one problem is the FP-cavity/laser lock loss when one moves a FP-cavity motor.

with acceleration = 0.01 units/s² and velocity = 50mm/s (here, the 'mm' unit seems strange as it is very fast), it's enough to make a move fast enough for small displacements (10 steps for example to center PZT position) and it seems that the cavity stays locked (only the FP-cavity/laser is locked).

=> to be checked when both FP-cavity/laser and FP-cavity/RF-reference feedback loops are running.

temperature stable around 21.7°C

today, the equivalent Smaract position corresponding to the 500.25MHz ring frequency is +147µm on MCS-1/ch0 (closed loop mode)

temperature since we moved to temperature probe (jump at the beginning of the plot) below the ring on a metallic base.

as the temperature is back to a normal value ~ 20.8°C, the injected power to the amplifier (PD_IN in the Alphanov software) is back to 3.15mW without doing any alignment.

this morning the lock was around 48.5kW with 30% laser amplifier ratio (16W) after CEP/alignment tuning.

Kevin moved the M4 mirror controller from the ISP controller to some ICEPAP controller.

the IHM to access this ICEPAP controller is accessible by launching 'jive' from any account ('operateur.thomx' for example).

once in the jive window, one has to select the 'device' tab, then select the OC=>OP=>OCH.02-MOT.03 device.

an AtkPanel is launched in which one can change the step values which are direclty the motor steps.

temperature stable around 20.8°C

today, the equivalent Smaract position corresponding to the 500.25MHz ring frequency is +158.4µm on MCS-1/ch0 (closed loop mode)

it is very difficult to maintain both loops in the same time as soon as it is needed to move one motor (CFP or Laser) because of the one element (RF reference or Laser or CFP) is drifting in frequency.

to try to understand why these 3 elements seems to drift so fast one from each other, we only measure the beating frequency between the RF reference and the free running laser (without lock of the CFP)... and we see a drift around several Hz by second of the beating signal => who is guilty ? Laser or RF synthesizer ?

one can compare their respective phase noise to have an idea of their relative phase/frequency stability :

the OneFive phase noise gives +40dBc/Hz  @10Hz offset in optical frequency which is 300000 (110dB) more than at 1GHz => 40 - 110 = -70dBc/Hz @10Hz offset @ 1GHz

to be compared to the SMA100A which gives -85dBc/Hz @10Hz offset @ 1GHz

conclusion : the RF reference should be more stable in long term and the drifts we see should come from the laser...

today, we locked the FP cavity at ~50kW with 30% laser amplifier ratio (16W) during almost all the day (from 10am to 4pm)
CEP optimized for MCS-1/Ch2 = -244µm at the end of the day.

once one finds the proper CEP value to reach to correct loops gain, the cavity lock and power are very stable:
one looses the lock only when one needs to use the Smaract motors to follow the long temperature drifts.
several elements of the ThomX machine have been powered ON progressively during the lock without any lock perturbation excepting for a very short time when switching ON the RF cavity (to be confirmed) and when one tried to inject electrons into the ring (loss of the electrons after few turns only)... but it's not very clear. the lock is still stable but some time one sees a lock loss without "reason"... could it be the electron loss or some bad compensation of the noise due to feedback, it's hard to say.

at the end of the day, I had to realign the FP-cavity injection and change the CEP more often than in the morning,
and surprisingly, the intra-cavity power drops a little bit at the begining of the lock (~50kW) and after some tenth of seconds (=> ~47-48kW).
it is not so much but it is very repeatable at each try.
I tried to optimize the CEP, the injection alignment, the PID parameters => it helps but at the end, I still have this slow power drop of few kW over tenth of seconds that I didn't see at the begining of the day... to be investigated.

today, the equivalent Smaract position corresponding to the 500.25MHz ring frequency is +156µm on MCS-1/ch0 (closed loop mode)

from several weeks, the maximum power stored in the FP-cavity was ~ 5kW.

today with Daniele, we finished to investigate the problem, and now the power inside the FP-cavity is back to ~50kW for 30% of laser amplifier ratio (~16W).

we optimized the signal received by the PDH photodiode by installing a large DET100 to collect more light.

if one installs a small photodiode (DET10) in the middle of the beam, the carrier signal when a FP-cavity crosses a resonance is larger because the photodiode "sees" only the part of the beam which is geometrically coupled to the cavity in its small active area, but :
1- once we will improve the geometrical coupling, the part of the incoming beam coupled to the FP-cavity will increase.
2- one need to work with a diffuser in front of this photodiode to precisely adapt the feedback loops gain : in that case the photodiode is sensitive to the whole input beam, whatever his active area size.
so, we decide to put a DET100 (which is given for 35ns rise time / 10MHz BW when connected on 50ohms).

see the scheme in attached file.
and a picture of the desktop with all the lock parameters :

the quality of the lock, seen on the reflected power signal is very good !
and the stability is only limited by the necessity to act on the laser Smaract motors to let the PZT in its working range.
dark blue : transmitted signal
green : PZT
pink : error signal
light blue : reflected signal

conclusion : it is not clear that the cavity Finesse have significantly increased during the last weeks, as we are roughly at the same level than before (47kW).
but as we precisely adapted the signal levels in the feedback scheme (PDH S/N ratio and Laselock parameters), the result is a more stable lock.
 

we confirmed the effect of the bunker temperature on the laser amplifier "beam pointing" fluctuations.
once the temperature is getting back to stable values, it doesn't happend again.

we bought a temperature data logguer to monitor them in the future: https://www.picotech.com/data-logger/tc-08/thermocouple-data-logger

this post close this thread.

