Tungsten carbide (WC) offers high-strength, high-melting points, and exceptional toughness, with critical applications in industries such as optical molding. Precision machining of WC typically uses grinding operations where tool wear is a significant issue, especially for small geometries required for the consumer electronics industry. Single-point diamond turning (SPDT) is another option for precision machining small features but excessive tool wear prevents this from being a viable option. Innovative approaches, such as in-situ laser-assisted diamond turning, have demonstrated remarkable potential in alleviating tool wear issues and generating optical-quality surface finishes. Laser-assisted techniques, leveraging laser energy for ductile mode machining, mitigate material cracks or fractures. This study delves into the intricate relationship between diamond tool geometry, particularly the rake angle, and ductile regime machining dynamics. Precise selection of diamond tool geometry and rake angle is crucial for desired surface quality. The experimental setup involves specialized equipment like a UMT Bruker tribometer with a modified OPTIMUS system to investigate the impact of tool geometry, specifically the rake angle, in micro laser-assisted material removal on tungsten carbide. The goal is precise and controlled material micro laser-assisted ductile mode removal while minimizing damage or subsurface defects. Results highlight that the rake angle significantly influences the critical depth of cut, with a -25° rake angle proving advantageous, especially when combined with higher laser power. Laser power and tool geometry are pivotal parameters for optimizing hard and brittle material machining, offering valuable insights for precision engineering applications.
Laser-assisted diamond turning has been shown to reduce tool wear, improve productivity, and achieve better surface specifications (including roughness and form) for traditionally diamond turnable materials for infrared optics. Amorphous glass being typically harder than IR materials, thus, diamond turning is less effective compared to traditional grinding and polishing methods. However, traditional grinding and polishing come with drawbacks, such as introducing significant subsurface damage ranging from 20-60 μm, necessitating removal during the polishing process, known as grey out. During grey out polishing, the optical axis can wander, leading to errors between the mechanical axis and optical axis when polishing aspheres. Moreover, sub-aperture polishing steps add mid-spatial frequency errors with each subsequent iteration before form convergence to a low irregularity. Laser-assisted diamond turning for amorphous glass shows promise as a method for rapidly producing near-net optics with minimal sub-surface damage.. This enables two critical gains for optics manufacturing: 1) glass optics can be polished to finished specifications much more quickly than with traditional grinding and polishing; and 2) mechanical tolerances such as wedge and sag can be maintained with precision, reducing manufacturing errors in aspheric optics. In this work, we present data showing that subsurface damage can be reduced to <3 μm for glass optics. Additionally, we demonstrate that form accuracy remains better than 500 nm for even after 10 or more diamond turning passes, indicating extended tool life and high level of conformity to near-net shape.
Tungsten carbide is a material of interest to the optical molding industry because of its suitable thermal properties in molding at higher temperatures. Tungsten carbide is typically ground and polished as tool wear from conventional machining is too high to be feasible. Laser assisted machining developed through Micro-LAM has allowed direct machinability of this material. A machinability study was performed on five grades of tungsten carbide that have been specially developed for glass lens molds. The primary difference between the grades studied is the grain size. With advances in material technology, there is an ability to provide finer grain structures in binderless alloys of tungsten carbide. Standardized trials were then performed across the different grades to evaluate machinability and surface roughness using Laser Assisted Machining (LAM) on a Single Point Diamond Turning (SPDT) platform. The trials proved that there is a strong dependence and correlation of grain size versus final achievable surface finish after LAM turning. Larger grain materials have larger voids and gaps which may cause larger pull outs. These voids then must be polished in post-processing to get beneath the sub-surface damage which is a function of the void depth. Laser assisted machining of fine grain tungsten carbide can achieve a mirror-like surface finish suitable for optical molding applications with minimal post-polishing. Using this technology allows for producing tungsten carbide molds through a more deterministic process. Also, given the range of diamond tool sizes, this method is suitable for complex geometries such as those used in the molding of collimation optics for 5G applications or biomedical applications such as endoscopes.
The discipline of optomechanical engineering has few dedicated courses and no complete programs at universities in the US. Yet, optomechanical engineering is a discipline that is vitally important to many sectors, including defense, aerospace, semiconductor, military, medical, and more. To truly turn optomechanical engineering into a complete discipline, educational programs must be established where the spine of the program is optomechanical engineering, rather than it simply existing as optional elective courses. This paper details the possible framework for such a program based on lessons learned from educating students at both the University of Rochester and the University of Arizona, which contain the two most prominent optics education programs in the US.
