in form of approximately 10 μm. The machining process is expected to have sub-micrometer error and the sources of this discrepancy are still being determined. Metrology of the glass optics is currently in progress.
We present experimental results from variable output refractive beam shapers based on freeform optical surfaces. Two freeform elements in close proximity comprise a beam shaper that maps a circular Gaussian input to a circular ‘flat-top’ output. Different lateral relative shifts between the elements result in a varying output diameter while maintaining the uniform irradiance distribution. We fabricated the beam shaping elements in PMMA using multi-axis milling on a Moore Nanotech 350FG diamond machining center and tested with a 632.8 nm Gaussian input. Initial optical testing confirmed both the predicted beam shaping and variable functionality, but with poor output uniformity. The effects of surface finish on optical performance were investigated using LightTrans VirtualLabTM to perform physical optics simulations of the milled freeform surfaces. These simulations provided an optimization path for determining machining parameters to improve the output uniformity of the beam shaping elements. A second variable beam shaper based on a super-Gaussian output was designed and fabricated using the newly determined machining parameters. Experimental test results from the second beam shaper showed outputs with significantly higher quality, but also suggest additional areas of study for further improvements in uniformity.
Germanium is commonly used for optical components in the infrared, but the high refractive index of germanium causes significant losses due to Fresnel reflections. Anti-reflection (AR) surfaces based on subwavelength “moth’s eye” gratings provide one means to significantly increase optical transmission. As found in nature, these gratings are conformal to the curved surfaces of lenslets in the eye of the moth. Engineered optical systems inspired by biological examples offer possibilities for increased performance and system miniaturization, but also introduce significant challenges to both design and fabrication. In this paper, we consider the design and fabrication of conformal moth’s eye AR structures on germanium freeform optical surfaces, including lens arrays and Alvarez lenses. Fabrication approaches and limitations based on both lithography and multi-axis diamond machining are considered. Rigorous simulations of grating performance and approaches for simulation of conformal, multi-scale optical systems are discussed.
While Alvarez lens prototypes have recently been manufactured and tested for visible wavelengths, there is little discussion of these types of components for infrared applications in the published literature. We present and characterize a germanium Alvarez lens for infrared imaging. Mathematical analysis for determining the required cubic surfaces is presented, and ray-based and wave-based optical simulations are performed to confirm and refine the expected variable-focus behavior. As part of the design study, we examine the effects of effective f-number of the Alvarez lens and gap between the freeform surfaces on image quality, modulation transfer function, and Strehl ratio. The germanium Alvarez lens pair is fabricated through freeform diamond micro-milling, and characterized using a custom-built imaging test station in the mid-infrared. The variable-focus and imaging capabilities of this lens are demonstrated experimentally and compared to predicted results with good agreement.
In 1967, Luis Alvarez introduced a novel concept for a focusing lens whereby two transmitting elements with cubic
polynomial surfaces yield a composite lens of variable focal length with small lateral shifts. Computer simulations have
demonstrated the behavior of these devices, but fabricating the refractive cubic surfaces of the types needed with
adequate precision and depth modulation has proven to be challenging using standard methods, and, to the authors'
knowledge, Alvarez lens elements have not been previously machined in infrared materials. Recent developments in
freeform diamond machining capability have enabled the fabrication of such devices. In this paper, we discuss the
fabrication of freeform refractive Alvarez elements in germanium using diamond micro-milling on a five-axis Moore
Nanotech® 350FG Freeform Generator. Machining approaches are discussed, and measurements of surface figure and
finish are presented. Initial experimental tests of optical performance are also discussed.
While mechanical ruling and single point diamond turning has been a mainstay of optical fabrication for many years,
many types of micro-optical devices and structures are not conducive to simple diamond turning or ruling, such as, for
example, microlens arrays, and optical surfaces with non-radial symmetry. More recent developments in machining
technology have enabled significant expansion of fabrication capabilities. Modern machine tools can generate complex
three-dimensional structures with optical quality surface finish, and fabricate structures across a dynamic range of
dimensions not achievable with lithographic techniques. In particular, five-axis free-form micromachining offers a great
deal of promise for realization of essentially arbitrary surface structures, including surfaces not realizable through binary
or analog lithographic techniques. Furthermore, these machines can generate geometric features with optical finish on
scales ranging from centimeters to micrometers with accuracies of 10s of nanometers. In this paper, we discuss
techniques and applications of free-form surface machining of micro-optical elements. Aspects of diamond machine tool
design to realize desired surface geometries in specific materials are discussed. Examples are presented, including
fabrication of aspheric lens arrays in germanium for compact infrared imaging systems. Using special custom kinematic
mounting equipment and the additional axes of the machine, the lenses were turned with surface finish better than 2 nm
RMS and center to center positioning accuracy of ±0.5 μm.
The use of diamonds to generate precision patterns and precision surfaces on a micrometer or nanometer scale has a history that dates back centuries. Uses of diamond in semi-automated machinery can be traced to ruling machines, pantographs, and ornamental turning with "diamond turning" dating back about a century. Poor behavior in machining more common materials (e.g. ferrous alloys) has limited diamond use in traditional industrial machining. The niche of
the single crystal diamond is its edge sharpness and the ability to produce near-optical finish in materials such as aluminum, copper and their alloys; however, due to machine limitations, diamond machining remained a novelty until relatively recently. A convergence of machine technologies developed for both weapons and commercial applications led to modern diamond turning. Current turnkey machines can produce contoured surfaces with surface finish in the range of 5 nm Ra and long range accuracy of micrometers or less. Macroscopic scale, three axis, diamond machining is a well-developed technology; machining of features on a micrometer and submicrometer scale is a new and rapidly developing application of single crystal diamond machining. The role of this technology in micro-optics replication has
yet to be fully defined.
A critical parameter in predicting tool wear during machining and in accurate computer simulations of machining is the spatially-resolved temperature at the tool-chip interface. We describe the development and the calibration of a nearly diffraction-limited thermal-imaging microscope to measure the spatially-resolved temperatures during the machining of an AISI 1045 steel with a tungsten-carbide tool bit. The microscope has a target area of 0.5 mm X 0.5 mm square region with a < 5 micrometers spatial resolution and is based on a commercial InSb 128 X 128 focal plane array with an all reflective microscope objective. The minimum frame image acquisition time is < 1 ms. The microscope is calibrated using a standard blackbody source from the radiance temperature calibration laboratory at the National Institute of Standards and Technology, and the emissivity of the machined material is deduced from the infrared reflectivity measurements. The steady-state thermal images from the machining of 1045 steel are compared to previous determinations of tool temperatures from micro-hardness measurements and are found to be in agreement with those studies. The measured average chip temperatures are also in agreement with the temperature rise estimated from energy balance considerations. From these calculations and the agreement between the experimental and the calculated determinations of the emissivity of the 1045 steel, the standard uncertainty of the temperature measurements is estimated to be about 45 degree(s)C at 900 degree(s)C.
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