Long Trace Profiler Research Articles

Stitching interferometry for long X-ray mirrors with lateral multi-shift-based absolute calibration.

To eliminate the reference mirror (REF) error of the Fizeau interferometer for measuring X-ray mirrors, the reference calibration method of lateral multi-shift measurements at the hundreds-micrometer pixel level is presented. Because of the high aspect ratio of long X-ray mirrors, by shifting the surface under test (SUT) along the tangential direction with integer multiple pixels, we extend the calibration method by using the difference between multiple shifted measurements to build an augmented multi-matrix for extracting the two-dimensional (2D) absolute surface. The method can be applied to arbitrary measurement regions of the test optics, and the measurement for both the benchmark sub-aperture and calibration of the REF is accomplished in a single measuring process. Furthermore, by adjusting the shift to the millimeter scale, reference-subtracted sub-apertures can be stitched to obtain the absolute 2D map of the X-ray mirror. Experimental results show that all the 2D discrepancies reach sub-nanometer repeatability, and comparison results between the long trace profiler (LTP) and the proposed method have been performed. Therefore, the results demonstrated that the proposed method meets the requirements of X-ray mirror measurements.

Application of a long-trace profiler in situ at TPS beamlines

To achieve an ultrahigh-resolution with soft x-ray beamlines at the Taiwan Photon Source (TPS), the slope error of a highly precise grating must be at the level of 0.1 μrad (RMS) under thermal loading with various curvatures. Some optics generally operate under a high power density from an undulator magnet on a beamline. The thermal load introduces a thermal bump in the optics profile and degrades the beamline performance, such as the energy resolution and beam size. Hence, active X-ray mirrors are emerging as primary solutions to meet the performance requirements of TPS. First, active X-ray optics were assembled in the laboratory and bent to the desired surface using an in-house long-trace profiler (LTP). When the optics were installed in the beamlines, LTP in situ (ISLTP) was used to fine-tune the mirrors under various operating conditions. This paper introduces the design and construction of ISLTP with a precision of 0.1 μrad (RMS) developed to measure the mirror profile in soft x-ray beamlines. The ISLTP can provide feedback perturbations owing to localised heat loading from the X-ray beam and thermal changes in the ambient environment, and its performance and in situ measurements are presented.

Nano-precision metrology of X-ray mirrors with laser speckle angular measurement

X-ray mirrors are widely used for synchrotron radiation, free-electron lasers, and astronomical telescopes. The short wavelength and grazing incidence impose strict limits on the permissible slope error. Advanced polishing techniques have already produced mirrors with slope errors below 50 nrad root mean square (rms), but existing metrology techniques struggle to measure them. Here, we describe a laser speckle angular measurement (SAM) approach to overcome such limitations. We also demonstrate that the angular precision of slope error measurements can be pushed down to 20nrad rms by utilizing an advanced sub-pixel tracking algorithm. Furthermore, SAM allows the measurement of mirrors in two dimensions with radii of curvature as low as a few hundred millimeters. Importantly, the instrument based on SAM is compact, low-cost, and easy to integrate with most other existing X-ray mirror metrology instruments, such as the long trace profiler (LTP) and nanometer optical metrology (NOM). The proposed nanometrology method represents an important milestone and potentially opens up new possibilities to develop next-generation super-polished X-ray mirrors, which will advance the development of X-ray nanoprobes, coherence preservation, and astronomical physics.

Metrology for optical components of Taiwan Photon Source beamlines.

To ensure the high performance of beamlines in Taiwan Photon Source, the metrology of the beamline optical components is essential to realize the theoretical optical simulation. The design and performance of a long-trace profiler (LTP) with moving optical head is described. A LTP is used as a metrological tool for the assembly and correction of optical components at the beamline. For a water-cooled mirror, a LTP serves to optimize the surface profile to a slope error of 0.1 µrad (rms) and adequate thermal conductance. To correct a bendable cooling mirror in a twisted condition, the sagittal slope is decreased by a screw adjustment through the LTP measurement. For the double-crystal monochromator (DCM) with flow-induced vibrations, we proposed an approach to stiffen the bellows hose partially inside the vacuum chamber. From an investigation of the vibrational spectra of the DCM before and after the treatment with the aid of the autocollimator, the current approach is confirmed to be an effective solution for suppression of vibration.

Development of a long trace profiler in situ at National Synchrotron Radiation Research Center.