Multiple wavelength interferometry has long been considered an option for the measurement of large aspheric slope departures. In particular, a synthetic wavelength offers a variety of approaches by which large phase excursions can be unwrapped. Using multiple wavelengths can create collimation and magnification mismatch errors between the individual wavelengths that arise during beam expansion and propagation. Here, we present and analyze alignment and calibration methods for a dual-wavelength interferometer that can significantly reduce both misalignment errors and chromatic aberrations in the system. To correct for misalignment, a general method is described for the alignment of a dual-wavelength interferometer, including the alignment of lasers, beam expanders, beam splitters for combining beams and for compensating errors in the reference surface, and the fringe imaging system. A Fourier transform test at the detector surface was conducted to validate that there is essentially no magnification difference between two wavelengths resulting from misalignment of optical system. For the chromatic aberration introduced by the optical elements in the system, a ray-trace model of the interferometer has been established, to simulate the chromatic effect that optical elements will have on the measurement results. As an experimental test, we examine an off-axis spherical mirror in a non-null condition using a highly aliased interferogram. The above alignment methods and the results are analyzed based on the simulated system errors. Using this method, we demonstrate a measured surface profile of deviation of λ/25 which is comparable to a direct measurement profile of the surface on axis using a Fizeau interferometer.
Intra-Tissue Refractive Index Shaping (IRIS) uses a 405 nm femtosecond laser focused into the stromal region of the cornea to induce a local refractive index change through multiphoton absorption. This refractive index change can be tailored through scanning of the focal region and variations in laser power to create refractive structures, such as gradient index lenses for visual refractive correction. Previously, IRIS was used to create 2.5 mm wide, square, -1 D cylindrical refractive structures in living cats. In the present work, we first wrote 400 μm wide bars of refractive index change at varying powers in enucleated cat globes using a custom flexure-based scanning system. The cornea and surrounding sclera were then removed and mounted into a wet cell. The induced optical phase change was measured with a Mach- Zehnder Interferometer (MZI), and appeared as fringe displacement, whose magnitude was proportional to the refractive index change. The interferograms produced by the MZI were analyzed with a Fourier Transform based algorithm in order to extract the phase change. This provided a phase change versus laser power calibration, which was then used to design the scanning and laser power distribution required to create -1.5 D cylindrical Fresnel lenses in cat cornea covering an area 6 mm in diameter. This prescription was inscribed into the corneas of one eye each of two living cats, under surgical anesthesia. It was then verified in vivo by contrasting wavefront aberration measurements collected pre- IRIS with those obtained over six months post-IRIS using a Shack-Hartmann wavefront sensor.
We present measurements of light scatter induced by a new ultrafast laser technique being developed for laser refractive correction in transparent ophthalmic materials such as cornea, contact lenses, and/or intraocular lenses. In this new technique, called intra-tissue refractive index shaping (IRIS), a 405 nm femtosecond laser is focused and scanned below the corneal surface, inducing a spatially-varying refractive index change that corrects vision errors. In contrast with traditional laser correction techniques, such as laser in-situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), IRIS does not operate via photoablation, but rather changes the refractive index of transparent materials such as cornea and hydrogels. A concern with any laser eye correction technique is additional scatter induced by the process, which can adversely affect vision, especially at night. The goal of this investigation is to identify sources of scatter induced by IRIS and to mitigate possible effects on visual performance in ophthalmic applications. Preliminary light scattering measurements on patterns written into hydrogel showed four sources of scatter, differentiated by distinct behaviors: (1) scattering from scanned lines; (2) scattering from stitching errors, resulting from adjacent scanning fields not being aligned to one another; (3) diffraction from Fresnel zone discontinuities; and (4) long-period variations in the scans that created distinct diffraction peaks, likely due to inconsistent line spacing in the writing instrument. By knowing the nature of these different scattering errors, it will now be possible to modify and optimize the design of IRIS structures to mitigate potential deficits in visual performance in human clinical trials.
Blue intratissue refractive index shaping (blue-IRIS) is a method with potential to correct ocular refraction noninvasively in humans. To date, blue-IRIS has only ever been applied to cat corneas and hydrogels. To test the comparability of refractive index change achievable in cat and human tissues, we used blue-IRIS to write identical phase gratings in ex vivo feline and human corneas. Femtosecond pulses (400 nm) were focused ∼300 μm below the epithelial surface of excised cat and human corneas and scanned to write phase gratings with lines ∼1 μm wide, spaced 5 μm apart, using a scan speed of 5 mm/s. Additional cat corneas were used to test writing at 3 and 7 mm/s in order to document speed dependence of the refractive index change magnitude. The first-order diffraction efficiency was immediately measured and used to calculate the refractive index change attained. Our data show that blue-IRIS induces comparable refractive index changes in feline and human corneas, an essential requirement for further developing its use as a clinical vision correction technique.
