Mini workshop on laser based alignment systems – minutes
30.01.2014
Welcome (Dr. Hermann SCHMICKLER)
09:00 – 09:10
Round the table and goals of the workshop (Dominique Missiaen)
09:10 – 09:30
Alignment requirements for CLIC and HL-LHC (Dr. Hélène MAINAUD DURAND)
09:30 – 10:00
Q&A
Q (A. GEIGER): How do you measure roll using the laser beam?
A (H. MAINAUD DURAND): There is no final solution yet, but few ideas. E.g. using
inclinometers or a second laser beam on the other side of the components.
Q (S. GUILLAUME): Do you still need HLS’ to model the vertical of wire systems?
A (H. MAINAUD DURAND): Yes, at the moment we are still based on HLS’
Q (S. GUILLAUME): Redundancy? What happens if a wire breaks? What’s the probability
of a wire break?
A (H. MAINAUD DURAND): There has been so far only one wire break in the LHC. This
was during a shutdown and because of a mechanical intervention.
A (A. HERTY): Probability of a wire break depends on the type of the wire which is used. At
the moment, the wires that are used are pretty fragile but we’ve been lucky so far.
Q (C. LASSEUR): Laser beam profile change along 200-300m?
A (D. MISSIAEN): We will see some results in the presentations.
Developping an iris diaphragm laser alignment system for Spring 8 storage ring
(Dr. Chao ZHANG)
10:30 – 11:00
Slide 9: It now takes 30s to measure all points, compared to 8min in the previous system.
This improves measurement accuracy because less laser drift occurs during a shorter
measurement time (cf. slide 10).
Q&A
Q (D. MISSIAEN): Do you have an estimation of the reproducibility of the iris when opening
and closing it?
A (C. ZHANG): Centre position reproducibility is approximately 3 µm after 100 repetitions.
Better results were expected.
A (D. MISSIAEN): These results are actually really good.
Q (M. SOSIN): Do you use off-the-shelf irises or did you develop them yourself?
A (C. ZHANG): Off-the-shelf. But you could design a more accurate one yourself.
Q (C. LASSEUR): What is the power of the laser and what kind of laser are you using?
A (C. ZHANG): 3.5mW. Diode laser coupled with optical fibre.
Q (D. MISSIAEN): Now you use irises. What would be the maximum number of irises that
could be used simultaneously and what is the maximum distance?
A (C. ZHANG): For the working range, see slide 6 (from 2m to 50m). For longer distance,
wider iris should be used.
Q (J. KEMPPINEN): There are 4 irises. Each one of them creates slightly different pattern on
the CCD. Are the irises optimized to minimize this difference do you use different analysis
algorithm for each pattern?
A (C. ZHANG): Try to get a uniform image by adapting the iris diameter with the distance.
Q (J-P. QUESNEL): The laser tube diameter is 22 mm. Are you sure that there are no
photons touching the tube walls?
A (C. ZHANG): The laser beam diameter we are using is 4-5 mm. The sensor is roughly 5
times bigger than laser beam. The smaller the beam size is, the less there is reflection from
the tube walls.
Q (V. BATUSOV): With which precision do you (manually) control the direction of the laser
beam itself?
A (C. ZHANG): It is not controlled. We chose a very stable and low drift laser tube. The
laser direction is not very important. The laser source and the CCD are shielded from air
flow.
Q (J-C. GAYDE): Your system is an imaging system. You create an image of the pattern
with the CCD. Are you concerned about the laser’s long term stability?
A (C. ZHANG): It is not taken into account. One measurement lasts 30 s and it is a four point
relative measurement.
Q (T. SUWADA): Do you take any care of the iris target’s tilt or rotation?
A (C. ZHANG): It is not measured or taken into account. It is very difficult to see or control
the tilt around the centre of the iris.
Propagation and stability characteristics of a 500 m long laser based fiducial line for
high precision alignment of long distance linear accelerators
(Tsuyoshi SUWADA)
11:00 – 11:30
Q&A
Q (J-P. QUESNEL): What is the size of the sensor?