To achieve an ultrahigh resolution of a beamline for soft X-rays at the Taiwan Photon Source (TPS), the profile of a highly precise grating is required at various curvatures. The slope error could be decreased to 0.1 µrad (rms) at a thermal load with a specially designed bender having 25 actuators. In the meantime, a long-trace profiler (LTP) was developed in situ to monitor the grating profile under a thermal load; it consists of a moving optical head, an air-bearing slide, an adjustable stand, and a glass viewport on the vacuum chamber. In the design of this system, a test chamber with an interior mirror was designed to simulate the chamber in the beamline. To prevent an error induced from a commercial viewport, a precision glass viewport (150CF, flatness 1/150 λ rms at 632.8 nm) was designed. The error induced from the slope error of the glass surface and the vacuum deformation was also simulated. The performance of the optical head of the LTP in situ (ISLTP) has been tested in the metrology laboratory. The sources of error of this LTP including the linearity and the glass viewport were corrected after the measurement. For the beamline measurement, an optical head was mounted outside the vacuum chamber; the measuring beam passed through the glass viewport to measure the grating profile in vacuum. The measurement of the LTP after correction of the above errors yielded a precision about 0.2 µrad (rms). In a preliminary test, an ISLTP was used to measure the grating profile at soft X-ray beamline TPS45A. The measured profile was for the bending mechanism to optimize the slope profile. From the measured energy spectrum, the slope error of the grating was estimated with software for optical simulation to be about 0.3 µrad (rms), consistent with our estimate of the ISLTP. In the future, it will be used to monitor the thermal bump under a large thermal load. In addition, an ISLTP was used to monitor the properties of optical elements-the twist and radius in the beamline during the installation phase.

The upgraded LTP-V at SLS

Since 2005 the Swiss Light Source (SLS) has been operating a Long Trace Profiler (LTP)-V from Ocean Optics in its metrology laboratory to measure the synchrotron optics for SLS. In 2012 we finished a significant upgrade to improve the accuracy, reliability and measurement efficiency in particular for the calibration of adaptive optics. Folding mirrors with figure errors <λ/100 and an additional linear encoder have been installed, the 1d CCD detector with 2048 pixels has been replaced by a 16 mega-pixel CCD camera with gigabit ethernet interface GigE, the monolithic software has been replaced by a modular, full- EPICS compatible system based on a new LTP plugin for the areaDetector software for image processing. The plugin allows slope determination in real time i.e. per frame.

Developmental long-trace profiler using optimally aligned mirror-based pentaprism

A low-budget surface slope-measuring instrument, the developmental long-trace profiler (DLTP), was recently brought into operation at the Advanced Light Source Optical Metrology Laboratory. The instrument is based on a precisely calibrated autocollimator and a movable pentaprism. The capability of the DLTP to achieve submicroradian surface slope metrology has been verified via cross-comparison measurements to other high-performance slope-measuring instruments when measuring the same high-quality test optics. Further improvement of the DLTP is achieved by replacing the existing bulk pentaprism with a specially designed mirror-based pentaprism, which offers the possibility to eliminate systematic errors introduced by inhomogeneity of the optical material and fabrication imperfections of a bulk pentaprism. We provide the details of the mirror-based pentaprism design and describe an original experimental procedure for precision mutual alignment of the mirrors. The algorithm of the alignment procedure and its efficiency are verified with rigorous ray-tracing simulations. Results of measurements of a spherically curved test mirror and a flat test mirror using the original bulk pentaprism are compared to measurements using the new mirror-based pentaprism, demonstrating the improved performance.

大口径大曲率半径光学元件的高精度检测

For lack of proper metrologic instruments to measure optical components with large-apertures and large radii of curvature,this paper investigates the feasibility to measure large radius of curvature(positive,negative) by using a Long Trace Profiler(LTP).The experiments are carried out to verify its advantages.The algorithmic precision of radius of curvature tested with the LTP is calculated,then the radius of curvature of full-aperture is figured out by a rotational measurement in a proper mechanical stage.Finally,obtained tested results are compared with those of a spherometer and a knife-edge tester.For the spherical surface with R=37.108 m and R=41.065 m,the repeatability of the LTP is in 0.05%,which means the relative discrepancy between the LTP and the spherometer or the knife-edge tester is under 0.05%.These results indicate that LTP is capable of measuring components with large radii of curvature in high-precision.

用长程轮廓仪测量大曲率半径元件的精度及相关分析

The long trace profiler (LTP) is proposed to measure radius of curvature (R) and surface figure of a longradius spherical surface in an optical shop. Equipped with a motorized rotary stage and a two-dimensional tilt stage, the LTP scans the full aperture and calculates the absolute radius of curvature of each scanning line based on the least square method. Nonlinear error and manufacture error difference between center and the edge are obtained by comparing R results. The R-limit is validated and expressed as D/R, where D is the aperture of the mirror under test. A full-aperture three-dimensional figure is also reconstructed based on triangle interpolation.

