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AdV-ALS (Installation)
gosselin, derossi, montanari, sposito, casanueva, vardaro, bersanetti, masserot, bulten - 20:26 Tuesday 26 January 2021 (50580) Print this report
fiber coupling between EIB and Laser lab atrium

Today we have been working on the coupling of the green light from EIB towards the beating box in the laser lab atrium.

This morning, we installed the two fibers between the EIB and the box.

This afternoon we have been working on the coupling.

First we removed the photodiode looking at the beam coming from the west arm and installed the collimator at its place.
We used the green coming from the SHGB of the atrium to do a backward propagation on EIB and try to superpose the beam coming from the collimator and the ones coming from the interferometer.

The first approach in order to maximize the coupling into the fiber, was to scan the VCO in order to see an average power at the output of the fiber and maximizing this one. Unfortunately the power at the input of the fiber was only of about 100 uW which was too low to allow any coupling improvement.

The second approach was to keep only the beam coming from north arm and rotate the HWP in front of PBS separating the beam coming from north arm and west arm. By doing that we were able to have some power both towards the collimator and towards the photodiode. We relocked the injection and thanks to the power on the photodiode we were able to lock the green. We had about 3.2 mW at the input of the collimator. The green was sent towards the atrium in one fiber and bring back next to the EIB thanks to the second fiber in order to ease the lecture on the powermeter.
Both the injection and the green were unlocking which made the alignement complicated but we finally managed to get up to 1.4 mW. Considering the losses of the 30 m of fiber and the additional fiber connection, this is already a relatively good coupling. We decided to stop trying to improve it for now. 

We removed the second photodiode of EIB (the one looking at north arm) and roughly installed the collimator.

Henk Jan rebalanced the EIB from remote. Then we had to slightly realigned the green into the fiber.

Finally, we relocked the injection.

Conclusion: the green coming from north arm is injected into the fiber and is connected to the beating box.

Comments to this report:
casanueva, masserot - 22:34 Tuesday 26 January 2021 (50584) Print this report

Once the signals in transmission were available, Alain has implemented the software changes needed to propagate the signals to the processes of ALS NEB. We re-tuned the triggers of the lock based on the new signals and we were able to engage the lock normally.

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