1. Optics Metrology at ALBA
Josep Nicolas
ALBA synchrotron light Facility

2. Optics Metrology at ALBA
Josep Nicolas
ALBA synchrotron light Facility
Outline
The Optics Lab
The NOM instrument
Some applications

3. Motivation… Mirrors and gratings must be measured…
Mirrors and gratings for beamlines are critical:
custom made for the beamline.
have long delivery time.
Expensive.
One cannot have spares.
In a 3rd generation light source, slope error of optics is usually the limit for optical performance of the beamline:
Flux density on SAMPLE
Spectral resolution
spot size on sample
100
500
1000
1500
2000
5000
10000
15000
20000
Energy (eV)
Total
Grating slope error
M2 slope error
M3 ast coma
Source
Cff=2 D4 = 4
Resolution Limit
150 l/mm
250 l/mm
400 l/mm
Contribution to the spectral resolution limit for a PGM, with slope errors of 0.5 μrad.

4. “If you can’t measure it, you can’t make it.”
…Motivation …
Most mirror polishers cannot measure mirror with accuracy better than 0.5 μrad
There are sources of error that canot be controlled by the polisher
Gravity sag
Holder clamping
Cooling manifolds
Calibration of mirror bender
Thermal bump
Bending approximation
Twist introduced by the clamping onto the mirror surface.
Also, a metrology lab opens the possibility to improve optics.
“If you can’t measure it, you can’t make it.”

5. Mirror 400 mm (toroidal)
Mirror 1200 mm (plane elliptic)
Diffraction grating (170 mm)
37 mirrors and diffraction gratings
14 bent onto plano elliptic
7 toroidal
7 longer than 600 mm
>85% better than 1 μrad
>50 % better than 0.5 μrad
Maximum radius specified 22 km
Minimum radius (sagittal) 35 mm
Best measured error 55 nrad
About 30% were initially out of specs

6. The optics and metrology lab
Experimental hall 23±1ºC
50 m2 Optics Lab
40 m2
17 m2
Class
Outside -5 to 35ºC
Metrology Lab
Annex
STM+AFM
21±0.2ºC
21±0.5ºC
Optics lab.
The optics and metrology lab is one of the perimetral labs of the expermiental hall.

7. FIZEAU INTERFEROMETER
NOM
NOM enclosure

8. Instruments Fizeau interferometer Alba-NOM Provides 1D information
Provides 2D information quickly.
Difficult to measure curved surfaces.
Limited accuracy.
Provides 1D information
Can measure curved surfaces
Ultimate accuracy
R=75 m
36 nrad RMS

9. The Alba NOM

10. A scanning deflectometer meridional slope only One line only
The Alba NOM
A scanning deflectometer
meridional slope only
One line only
Versatile (any figure)
Ultimate accuracy

11. Unstability. Vibrations, turbulence
The Alba NOM
Sources of error
Unstability. Vibrations, turbulence
Guidance error and pentaprism alignment
Autocollimator errors

12. Thermal stability
The NOM is placed a passive isolation enclosure, with 100 mm thick high density polyuretane walls. Power disipation inside is minimized.
The lab itself is stablized to <0.3ºC (upgrade pending)
ΔT < 50 mK in 4 days

13. (pitch between platforms)
Ground vibrations
Although the amplitude of the vibration in the ground is 50 nm RMS, the measurement on top of the NOM is 13 nrad RMS 20 40 60 80
100
0.1 1 10
Frequency (Hz)
Spectral Amplitude (nm RMS) 20 40 60 80
100 10 2
Frequency (Hz)
Amplitude spectral density (nm/Hz)
Ground vibration
(vertical)
NOM
(pitch between platforms)
13 nrad RMS in 6:40 min

14. Pentaprism damping
The pentaprism compensates most of the noise induced by vibrations, being turbulence the remaining source of stability
Noise along the optical path of the beam, and the corresponding spectrum

15. Motion performance
The use of a differential interferometer allows an accurate characterization of the motion performance of the bench
Backlash
Accuracy
Repeatability
Resolution
Position
100 nm
3.5 um
77 nm
20 nm
Resolution, a 40 nm step
Positioning error
40 nm

16. Guidance measurements
Backlash
Accuracy
Repeatability
pitch
2 nrad
7.1 urad
200 nrad
yaw
390 nrad
10.1 urad
94 nrad
roll
<115 nrad
6.1 urad
<200 nrad
flatness
14 nm
1.1 um
42 nm
straightness
2 nm
2.1 um
47 nm z x z x
Pitch
Yaw
Flatness
Straightness

17. Raytracing of the pentaprism
Pitch
Yaw
Roll
Flatness
Double pass raytracing of the scanning pentaprism is used to determine the influence of guidance error on the measurement

18. Guidance accuracy
R = 100 m
Contribution of the guidance error to the LTP error

19. Guidance accuracy Pentaprism error – R=100 m Different spheres
Different spheres – Position LUT
Pentaprism roll – R=100 m

