KAGRA Logbook

IOO (IMC)

masayuki.nakano - 22:11 Saturday 02 February 2019 (7942)

IMC high power test

[Enomoto, Nakano]

We tested the high power injection onto the IMC. We increased the laser power up to 20 W but at that power, we could not lock the FSS loop. I will check it again at some point.
And also, the transmitted ratio of the IMC getting lower when the injection power gets larger. When the injection power is 1W, the transmittance is around 80%, although it was around 60 % for the 5 W injection. We guess that might be because the MCL loop has not enough gain to overcome the radiation pressure. I gonna calculate the radiation pressure effect for the LSC and also ASC again.

At the same time, the PD signal of the MCE TRANS was calibrated to the reflected power from the IMMT1. The attached figure shows the calibration curve.

Images attached to this report

Comments to this report:

kiwamu.izumi - 2:26 Tuesday 05 February 2019 (7963)

Hey Masayuki,

Your hypothesis of radiation pressure being too high sounds a bit strange to me. Assuming that the MCL loop has an integrator in its digital filters, I expect that the coils on the MCe would saturate at the DAC output if the DC radiation pressure is too large to compensate for. Although I am not sure if the MCL loop has been regularly engaged these days.

Could this be due to a thermal lensing somewhere which messes up the mode matching?

masayuki.nakano - 11:40 Tuesday 05 February 2019 (7968)

Kiwamu,

Thanks for your comment.

Yes, the MCL loop has the integrator, but the thing is that I forget to turn it on during the high power test. Although the guardian treats it usually, I locked the IMC by myself.
About the thermal lens, I'm not sure but it would be one possibility. I saw the REFL image during the IMC gets locked and it seems circle-shape beam. This means there is two (or maybe more) candidate, one is the 0th-order mode because the control point is not really on resonance, another is the 2nd-order mode because of the thermal lens miss mode matching.