NAOJ GW Elog Logbook 3.2
Participants : Eleonora, Marc, Yuefan, Yuhang
Since last week there were few attempts to characterize the green beam reflected by the FC.
One first task was to design a periscope able to extract the beam going inside the photodiode used to lock the FC.
As Matteo Tacca requested, this periscope was installed before the lens of the photodiode.
We couldn't find any green BS so we used a IR BS as the bottom mirror of the periscope.
We still need to use 2 optical density in order to reach power values similar to the previous configuration.
This let around 200 uW going inside the photodiode which is enough to lock
[ By the way one attempt let only 18 uW inside the photodiode and lock wasn't possible.]
Another difficulty was to avoid the clipping of the reflected beam at the FI output (face toward the FC) which needed to change a bit the alignment condition of the FC.
There are also some strange big features on this beam which might be investigate a bit.
We then installed a steering mirror at the output of the periscope in order to have enough space to characterized the reflected green beam.
The characterization is Fig.1 attached to this entry.
The first points seems to suggest that the beam is collimated around 2.2 mm.
However the last points are quite differents in size.
This might come from the fact that the FC unlocked several times during this measurement.
We started from a quite good alignment (1.2V transmission) to end with a poor one (0.8 V).
Here are the data :
z = [ 0 1 2 3 4 5 6 15]*2.5+40 [number of holes]+position of first measurement
wW = [2170 2192 2181 2137 2239 2183 2214 2380]/2
wV = [ 2169 2207 2216 2211 2207 2202 2112 2114]/2
Last Friday we wanted to do this characterization again because it seems that our previous points were still inside the Rayleigh range of the beam (which should be few meters)
The solution is to add a lens and recover the beam parameters without the lens.
We faced few issues during this measurement :
Faraday Isolator
- The Faraday Isolator output (facing the FC) is clipping probably higher order mode which are quite large w.r.t. the FI aperture
- The FI PBS which reflects the beam toward the locking photodiode is a bit broken ( it is the cause of the "strange" behavior of the beam we could observe. However, with proper alignment it is possible to avoid hitting the broken part of the PBS and have a good beam ( meaning we can avoid to have black line on the beam)
For now it seems we can keep going on with this FI at least for the beam characterization.
It may be useful in the future to change this FI for a larger aperture one or at least change the broken PBS.
BS
- We found out that the beam was shaking a lot ( ~40 urad rms ) while performing this measurement. This is due to the fact that the BS pitch correction is close to saturation (-10V) and some peaks lead to correction saturation.
The solution is to move the BS pitch picomotor. Matteo also recommends to follow the GEO procedure : Every ~2,4 weeks, use picomotors to put mirrors on good positions in order to avoid this kind of saturation.