this morning with Daniele, we did a quite long run with this motor position and it seems we don't lose the lock too much.

the motor positions are:

MOT.03 (M4): -186 500 steps

MOT.06 (M1): -797 500 steps

we got 85kW after CEP optimization and rough alignment optimization.

Fiber injection is finished with the new ThomX stretching CVBG in double pass (+50% injection power compared to K-BOX CVBG in simple pass).

- Output Onefive: 27 mW
- After Onefive + lambda/2 + isolator: 25.8 mW
- After Onefive + lambda/2 + isolator + lambda/2 + PBS + CVBG*2 + PBS: 9.6 mW
- After injection into fiber and EOM (+all preceding): 3.2 mW
- Input 99% of the amplifier: 2.86 mW

> 660 mW on 1% photodiode DET100 @1Mohm

today, we tried to lock the FP cavity with the Smaract motors ON (with option -LV).
we know the Smaract controllers produce some noise and the lock is very bad or impossible when the controller in ON (whatever the displacement mode is : closed or open loop, or piezo scan).
then we need to do a fast feedback loop on the EOM inside of the Onefive laser.
the problem is we cannot fill directly the error signal (~ 300mVpp) as the signal level is too low to produce some effect.

1- we tried to use the AC-coupled homemade amplifier alone but the output range is too low (+/- 3V)

2- we tried to use the M250 video amplifier for EOM with AC-coupling but the output range is still to low (+/-30V ? => to be confirmed)
but we saw an improvement in the locking.

3- we tried to combine AC-coupled homemade amplifier + M250 video amplifier for EOM with AC-coupling.
we are able to lock (~ 30% coupling) but the lock quality is very poor : we clearly see that we oscillate around the maximum of the Airy peak.
we tried several combination of the global gain (fast feedback + Laselock) using the diffuser, of the Laselock PID parameters but it seems we are not able to lock properly.

we measured a global delay of this double amplification stage of 80-90ns for the homemade amplifier and 150ns for the homemade+M250 amplifiers.
this delay is compatible with ~ 500kHz BW for the feedback => it seems it is not the reason...

we measured also the linearity of the homemade amp => there is ~ 30dB between a frequency and its 1st harmonic even at low signal...
the amp scheme is not very linear.
in comparison, the HV M250 amplifier exhibit > 80 dB of linearity !
we will try to remplace the homemade amp by a commercial FEMTO amplifier to imrpove the linearity and see if it improves also the lock.

we measured also a 100Hz AM modulation on the output signal of the homemade amp+M250.
=> we can try to work in differential (HV+ - HV-) to see if it helps to remove this modulation.

today, after the several unsuccessful attempt yesterday to get an improvement in the lock,
we decided to "rebuild" the error signal electronics block by block and step by step :
the scheme is basically :
- DET36 photodiode
- followed by a 10MHz low pass filter to remove the laser frequency harmonics and keep only the modulation frequency at 8.4MHz.
- connected to a FEMTO HPVA AC-coupled 40dB gain amplifier with 50ohms in parallel on its input (which is connected to the photodiode).
- connected to a Minicircuit mixer which is also demodulated by the generator at 8.4MHz
- followed by a 1.9MHz low pass filter to remove image frequencies

when this signal is sent to the Laselock box, the lock of the cavity is possible but very noisy.
we need to put a large D values in the PID to maintain the lock at the price of oscillations and gain loss ! :-(

when this signal is connected also to the M250 video EOM amplifier (which is 50 ohms), but this amplifier is not used,
we suddenly got a much better lock (see the attached pictures), certainly due to the 50 ohms connected to the input of the Laselock system => to be verified.
one could have some noise current at the Laselock input which produces less noise at the output when the input impedance is 50 ohms, instead of the several kohms when the input is unloaded...

then, we were able to get a stable lock at ~40kW with 30% coupling and 30% of amplifier :
see this post with the same values : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/227
but now, the lock has been done WITH ALL the motors controllers ON !!! :-)))
now, we can try to improve the lock with the fast feedback loop.

and the only real problem is the mode degeneracy we need to block with the L-shape.

