Shinan Qian

The penta‐prism LTP: A long‐trace‐profiler with stationary optical head and moving penta prisma)

Metrology requirements for optical components for third generation synchrotron sources are taxing the state‐of‐the‐art in manufacturing technology. We have investigated a number of effect sources in a commercial figure measurement instrument, the Long Trace Profiler II (LTP II), and have demonstrated that, with some simple modifications, we can significantly reduce the effect of error sources and improve the accuracy and reliability of the measurement. By keeping the optical head stationary and moving a penta prism along the translation stage, the stability of the optical system is greatly improved, and the remaining error signals can be corrected by a simple reference beam subtraction. We illustrate the performance of the modified system by investigating the distortion produced by gravity on a typical synchrotron mirror and demonstrate the repeatability of the instrument despite relaxed tolerances on the translation stage.

Precise measuring method for detecting the in situ distortion profile of a high-heat-load mirror for synchrotron radiation by use of a pentaprism long trace profiler.

A precise measuring method for detecting the in situ distortion profile of a high-heat-load mirror for synchrotron radiation by use of a pentaprism long trace profiler (LTP) is presented. A maximum distortion of 0.47 microm across a length of 180 mm was measured for an internally water-cooled mirror on an undulator beam line at ELETTRA while exposed to a total emitted power of 600 W. This first successfully tested in situ distortion profile points out the importance and need for this method. Two configurations for performing in situ LTP tests are discussed. For this measurement the configuration with all the equipment external to the vacuum chamber was used. The experiment has an accuracy and a repeatability of 0.04 microm. Suggestions for improving the accuracy and stability are presented.

In situ surface profiler for high heat load mirror measurement.

High heat loads on optical components in next-generation synchrotron radiation sources will require the use of sophisticated methods to prevent surface distortion that would degrade the intrinsic source brightness. In some cases it is desirable to be able to measure the mirror figure under actual operating conditions in ultrahigh vacuum. We propose to modify the standard long-trace profiler configuration to enable scanning profiler measurement of mirrors under actual high heat load conditions. The modification entails the use of a penta prism on a translation stage inside the vacuum chamber, with the optical head mounted outside the chamber. This configuration is similar to the original pencil-beam interferometer system developed by von Bieren, but it contains a number of modifications that enhance its accuracy.

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.

Wave front-splitting phase shift beam splitter for pencil beam interferometer

A beam splitter to create two separated parallel beams is a critical part of the pencil beam interferometer. Beam splitters can be separated into two categories based upon their operating principle: amplitude splitting or wave front splitting. A wave front-splitting phase shift beam splitter (WS-PSBS) with a monolithic structure and equal optical path is described that is well suited to the stability requirements of a pencil beam interferometer. Several techniques to produce a WS-PSBS are presented. In addition, the WS-PSBS using double thin plates or a single plate, made from microscope cover plates, has great advantages of economy, convenience, availability, and ease of adjustment over other types of beam splitters. A comparison of stability measurements made with the WS-PSBS and other equal optical path beam splitters is presented. The pencil beam interferometer using the WS-PSBS can achieve a stability of 0.45 μrad root mean square over 15–24 h periods.

Accuracy Limitations in Long-Trace Profilometry

As requirements for surface slope error quality of grazing incidence optics approach the 100 nanoradian level, it is necessary to improve the performance of the measuring instruments to achieve accurate and repeatable results at this level. We have identified a number of internal error sources in the Long Trace Profiler (LTP) that affect measurement quality at this level. The LTP is sensitive to phase shifts produced within the millimeter diameter of the pencil beam probe by optical path irregularities with scale lengths of a fraction of a millimeter. We examine the effects of mirror surface “macroroughness” and internal glass homogeneity on the accuracy of the LTP through experiment and theoretical modeling. We will place limits on the allowable surface “macroroughness” and glass homogeneity required to achieve accurate measurements in the nanoradian range.

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.

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.

Penta-Prism Long Trace Profiler (PPLTP) for measurement of grazing incidence space optics

The long trace profiler (LTP) is in use at a number of locations throughout the world for the measurement of the figure and mid-frequency roughness of x-ray mirrors. The standard configuration requires that the surface tested lie in a horizontal plane as the optical head is scanned along a horizontal line. For applications where gravity-induced sag of the surface cannot be tolerated, such as in x-ray telescope mirror metrology, it is desirable to measure the mirror as it is mounted in a vertical configuration. By making simple modifications to the standard LTP system, we have demonstrated that it is possible to use the LTP principle to measure the surface of x-ray mirrors and mandrels mounted in the vertical orientation. The major change in the LTP system is the use of a penta prism on a vertical translation stage to direct the probe beam onto the surface and the addition of a precision rotation stage to hold the test object. A 3D map of the surface topography of the complete cylindrical asphere can be generated quite easily with this technique. Measurements with a prototype system indicate a slope error accuracy of better than 1 microradian is possible, with a figure error repeatability of better than 50 nm.

Large-mirror figure measurement by optical profilometry techniques

In most cases, conventional interferometric methods can be used to test the figure of large spherical and flat optical components. There are, however, certain types of unconventional surfaces, such as those used in grazing incidence x-ray applications, that are nearly impossible to test by conventional means. These cylindrical aspheres are usually tested by some type of scanning optical profiler. We discuss the use of a versatile slope measuring scanning interferometer system, the Long Trace Profiler (LTP), in measuring the figure error of large surfaces, particularly those that have extremely long radii of curvature in the tangential direction. Use of this instrument in different configurations has permitted measurements to be made on cylindrical asphere segments that are over one meter long, on water-cooled high heat load mirrors in ultra high vacuum synchrotron beam lines under actual operating conditions, and on complete x-ray telescope mirror shells and mandrels that are mounted in a vertical configuration to minimize gravity sag errors. Each of these different configurations has its own particular advantages and shortcomings. The ultimate performance of the LTP depends upon the thermal stability of the local environment. We illustrate the effects that temperature variations on the order of plus or minus 0.1 degrees Celsius have on the errors in the measurement of a long radius sphere.