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LAS (bKAGRA laser)
kenta.tanaka - 20:09 Friday 09 August 2024 (30806) Print this report
RFAM reduction trial: Day 8

Miyakawa, Tanaka

## Abstract

We're so sorry the RAM offset issue is not solved with the high-power laser at this time.

## What we did

  • We tried to reduce the RF45 offset on REFL PD1 and PD3. This time, the power on REFL PD1 increased more than usual maybe because the REFL HWP was rotated. However, the sensitivity of the RFAM offset for each angle of WPs got high rather the power on REFL PD was high. So we left it.
  • First, we rotated the QWP in front of EOM1 to minimize the RFAM offsets of RF45. Fig.1 and Fig.2 are the angles of QWP before and after the rotation. After that, we rotated the QWP between EOM1 and EOM2 to minimize the RAM offsets of RF17 and RF56. Fig.3 and Fig.4 are the angles of QWP before and after the rotation. FIg.5 is the result. The RF45 RAM offset got almost 0.
  • Then, we were out of PSL and monitored the temperature dependence for about 1 hour. fig. 6 is the time trend of the offsets. RF45 offsets seem to be not so bad. However, RF17 and RF56 seem to get worse in terms of the fluctuation amplitude. So we reentered PSL and adjusted the QWP between EOM1 and EOM2. However, the situation has not improved.
    • We assumed that when the offset is 0 by rotating only the QWP in the case that the input beam has some ellipticity and this polarization axis misaligns the crystal axis, the AM component from the misalignment just eliminated by the AM component from some ellipticity of the input beam. So we can't distinguish between this cancellation and just vanishment of the ellipticity effect by getting the offset 0 because we are not sure whether the polarization axis and the ellipticity of the input beam change or not with the 20 W power. (Originally, I wonder if the polarization axis and the ellipticity of the PMC output are changed or not?)
    • So we may need to measure the ellipticity and the polarization axis of the high-power beam...
  • Anyway, fig.7 shows the final result. we gave up for this time.

## Next ?

  • implement a remote rotation function to each WP to adjust the angle without changing the room temperature
  • measure the ellipticity or the polarization axis of the other PMC output beam when the input power is 20W (I'm not sure the other output beam from PMC has the same as the main beam. if so, it may be the clue to solve this issue.)
Images attached to this report
Comments to this report:
satoru.takano - 6:36 Friday 16 August 2024 (30845) Print this report

I came up with an idea why RFAM is large when the output power of the main laser is large.

To make the issue simple, consider the situation that the input beam to EOM1 is perfect and we want to know the effect from EOM2 only. The amount of RFAM is given by (ref: JGW-G2415900)

P(\omega_{RF}) = P_0J_1(M)\cos\beta\cos\gamma\sin\beta\sin\gamma\sin\Delta\phi.

Here, if the optical axis of EOM2 is well close to s and p-pol, γ corresponds to almost the angle of HWP3 in PSL because there is no polarization optics between EOM2 and the pair of TFPs.

For the low power case, we set the angle γ to almost 90 deg because we want to inject as high power as possible. Considering the RFAM formula above, with this angle setting RFAM is close to 0.

For the high power case, we set γ to some angle to throw away most of power before PSL output so that the power injected to IMC is around the nominal value. For this case, we have finite value of RFAM, and the value may be larger than that for the low power case. To make it worth, RFAM would get larger if we rotate HWP3 to increase the input power to IMC and could interrupt the control of main interferometer.

One way to avoid this effect is to put a polarizer between EOM2 and the HWP3.

I haven't checked this theory quantitatively, so of course it may not explain the current RFAM issue. But for future high power operation we should keep it in our mind.

 

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