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yesterday morning with Manar,
1) we fixed the threshold level of the laser input signal which makes the measurement of the repetition rate.
(if the rep rate is not measured properly, the amplifier safety interlock stops immediately the amplification).
we followed a tutorial from Guillaume Machinet.
when you start the Alphanov control software, 4 panels pops up : central control, 2nd stage amplifier, and 2 panels for controlling the pumps of the 3rd stage amplifier.
!!! Warning !!! ..... if you switch ON the amplifier using the central control panel, the amplifier starts immediately AT FULL POWER..... !!! Warning !!!
it is not mentionned in the tutorial document.
to prevent this problem, you first need to switch "Laser OFF" on the 2 panels controlling the pumps of the 3rd stage amplifier and let "Laser ON" only on the panel controlling the 2nd stage.
after starting the amplifier, we tried to find the median value of the threshold to get ~33MHz instead of 100MHz measured previously.
the initial voltage threshold on the photodiode measuring the rep rate was 0.4V which leads to get 100MHz.
first, we changed step by step the threshold to reach 0.87V => one gets 33MHz.
we tried the find the maximum threshold but once the level is above the signal maximum value, it triggers an alarm and stops immediately the amplifier.
then, one needs to restart completely the software which can be tedious due to connections/alarms issues.
finaly, we found out that the low and high level thresholds to get 33MHz were not reproductible at each start of the amplifier... :-(
then, we put the threshold at 1V following the turial procedure which seems to work.
2) we installed the optical scheme to make the beam profile measurement : see attached image
we used the HR mirror close to the amplifier output (as putting a wedge at that place is not convenient if you want to properly dump the transmission and reflections).
and then, we placed 2 wedges, using the front reflection to get low power beam profile.
the Thorlabs LB2 has been used to dump the transmission of the first wedge, it can handle 25kW/cm² and 25J/cm².
we used black aluminium screens to dump the secondary beams.
we measured after the 2 wedges :
amplifier at 20% => 71µW
amplifier at 30% => 139µW
amplifier at 40% => 200µW
it has to be compared with the amplifier power :
amplifier at 20% => 8.7 W
amplifier at 30% => 17 W
amplifier at 40% => 26 W
which leads to a reflection coefficient of the wedge of 0.28%.
=> OK, as one uses PS811-B Thorlabs 4° wedges with B coating.
with B coating, the reflectivity given by the manufacturer is around 0.3% @ 1030nm.
unfortunately, we had a "case temperature alarm" coming from the Alphanov software when we reached 40% for the amplification level.
this alarm stopped immediately the amplifier.
we saw that the chiller was in warning state too and the "present temperature PV" was not stable at all, flutuating by 3-4 degrees after the amplifier has been turned off...
we restarted several times the chiller to see if the problem disapears but it was still there ! => to be investiguated !
PS : we changed the USB cable between the amplifier controler and the PC to try to fix the several "connexion lost" problems but it didn't help...
PS2 : after discussing with Sophie Chance and Marie Jacquet, ThomX suffered a water circuit leakage on Monday:
the full circuit has been emptied and they had to remplace the water by some common water and not demineralized water....
it can be related to the chilller issue observed yesterday !
| Ronic Chiche wrote: |
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today with Manar, we brought everything to make the profile measurement.
the chiller was in error because of the water level : we filled it in.
! warning ! there is no alarm signal, only a message on the screen.
we checked the incoming power measured by the software : 5.5 mW => OK
and we measured the output power (with pump) for :
0% => ~ 300 mW
10% => ~900 mW
20% => 8.65 W
30% => 17W
=> same as before.
the first HR mirror at the output of the amplifier was slightly scratched => we replaced it.
for sake of simplicity, we plan to use first a HR mirror and then 2x wedges for the profile measurement => to be checked next time.
we have to remove secundary beam reflections, then wedges are easier to use than AR/AR mirrors (as the 2 reflected beams are parallele).
at full power, the output power is ~ 70W
after 1st wedge : 2.8W
after 2nd wedge : 112 mW => the power should be low enough to use absorptive filters in front of the beam profiler.
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