A (T. SUWADA): The diameter is 10 mm.
Q (J-P. QUESNEL): How do you guarantee spotting of the whole laser profile? The laser
beam diameter is 30 mm whereas the sensor diameter is only 10 mm.
A (T. SUWADA): The results show that there is enough sensitive area on the sensor. Only
the centre of the pattern is important.
Q (J-C. GAYDE): Sometimes the bell curve is flatter. You measure only the top part of the
profile which is not always stable. What is the main source of instability? Is it the laser beam
or the instability of the top region of the bell curve?
A (T. SUWADA): It is mainly caused by the pointing stability.
Q (V. BATUSOV): Did you try using transparent photodiodes or have you always used a
standard one which absorbs the whole beam power?
A (T. SUWADA): We have used only the standard ones.
Q (V. BATUSOV): How long does a series of measurements last?
A (T. SUWADA): Few minutes.
Q (S. GUILLAUME): Where is the materialization of the straight line? If the highest point of
the bell curve is chosen, we assume that the spot is symmetrical. On your slides we can see
that it is not (density is in the left). Yet you have repeatability of 10 µm. How did you
estimate it?
A (T. SUWADA): Symmetrical Gaussian function was used.
Q (D. MERGELKUHL): On slide 20, the systematic error is just the alignment system’s
error. What about fiducialisation?
A (T. SUWADA): Accuracy of the mechanical assembly (mechanical arm, shown on slide
25) is very accurate, 25 µm.
Q (C. LASSEUR): How did you isolate the optical table from the floor? (To estimate the
stability of the box containing the laser and the optics)
A (T. SUWADA): The optical table is mounted on an iron plate and an iron girder.
Q (D. MISSIAEN): (Referring to slide 16) What is done to the first QDP when the second
one is being measured?
A (T. SUWADA): It is removed manually. Reproducibility of this manoeuver is 30 µm.
Q (C. LASSEUR): Why haven’t you considered semi-transparent silicon amorphous
detectors? You could measure several points simultaneously without stopping the laser beam.
A (M. SOSIN): They bend the laser beam.
Q (J-P. QUESNEL): Have you tested the vacuum stability influence in the laser beam
stability?
A (T. SUWADA):
Laser Beam Fiducial Line Application for metrological purposes
(Mikhail LYABLIN et al.)
11:30 – 12:00
Q&A
Q (J-C. GAYDE): What are the results you obtain with stabilized white noise?
A (M. LYABLIN): Relative values.
Q (J-C. GAYDE): You try to stabilize your laser beam by closing it in a tube?
A (M. LYABLIN): The results are 155 times better than in open air.
Q (S. GUILLAUME): How do you model the profile of the beam, considering that you
refract the beam (no vacuum  air has different refraction index in different places)?
A (M. LYABLIN): The effect is less than 1 µm on 150 m.
A (S. GUILLAUME): This is not sure.
Q (A. GEIGER): What is the trend of the seismic measurement plot?
A (M. LYABLIN): It was measured in a normal office. There is a slope of 10-7 rad.
Q (D. MISSIAEN): On slide 4, what is the accuracy of the point measurement?
A (M. LYABLIN): There are no results, yet.
Q (S. GUILLAUME): On slide 9, how accurate is the plate parallel?
A (M. LYABLIN): At the moment 10-6 rad, in the future 10-8 rad.
Status report on the laser based Straight Line Reference System @ DESY
(Dr. Johannes PRENTING)
14:00 – 14:30
Q&A
Q (H. MAINAUD DURAND): How many spheres can be put on the laser beam line?
A (J. PRENTING): At the moment 16-17, evenly distributed 2 per position. 6 positions every
24 m.
Q (H. MAINAUD DURAND): What is the needed vacuum? Why such a value?
A (J. PRENTING): 10-2 mbar. After a general calculation, we just tried it and it worked.
