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This morning with Manar, we continued the strecher CVBG alignment and fiber injection procedure :
- we checked the power at the output of the laser and confirmed the measurement after the Isolator : ~ 36 mW
- we tried to improve the power after the strecher CVBG by rotating the quarter-wave plate but it seems we were already at the maximum : ~ 10.5 mW
- we aligned the 2 fiber injection mirrors + schafter-kirchoff mount z-axis : we saw 5 mW at the output of the fiber but it is very difficult to keep the power after swcrewing the 2 z-axis fixing screws of the schafter-kirchoff mount.
we decided to replace the 750mm focusing lens installed by a 1000mm lens.
- we aligned again the 2 fiber injection mirrors + schafter-kirchoff mount z-axis : now, we have ~ 6.2 mW stable at the output of the fiber after screwing the 2 z-axis fixing screws of the schafter-kirchoff mount.
the procedure for aligning the schafter-kirchoff mount is :
- unscrew the z-axis and tilt fixing screws (5 screws in total).
- improve the injection with the z-axis knob and the tilt screws
- tighten very softly all the fixing srews once the optimization is finished => you will lose a part of the alignment but not completely (~ 1 mW level)
- redo a part of the alignment with the 2 aligning mirrors => you should find back the values after optimization.
we let the power-meter at the output of the fiber to check in the next days if the injected power in the fiber changes or not....
it is important to not "kick" the fiber injection box or put anything on it ! as the schafter-kirchoff mount adjustment is soooo touchy...
| Ronic Chiche wrote: |
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Manar + Victor, today in the morning we aligned the CVBG of the stretcher.
The double path using the mirror mounts, reached up to 5.4 mW just before the injection into the fibers
then we rotated the quarter wave plate and the power increased by a factor of ~ 2 to 10.3 mW
we started with the injection into the fiber by alignment of the 2 mirror mounts.
1) fiber not connected and have everything pass through the mount
2) connect fiber loosely and increase the power in fiber bit by bit until fiber fixed fully
reached power up to 438 uW
alignment not finished using the mounts, there is still also schafter-kirchoff mount.
| Ronic Chiche wrote: |
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I used the 33MHz spectrum measurement : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/153
fitted by P=P0*sech²((f-f0)/df)
f0 = c/1030m
df = (c/1030nm^2) * 2.5nm
and the strecher CVBG measured data (in attached file with reference D24-02)
to estimate the expected power to be coupled into the fiber.
the corresponding plot shows 3 curves:
- black : the 33MHz laser spectrum "manually fittted" with the sech² function (mentionned at the begining of this post) to match the measured spectrum from the elog.
- blue : the strecher CVBG reflectivity curve from the Excel measured datasheet in attachement.
- red : the corresponding output power after a double path into a CVBG (the reflectivity is applied twice).
with this simulation, one can estimate the power after CVBG to be 12.5 mW for 35 mW of input power.
or 9.6 mW after CVBG for 27 mW of input power which is exactly what has been measured in a previous post by Loic : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/70
| Ronic Chiche wrote: |
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This morning with Manar, we measured the power at different points:
- direct measurement at the output of the 33MHz laser : 35 mW
inside the injection box :
- after the Isolator + focalization lens : 35 mW
- just at the input of the strecher CVBG : 35 mW
- atfer optimizing the alignment of the double path CVBG, in between the 2 fiber injection coupling mirrors : 5,7 mW
(the power is measured after going through the quarter-waveplate and PBS)
I found an old post from Loic claiming that with 27mW input power, we got 9.6 mW after the PBS instead of 5,7 mW !
https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/70
we have to check if the laser wavelength shifted, if the alignment could be improved, if the quarter-waveplate has the right angle,....
then, we tried to couple this 5,7mW inside the fiber using the schafter-kirchoff mount (SKM) but it is a nightmare.
changing the focus and the internal fiber angle is very sensitive, not always predictable and rarely reproductible...
I have to ask Guillaume how he used this mount...
the best power we saw in the fiber is 1mW but after screwing the fixing screws of the SKM, we lose almost all the alignment and we have something around 100-300µW...
| Ronic Chiche wrote: |
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In preparation for tomorrow morning, I did a test with a 45° mount to have a power pickup inside the injection box.
I used the 133MHz laser.
output power measured with the powermeter : 45mW
output power measured with the powermeter + OD2 : 2mW
output power measured with the 45° mount with BB1-E03 mirror with the powermeter + OD2 : 2mW
as expected the BB1-E03 mirrors have a very good reflectance for AOI=45°
the specs give >99% @ 1030nm for both S and P polarizations.
| Ronic Chiche wrote: |
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This morning with Viktor, we started the alignment of the CVBG and fiber.
we did a better alignment of the 2-pass CVBG.
we are able to the see a spot after PBS (after the 2 CVBG pass) which means the alignment is OK even if it can be improved.
then, we started the fiber injection alignment with the 2 last mirrors (7 & 8 on Alphanov documentation).
we saw that if we unswcrew the fiber to play on the focal position, we are able to improve a lot the power in the fiber.
2-3µW with the fiber screwed => 500µW with the fiber unscrewed.
it means the beam is not enough focused.
I will ask Guillaume Machinet his advice when injecting the 33MHz... do we need to replace the long focal lens just after the Isolator ?
and with which value approximatively ?
we played also with the Schafter+Kirchhoff mount of the fiber colimator (see attached documentation).
we loosen the 2 screws around the eccentric key which adjust the focal position + play on this eccentic key + tighten the 2 screws again.
now, we reached 330µW with the fiber properly screwed.
we have to check the available power before the fiber injection but we have very few place to place the powermeter.
maybe with a small mirror ?
| Ronic Chiche wrote: |
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Today, after laser was modelocked, we checked the power inside the fiber coming from the Alphanov Strecher box.
it was almost zero, at the nW level.
after doing some very rough alignment, we clearly saw some power (with the powermeter) correlated with the position of mirrors, when turning the alignment knobs.
this is a first step but the output power in the fiber is still very low, about 20nW !
an additionnal 2mm L-shaped hex key is needed to do some walking alignment on the mounts.
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