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.

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

Author(s): McKinney, Wayne R. | Abstract: Brightness preservation requirements for ever brighter synchrotron radiation and free electron laser beamlines require surface slope tolerances of x-ray optics on the order of 0.2 mu rad, or better. Hence, the accuracy of dedicated surface slope metrology must be 0.1 mu rad, or even less. Achieving this level of measurement accuracy with the flagship instrument at synchrotron radiation metrology laboratories, the Long Trace Profiler (LTP), requires all significant sources of systematic, random, and instrumental drift errors to be identified, and reduced or eliminated. In this respect, the performance of certain components of the Advanced Light Source LTP-II design [Kirschman, et al., Proc. SPIE, 7077, 70770A-12 (2008)] is analyzed, considering the principal justification for inclusion of each component, possible systematic error due to the quality of its optical material, and drift effects due to generated heat, etc. We investigate the effects of replacement of the existing diode laser with a fiber-coupled laser light source, and demonstrate that reducing the number of components by using a single beam on the surface under test (SUT), rather than an original double beam maintains, or even improves the accuracy of measurement with our LTP. Based on the performance of the upgraded LTP, we trace the further steps for improving of the LTP optical system.

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Studies in Optimal Configuration of the LTP

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.

Design of multiple-function long trace profiler

For absolute measurement of groove density of diffraction gratings, we suggest a simple method that utilizes two parallel beams, a rotary beam splitter and single-beam long trace profiler. We use the fact that two parallel beams are focused on a point in the focal plane of a Fourier transform lens. Especially, with elimination of the eccentricity effect, the groove density of a diffraction grating was measured for higher precision than ever before. The eccentricity effect is a shift of the interaction points due to the off-centered rotating mount of grating, which is the essential error source in previous measurements. The method was applied to measure the groove density of a varied line spacing plane grating with a central groove density of 400 line/mm. The groove density error was measured to be 0.01 line/mm rms, with a period measurement error of 0.3 nm rms.

Technique for measuring the groove density of a diffraction grating with elimination of the eccentricity effect

A beam splitter to create two separated parallel beams is a critical unit of a pencil beam interferometer, for example the long trace profiler (LTP). The operating principle of the beam splitter can be based upon eitehr amplitude-splitting (AS) or wavefront-splitting (WS). For precision measurements with the LTP, an equal optical path system with two parallel beams is desired. Frequency drift of the light source in a non-equal optical path system will cause the interference fringes to drift. An equal optical path prism beam splitter with an amplitude-splitting (AS-EBS) beam splitter and a phase shift beam splitter with a wavefront-splitting (WS-PSBS) are introduced. These beam splitters are well suited to the stability requirement for a pencil beam interferometer due to the characteristics of monolithic structure and equal optical path. Several techniques to produce WS-PSBS by hand are presented. In addition, the WS-PSBS using double thin plates, made from microscope cover plates, has great advantages of economy, convenience, availability, and ease of adjustment over other beam splitting methods. Comparison of stability measurements made with the AS-EBS, WS-PSBS, and other beam splitters is presented.

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Dacheng Li

Papers

Principle of π-phase plate long trace profiler for synchrotron radiation optics