The search for and registration of the superweak angular ground motions
(Mikhail LYABLIN)
14:30 – 15:00
Q (M. SOSIN): On slide 3, how do you calibrate the experimental setup?
A (M. LYABLIN): There is a calibration screw. Fixed angle at 10-5 rad, measured with a 32
bit ADC with 10-9 rad resolution.
Q (M. SOSIN): On slide 3, the sensor S is the liquid surface?
A (M. LYABLIN): Yes, only 3 mm layer of liquid to prevent having waves in the vessel (just
inclination).
Q (A. GEIGER): It’s water?
A (M. LYABLIN): It’s oil because it has a better viscosity.
Q (J-P. QUESNEL): What is the material?
A (M. LYABLIN): Aluminium tower.
Q (J-P. QUESNEL): Is the test area acclimatized?
A (M. LYABLIN): No.
Q (J-P. QUESNEL): For long term stability, you need very stable instrumentation?
A (M. LYABLIN):
Q (S. GUILLAUME): Do you have long term measurement series?
A (M. LYABLIN): Yes, 5 days.
Q (S. GUILLAUME): Do you see the tides? Because there should be an impact of 10-7 rad.
A (M. LYABLIN): 10-7 rad difference between day and night.
(S. GUILLAUME): You should see two peaks per day.
Q (S. GUILLAUME): Is your goal to measure high frequency inclination or long term?
A (M. LYABLIN): 1 Hz, it is interesting in the future linear accelerators.
Q (D. MARTIN): On slide 5, what is this phenomenon?
A (M. LYABLIN): Not known, it is seen once a week.
Q (A. GEIGER): What is the orientation of your setup?
A (M. LYABLIN): North-South.
Q (M. SOSIN): Where is your setup installed?
A (M. LYABLIN): In my office in 926 at CERN.
Laser Alignment multipoint Based-Design Approach (LAMBDA)
(Guillaume STERN)
15:00 – 15:30
Q&A
Q (D. MERGELKUHL): Did you use other colors or materials than aluminum and black
paper?
A (G. STERN): Yes, but not all colors and materials had targets around. It seems that color
doesn’t have a big effect.
Q (J-P. QUESNEL): What is the target position accuracy?
A (G. STERN): The metrology laboratory measured them with +/- 5 µm accuracy.
Q (J-P. QUESNEL): How long did the 2 mm movement take?
A (G. STERN): 2 - 2.5 hours.
(J-P. QUESNEL): There might have been deviation in the laser beam due to the long
measurement time.
Q (D. MERGELKUHL): What happens when the accelerator’s length changes? E.g. because
of temperature change.
A (A. GEIGER): This has no effect as the complete reference information for the camera is
on the target plate (that has to be attached to the component to be aligned).
Q (J-C. GAYDE): What about rotations?
A (H. MAINAUD DURAND): Roll has to be measured using inclinometers etc.
Q (J-P. QUESNEL): Instead of circular targets, I’ve used crosses.
A (S. GUILLAUME): It’s better to fit an ellipse than a cross. A cross has two lines that have
to be crossed which makes it complicated for the fitting.
Q (D. MISSIAEN): To which distance it is possible to do a measurement?
A (G. STERN): It’s possible to do the fitting up to at least 200 m but the standard deviation
grows significantly with distance in open air (std is already 2 mm at 200 m).
Q (C. LASSEUR): Have you tried reflective targets and taking photos with a flash?
A (G. STERN): No. So far, only passive targets have been used because it was a bit easier
regarding the light.
Discussion about CCD matrix size:
(M. LYABLIN): What if you used a CCD with bigger matrix (e.g. 20 Mpix)?
(D. MERGELKUHL): Now you use a 1.3 Mpix CCD  there’s surely room for
improvement. But you would have to change the whole setup (CCD, chip, optics), not just the
CCD.
(G. STERN): There are also other challenges (laser beam stability on long distance etc.)
