here is the picture of the L-shape arm and the inside of the vessel.

this image corresponds to the "zero" position on each positionner.

redoing the PDH error signal scheme with discrete components is more flexible and it is easier to check the signal/noise ratio.
now, we are able to get a quite clean lock only with the PZT correction (w/o fast feedback correction using the EOM) even with both motors controllers ON (cavity and laser).

then, one can consider this part is over, even if one can still improve the lock with the EOM.
see this post for that part and some details on the new PDH signal scheme : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/240

yesterday with Daniele, we tried again to move the arm horizontally and vertically from the "zero position" to have a better understanding of the beahvior,
but it is roughly the same :
- a clear beam power reduction (=> cut of the beam) when one moves the horizontal axis
- a very small beam power reduction when one moves the vertical axis

as there are no end-position limits on the axis and as it is possible to "touch" the motors or different mecanichal pieces, we prefered to be conservative:
after several tries, one placed the arm at a position where the HOMs seem to be pretty well suppressed.

for the moment, we will consider this arm in a proper position.

This afternoon, I connected the HV output of the Laselock (instead of the standard 0-10V output) to the FP-cavity PZT to increase the locking range.
I didn't see any degradation of the FP-cavity/synthesizer lock.

this morning, we tried to find the origin of the 20Hz oscillation.

- we switched OFF the laser Smaract motors controller => no change

- then, we addionally disconnected the FP-cavity PZT cable from the Laselock (we put a charge of 1kohm) => no change

- then, we switched ON the laser Smaract motors controller and switched OFF the FP-cavity motors controllers => no change

in conclusion, we don't really know where this instabillity comes from.
the amplitude is roughly 1Vpp (when the oscillation is at its maximum) on the laser PZT <=> length oscillation of ~20nm pp

could it come :
- from the air cooling regulation with pressure variation ?
- from vibrations of the hexapod below the table ?
or is it from inside of the laser or FP cavities ?

see these posts for the first measurements on this issue: https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/257

this afternoon, we did 2 tests to better understand this 20Hz oscillation:

- we locked the amplified laser directly to the 500MHz ring reference oscillator, without any intermediate locking to the FP-cavity => no change
the 20Hz oscillation is still present in the correction signal of the laser PZT.

- we switched OFF the controller of the hexapod => no change.

conclusion:
the 20Hz oscillation is coming from the laser cavity
or is coming from "outside" and could be measured, maybe at a higher level, with an external "noises & vibrations measurement system".

this afternoon, we tried to better understand how to drive properly the EOM to kill high frequency noise.

we locked to laser on the FP-cavity as usual.

then we injected a 0-10V square signal on the laser EOM @ 1KHz (with fast rise and fall times ~ 10ns)

we clearly see a small drop on the cavity transmitted power, but much like a sine wave in phase with square signal, because of the small bandwidth of the cavity ~ 1kHz.
then it is difficult to deduce a time response of the system when one injects a signal on the EOM.

because of compensated noise on the PZT signal, one does not see any variation on this signal

because of the bandwidth of the feedback (~10kHz => 100µs period), the possibly visible effect of the square input signal on EOM is compensated quickly,
in addition, the effect with 0-10V input signal is small and superposed with other noise sources => one does not see a clear correlation.
we planned to work with 0-100V input signal but we add a strange issue at this moment BEFORE increasing the voltage on the EOM
=> see next post : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/261

suddenly, BEFORE switching the HV amplifier ON and BEFORE connecting it to the EOM (in order to explore the HV effect on the EOM for the fast feedback loop),
we lost the lock between the laser and the FP-cavity: in attchement a plot a the lock with the "best" PID parameters.
- yellow: FP-cavity transmission signal
- green: PZT signal
- pink: PDH error signal

the lock was pretty "normal" except that we observed that the intra-cavity power is always slowly increasing from 40kW to ~44kW during a lock, all along this last week.
there are several possiblities for that :
- a slow increasing of the input power (we see the effect on the reflected signal when the FP-cavity is not locked)
- fluctuations of the CEP to the "good" value,
- or more supprisingly an improvement of the Finesse.

after this issue, we tried to change the laser input power (30% to 25%), or the change the CEP => we always get the same result => the lock is either too "weak" (not enough gain) or too "strong" (the system is unstable and goes in oscillations visible on the picture a the end of each lock period).
we tried to change the PID parameters quite a lot to try to compensate a change in the FP-cavity transfer function without any effect => impossible to have a proper lock as before.
we tried to check if the PDH phase, to produce a proper error signal, has changed => no, it was the good one.
we tried to correct the laser alignement to the FP-cavity, but it was more or less correct and we didn't see any change in the locking.

then, one possibility could be that one dust have been suddenly removed from the cavity mirrors by the high power and the Finesse suddenly increased substancially.
=> more Finesse => less bandwidth => high frequency noise are less "visible" in the error signal and we get less bandwith for the feedback => more difficult to lock.
we thought that this kind of problem could be solved by changing the CEP, but in this case, it didn't succeded to lock.

the other possibility is that just before having this issue, we were doing tests on the EOM with a 0-10V signal.
=> could it be possible that the static polarisation of the laser has changed ?
then, we would need to adjust the waveplates in the laser path to adjust the correct polarisation ?
=> not for sure... as the maximum transmitted power at the begining of the lock is the same as before... and compatible with ~ 43 - 44kW in the FP-cavity.

could it be also that the intermediate signals of the PDH scheme are saturated (because of the increasing power : 40kW to 44kW) and produce a "false" error signal leading to instability ?

or could it be a bug in the Laselock ?
=> we could restart it to confirm....

the goal is to estimate what could be the frequency drift at 500MHz for the SMA100A: see phase noise datasheet in attachement

Sphi(f) = FFT ( Rphi(T) ) = FFT ( < Phi(t) Phi(t+T) > )

at low frequency, Sphi(f) ~ A / (f^n) = A*(2pi)^n / (i2pi*f)^n => Rphi(T) = A*(2pi)^n*T^(n-1) / (n-1)!

for the SMA100A : n ~ 2 and A =10^-7 at f0=1GHz with B22 option

=> Rphi(T) ~ 4e-6*T

this morning, we tried:

- to move a bit the arm of the L-shape off the beam axis (in case of it could have touch something and induce vibrations) => no effect

- to change the CEP to get an equivalent lower Finesse => weak improvement

- to add a diffuser in front of the PDH photodiode and check the saturation level after the FEMTO amplifier to avoid non linearity effects in the PDH signal => weak improvement

- to move the half and quarter waveplates in the incoming beam path => no effect

then, we have to work with the laser intracavity EOM to try to cancel high frequencies noise

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.

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.

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"

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 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...

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.

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

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.

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.
 

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