For vortex beams, characterization and optimization of the optical system are important. However, wavefront measurements on focused vortex beams are difficult because they have complex phase and intensity distributions. As a measurement method, we proposed the use of ptychography, in which the intensity and phase of the beams are retrieved using several far-field diffraction patterns. We constructed an optical system with a He-Ne laser light source to clarify the usefulness of ptychography. Test vortex beams were produced by a spatial light modulator (SLM) and focused by a plano-convex lens. A pinhole was scanned on the focal plane for collection of the diffraction intensity profiles. The phase and intensity of the vortex beams on the focal plane were retrieved so that the calculated beams were consistent with the intensity data. The retrieved intensity and phase distributions were compared with distributions predicted using the inputs for the SLM. They agreed well, indicating that the ptychographic phase retrieval method can be used for precise characterization of vortex beams. This method is valuable for improving the performance of applications using vortex beams.
Focusing and imaging optics can be characterized by evaluating the wavefront error of the focused beam. We have bean developing a ptychographic phase retrieval method using a visible laser to measure the wavefront error. In this study, the measurement accuracy of the method is increased by improving both the phase retrieval algorithm and the experimental setup. The system is applied to the characterization of an ellipsoidal mirror used for the focusing of soft X-rays. The posture of the mirror can be measured with a resolution of 1.4 μrad. The wavefront error originating from the surface profile error can be detected with an accuracy of 0.01λ (root mean square).
It is possible to achieve soft X-ray nanofocusing with a high efficiency and no chromatic aberration by using an ultraprecise ellipsoidal mirror. Surface figure metrology is key in the improvement of surface figure accuracy. In this study, we propose a ptychographic phase retrieval method using a visible light laser to measure the surface figure error profile of an ellipsoidal mirror. We introduce a simple experimental system for ptychographic phase retrieval and demonstrate the basic performance of the proposed system. Obtainable wavefront information provides both the figure error and the alignment of the ellipsoidal mirror that yield the best focusing. This developed method is required for offline adjustments when an ellipsoidal mirror is installed in the beamline of synchrotron radiation or X-ray free-electron laser light sources.
Mirrors are key devices for creating various systems in optics. Focusing X-ray and extreme ultraviolet (EUV) light requires mirror surfaces with an extremely high accuracy. The figure of an ellipsoidal mirror is obtained by rotating an elliptical profile, and using such a mirror, soft X-ray and EUV light can be focused to dimensions on the order of nanometers without chromatic aberration. Although the theoretical performance of ellipsoidal mirrors is extremely high, the fabrication of an ideal ellipsoidal mirror remains problematic. Based on this background, we have been working to develop a fabrication system for ellipsoidal mirrors. In this proceeding, we briefly introduce the fabrication process and the soft X-ray focusing performance of the ellipsoidal mirror fabricated using the proposed process.
The ellipsoidal mirror is one of the most effective achromatic focusing optic with large aperture and nanofocusing ability.
Because of the large aperture of mm-order size, this optic is suitable for a laboratory-based light source that has a large
divergence angle. Recently, soft X-rays produced by high-order harmonics have become available. Such a beam has high
spatial coherency but relatively large divergence angle. This light in combination with an ellipsoidal mirror will generate
a highly intense focusing nanobeam that will contribute to various experiments and analyses such as those of
photoelectron spectroscopy and nonlinear optical phenomena. In this paper, we present the optical design for a lab-based
soft X-ray beamline and the results of optical simulation considering the parameters of the source. Finally, we introduce
a two-stage focusing system with an axisymmetric mirror as a promising soft X-ray focusing system.
An ellipsoidal mirror is a promising type of X-ray mirror, because it can focus X-rays to nanometer size with a very
large aperture and no chromatic aberration. However, ideal ellipsoidal mirrors have not yet been realized by any
manufacturing method. This is partly because there is no evaluation method for its surface figure profile. In this paper,
we propose and develop a method for measuring surface figure profile of ellipsoidal mirrors using phase retrieval. An
optical design for soft X-ray focusing, the employed phase retrieval method and an experimental optical system
specialized for wavefront measurement using a He-Ne laser are reported.