(D. MERGELKUHL): But on short distance (3 m) you can surely have better results with a
better camera.
(A. HERTY):
(G. STERN):
(M. SOSIN): I have a doubt that adding pixels would improve accuracy. Instead, smaller
pixels would be needed.
Validation of laser alignment systems by systems referred to gravity
(Sebastien GUILLAUME)
16:00 – 16:30
S. GUILLAUME thinks that different laser techniques should be well benchmarked and
compared to HLS’ and wire systems in a suitable setup. In addition, he thinks that if there
were a working micrometric laser alignment system, it would be very useful also in geoid
measurements.
Q&A
Q (D. MARTIN): Comparison is necessary. Have you done it so far?
A (S. GUILLAUME): I haven’t had the possibility to do it.
A(H. MAINAUD DURAND): There is an on-going campaign to compare RasNIK and wire
system on 140 m. However, there are some issues with RasNIK integration to the setup and
with humidity in the test tunnel.
The laser reference line method and its comparison to a total station in an ATLAS like
configuration
(Dr. Vitali BATUSOV)
16:30 – 17:00
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General design slide:
o 50 µm precision was reached in the ATLAS calorimeter installation
o We wanted to improve this system
 Test installation in a tube in the corridor of CERN building 926
 Tube diameter 50 mm and length 50 m
 Better results with closed tube ends (due to standing acoustic waves)
Adapter mechanical manufacturing accuracy approximately 20-30 µm
Calibration function has a linear and a non-linear part
o Linear part up to 0.5 mm from photodiode centre  worse results when the
laser beam centre is located over 0.5 mm away from the photodiode’s centre
Better results expected because now:
o AT401 available
o feedback control of the target
o better mechanical stability of the laser support
Q&A
Q (D. MISSIAEN): Which total station did you use before the AT401?
A (V. BATUSOV): TC 2002
Q (D. MISSIAEN): Wouldn’t it be better to compare with a stretched wire (total station’s
vertical measurement is not very accurate)?
A (V. BATUSOV): Why not in the future.
(D. MERGELKUHL): Total station measurements are easier to implement than stretched
wire.
Q (A. GEIGER): Does your system have to be perfectly aligned compared to the tube?
A (V. BATUSOV): It has to be aligned compared to a surface, in this case it has to be in the
tube.
(C. LASSEUR): Measurements were performed in very difficult conditions.
Discussion about the use of theodolites.
Q (T. SUWADA): How big is your laser spot size?
A (V. BATUSOV): We’re using a 1 mW red visible semiconductor laser. Wavelength
650 nm and spot size is 10 mm at 50 m.
Q (T. SUWADA): Have you tried ellipsoidal spot?
A (V. BATUSOV): Spot form depends on the collimator. We had circular spot, not
ellipsoidal. We can track a move of laser of 10 µm with +/- 3 µm accuracy.
Discussion about the use of theodolites continued.
Summary of all developments and discussion
(Dominique MISSIAEN)
17:00 – 18:30
(J-P QUESNEL): Nobody spoke about the quality of the laser beam (only sensor quality was
addressed). So the development in the straight line reference has been more in the sensor
side.
(S. GUILLAUME): I agree. The only way to benchmark the straightness of the laser beam
line is to compare it properly to HLS’, stretched wires etc.
(J-P QUESNEL): Physicists can give theoretical estimation of photon trajectories.
(S. GUILLAUME): In vacuum, perfect laser gives a perfect Gaussian profile.
Q(C. LASSEUR): What is the deadline to choose a technology for the HL-LHC?
A(H. MAINAUD DURAND): We still have a bit of time. Installation 2023.
(C. LASSEUR): Now would be the time to see what happens in a real accelerator
environment. E.g. do we need a shielding box like KEK is using.
(S. GUILLAUME): Does someone have experience of a telecentric objective? There is no
scale change. You see only what is inside a cylinder, without scale change with distance.
Possibly interesting.