Today, the power measured at the input of the amplifier (PD_IN on the LAL amplifier software) is 2.5mW instead of >3mW generally measured

The power coming from the NKT/Onefive Origami oscillator is still >37mW (measured directly with the powermeter at the laser output without OD2).

thus, the problem should come from:

- the strecher/fiber alignment.

- or maybe from a wavelength shift of the oscillator

Last monday with Victor we have cheked the stability of de pointing of the laser FP.

The climatisation was operating since 3 days in satble way. The laser pointing was very stable before to inejct in to the cavity (picture 1) and also for the reflexion from the cavity (picture 2).

That means thant it is necessary to have a stable climatisation operation.

This morning FP Laser was operating well locked at 30 KWatts in stable vay.

There are some fluctuations due tu pointing instabilities probably dues to temperature fluctuations in de bunker.

In the attacced picture are reported the lock parameters and signals.

By adjusting the position of laser caviti length the lock old all de morning.

this morning, I locked the cavity to ~25kW without any problem.

but this afternoon, the reflected power exhibited low frequency (~1Hz) fluctuations of about 10% without any lock.
the reflected PhD is a DET10 which has a small surface.

we checked the OneFive oscillator power which is perfectly stable.

we changed the DET10 PhD for a DET100 PhD with ~1cm surface : we don't see any power fluctuation => the amplifier power seems stable.

we put back the DET10 PhD : we see these fluctuations of about 10% => it could be some pointing effect !!!
when one locks the FP-cavity, we clearly see exactly the same power fluctuations at the Transmission PhD but complementary => the sum is constant.
so, it seems clear that the beam coupling to the cavity is fluctuating due to some pointing fluctuation of the incoming beam.

a reason of these fluctuations could be the thermal jump done today because of the air cooling system of the bunker :
the temperature jumped from 25°C yesterday (and maybe still this morning ?)  to 18°C this afternoon !!!
thus, some mechanical parts (the compressor CVBG ?) could be moving and then could produce these pointing fluctuations...

after a sudden FP-cavity Finesse increase, we had to install the Fast Feedback Loop in order to continue to lock the cavity.

this afternoon, I was able to lock both the FP-cavity and the Laser on the 500.25MHz oscillator reference.

I had to change the CEP to reduce the effective Finesse and help to get a better stabililty => P ~ 25kW inside the FP-cavity for 30% of laser amp. power ratio.

the 20Hz oscillation is still there on the PZT signal and can be seen also on the reflected signal when the amplitude of the PZT signal is large or when the lock is lost or almost lost.
this 20Hz signal is modulated by a higher frequency of the reflected signal => to be investigated...

the posts, specific to the 20Hz oscillation noise, are here : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/258

this post closes this thread.

I copy the post about Fast Feedback loop : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/265

This morning,

- I installed the fast feedback loop.
now, the error signal goes to the Laselock AND
to a FEMTO DHPVA amplifier which is connected to the Leysop M250 HV amplifier connected through its HV+ output to the EOM.
one can set the gain of this loop thanks to the DHPVA gain potentiometer and to a 30dB attenuator.
it allows to have a fast and stable lock ONLY IF one reduce the FP-cavity gain using the CEP.
I will check later if I'm able to lock at the maximum gain but today, the cavity power is ~ 30kW, to be compared to the 45kW we were able to get before the "mirror cleaning event".

- I had to swap the sampling frequency of the Laselock from 250kHz to 2.5MHz to reduce the latency and improve the Slow feedback loop stability.
with the previous sampling frequency, the fast feedback loop was almost uneffective...

Conclusion : now, this Fast feedback loop is mandatory to lock the cavity at high Finesse or high Gain (G > 10k)

this afternoon, in adjusting the CEP and improving the parameters of the lock, I was able to get ~ 47kW stable inside the cavity, always for 30% of laser amplification ratio.

it confirms that the Finesse, and then the gain of FP-cavity, has suddenly increased thanks to some "mirrors surface cleaning"

This morning,

- I installed the fast feedback loop.
now, the error signal goes to the Laselock AND
to a FEMTO DHPVA amplifier which is connected to the Leysop M250 HV amplifier connected through its HV+ output to the EOM.
one can set the gain of this loop thanks to the DHPVA gain potentiometer and to a 30dB attenuator.
it allows to have a fast and stable lock ONLY IF one reduce the FP-cavity gain using the CEP.
I will check later if I'm able to lock at the maximum gain but today, the cavity power is ~ 30kW, to be compared to the 45kW we were able to get before the "mirror cleaning event".

- I had to swap the sampling frequency of the Laselock from 250kHz to 2.5MHz to reduce the latency and improve the Slow feedback loop stability.
with the previous sampling frequency, the fast feedback loop was almost uneffective...

in attachement, the Laselock parameters and a picture of a lock.
yellow: FP-cavity transmission signal
blue : FP-cavity reflection signal
green: PZT signal
pink: PDH error signal

One has to move again the L-shape arm to remove the HOM
and check if we are able to lock in the same time the FP-cavity on the 500.25MHz reference oscillator... to be done this afternoon.

it could be also an optical unstability, as when the intra-cavity power increases, the radius of curvatures of the mirrors increases too due to thermal effect, and then one could go in the instability region.

but if it was the case, by reducing the power in the FP-cavity, we would also reduce the thermal effect and then, we would come back in the stability region...

and it is not the case : the system is unstable even with 20kW instead of 40kW.