Speaker
Description
We report on a fast and reliable method for wavefront sensing in the soft X-ray domain, developed for the characterization of rotationally symmetric optical elements, like an ellipsoidal mirror shell. In our laboratory setup, the mirror sample is irradiated by an electron-excited (4.4 keV), micron-sized ($\approx 2\,\mu\textrm{m}$) fluorescence source (Carbon K$_{\alpha}$, 277 eV). The near-focal, 3-D intensity distribution $I(\vec{r})$ is recorded by a CCD camera ($512\times 512$ pixels à $13.5\,\mu\textrm{m}$) at multiple positions along the optical axis, displaced by $(20-25)\,\%$ from the focus. The transport-of-intensity equation is interpreted in a geometrical sense from plane to plane and implemented as a ray tracing code in Mathematica$^{\tiny{TM}}$ / Optica$^{\tiny{TM}}$, to retrieve the phase $\phi(\vec{r})$ from the radial intensity gradient on a sub-pixel scale. 15 intra-focal CCD image pairs are evaluated in this way and averaged to an annular 2-D map of the wavefront error. In units of the test wavelength (C K$_{\alpha}$), we find $\sigma=\pm 47\,\lambda$ (rms) and a P-V of $\pm 118\,\lambda$. The wavefront can be used in a threefold purpose: First, the focus is predicted with a result of $48.3\,\mu\textrm{m}$ (rms), in reasonable agreement with the direct experimental observation of $55.3\,\mu\textrm{m}$ (FWHM). Secondly, the combined figure and alignment error of the ellipsoid is reconstructed $-$ and again, the statistical mean of $\pm 9.4$ arcsec (rms) roughly coincides with independent estimations from the measured focal intensity distribution ($\pm 11.8$ arcsec). At last, a diffractive wavefront corrector may be computed and fabricated, for wavelength-dispersive spectroscopy with high efficiency and optimized resolution.
| Journal of Synchrotron Radiation Special Issue: will you submit your contribution? | yes |
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