Mini workshop on laser based alignment systems – minutes
31.01.2014
Brainstorming
(H. MAINAUD DURAND): This is a wrap-up session where we try to address the questions which were
not yet issued during the workshop. (cf. D. MISSIAEN’s slides from Thursday’s conclusion session)
1st issue: Attach sensor to the vacuum and create a link between the target position in the vacuum
and the object to be aligned
(H. MAINAUD DURAND): DESY has found a way using a flange. But it is not clear whether the vacuum
of 10-2 mbar adds constraints.
(R. KAEMPFER): If the vacuum is not very strong, you can use a flexible tube
- You can put your sensor in the tube, attach it on the tube and attach the tube to the
structure to be aligned.
- I use bellows  this adds mechanical constraint (10-20 µm additional error in repeatability)
(V. BATUSOV): Use non-destructive sensors in the vacuum. We use parallel planes which allow the
laser beam to go through them.
- How many sensors can be added?
- At the moment, this is just a proposal
- Does it refract the laser beam?
- A flexible system proposed by M. LYABLIN could potentially be installed everywhere
(J-C. GAYDE): Difficult to answer, depends on the design.
(R. KAEMPFER): Link between inside and outside of the vacuum could also be optical (not necessarily
mechanical)
- Very hard to make
(J-P. QUESNEL): Take LAMBDA for an example. It doesn’t matter where the camera is, just the laser
beam and the target have to be inside the vacuum.
(J-C. GAYDE): This does not solve the problem of linking the laser beam spot position to the
component’s mechanical centre.
(M. SOSIN) proposes:
-
Use bellows, flexural components, springs
o For roll, you need an inclinometer etc.
Comments:
o R. KAEMPFER thinks that this design creates too many mechanical constraints
o M. SOSIN thinks that the only force affecting is the vacuum pulling the sensor
towards the vacuum tube. This can be diminished.
(M. LYABLIN) proposes:
-
-
Proposal consists of mirrors
o The laser source and the sensors are outside of the vacuum. The mirrors reflect the
laser beam in and out of the vacuum.
Comments:
o (D. MERGELKUHL): Where is the component that you want to align? What is the link
between the mirrors and the component to be aligned?
Not related to previous proposal, H. MAINAUD DURAND asks T. SUWADA how they determined that
5 Pa vacuum is strong enough.
A(T. SUWADA): We tested it, variation of laser beam size compared to vacuum levels.
(S. GUILLAUME): To calculate the needed vacuum level, the most important parameters are the
refraction index of the medium, the level of turbulence and the atmospheric refraction cancellation.
- Knowing these parameters, you can make the calculations. It is more difficult to predict in a
vacuum than on open air.
(R. KAEMPFER): proposes:
-
Flexible tube, no bellows, sensor attached rigidly to the tube
(P. BESTMANN): proposes:
-
-
Separate laser beam tube for each component to be aligned. The laser tubes are
interconnected with bellows. This creates additional mechanical constraints but does not
affect the measurements. The particle beam vacuum already creates mechanical constraints.
Comments:
o (S. GUILLAUME): Check with vacuum experts whether this is possible (a lot of joints)
o (P. BESTMANN): already done.
o (S.GUILLAUME): I have experience with the deflectometer (10-3 vacuum) and there
were already problems with joints and bellows.
Not related to the previous proposal, D. MISSIAEN says he likes the DESY solution involving a flange
and metrological targets outside the vacuum.
(D. MERGELKUHL): You need double amount of sensors (costly and complicated)
(M. LYABLIN): proposes:
-
A vacuum mover, position of which is measured compared to a point outside the vacuum.
Not related to previous proposal, C. LASSEUR goes back to D. MISSIAENS suggestion that a DESY like
solution is good. A lot of discussion about this.
R. KAEMPFER sums up that the good solution depends a lot on the sensor type which is used.
2nd issue: Straightness of the laser beam
(H. MAINAUD DURAND): In the presentations, we have seen photos which show that the laser beam
is not really straight/homogeneous.
M. SULC is asked to give his opinion about the best laser technology for use in a straight line
reference.
(M. SULC): He-Ne is the best when you don’t need high intensity (1-2 mW). It gives a “perfect”
Gaussian profile.
(S. GUILLAUME): What do you think about blue laser?
(M. SULC): It is theoretically good (because the diameter is smaller than a red one) but there are no
blue gas lasers. There are only blue semiconductor lasers and they are worse.
(M. LYABLIN): There is blue cadmium (as good as gas) laser. He-Ne heats up a lot which creates drift.
(J-P. QUESNEL): He-Ne on 200 m, what is the beam size?
(M. SULC): Depends on the spot size in the beginning.
(J-C. GAYDE): There might be different modes (frequency) which might perturb the measurements.
You think that with He-Ne you can have just one mode?
(M. SULC): Exactly
(M. LYABLIN): If we use an optical fibre with only one collimator lens, we just keep the fundamental
T00 mode and cancel all other modes.
(M. SULC): If you have just one collimator and then let the laser travel several hundreds of meters,
there might be problems.
Discussion about the SPRING-8 system:
(J-C. GAYDE): It was interesting to learn yesterday that in the SPRING-8 system, you don’t need to
care about the laser beam stability.
