[Kokeyama, Michimura]
On March 17 at 20:10 JST the two commissioners of KAGRA simultaneously observed a 3-km Michelson fringe signal.
What we did:
0. Modified the LSC model to include REFL DC PD (channel 12 of ADC0; see klog #1039)
1. After the initial alignment of ETMY, we unlocked IMC and found the REFL DC PD (Thorlabs PDA100A) had ~1300 counts background with 30 dB gain.
2. Covered the REFL port with aluminum foil (see reflfoil.jpg) to get ~-300 counts background with 70 dB gain.
3. Misalinged ETMX and swept ETMY randomly, and immediately found the reflected beam from ETMY at REFL PD.
4. Tuned the ETMY alignemnt to maximize the REFL PD output. During this tuning, we turned the gain of REFL DC PD fron 70 dB to 0 dB.
5. Checked the ETMY reflected beam at REFL port (see reflbeamfromy.jpg). It didn't show the striped pattern as in the ETMX reflected beam.
6. Aligned the ETMX back again and swept ETMY in length with amplitude of 30000 counts.
7. Observed Michelson fringing (see fringes.png; movie will be uploaded later).
Next:
1. There was a large amount of length to alignment coupling for ETMY. We should balance the coils for the BS, ETMX, and ETMY.
2. 30000 counts sweep (almost the DAC limit) in ETMY length gave only one fringe. We should check the actuation range.
3. Alignment is not perfect. Also, we should have oplev damping for the BS, ETMX, and ETMY.
Nice!
It is a great news that the Y beam comes back to REFL.
Regarding the fringes, I'm not 100% sure if this is actual interference or due to the beam pointing fluctuation of the Y beam.
The fact that the REFL count goes to near zero indicates it is due to interference. But there are other places where the REFL count goes flat to zero. Let us check this more in detail.
Anyway, certainly we are making a good progress.
Conglatulations!!
And I am happy to hear that IFI may correctly be working :)
Aso-san, I believe it is real. We saw the interference pattern on a sensor card and that was obviously it. I think Michimura-kun will upload its video sometime soon, and we will install camera at REFL
Yep, as we don't have local length sensors for ETMX and ETMY, we need to rely on only oplevs for the purpose but they are not such sensitive to length changes. In order to say this is the fringe by relative length changing btwn X and Y, we also need to see the sine-wave like signal change at least... As we don't have AS port this time, it would not be possible to make a circle diagram.
Here is a fringe video taken by YM.
Aso-san, about REFL PDA being zero on the StripTool plot, it was when YM was blocking the light to see it by himself on the sensor card.
AS Shoda-san said, please be careful for your health.
Great news!!
Congratulations to all the commissioners!