Upgrade of long trace profiler for characterization of high-precision X-ray mirrors at SPring-8

The long trace profiler (LTP) at SPring-8 has been upgraded to improve stability and resolution of slope measurement. The performances of the upgraded LTP at SPring-8 are presented by cross-checking measurements on a flat mirror with data obtained using Nanometer Optical Component Measuring Machine (NOM) at the Helmholtz Zentrum Berlin / BESSY-II.

SPring-8 におけるX 線ミラーのための表面形状計測装置 LTP(Long trace profiler)の開発

SPring-8 におけるX 線ミラーのための表面形状計測装置 LTP(Long trace profiler)の開発

Long-trace profiler for neutron focusing mirrors

A long-trace profiler (LTP) optimized for measuring the shape of large neutron supermirrors has been designed and built. This LTP can measure 1.6 m long mirrors in both vertically and horizontally deflecting geometries, which is essential to achieve best performance from bendable mirrors. The LTP suppresses the influence of angular deviations of the linear-stage carriage during translation with a pentaprism and a cylindrical lens. The stationary optical head and the carriage-mounted pentaprism are precisely aligned to rotate about the optical axis between the two components. This feature allows measurements to be made on mirrors mounted vertically, horizontally or at any angle in between. The LTP software allows for rapid optimization of parameters for dynamically bent elliptical mirrors. Here we describe the motivation for the LTP, the design, and a first application of the LTP to study the effect of gravity on a bent microfocusing neutron supermirror.

A Shack–Hartmann measuring head for the two-dimensional characterization of X-ray mirrors

The recent development of short-wavelength optics (X/EUV, synchrotrons) requires improved metrology techniques in terms of accuracy and curvature dynamic range. In this article a stitching Shack-Hartmann head dedicated to be mounted on translation stages for the characterization of X-ray mirrors is presented. The principle of the instrument is described and experimental results for an X-ray toroidal mirror are presented. Submicroradian performances can be achieved and systematic comparison with a classical long-trace profiler is presented. The accuracy and wide dynamic range of the Shack-Hartmann long-trace-profiler head allow two-dimensional characterizations of surface figure and curvature with a submillimeter spatial resolution.

Design of multiple-function long trace profiler

The long trace profiler (LTP) is a useful optical metrology instrument for measuring the figure and slope error of cylindrical aspheres commonly used as synchrotron radiation (SR) optics. A multiple-function LTP (LTP-MF) is developed at the optical metrology laboratories of Brookhaven National Laboratory (BNL) with a goal of increasing the accuracy of the measurement and extending the range of applications to other areas. Characteristics of the LTP-MF are a very compact and lightweight optical head, a large angular test range (±16 mrad), and high accuracy. The LTP-MF can be used in various configurations: as a laboratory-based LTP, as an in situ LTP or penta-prism LTP, as an angle monitor, as a portable LTP, and as a small-radius-of-curvature test instrument. The system configuration, designs of the compact optical head and slide, and analysis of different measurements modes and systematic error correction methods are introduced. Preliminary comparison measurements between the LTP-MF and the LTP-II are presented.

Determination of groove spacings for concave diffraction gratings

A long trace profiler (LTP) based method has been reported to measure the groove density of a concave grating with a curvature radius 14346.8 mm. It is appropriate for gratings with a sufficiently large curvature radius. Measurements of the groove density of a concave or convex grating with a small curvature radius could be problematic. A system based on a diffraction method and developed to check the groove variation of variable line spacing (VLS) gratings was used to measure a concave grating at NSRL recently and is the subject of this paper. For a grating with a small radius of curvature, the off-axis errors become dominant factors determining the system uncertainty. The errors are evaluated, and a correction algorithm is developed. A commercially available concave grating with a curvature radius of 750 mm was measured. The maximum relative groove density error (ΔN/N) was 3×10−5.