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

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 with Daniele, we operated the FP-cavity to be sure there is no issue due to the Linac section changing operation :
- opening/closing the valves due to air pressure break down
- opening/closing the bunker roof
- large temperature change
- etc...

the temperature in the bunker dropped to 18°C instead of 21.5°C, so we had to do some FP-cavity alignment.
but after obtaining resonances and changing the CEP, we got more than 50kW for 33% of amplifier ratio => OK !

This morning, I locked again the FP-cavity at more than 50kW with 33% of laser amplifier ratio.

the temperature is measured today at 20.5°C despite the fact the air conditionning is still OFF.
the temperature, rising from 18°C, started monday at 11AM roughly.
maybe, the difference comes from the external doors of the Igloo which have been closed recently ? to be confirmed.
as the temperature is measured inside the airflow housing, all the powered equipements are disspating some heat, rising the inside temperature compared to the bunker temperature.
EDIT : Sophie Chance told me the water temperature cooling of ThomX was defective... the water temperature went to 29°C ! it is being fixed by Dalkia.
careful, we don't have a temperature measurement of the water going to the chiller...

to find resonances, I moved the OC/OP/OCH.01-MOT06 motor from position -770 000 steps to position -788 600 steps which corresponds to ~100µm !
it can be explained by the temperature change and also because during the lock last week, I let the MOT06 motor in position after the cavity was at 50kW.
I had also to change the CEP motor position from -170µm to -420µm and to do some alignment (with walking procedure).

accessing the motor OC/OP/OCH.01-MOT03 from the ATK panel, produces an error : "connection to device failed"... to be solved by Kevin ?

from now on, the transmission factor for calculating the FP intra cavity power is 1/1.75ppm ~ 570 000 (changed in the Powermeter software)

this morning the temperature of the water cooling circuit is still very high said Sophie, and the laser amplifier temperature "Temp Amp1" is at 29°C instead of ~23-25°C
then, when I start the laser amplifier, it stops rapidly with a warning error.
I will check the chiller status as the Dalkia should fix the water temperature problem at some time (Sophie + Alice working on this point)

the air temperature in the bunker is back at 19.5°C from last Friday afternoon and is stable at less than 0.5°C.

after restarting the chiller, the "Temp Amp1" is now at 25°C before starting the power in the amplifier, it rises at 28.5°C after some minutes
I found the resonances with the motor OC/OP/OCH.01-MOT06 at position -783 460 steps.
and I had to tune the CEP to get the maximum resonances at -198µm.

with the new transmission factor, I got almost 90kW for 33% laser amplifier ratio after a simple alignment procedure.

the motor OC/OP/OCH.01-MOT03 is sill in error.

this morning, I had to restart the chiller. Temp Amp 1 is at 27.5°C after having started the amplifier at 33% ratio.

I removed the 100kohms resistor on the Transmission channel, on the scope (now, it is 1Mohms), to get more sensitivity.

the air temperature is around 21°C and is stable during the last days.

I don't see any power coming from the amplifier despite it is at 33%... I think the MPS prevent us to work and the amplifier shutter is blocking the beam.
I will send an email to Sophie to check that point.

EDIT : Kevin showed me how to acknowledge the error on the MPS
=> go to "Detail" and then acknowledge the "vacuum error".

I saw the FP-cavity resonances but I didn't have time to optimize them as the water temperature is still too high and the chiller is not working good enough.
=> the amplifier did a safety stop.
one needs to wait this issue is fixed.

OC/OP/OCH.01-MOT06 at position -790 000 steps.
CEP motor = -300µm but far from the optimum CEP position.

this morning, the computer was restarted and I had to launch all the applications.
no problem except the powermeter for which I had to do a power switch OFF and ON remotely.

the chiller was in error and I had to restart it also.

the air temperature seems to be not properly controlled (see the picture)
one can see a constant temperature drop from Tuesday 9 April 10:30am.

it seems the water cooling is also not working properly as I saw the laser amplifier temperature (Temp Amp 1) increased constantly during the run from 24°C to 30°C (in 30 mintues).
=> it's not normal, it should be regulated at 25°C !

I got 89kW in the FP-cavity with 33% of laser amplifier ratio after tuning the CEP and the alignment.