A refractometer system using four modified Wu-type heterodyne interferometers with a variable length vacuum cell is presented. The proposed system has two working modes: (1) a moving mode for measuring the absolute air refractive index at the start of a measurement and (2) a static mode for monitoring the air refractive index fluctuation with the same bandwidth as a traditional displacement interferometer. The system requires no gas filling or pumping during the measurement and can be used for real-time refractive index compensation. Comparison experiments with empirical equations are conducted to investigate the feasibility and performance of the proposed system. The standard deviation of the measurement difference between the proposed system and empirical equation is 2.8 parts in 107, which is close to the uncertainty of our refractive index reference based on the accuracy of the environmental sensors. The relative refractive index tracking is a few parts in 108 with a bandwidth of 10 Hz, but high bandwidths are readily achievable.
In this proceedings, we present a 3DoF (one linear, two angular) optical probe for measuring freeform optics in conjunction with an optical coordinate measuring machine (OCMM). This probe uses homodyne interferometry in a Michelson configuration and position sensing detection to simultaneously measure displacement, tip, and tilt. The goal of this work is to investigate point-to-point methods for measuring freeform optics and establish a probing methodology that can perform self-alignment with respect to the local optical surface. We present the design and preliminary benchtop validation of the probe's performance. Benchtop validation shows successful measurements with 5 nm linear and 20 μrad angular noise levels, with a 15 μm spot size. A CMOS sensor is used for visual confirmation of proper focus on measurement surface to minimize initial defocus error. A PSD detects linear horizontal and vertical displacement of the reflected beam from the measurement surface using autocollimation. In-phase and quadrature signals are measured by two photodetectors and post-processed to obtain displacement information. Periodic error caused by polarization effects and beam mixing is compensated by FPGA-based signal processing.
Experience at the Laboratory for Laser Energetics has shown that broadband base vibrations make it difficult to position cryogenic inertial confinement fusion targets. These effects must be mitigated for National Ignition Facility–scale targets; to this end an active vibration stabilization system is proposed. A single-mode optical fiber strain probe and a novel fiber contained heterodyne interferometer have been developed as a position feedback sensor for the vibration control system. A resolution limit of 54.5 nƐ; is measured with the optical strain gauge, limited by the lock-in amplifier. Experimental measurements of the sensor that show good agreement with reference resistive strain gauge measurements are presented.
A theoretical model has been developed to analyze the output polarization state of a total internal reflection-based retroreflector as a function of pitch and yaw motions. There are six different beam paths in the retroreflector, and thus output polarization states, for a given pitch or yaw misalignment. This polarization model discusses the electric field changes of the laser beam based on Fresnel equations for phase and polarization change on reflection. Jones matrices are computed based on Snell’s law, Fresnel equations, the solid geometry, and coordinate transformations to obtain a Jones matrix model of the retroreflector for a given misalignment. Modeling results show that there is always a rotation to the input beam’s polarization and there are specific input regions that are not sensitive to pitch motions but are sensitive to yaw motions. Validation of the model is also presented, using both theoretical and experimental results published by Kalibjian in 2004.
It is known that far-field scattered light requires a priori sample information in order to reconstruct nm-scale information such as is required in semiconductor metrology. We describe an approach to scatterometry that uses unconventional polarization states in the pupil of a high NA objective lens. We call this focused beam scatterometry; we will discuss the sensitivity limits to this approach and how it relates to micro-ellipsometry as well as low-NA scatterometry.
Point spread function engineering is usually accomplished by controlling the amplitude, phase and/or polarization of the pupil fields. We analyze and test an optical design for full amplitude, phase, and polarization control of the pupil fields using a single spatial light modulator. In our scheme, the beam is spatially split into four components whose relative phases provide the four degrees of freedom necessary for amplitude, phase, and polarization control.
This Field Guide provides a practical treatment of the fundamental theory of displacement measuring interferometry, with examples of interferometry systems and uses. It outlines alignment techniques for optical components, signal processing systems for phase measurements, and laser stabilization for homodyne and heterodyne sources. The concept of displacement measurement uncertainty is discussed with a practical example of calculating uncertainty budgets. For practicing engineers, this Field Guide will serve as a refresher manual for error sources and uncertainty budgets. For researchers, it will bring new insight to the way in which this technology can be useful in their field. For new engineers, researchers, and students, it will also serve as an introduction into basic alignment techniques for breadboard-based optical systems.