20. Position LUT solves main errors
Guidance accuracy
Different spheres
Pentaprism error – R=100 m
Accurate + aligned pentaprism is essential ( + unexpensive)
Different spheres – Position LUT
Pentaprism roll – R=100 m
Position LUT solves main errors

21. Calibration of the optics
Redundant-independent datasets
We use an error model to combine redundant-independent measurements of one mirror, and that allow to estimate the slope profile and the linearity error of the instrument.
The results are as accurate as the error model.
We have estimated by Monte Carlo simulation the dependence on measurement parameters

22. Calibration: Experimental results
A 100 mm, R=75 m sphere has been used to cover 12 mrad range of the NOM.
Each reconstruction is based on 54 scans
Residual aberration + periodic subpixel interpolation error found for two different roll positions of the pentaprism.
R=75 m
36 nrad RMS

23. Other aspects… Facing up Facing side
The ALBA NOM can be set to measure mirrors in working orientations
It is integrated to the TANGO control system
Fully automated.  1700 scans in 2010 in about 1600 hours (2.5 km)
Facing down

24. Site acceptance tests of purchased optics
The Alba NOM
Applications
Site acceptance tests of purchased optics
Collaboration with mirror manufacturers
Installation of optics on their holders
Calibration of mechanical benders
Optimization of figure error

25. Spring actuators (by SESO)
Plano elliptic
Facing down
R0 = 129 m
L = 800 mm ( 6 mrad)
0.234 μrad RMS
The mirror figure was optimized by spring actuators.
Accuracy is required for convergence of the optimization process, on both the measurement and on the spring adjustment

26. Optimization of slope
By introducing a deformation of the mirror bulk, one can partially compensate the polishing error of the mirror.
Mirror before optimization
Polished surface with slope error
Mirror bulk
It requires:
Accurate metrology
Accurate deformation model
Accurate mechanics
Mirror after optimization
Pulling forces
Elastically deformed mirror bulk
Pushing forces

27. Optimization method Based on the elastic beam theory.
Separate bending (motorized) from correction (weak adjustable springs)
Optimize forces and positions
Use as few actuators as possible
Minimization using simplex (with boundaries and random initial conditions)
Iterative (because spring force cannot be directly set)
Achieved slope as a function of the applied actuators, with error on the applied forces.

28. 300 mm long mirror by InSync on a 2-couple bender by Irelec.
Springs

29. Optimization results After optimization Before optimization
Center stripe improved from μrad RMS to μrad RMS with 2 actuators
Agreement between prediction and measurement is 0.18 nm RMS

30. A second case: HFM A second mirror was optimized: 600 mm long mirror
Horizontal deflection
4 spring actuators (pulling and pushing)
Initial error: μrad RMS
Final error: μrad RMS
Limited by the poor control of the applied force by the springs (designed for corrections in the order of 0.5 μrad)

31. Focus results
Beamline measurements indicate that the spot can be expanded vertically by a factor 20 with very little inhomogeneities.
Defocused beam
Focused beam
110 μm
10 μm
50μm
220 μm
The YAG diagnostics does not allow an accurate measurement of the spot size when focused.

32. In-situ measurement
The slope error of the VFM was measured using the pencil beam method. Showing excellent agreement with the figure measured in the laboratory 2 years before and after many bending-unbending, cycles.
∆𝑧=2𝑞 ∆𝑛(𝑢)
𝑢= 𝑧 𝑐𝑜𝑠𝛼
0.055 μrad
0.045 μrad

33. Sub-nanometer correction
Bending actuators
Bending actuators include high resolution strain gauge to provide reliable feedback on the bent ellipse.
Stepper motors
Encoder+homing
Strain Gauges
Spring actuators
Correction independent of ellipse
High force resolution
High dynamic range
Minimum distance 22 mm
Pulling or pushing.
Motorized for optimization at the NOM

34. First prototype finished last week.
Measurements in progress

35. Summary
Optical metrology is essential to guarantee the optimal performance of beamlines. About 30% of our optics have benefited from our Metrology.
The Alba-NOM instrument, has been carefully characterized in terms of sensitivity and accuracy. For this we combined, accurate motion metrology, ray-tracing of the measurment process and numerical methods.
The measurements provided by the NOM are accurate enough to optimize the mirror figure. We are running a project to develop subnanometer accuracy for long mirrors.

36. …Acknowledgements Acknowledgements
Administration
Laura Campos …
Computing and Controls
Zbigniew Reszela
Sergi Blanch
Guifré Cuní
Sergi Pusó
Ramon Escribà
Engineering
Carles Colldelram
Edmundo Fraga
José Ferrer
Ricardo Valcárcel
Claude Ruget
IRPI (France)
Alba beamlines
All beamline scientists
Optics Group
Team
Juan Carlos Martínez
Josep Vidal
Igors Sics
Iulian Preda
Juan Campos (UAB)
We are a small group but we have strong support from many….
Other facilities…
Frank Siewert (HZB)
Daniele Cocco (LCLS)
Mourad Idir (NSLS II)
Muriel Thomasset (Soleil)
Amparo Vivo (ESRF)
Valeriy Yashchuck (ALS)
Ray Barrett (ESRF) …