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Entry   Increasing the FP-cavity power , posted by Ronic Chiche at ThomX igloo about lasers and optics 8x
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                Reply   Increasing the FP-cavity power , posted by Ronic Chiche at ThomX igloo about lasers and optics 
Message ID: 469     Entry time: Mon May 18 11:33:46 2026     In reply to: 468     Reply to this: 470
 Author: Ronic Chiche 
 Status: Under Process 
 Type: report 
 Category: lasers and optics 
 Location: ThomX igloo 
 Title: Increasing the FP-cavity power  

this morning with Daniele, we set the amplifier ratio to 33% => 97kW

then, we set the amplifier ratio to 50% => 124 kW (position of the diffuser on Axis 18 : +11775)

we changed the waveplates axis => no effect

we changed the position of the L-shape :
- lower button / clockwise / 1/20 of a turn (one has to decrease the laser motor position to compensate) => no effect but the misalignement is very sensitive for this axis

- upper button / anti-clockwise (the Lshape position is getting down) / we went to the limit (not very sensitive in alignment) => no effect ! (no beam cut nor HOM apparition)

=> conclusion : now, the L-shape is badly positionned (it's off the beam) but it cannot be the reason for a lake of power in the cavity.

PS : the lock seems very stable for long term (~ 130kW)

 

Ronic Chiche wrote:

I installed a stronger optical filter on the reflection photodiode and loaded it on 1kOhms.

70% amplifier ratio => 575 mV => 575 µA => no more photodiode linearity issue

amplifier ratio (%) => Vr unlocked (mV)
10 => 14
20 => 156
30 => 295
40 => 412
50 => 491
60 => 545
70 => 575

to be compared to : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/378

=> still a big discrepency => could we have some clipping in the reflective wedge ?
=> I tried in realigning the wedge but got roughly the same result.
=> could it be the size of the PhD (DET36 presently) ?

=> we should also check the amplifier power at FPC input before increasing power

 

Ronic Chiche wrote:

new power increase after alignment of the waveplates and iris.

amplifier ratio (%) => FPC cavity power (kW) => FPC cavity coupling (%) => Vr unlock (mV)
33               => 98                      => 57              => 260
35               => 101                     => 56             => 280
40               => 107                    => 53             => 320
45               => 109                    => 49             => 354
50               => 114                    => 48             => 381
55               => 114                    => 44             => 403
60               => 110                    => 40             => 420
70               => 100                    => 31             => 453

We still observe a discrepancy between the measured and estimated input power (using the unlocked reflection photodiode) as the amplifier ratio increases.
But this time, the estimated FPC power is also badly estimated.
So, it is possible that the reflection photodiode is operating in a nonlinear regime, as 300 mV across 50 Ohms is equivalent to 6 mA, which may be too much to remain in the linear regime.
It could be interesting to reduce the photodiode's power (add an optical filter) and use a 1k-Ohm load impedance.

 

Ronic Chiche wrote:

today with Alice and Daniele,

we checked the distance of the optical setup :

from the amplifier output (the fiber output) to the 2nd lens (+200 mm) :
131.5+41+17+28 cm = 217.5 cm

from the 2nd lens (+200 mm) to the FP cavity window :
133+97+70+14+22 cm = 336 cm

1st lens : -100 mm
2nd lens : +200 mm
distance between them : 11 cm

we checked also the alignment of the different iris and waveplates of the optical setup :
we had to realign a little bit the 2 waveplates.
after the realignment, we obtain the 96-97kW in the FPC for 33% of amplifier ratio !

Ronic Chiche wrote:

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:

  1. Amplifier ratio = 35%
    Vr unlocked = 248 mV
    Vr locked = 103 mV
    Coupling = 58.5%
    Stored power = 90 kW
    Vpdh = 50 mV rms
     
  2. Amplifier ratio = 40%
    Vr unlocked = 282 mV
    Vr locked = 130 mV
    Coupling = 54%
    Stored power = 97 kW
    Vpdh = 52 mV rms
     
  3. Amplifier ratio = 45%
    Vr unlocked = 311 mV
    Vr locked = 152 mV
    Coupling = 51%
    Stored power = 103 kW
    Vpdh = 55 mV rms
     
  4. Amplifier ratio = 50%
    Vr unlocked = 328 mV
    Vr locked = 171.5 mV
    Coupling = 47.7%
    Stored power = 103 kW
    Vpdh = 51 mV rms
     
  5. Amplifier ratio = 55%
    Vr unlocked = 353 mV
    Vr locked = 204 mV
    Coupling = 42%
    Stored power = 96 kW
    Vpdh = 51 mV rms
     
  6. Amplifier ratio = 60%
    Vr unlocked = 375 mV
    Vr locked = 227 mV
    Coupling = 39.5%
    Stored power = 95 kW
    Vpdh = 53 mV rms
     
  7. 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

 

 

 

 

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