The next generation of synchrotrons and free electron lasers requires x-ray optical systems with extremely high-performance,
generally, of diffraction limited quality. Fabrication and use of such optics requires highly accurate
metrology. In the present paper, we discuss a way to improve the performance of the Long Trace Profiler (LTP), a slope
measuring instrument widely used at synchrotron facilities to characterize x-ray optics at high-spatial-wavelengths from
approximately 2 mm to 1 m. One of the major sources of LTP systematic error is the detector. For optimal functionality,
the detector has to possess the smallest possible pixel size/spacing, a fast method of shuttering, and minimal nonuniformity
of pixel-to-pixel photoresponse. While the first two requirements are determined by choice of detector, the
non-uniformity of photoresponse of typical detectors such as CCD cameras is around 2-3%. We describe a flat-field
calibration setup specially developed for calibration of CCD camera photo-response and dark current with an accuracy
of better than 0.5%. Such accuracy is adequate for use of a camera as a detector for an LTP with performance of ~0.1
microradian (rms). We also present the design details of the calibration system and results of calibration of a DALSA
CCD camera used for upgrading our LTP-II instrument at the ALS Optical Metrology Laboratory.
Micro-focusing is widely applied at soft and hard x-ray wavelengths. One typical method, in addition to zone plates, is to
split the focusing in the tangential and sagittal directions into two elliptically cylindrical reflecting elements, the so-called
Kirkpatrick-Baez (KB) pair. In the simplest case each optic is made by grinding and polishing a flat, and applying
unequal bending couples to each end. After briefly reviewing the nature of the bending, we show two new methods for
optimal adjustment of these mirror systems using our surface normal slope measuring instrument, the long trace profiler
(LTP). First, we adapt a method previously used to adjust mirrors on synchrotron radiation beamlines. We measure the
slope of the surface before and after a single small adjustment of each bending couple. This permits an approximation to
the functional dependence of slope on the adjustments, and allows, by applying the results of a simple matrix calculation,
direct adjustment to a nearly final setting. Typically, the near linearity of the problem determines a very fast convergence
of the adjustment procedure. Second, we subdivide the slope data from the LTP into three regions on the mirror, and fit a
circle to each sub-region by regression. This method also allows rapid iterative adjustment of both bending couples. We
show that this method is a particular case of the first one. As an overall indicator of predicted performance, we ray trace,
using profiler data, predicting the exact optical performance to be expected during use of the system.
In this work, we investigate the effect of air convection on laser-beam pointing noise essential for the long trace profiler (LTP). We describe this pointing error with noise power density (NPD) frequency distributions. It is shown that the NPD spectra due to air convection have a very characteristic form. In the range of frequencies from ~0.05 Hz to ~0.5 Hz, the spectra can be modeled with an inverse-power-law function. Depending on the intensity of air convection that is controlled with a resistive heater of 100 to 150 mW along a one-meter-long optical path, the power index lies between 2 and 3 at an overall rms noise of ~0.5 to 1 microradian. The efficiency of suppression of the convection noise by blowing air across the beam optical path is also discussed. Air-blowing leads to a white-noise-like spectrum. Air blowing was applied to the reference channel of an LTP allowing demonstration of the contribution of air convection noise to the LTP reference beam. The ability to change (with the blowing technique presented) the spectral characteristics of the beam pointing noise due to air convection allows one to investigate the contribution of the convection effect, and thus make corrections to the power spectral density spectra measured with the LTP.
The consistency of different instruments and methods for measuring two-dimensional (2D) power spectral density (PSD) distributions are investigated. The instruments are an interferometric microscope, an atomic force microscope (AFM) and the X-ray Reflectivity and Scattering experimental facility, all available at Lawrence Berkeley National Laboratory. The measurements were performed with a gold-coated mirror with a highly polished stainless steel substrate. It was shown that these three techniques provide essentially consistent results. For the stainless steel mirror, an envelope over all measured PSD distributions can be described with an inverse power-law PSD function. It is also shown that the measurements can be corrected for the specific spatial frequency dependent systematic errors of the instruments. The AFM and the X-ray scattering measurements were used to determine the modulation transfer function of the interferometric microscope. The corresponding correction procedure is discussed in detail. Lower frequency investigation of the 2D PSD distribution was also performed with a long trace profiler and a ZYGO GPI interferometer. These measurements are in some contradiction, suggesting that the reliability of the measurements has to be confirmed with additional investigation. Based on the crosscheck of the performance of all used methods, we discuss the ways for improving the 2D PSD characterization of X-ray optics.
