KAGRA Logbook

IOO (Laser Bench)

yutaro.enomoto - 22:02 Monday 22 May 2017 (2786)

Alignment for the Reference Cavity

[Nakano, Kasuya, Enomoto]

= Summary =
Today we did alignment work for RefCav, saw the fundamental mode light resonate in RefCav , and maximized the peak of the transmission of the fundamental mode light by changing the alignment of two steering mirrors (mode matching not yet optimized).

= How we did =
- checked by eye where the centers of the cavity mirrors are with respect to the centers of holes of the box covering RefCav (this step was critcal)
- injectd HeNe laser beam from the end of RefCav and put the beam so that the beam resonates in the cavity
(In this step, information of where the centers of the cavity mirrors are was quite beneficial. HeNe laser was useful because it is visible and the finesse for HeNe is very low (we guess it is below 10).)
- aligned mirrors before RefCav so that the paths of HeNe and IR coincide
- at this point we found resonance of very high order modes (of IR light) in the transmission light with a CCD camera.
- optimized the alignment by try-and-error with the frequency of the main laser sweeped by both PZT and temperature control.
- once we found the foundamental mode resonating in the cavity, optimized the alignment by maximizing the peak power of the transmission light of the fundamental mode with transmission light monitored by a PD

= Next =
To lock RefCav, we are going to first prepare necessary cables, circuits (almost ready?), digital models, and so on. Then we lock RefCav, hopefylly tomorrow

Comments to this report:

yutaro.enomoto - 20:52 Tuesday 23 May 2017 (2791)

[Nakano, Kasuya, Enomoto]

Today we did the following preparation work for locking RefCav; re-ordering circuit boards as designed in the racks in PSL room, preparing cables to be used, applying voltage to EOM for RefCav lock and installing RF PD to detect the reflection of RefCav.

We were able to sucessfully observe the PDH-shaped error signal (that indicated the EOM and RF PD work properly), but the signal was too dirty due to bad mode matching so that we gave up trying to lock the cavity.

Next thing we are going to do is to reshape the beam profile by putting one or more temporary lenses to make the mode matichg ratio better.

yutaro.enomoto - 18:22 Wednesday 24 May 2017 (2794)

[Nakano, Kasuya, Enomoto]

Dirty error signal of RefCav was due to interference effect which was most likely caused by birefrengence of AOM crystal (we had injected circular polarized light into AOM, this AOM is used as a double path AOM).
We swapped the order of QWP and AOM so that we can inject linear polarized light into the AOM. Then we optimized the angle of QWP by minimizing the amplitude of the interference; at the optimal angle, the transmittance of PBS was maximum.
Currently this problem was solved. Inject linear polarized light into AOM!!