Profile coating and its application for Kirkpatrick-Baez mirrors

For microfocusing x-ray mirrors, an elliptical shape is essen- tial for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. A differential coating method to con- vert a cylindrical mirror to an elliptical one has been previously reported. The coating was obtained by varying the sputter source power over a moving substrate. Here we report a new method of profile coating: the sputter source power is kept constant, while the substrate is passed over a contoured mask at a constant speed to obtain a desired profile along the direction perpendicular to the direction of substrate motion. The shape of the contour depends on the desired profile and the thickness distribution directly above the gun at the substrate level. The thickness distribution was measured on films coated on Si wafers using a spectro- scopic ellipsometer with computer-controlled X-Y translation stages. A model was developed to fit the measured thickness distribution, which determines the relative thickness weightings. When the substrate moves during a deposition, the film thickness is proportional to the length of the opening on the shield can along the direction of motion. By equating the sum of relative weightings to the required relative thickness at the same position, the length of the opening at that position can be determined. By repeating the same process for the whole length of the required profile, a contour can be obtained for a desired thickness profile. The number of passes and the speed of the substrate are determined according to the required thickness and the growth-rate calibration of a test run. The mir- ror coating profile is determined from the difference between the ideal surface figure of a focus ellipse and the surface figure obtained from a long-trace profiler measurement on the substrate. A Kirkpatrick-Baez (KB) mirror pair was made using Au as a coating material and cylindri- cally polished mirrors as substrates. Synchrotron x-ray results using this KB mirror pair showed a focused spot size of 0.430.4 mm 2 . © 2003

Technique for measuring the groove density of diffraction gratings using the long trace profiler

Diffraction gratings are the most efficient systems for dispersing radiation in the soft x-ray and extreme ultraviolet region. They can have a constant periodic structure or a variable line spacing. The performance of an instrument adopting optics depends on the grating manufacturing errors. These could be both slope errors (routinely measured by various instruments) and undesired groove density variation. The measurement of the groove density is mainly performed with home-made setups, which use the diffraction pattern generated by a laser or, sometimes, the images collected with an atomic force microscope or a scanning tunneling microscope. With the use of a long trace profiler (LTP), an instrument developed to measure the longitudinal profile of a mirror, a method to measure the groove density (and the groove density variation) of a surface diffraction grating was developed. It consists of the measurement of the angle of a particular diffraction order, taking advantage of the extreme angular precision of the LTP. An accuracy of one part per million, or better, in the determination of the groove density constancy/variation, is reached. This method demonstrates the great versatility of the LTP and, in particular, its infinite depth of field.

Equal optical path beamsplitter for a pencil beam interferometer and shearing interferometer

To produce stable beams used by pencil beam interferometers, a new equal optical path beamsplitter (EBS) is developed. The beamsplitter divides one incident pencil beam into two parallel exiting beams with a fixed separation. The new EBS is a monolithic unit comprised of two right-angle trapezoidal prisms with a thin film beamsplitting coating on the faces, which are cemented together. Adjusting the dimensions of each element according to a simple algebraic relation produces two identical, parallel beams separated by a fixed distance with a zero optical path difference between them. The monolithic EBS is very simple and stable. It has applications in interferometers that require the use of a pencil beam source, such as the long trace profiler (LTP) and the precise angle monitor. If a large-diameter wavefront is incident on the EBS, it can be used in shearing interferometer applications, such as the lateral translation shearing interferometer and the rotational shearing interferometer. Use of the EBS in all of these configurations significantly improves instrument stability and accuracy. Some application examples and test results are presented.

Long-trace profiler with cyclic optical configuration

The results of the development of a much simpler optical configuration for the long-trace profiler (LTP) are presented. The current technique employs a cyclic optical configuration to achieve zero optical path difference for the closely spaced, laterally separated laser beams. The accuracy of measurement is found to remain as good as that in the case of the widely used LTP, although the geometrical alignment problem is simplified significantly.

Portable long trace profiler: Concept and solution

Since the early development of the penta-prism long trace profiler (LTP) and the in situ LTP, and following the completion of the first in situ distortion profile measurements at Sincrotrone Trieste (ELETTRA) in Italy in 1995, a concept was developed for a compact, portable LTP with the following characteristics: easily installed on synchrotron radiation beam lines, easily carried to different laboratories around the world for measurements and calibration, convenient for use in evaluating the LTP as an in-process tool in the optical workshop, and convenient for use in temporarily installation as required by other special applications. The initial design of a compact LTP optical head was made at ELETTRA in 1995. Since 1997 further efforts to reduce the optical head size and weight, and to improve measurement stability have been made at Brookhaven National Laboratory. This article introduces the following solutions and accomplishments for the portable LTP: (1) a new design for a compact and very stable optical head, (2) the use of a small detector connected to a laptop computer directly via an enhanced parallel port, and there is no extra frame grabber interface and control box, (3) a customized small mechanical slide that uses a compact motor with a connector-sized motor controller, and (4) the use of a laptop computer system. These solutions make the portable LTP able to be packed into two laptop-size cases: one for the computer and one for the rest of the system.