A procedure and software have been developed to transform the area distribution of the residual surface heights available from the measurement with the Micromap interferometric microscope into a two-dimensional (2D) power spectral density (PSD) distribution of the surface height. The procedure incorporates correction of one of the spectral distortions of the PSD measurement. The distortion appears as a shape difference between the tangential and sagittal PSD spectra deduced from the 2D PSD distribution for an isotropic surface. A detailed investigation of the origin of the anisotropy was performed, and a mathematical model was developed and used to correct the distortion. The correction employs a modulation transfer function (MTF) of the detector deduced analytically based on an experimentally
confirmed assumption about the origin of the anisotropy due to the asymmetry of the read-out process of the instrument's CCD camera. The correction function has only one free parameter, the effective width of the gate-shaped apparatus function which is the same for both directions. The value of the parameter, equal to 1.35 pixels, was found while measuring the 2D PSD distribution of the instrument self-noise, independent of spatial frequency. The effectiveness of the developed procedure is demonstrated with a number of PSD measurements with different X-ray optics including mirrors and a grating.
A data acquisition technique and relevant program for suppression of one of the systematic effects, namely the 'ghost' effect, of a second generation long trace profiler (LTP) is described. The 'ghost' effect arises when there is an unavoidable cross-contamination of the LTP sample and reference signals into one another, leading to a systematic perturbation in the recorded interference patterns and, therefore, a systematic variation of the measured slope trace. Perturbations of about 1-2 μrad have been observed with a cylindrically shaped X-ray mirror. Even stronger 'ghost' effects show up in an LTP measurement with a mirror having a toroidal surface figure. The developed technique employs separate measurement of the 'ghost'-effect-related interference patterns in the sample and the reference arms and then subtraction of the 'ghost' patterns from the sample and the reference interference patterns. The procedure preserves the advantage of simultaneously measuring the sample and reference signals. The effectiveness of the technique is illustrated with LTP metrology of a variety of X-ray mirrors.
We report the experience of the Advanced Light Source group in designing and building a series of nine electroless nickel-plated invar mirrors. The first four mirrors constructed appeared initially to be good but later it became evident that the nickel plating on all nine had been done improperly. The problem first appeared as blister-like defects about half a micron high and one to three centimeters wide. The cause turned out to be local separation of the plating from the substrate. In this paper we discuss the technical issues involved in building mirrors from invar and in preparing for and applying the needed electroless nickel coatings. We describe the studies that we carried out to evaluate the questions of adhesion, stress and polishability and report broad success in remanufacturing four of the mirrors. At time of writing one of the four has met specification showing good figure (0.8 μr rms) and finish (6 Å rms).
Today the Long Trace Profiler (LTP) is widely accepted as a viable way to measure X-ray mirrors, and at some institutions is the only instrument available for measuring long, high- curvature aspheres. Although some questions of absolute accuracy over the entire LTP measurement range remain unanswered, a comparison of LTPs can still be made to assess measurement variation. Recently a round robin survey of some LTPs within the United States has been made using a single set of mirrors. These mirrors were used to characterize the performance of an LTP over its advertised range of operation. The results of this survey are presented here.
Error reduction techniques for the long trace profiler are presented. Techniques that have been used for years are critiqued, and new methods are suggested.
The toroidal, silicon mirror on microdiffraction beamline 7.3.3 at the Advanced Light Source provides a 1:1 focus of the bend magnet source. The mirror is bent by two lead springs that are bolted to it through a pari of adhesive bonded end blocks. Because of the high loads that these adhesive joints must carry, three specific features of the bonds were tested: bondline geometry of the mating end blocks, surface preparation of the adherends, and strength of the adhesive. Bond strengths were evaluated by loading small test mirrors to failure using two epoxies under two different conditions of surface preparation - acid etching and simple UHV cleaning. In addition, the mirror's temperature distribution and figure errors were calculated with an Ansys Finite Element Model. The model's predictions were correlated to long trace profilometry as well as x-ray focus measurements.
The Long Trace Profiler, an instrument for measuring the slope profile of long X-ray mirrors, has been used for adjusting bendable mirrors. Often an elliptical profile is desired for the mirror surface, since many synchrotron applications involve imaging a point source to a point image. Several techniques have been used in the past for adjusting the profile measured in height or slope of a bendable mirror. Underwood et al. have used collimated X- rays for achieving a desired surface shape for bent glass optics. Nonlinear curve fitting sing the simplex algorithm was later used to determine the best fit ellipse to the surface under test. A more recent method uses a combination of least squares polynomial fitting to the measured slope function in order to enable rapid adjustment to the desired shape. The mirror has mechanical adjustments corresponding to the first and second order terms of the desired slope polynomial, which correspond to defocus and coma, respectively. The higher order terms are realized by shaping the width of the mirror to produce the optimal elliptical surface when bent. The difference between desired and measured surface slope profiles allows us to make methodical adjustments to the bendable mirror based on changes in the signs and magnitudes of the polynomial coefficients. This technique gives rapid convergence to the desired shape of the measured surface, even when we have no information about the bender, other than the desired shape of the optical surface. Nonlinear curve fitting can be used at the end of the process for fine adjustments, and to determine the over all best fit parameters of the surface. This technique cold be generalized to other shapes such as toroids.
