1. Thin-disk laser multi-pass amplifier
K. Schuhmann
for the CREMA collaboration
ETH Zurich, Switzerland
Paul Scherrer Institute, Switzerland
Hi I am Karsten Schuhmann / Thanks, for the kind introduction,
I want to present you our thin disk multi pass amplifier.
Our laser system was developed for the measurement of nuclear charge radii of muonic atoms.

2. Muonic Atoms
Muons are like electrons but 200 times heavier Much higher overlap to nucleus
Energy level shifted by finite size effect ∆𝐸~ 𝑚 3 ∙ 𝑅 𝑁 Measure 2S-2P splitting RN
A muonic atom where the electrons are replaced with a negative muon.
As muons are 200 times heavier then electrons they are orbiting very close to the nucleus experiencing its finite size
We used this effect to determine the charge radius by the mean of laser spectroscopy of the 2S-2P energy splitting or lamb shift
This experiment was performed at the Paul Scherrer Institute in Switzerland as it provides a very powerful proton beam
11/7/2018
Karsten Schuhmann

3. Setup at PSI Schematic drawing of our experiment:
The high current proton of PSI
Hits carbon target
generates exotic particles
Pions are selected and caught in cyclotron trap
Slowed down and decay to muons
Muons are transported in the momentum Filter
Muon are stopped in the gas target
Muonic atoms
Before entering the gas target muons trigger the laser system
Frequency doubled Thin-disk laser
Ti:Sa illuminating the target and exxiting the transition
11/7/2018
Karsten Schuhmann

4. Laser requirements Trigger on incoming muon Stocastical trigger
Decay time of muonic 2S state ≈ 2 µs Latency ≤ 1 μs
Disk oscillator
The laser is triggered on incoming muons
These muons appear at random times.
The live time of muons is just 2 µs
So in order not to shoot on an empty target it is crucial to have a trigger to laser pulse delay of below 1 μs
11/7/2018
Karsten Schuhmann

5. Thin disk laser Efficient cooling high power cw. pumping
small thermal lens
Long upper state lifetime
high pulse energy
11/7/2018
Karsten Schuhmann

6. Laser scheme Oscillator-amplifier design with SHG CW-pumping Prelasing
Disk amplifier
8 Reflections
Disk oscillator
SHG LBO
Dilas, 969 nm pump diode
Laserline, 940 nm pump dide
1030 nm, 30 mJ
1030 nm, 100 mJ
515 nm, 50 mJ
400 W
500 W
Oscillator-amplifier design with SHG
CW-pumping
Prelasing
Disk oscillator
Explain the page!
I do not want to go into any detailes
As the subject of my talk, is the multi-pass amplifier.
11/7/2018
Karsten Schuhmann

7. Gaussian beams
11/7/2018
Karsten Schuhmann

8. Lenses and Gaussian beams
11/7/2018
Karsten Schuhmann

9. Relay imaging
M= −1 0 0 −1
11/7/2018
Karsten Schuhmann

10. Relay imaging 8 pass propagation
11/7/2018
Karsten Schuhmann

11. Relay imaging propagation
Including a thermal lens effect
11/7/2018
Karsten Schuhmann

12. Design of the our propagation
The plot shows the beam waist along the propagation of the amplifier
You will recocnice the first 3000mm are identical to our model cavity.
In order to complete a cavity roundtrip an inverted version of the cavity is attached.
At the end 4 cavity roundtrips are unreeled and form our multi pass amplifier.
Significant changes for different thermal lenses can be seen
To judge the impact of these our amplifier has to be compared with other feasible propagations
11/7/2018
Karsten Schuhmann

13. Propagation in the amplifier vs. corresponding optical resonator
11/7/2018
Karsten Schuhmann

14. Propagation in the amplifier
11/7/2018
Karsten Schuhmann

15. Our multi-mirror array vs. Thorlabs KMS
Deviation from model cavity grow quadratically with the size of the array
Propagation length
Astigmatism
Mirror to mirror distance of 31mm
new
Earlier version based on commercially available compact mirror holder
Allowed for tight placing with a mirror to mirror distance of just 31mm
Alignment precision not sufficient
Development of our own mirror holders
Provide Same mounting distance
Their superior performance is based on….
old
11/7/2018
Karsten Schuhmann

16. 145 mJ Amplifier tested up to 145 mJ All beam passes M2 ≤ 1.03
2 years of operation ≈ 90 mJ
Higher pulse energy is expected for higher input pulse energy and larger pumping spot.
For our laser long term stability was more important than higher output power.
11/7/2018
Karsten Schuhmann

17. 145 days without realignment
As you can see, the amplifier gain stayed nearly unchanged for more than a third of a year.
This long term stability was not only caused by the precise design of the mirror holder or the high temperature stability.
In fact the time includes cooling water interrupt leading to room temperatures around 35°C and the drop out of a cooling water pump.
The long term stability was mainly caused by the low alignment sensitivity of our amplifier concept.
11/7/2018
Karsten Schuhmann

18. Misalignment study of our propagation
The misalignment plot in comparison to the Gaussian beam waist.
All misalignments of the thin disk are compensated after 8 passes
Misalignment of in coupled beam of 100 µrad
Misalignment of thin disk 25 µrad
Deviation of beam axis from optical axis
Plotted against the propagation length Z
Blue line in coupled beam tilted
Red line disk tilted
Full compensation of any deviation after 8 reflections on the disk
As at IFSW associated Institute plane parallel design
With introduced a very interesting modification
11/7/2018
Karsten Schuhmann

19. Misalignment of our propagation + 45° mirror pair
propagation in the xz-plane stays unaffected
Inversion of angle and excursion in the yz-plane
Misalignment of in coupled beam of 100 µrad
Misalignment of thin disk 25 µrad
Tilt compensation after 8 reflections on the disk
𝑀 𝑥 =
𝑀 𝑦 = −1 0 0 −1
Same plot as before however the second end mirror was replaced by a pair of 45° mirrors
No effective compensation for a tilt of the in coupled beam
Halving the the misalignment due to tilt of the disk
In order to validate this in reality we performed some misalignment measurements.
11/7/2018
Karsten Schuhmann

20. Misalignment stability of our amplifier
11/7/2018
Karsten Schuhmann

21. Summary Multi-pass amplifier with
low sensitivity to thermal lens and pointing
Operation for months without the need for realignment.
Determination of Lamb shift in Muonic He4 and He3
A retro reflector reduced the effect of thin-disk tilt by a factor of 4
This work is supported by the SNF_200021L , DFG_GR_3172/9-1 and the ERC StG. #
11/7/2018
Karsten Schuhmann

22. Exit-beam parameters for both amplifier concepts
Input beam
Our amplifier
4f-amplifier
w = 2.66 mm
8 reflections on the thin-disk
Explain the page!
You can see the benefit of the concatenation of stable optical segments.
To realize the propagation with a minimal number of mirrors and highest stability
11/7/2018
Karsten Schuhmann

23. Multipass pump optics Low pump light absorption
Multi pass pumping optics
Relay imaging design
11/7/2018
Karsten Schuhmann