Sub-Angstrom surfaces are frequently specified by customers requiring increased optical and energy efficiency, with most stringent requirements from applications in the UV and from certain high power laser systems. Much research has been performed to understand related surface dynamics, with considerable amounts of information published on precision finishing of optical glass. Many suppliers of low roughness surfaces rely on finishing methods that create subsurface damage through the aggressive removal of material. While the magnitude of this damage can be minimized through the use of progressively finer abrasives, detectable levels of latent structure are still evident. However, finishing processes that merely wipe clean the Beilby layer without disturbing the substrate produce nearly perfect surfaces with little to no subsurface damage. This has been the focus of development efforts at Edmund Optics. By careful management of process variables, extremely smooth surfaces lacking subsurface damage have been demonstrated in fused silica and N-BK7 materials. Applications for optics having these characteristics are found in automotive, defense, medical, and industrial domains. This paper discusses results achieved for producing sub-Angstrom surfaces on fused silica and N-BK7 glass. Surfaces with measured roughness of 0.5Å have consistently been demonstrated on fused silica, with results of around 0.8Å shown for N-BK7. Types of processes useful for achieving these results will be discussed, along with basic metrology methods for producing reliable sub-Angstrom measurements.
High efficiency surfaces for low loss or high-power laser applications require extremely sensitive instrumentation to measure. Because these sub-angstrom surfaces push the limits of current optical profilers and atomic force microscopes great care must be taken to ensure the accuracy of surface roughness measurements. This paper will explain the techniques required to optimize the performance of the optical profiler and, most importantly, will explain the absolute necessity of understanding the spatial frequency bandwidth which creates the roughness value. It will be shown that this bandwidth defines instrument capabilities and facilitates data correlation between vastly different metrology instrumentation.
The demand for infrared transmitting materials has grown steadily for several decades as markets realize new
applications for longer wavelength sensing and imaging. With this growth has come the demand for new and
challenging material requirements that cannot be satisfied with crystalline products alone. Chalcogenide materials,
with their unique physical, thermal, and optical properties, have found acceptance by designers and fabricators to
meet these demands.
No material is perfect in every regard, and chalcogenides are no exception. A cause for concern has been the
relatively low fracture toughness and the propensity of the bulk material to fracture. This condition is amplified
when traditional subtractive manufacturing processes are employed. This form of processing leaves behind micro
fractures and sub surface damage, which act as propagation points for both local and catastrophic failure of the
Precision lens molding is not a subtractive process, and as a result, micro fractures and sub surface damage are not
created. This results in a stronger component than one produced by traditional methods. New processing methods
have also been identified that result in an even stronger surface that is more resistant to breakage, without the need
for post processing techniques that may compromise surface integrity.
This paper will discuss results achieved in the process of lens molding development at Edmund Optics that result in
measurably stronger chalcogenide components. Various metrics will be examined and data will be presented that
quantifies component strength for different manufacturing processes.
Achromatic diffractive features on lenses are widely used in industry for color correction, however there is not a welldefined standard to quantify the performance of the lenses. One metric used to qualify a lens is the sag deviation from the nominal lens profile. Imperfections in the manufacturing of the diffractive feature may cause scattering and performance loss. This is not reflected in sag deviation measurements, therefore performance measurements are required. <p> </p>There are different quantitative approaches to measuring the performance of an achromatic diffractive lens. Diffraction efficiency, a measure of optical power throughput, is a common design metric used to define the percent drop from the modulation transfer function (MTF) metric. The line spread function (LSF) shows a layout of the intensity with linear distance and an ensquared energy specification can be implemented. The MTF is a common analysis tool for assemblies and can be applied to a single element. These functional tests will be performed and compared with diffractive lenses manufactured by different tool designs. <p> </p>This paper displays the results found with various instruments. Contact profilometry was used to inspect the profile of the diffractive elements, and a MTF bench was used to characterize lens performance. Included will be a discussion comparing the results of profile traces and beam profiles to expected diffraction efficiency values and the effects of manufacturing imperfections.
Finished lens molding, and the similar process of precision lens molding, have long been practiced for high volume, accurate replication of optical surfaces on oxide glass. The physics surrounding these processes are well understood, and the processes are capable of producing high quality optics with great fidelity. However, several limitations exist due to properties inherent with oxide glasses. Tooling materials that can withstand the severe environmental conditions of oxide glass molding cannot easily be machined to produce complex geometries such as diffractive surfaces, lens arrays, and off axis features. Current machining technologies coupled with a limited selection of tool materials greatly limits the type of structures that can be molded into the finished optic.<p> </p> Tooling for chalcogenide glasses are not bound by these restrictions since the molding temperatures required are much lower than for oxide glasses. Innovations in tooling materials and manufacturing techniques have enabled the production of complex geometries to optical quality specifications and have demonstrated the viability of creating tools for molding diffractive surfaces, off axis features, datums, and arrays. Applications for optics having these features are found in automotive, defense, security, medical, and industrial domains. <p> </p>This paper will discuss results achieved in the study of various molding techniques for the formation of positive diffractive features on a concave spherical surface molded from As2Se3 chalcogenide glass. Examples and results of molding with tools having CTE match with the glass and non CTE match will be reviewed. The formation of stress within the glass during molding will be discussed, and methods of stress management will also be demonstrated and discussed. Results of process development methods and production of good diffractive surfaces will be shown.
Fabrication of fused silica optics for high-powered Nd:YAG laser applications commonly employs grinding and
polishing processes to generate smooth, specular surfaces. The industry often describes such surfaces as "laser quality"
after assessment against such gauges as surface roughness or scratch-dig standards; however, surfaces deemed
acceptable have performed variably when actually exposed to high-powered laser illumination. Traditional processes to
prepare such surfaces have often relied on rules of thumb, but we have found a convenient and simple method to help the
fabricator optimize expressly for a desired performance metric, that of low subsurface damage. Subsurface damage often
has immediate impact on susceptibility to destruction by high-power laser illumination, and we find that this damage is
not universally related to surface roughness. In addition, we show that surface roughness measurements may vary
depending on the measurement method used, such as white light interferometry (WLI), variable angle spectroscopic
ellipsometry (VASE) or atomic force microscopy (AFM).