An elliptically bent mirror of total length 1.25 m has been developed at the Advanced Light Source (ALS) for focusing soft x-rays. The mirror is used to produce a small, high flux density illuminated field of view for a Photo-Emission Electron Microscope. The requirement to collect the maximum horizontal aperture with the need to highly demagnify the source leads to a mirror with a wide range of curvatures along the surface. This combined with the need to produce a low slope error surface at a reasonably low cost has required us to develop a mirror based on the controlled bending of a flat substrate. This is an extension of several other mirror projects at the ALS where controlled bending of glass and metal substrates has been used in micro-focusing applications. Those mirrors however are a maximum of 200 mm long, and in this paper we describe the new challenges we have faced and the solutions we have adopted in developing a long and highly elliptical mirror. The mirror described here is manufactured from a low carbon steel (1006) which is capable of good dimensional stability, it is electroless nickel plated for polishing, and is bent into an elliptical shape by the application of unequal couples. We describe the mirror fabrication process, the mechanical details of the bending mechanism and the experimentally measured slope error from an ellipse. The final mirror has an rms roughness of 6 angstroms (rms), a full aperture (1.1 m) slope error of 14 (mu) rad (rms), and a slope error of < 3 (mu) rad when optimized over approximately 2/3 of the required optical length (0.917 m).
There are a number of ray trace programs currently used for the design of synchrotron beamlines. While several of these programs have been written and used mostly within the programmer's institution, many have also been available to the general public. This paper discusses three such programs. One is a commercial product oriented for the general optical designer (not specifically for synchrotron beamlines). One is designed for synchrotron beamlines and is free with restricted availability. Finally, one is designed for synchrotron beamlines and is used primarily in one institution. The wealth of information from general optical materials and components catalogs is readily available in the commercial program for general optical designs. This makes the design of an infrared beamline easier from the standpoint of component selection. However, this program is not easily configured for synchrotron beamline designs, particularly for a bending magnet source. The synchrotron ray trace programs offer a variety of sources, but generally are not as easy to use from the standpoint of the user interface. This paper shows ray traces of the same beamline using Optikwerks, SHADOW, and RAY, and compares the results.
Although x-ray micro-foci can be produced by a variety of diffractive methods, grazing incidence mirrors are the only route to an achromatic focus. In this paper we describe our efforts to produce elliptically shaped mirrors with the very high figure accuracy necessary for producing a micro-focus. The motivation for this work is provided by the need to produce achromatic foci for a range of applications ranging from tunable micro-focus x-ray photoelectron spectroscopy ((mu) -XPS) at soft x-ray energies to micro-focus white beam x-ray diffraction ((mu) -XRD) at hard x-ray energies. We describe the methodology of beam bending, a practical example of a system we have produced for (mu) -XRD, and results demonstrating the production of a surface with micro-radian figure accuracy.
The long trace profiler (LTP) has been used to assure the quality of x-ray beamline mirrors with fixed radii of curvature after they have arrived from the manufacturer and before they are permanently set in the beamline. In the case of some adjustable radius mirrors, the adjustment mechanism may require setting outside of the beamline. The bending mechanism often bends the mirror into a tangential cylinder, which is difficult to measure by interferometric methods. Measuring medium- to long-radius cylinders is a routine task for the LTP. Thus, a bendable (adjustable radius) mirror may be adjusted in the metrology laboratory before the mirror is placed in the beamline. This paper describes the method of adjustment and surface quality assessment for bendable, adjustable radius mirrors in general, and shows results for a small mirror that is bent into an elliptical cylinder.
We give a brief summary of the requirements for water cooled optical components for the Advanced Light Source (ALS), a third generation synchrotron radiation source under construction at Lawrence Berkeley Laboratory (LBL). Materials choices, surface figure and smoothness specifications, and metrology systems for measuring the plated metal surfaces are discussed. Results from a finished water cooled copper alloy mirror will be used to demonstrate the state of the art in optical metrology with the Takacs Long Trace Profiler (LTP II).
Over the last several years the long trace profiler (LTP) has been evolving into a sophisticated machine capable of measuring surface profiles of very long dimensions. This report explains improvements, both hardware and software, that have helped to achieve accuracies and ranges in surface profiling that have been unobtainable until now. A comparison made by measuring standard optical surfaces on other instruments corroborates these accuracies.
The continuing effort to develop, design, construct, and evaluate the performance of an airborne autonomous wavemeter for tuning solid state lasers is summarized. The wavemeter supports tunable solid state lasers that are used for an atmospheric remote sensing technique. One atmospheric species, water vapor, is measured by tuning one laser to precisely the line center of a water vapor line and by tuning another laser off the line. The two sets of received backscattered radiation are ratioed and corrected to determine the vertical profiles of water vapor. On a spacecraft platform, an advanced system could monitor the vertical water vapor profiles. This would provide a technology improvement for meteorological forecasting.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.