UltraForm Finishing (UFF) is a deterministic, subaperture, computer numerically controlled, grinding and polishing platform designed by OptiPro Systems. UFF is used to grind and polish a variety optics from simple spherical to fully freeform, and numerous materials from glasses to optical ceramics. The UFF system consists of an abrasive belt around a compliant wheel that rotates and contacts the part to remove material. This work aims to measure the stiffness variations in the system and how it can affect material removal rates. The stiffness of the entire system is evaluated using a triaxial load cell to measure forces and a capacitance sensor to measure deviations in height. Because the wheel is conformal and elastic, the shapes of contact areas are also of interest. For the scope of this work, the shape of the contact area is estimated via removal spot. The measured forces and removal spot area are directly related to material removal rate through Preston’s equation. Using our current testing apparatus, we will demonstrate stiffness measurements and contact areas for a single UFF belt during different states of its lifecycle and assess the material removal function from spot diagrams as a function of wear. This investigation will ultimately allow us to make better estimates of Preston’s coefficient and develop spot-morphing models in an effort to more accurately predict instantaneous material removal functions throughout the lifetime of a belt.
Freeform and conformal optics represent the next generation of optical systems where their utilization leads to more compact, lighter, and higher performance systems for solar collectors, consumer optics, and defense applications. Optical coordinate measuring machines present one option for accurate metrology of freeform components but have two limitations: metrology system errors and optical probe errors. In this work, we address the latter of the two by demonstrating a compact optical probe capable of fiber delivery and fiber detection to remove potential heats sources away from measured optic. A bench top demonstrator has yielded a displacement resolution below ±10 nm and has a noise floor of approximately ±18 μrad for surface slope in two orthogonal directions. In this Proceedings, we discuss our probe concept, operating principle, and preliminary measurements with a bench top proof-of-concept system. The goal of this work is to ultimately integrate this probe into OptiPro’s UltraSurf, a 5-axis optical coordinate measuring machine for measuring freeform and conformal optics.
Optomechanical design considerations are presented in the development of a fiber-delivered three degree-offreedom displacement measuring interferometer. The tool can be used to simultaneously calibrate the linear motion and rotational errors of a translating stage using a single measurement beam incident on a plane mirror target. This novel interferometer incorporates a quadrant photodiode to measure four spatially separated interference signals all within a single optical interference beam, otherwise known as differential wavefront sensing. In post processing, a weighted phase average is created over symmetrically adjacent pairings of detector elements to decouple and measure displacement and changes in pitch and yaw. Design considerations include a custom displacement interferometer architecture, mechanical analyses and qualification testing of a working prototype. This interferometer has the potential for providing multi-DOF calibrations for precision motion stages.
UltraForm Finishing (UFF) is a deterministic sub-aperture computer numerically controlled grinding and polishing
platform designed by OptiPro Systems. UFF is used to grind and polish a variety of optics from simple spherical to fully
freeform, and numerous materials from glasses to optical ceramics. The UFF system consists of an abrasive belt around
a compliant wheel that rotates and contacts the part to remove material. This work aims to accurately measure the
dynamic coefficient of friction (μ), how it changes as a function of belt wear, and how this ultimately affects material
removal rates. The coefficient of friction has been examined in terms of contact mechanics and Preston’s equation to
determine accurate material removal rates. By accurately predicting changes in μ, polishing iterations can be more
accurately predicted, reducing the total number of iterations required to meet specifications. We have established an
experimental apparatus that can accurately measure μ by measuring triaxial forces during translating loading conditions
or while manufacturing the removal spots used to calculate material removal rates. Using this system, we will
demonstrate μ measurements for UFF belts during different states of their lifecycle and assess the material removal
function from spot diagrams as a function of wear. Ultimately, we will use this system for qualifying belt-wheel-material
combinations to develop a spot-morphing model to better predict instantaneous material removal functions.
The University of Rochester is well known for the Institute of Optics as well as a strong Mechanical Engineering
program. In recent years, there has been collaboration between the two departments on a variety of topics, including a
new joint faculty position. There are new cross-listed courses in Optomechanics and Precision Engineering (Spring
2012), which are described in this paper. As yet, there is no formal specialization in Optomechanics, but many students
create their own program from available courses combining optics and mechanics with other disciplines. Students have
the opportunity to participate in the several research areas which cross discipline boundaries. For example, a student
design team is building a 16" telescope which they hope can become the basis of an intercollegiate design contest. In
addition to full semester courses, there is a summer program of short courses available to working engineers as both
refresher courses or as introductory courses on new topics.
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