The optics group metrology laboratory (OML) is responsible for characterizing the optics mounted on the beamlines.
As smaller and smaller beams are required, there is a need for optics with ever greater curvatures and smaller slope errors.
The exacting specifications of the M4A mirror on the MICROFOCUS line responsible for making the image of the exit slit of the monochromator on the entrance of a spectrometer motivated the MLO group to review the measuring procedures. M4A is a strongly curved ellipse (R<25m) with a slope error specification of < 0.5 µrad instead of < 1 µrad normally required for such a curvature.
The Long Trace Profiler (LTP) at SOLEIL is an instrument that measures mirror slopes 1,2 directly and therefore the radius of curvature. Its reproducibility is 0.2 µrad, with a sensitivity of 0.12 µrad. With this type of mirror, simple calibration of the instrument is not enough: nonlinearity effects due to imperfections in the system’s optics also have to be taken into account. Based on the same principle as multivariate analysis3, the MLO group has developed a linearity error elimination procedure (LEEP) for the LTP, which consists of separating the errors introduced by the instrument, from the measurements of the optical surface (4).
Figure 1: Mirror measurements before and after using LEEP.
In order to evaluate its reliability, Amparo Rommeveaux, Head of the metrology laboratory of the ESRF optics group, applied this method to the same M4A MICROFOCUS mirror, using an LTP developed independently of the one at SOLEIL. The slope difference between measurements was less than 0.2 µrad, which corresponds in this case, to an upper margin of 1.6 nm.
The next stage will be to reach a measurement agreement of less than 1 nm. In fact, future nanofocusing beamlines with only diffraction limited optics will require this level of precision.
Figure 2 : Figure 2: Comparison of the LEEP method on the LTP both
at SOLEIL and ESRF.
Figure 3 : Measurement of M4A Microfocus mirror with the LTP at SOLEIL
Acknowledgements:
We would like to thank Amparo Rommeveaux from ESRF for his cooperation and trust in the project during the comparative measurements.
We would also like to extend our thanks to the members of the Microfocus group for their cooperation and permission to use the M4A mirror for our tests.
Further information:
1. A. Rommeveaux, M. Thomasset and D. Cocco, « The Long Trace Profiler”, Modern developments in X-Ray and neutron Optics, COST, Springer, p 181.
2. D. Cocco and M. Thomasset, “Measurement of groove density of diffraction gratings”, Modern developments in X-Ray and neutron Optics, COST, Springer, p 207.
3. P. Dumas, “Introduction à l’analyse multivariée”, Séminaire Section Scientifique Mesures, Methodes et Instrumentation, juillet 2008.
4. F. Polack, M. Thomasset, S. Brochet, A. Rommeveaux, “An LTP stitching procedure with compensation of instrument errors: Comparison of SOLEIL and ESRF results on strongly curved mirrors”, (2009), doi:1013/j.nima.2009.10.13.
