Last Friday, we tried to increase the FP-cavity power by just increasing the amplifier ratio from 35% to 70%.
It took a bit longer than expected because it started with the surprise of finding our entire setup powered off…
The power had tripped, and the switch that allowed us to remotely reset the power supplies had been damaged.
The switch was bypassed, and the instruments restarted → OK.
As usual, the locks were pretty unstable (as has been the case for several weeks), and we couldn’t hold them for more than 10–20 seconds…
So I removed the HV amplifier between the LaseLock and the laser PZT.
Since the Smaract MCS2 controller is much less noisy than the MCS1, we can use it directly without unlocking the cavity or going through "piezo-scan" mode.
So basically, having a larger range on the laser PZT is less useful.
Result: perfectly stable locks in the short term (as before installing the amplifier) AND in the long term as well… no more random unlocks!!!
So the amplifier must have been picking up parasitic noise that was being reinjected into the loop and causing regular unlocks… it works much better now! :-)
Actual power ramp-up was done entirely from the control room:
Previously measured relation between amplifier ratio and input power to the cavity:
Amplifier ratio = 10 20 30 40 50 60 70 %
Amplifier Pin = 0.8 8 15.8 24 32.5 39.5 45 W
We performed 7 measurement points:
- Amplifier ratio = 35%
Vr unlocked = 248 mV
Vr locked = 103 mV
Coupling = 58.5%
Stored power = 90 kW
Vpdh = 50 mV rms
- Amplifier ratio = 40%
Vr unlocked = 282 mV
Vr locked = 130 mV
Coupling = 54%
Stored power = 97 kW
Vpdh = 52 mV rms
- Amplifier ratio = 45%
Vr unlocked = 311 mV
Vr locked = 152 mV
Coupling = 51%
Stored power = 103 kW
Vpdh = 55 mV rms
- Amplifier ratio = 50%
Vr unlocked = 328 mV
Vr locked = 171.5 mV
Coupling = 47.7%
Stored power = 103 kW
Vpdh = 51 mV rms
- Amplifier ratio = 55%
Vr unlocked = 353 mV
Vr locked = 204 mV
Coupling = 42%
Stored power = 96 kW
Vpdh = 51 mV rms
- Amplifier ratio = 60%
Vr unlocked = 375 mV
Vr locked = 227 mV
Coupling = 39.5%
Stored power = 95 kW
Vpdh = 53 mV rms
- Amplifier ratio = 70%
Vr unlocked = 396 mV
Vr locked = 272 mV
Coupling = 31.3%
Stored power = 82 kW
Vpdh = 53 mV rms
Normally, the unlocked Vr voltage should be proportional to the incident power on the cavity.
But it clearly isn’t at all!
We need to verify that this photodiode remains well aligned as power increases.
Or whether we might be clipping on a lens edge as the power increases.
Or whether the power still follows the amplifier ratio/power relation measured several months ago.
To be checked during the power ramp-up from the bunker.
We didn’t go above 103 kW!!! :-(((
I think the main reason is that we are probably hitting a mount edge, and the coupling drops so quickly that it dominates
→ so we absolutely need to redesign a proper telescope.
We also took beam images for different Pin values, but since the power in the cavity barely changes, it’s not very informative.
The last plot shows that the signal from the reflection photodiode indeed corresponds to the incident power in the cavity.
There is a strong chance we are hitting something when changing the power… to be checked in the bunker!!!
Have a nice weekend
Daniele & ronic
|