(J-P. QUESNEL): But the distance is only 10 m.
(S. GUILLAUME): I think that the limiting factor (for precision) is space coherence.
(M. LYABLIN): I don’t think that the Japanese system is feasible for a fiducial line. The successive
irises create parasite diffractions.
(J-P QUESNEL): I think that there should be a study of beam stability/profile and of the effect of
multiple sensors on long distance (200 m).
(C. LASSEUR): and intensity
(T. SUWADA): Semiconductor laser does not have aspect ratio of one (different beam diameter in
vertical and horizontal). In addition, its coupling coefficient is below 30% (more powerful laser is
needed). Also, optical fibres cut phases whereas uniformity is important.  Semiconductor lasers
are ok with distances below 100 m but not with longer distances.
(T. SUWADA): Coupling He-Ne to an optical fibre creates intensity fluctuation. This is why we don’t
use optical fibre at KEK.
T. SUWADA shows a slide about their near future as well as images from their research paper.
- They want to replace QPD’s with Fresnel plates  radiation hardened, transparent and
very stable without the need for feedback control in laser positioning (because if there was a
drift of the laser beam, this would not have any effect on the diffraction pattern).
- D. MISSIAEN points out that Fresnel plates are not transparent, you still need to remove
the previous ones from the laser beam line to be able to measure the next ones.
- H. MAINAUD DURAND asks how many zones you need in a Fresnel plate.
- T. SUWADA replies that it depends on the total measurement length
- H. MAINAUD DURAND asks about the supported radiation level but it is not known.
Questions to M. LYABLIN:
(D. MISSIAEN): How do you insert acoustic waves to your laser tube?
(M. LYABLIN): They are there automatically.
(C. LASSEUR): In 2008, you put some ventilators in the beam pipe.
(M. LYABLIN): We wanted to see if the turbulence affects the laser beam. It doesn’t really on 50 m,
temperature variation has bigger effect. Actually originally the idea to put the windows in both ends
of the tube was to prevent temperature variations.
(H. MAINAUD DURAND): H. VAN DER GRAAF wanted to replace vacuum with a helium flow. Ideas?
(T. SUWADA): An idea to check the straightness of the laser beam:
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The laser beam has to be in vacuum in this proposal.
Comments:
o (M. LYABLIN): What about in open air?
o (M. LYABLIN): There should be a comparison between a laser beam in vacuum and
another one in open air (distance between them along the beam lines)
 (D. MERGELKUHL): It is known that the laser is not straight in open air. More
interesting to know is whether it is straight in vacuum. E.g. deviations in
intensity along the beam.
o (H. MAINAUD DURAND): Maybe your idea with two sensors per measurement point
(one per beam) instead of moving one sensor could be better.
o (S. GUILLAUME): Does it have to be the same beam with mirror reflection? Maybe
use a beam splitter to have two parallel beams from the same laser?
o Discussion between S. GUILLAUME, J-C. GAYDE and R. KAEMPFER.
o (J-C. GAYDE): With two different beams, you can’t see external disturbances (same
measurement point). Only internal ones. With SUWADA’s proposal, both can be
seen.
(R. KAEMPFER): Don’t you think that the laser manufacturers already measure/verify their laser
beam profiles/straightness?
S. GUILLAUME and H. MAINAUD DURAND point out the inter-comparison is definitely needed.
(H. MAINAUD DURAND): There will be inter-comparison at CERN in a 500-m-long tunnel. The design
starts soon and let me know if you want to participate.
Discussion about data acquisition and image processing:
(J-P. QUESNEL): Data acquisition, software and computing are very different with lasers than with
e.g. wire systems. Image processing is much slower than WPS acquisition.
(D. MERGELKUHL): For lasers, the acquisition (of the camera) is timely but image processing is
nowadays rather fast.
(S. GUILLAUME): I prefer open-source, in-house image processing software to a commercial one.
(M. SOSIN): Nowadays software can be optimized to be really fast with reasonable accuracy. For
example the Renishaw optical ruler uses a fast low res camera but processing is fast.
(M. SOSIN): Xilinx has open source software libraries for image processing.
(S. GUILLAUME): There is no off-the-shelf software which gives 2D Gaussian fitting, residuals,
elliptical fitting etc. with the precision we need.
M. LYABLIN proposes a method to check the straightness of the laser beam.
(D. MISSIAEN): We haven’t talked about sensors this afternoon. What about PSD’s?
(D. MARTIN): I have experience of PSD’s, but haven’t gotten below 75 µm accuracy.
(D. MISSIAEN): Maybe there are new, more accurate models in the market nowadays. But aren’t
they expensive?
(D. MARTIN): They are cheap, 20-30 euros. We haven’t had any problems with them in the ESRF
synchrotron (radiation hard).
Conclusions:
M. LYABLIN and T. SUDAWA will test the laser beam straightness and they seemed willing to
collaborate.
S. GUILLAUME and G. STERN are working on the image processing software.
M. SOSIN proposes to work in the future on the linking between inside and outside of vacuum.
Study of the needed vacuum level to ensure laser beam quality (S. GUILLAUME will ask the industry)
S. GUILLAUME thinks that the theoretical basis of the laser physics in a vacuum tube (e.g. what
exactly happens when you rotate or move a Fresnel plate a little) should be studied.