[Kitazawa, nakano]
Finally, we have closed ISS loop stably, and ISS contnued to be locked for more than 5 hours!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Also, the noise budget has been done. The detal will be repoted soon.
ISS loop was not closed by below factors.
1, Actuator of ISS( AOM ) alignment was not good.
2, ISS servo gain was inappropriate.
We made a change these and remeasured open loop gain(below figure) (UGF=63kHz).
Next, we measured RIN(below figure) with ISS in-loop PD and ISS out-of-loop PD.
Yellow line is Free run, red line is in-loop, blue line is out-of-loop.
It seems that RIN of in-loop and out-of-loop limited by something noise around 10-7/rHz.
At this time, main laser power was 3.1 W ,input power of in-loop PD was 3.45 mW and out-of-loop PD was 3.17 mW and koach filter was off.
ISS loop was not closed by below factors.
1, Actuator of ISS( AOM ) alignment was not good.
2, ISS servo gain was inappropriate.
We made a change these and remeasured open loop gain(below figure) (UGF=63kHz).
Next, we measured RIN(below figure) with ISS in-loop PD and ISS out-of-loop PD.
Yellow line is Free run, red line is in-loop, blue line is out-of-loop.
It seems that RIN of in-loop and out-of-loop limited by something noise around 10-7/rHz.
At this time, main laser power was 3.1 W ,input power of in-loop PD was 3.45 mW and out-of-loop PD was 3.17 mW and koach filter was off.
The result of noise budget is this figure. We can not measure the servo noise,so it is not contain elec noise.
In-loop : High frequency side is limited by laser noise, low frequency may be limited by elec noise(when servo noise is measured,it will be made clear.).
Out-of-loop : High frequency side is limited by laser noise and low frequency side is limited by elec noise.
The RIN spectrum measured at the signal conditioning filter is attached. Around 100 Hz, there is no additional noise and reach the shot noise limit.
This means the noise limiting the out-of-loop intensity stability is the noise at the signal conditioning filter.
From the Noise budget plot in klog6870, the PD dark noise and this signal conditioning filter have the same shape signal. That would mean that the opamps at each filter have common noises. If so, the DC power supply would be the most likely noise source of the opamp.
Mmm, my expectation was not true.
I used the stand-alone DC supply as the power supply for the ISS-PDs, but the noise remains.
Now, I don't have any idea.
