We investigated the MNV oplev at Yend.
Conclusion :
We found the clipping of the reflection beam.
So we tweaked the injection part in Yaw direction and set the good injection point.
Motivation :
As described in klog26149,
MNV oplev SUM value became smaller with decreasing the temperature.
Measurement :
(1) I measured the input power by power monitor (set 635 nm)
Input: 0.822 mW with 2.5 mW displayed
Actual beam power compared with the displayed one became half, but only half, not 1/100.
I also set the displayed beam power to 0.75 mW and the output power was 0.2226 mW (Previously 0.5 mW)
Output: Too small (This is reasonable that QPD SUM is 13000 with about 0.4mW, now QPD SUM is about 100, which is corresponding to 4uW)
I concluded the input beam power was not a problem, so we skipped
- Turned on/off light source
- Changed the displayed power and measure with the power monitor
- AC monitoring
- Changing the light source
- Changing the 100V AC outlet(plug?)
and so on
(2) Check the input status by my eye and iPhone camera
Fig.1. shows the photo image from the MNV viewport.
As you can see, some scattered beam can be seen (And could not find the reflection beam)
So after discussing with Ushiba-san, we touched the injection part in Yaw and check the scattered light
(3) Touched the oplev injection port in Yaw direction
Finally, I found the reflection beam from MNV mirror was clipped at the viewport of the radiation shield.
When we moved the injection port to left direction, the QPD SUM value became larger.
When we moved the injection port to left direction more, the clipping to the injection jig was detected.
So in the Today situation, we set the center point for good QPD SUM value.
(4) Set the displayed power
To set the QPD SUM about 10,000cnt, we reduced the displayed beam power to 1.3 mW.
Input power was 0.413mW and output power was 0.315 mW