OC/OP/OCH.01-MOT06 roughly at position -782 000 steps.
CEP motor = -425µm

the OC/OP/OCH.01-MOT03 is still faulty => to be fixed by Kevin.

this morning, the chiller was in error and I had to restart it.

the air temperature is around 21°C.

the water cooling has been repaired but the initial Temp Amp 1 is at 28°C.

I got 86kW in the FP-cavity with 33% of laser amplifier ratio after tuning the CEP and the alignment.
but the amplifier stopped after an error after 30 minutes => the Temp Amp1 reached 30°C => it's not normal.
the chiller was again in error. I tried to restart it but I got an ERROR14 : Thermostat error, the flow rate is zero !!!
the tubes for the inner circuit going to the amplifier are a little hot by touching them with my hand.
the tubes for the outer circuit going to the ThomX water circuit is hot for the "blue" tube et cold for the "yellow" tube.
it means there is no flow in the outer circuit.
Jean-Noel just told me it's normal because he stopped the ThomX water pump because of a too high temperature => Dalkia should fix the problem today.

OC/OP/OCH.01-MOT06 roughly at position -792 600 steps.
CEP motor = -211µm

the OC/OP/OCH.01-MOT03 is still faulty => to be fixed by Kevin.

Yesterday, the ThomX water circuit should have been repared,
and yesterday aftenoon, Daniele restarted the chiller and I checked it this morning without any error.
BUT the pump of the ThomX water circuit is again in default...
Dalkia has been called to fix the problem.

despite this point, I restarted the laser amplifier this morning.
everything seems as usual and I can see some small resonance at the output of the Fabry-Perot cavity
BUT the reflection photodiode level doen't change when the amplifier is ON.
I need to check if the photodiode switch is set ON or if it is misaligned or dead.... to be checked.
in between, I prefer to stop the laser amplifier.

edit : the photodiode problem was coming from the PhD power supply which was OFF.
I just turned it ON.

the air temperature is between 21°C and 22°C.

I tried to lock the cavity but it was very instable with only short time locks.
I had to increase the parameters of the PID:
P = 0.07
I = 0.0007
D = 0.706
after that, the lock was stable but we can see something in transmission (dark blue curve) which seems to be a degeneracy.

maybe because of the water cooling temperature, after stopping the laser amplifier, I was not able to restart it.
I always got an error (an alarm was triggered by bad sequence) when I want to switch ON the preamplifier at 0%.
the Temp Amp 1 is about 25-26°C... it is stange.
to be tested after the water cooling is repaired.

today, I tried to restart the amplifier at 0% (without 3rd stage) but it didn't start... still the same error message.

but the PD_IN power and the PD_PULSE frequency seems correct.... I will try to power the amplifier OFF and ON later to see if one can reset this error.

edit : finally, a the end of the day, I switched OFF and ON several times the amplifier but I always get the same error message "bad sequence error".
I tried to use the Alphanov software to see if we can get more information about the error :
the watchdog LED is RED => I have to check if it is normal or not before starting the amplifier
and the PD_CRI LED is RED => normal because the preamp stage is not started (and it does not want to start...)

PD_IN is at 3.1mV but a previous post says that the amplifier worked with 2.7mV.
we can try to increase to 3.2-3.3 mW and see if it works....

edit : it seems we already had this kind of error before.
Guillaume suggested to check the average power of the seeder at the output of the fiber, the repetition rate and the stability of the signal.
previous post (https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/179) have shown an input power (PD_IN) around 5.7mW (see the attached image) !
this power was maybe obtained without any EOM which divide the power by ~2.
=> we can try to remove it temporarilly to check if the amplifier is able to restart in this condition... to be done

today with Daniele, we disconnected the EOM to increase the injected power of the amplifier, we were at 4.8mW but we still have the "bad sequence" error => email to Guillaume
we tried also several time again, to stop and restart the amplifier, but it didn't help.
I connected back the EOM and the power dropped to 2.7mW instead of 3.1mW => fiber injection alignement to be done

this morning I monitored the frequency shift between the FP-cavity frequency (the laser is locked on it) and the RF reference frequency @ 500MHz,
during the warm-up time, for 50kW stored in the cavity (the recording started ~ 9h20).

one observes on fig.1 a simple exponential behavior with 500Hz frequency shift @500MHz (equivalent to ~ 10µm) for 50kW stored. 
the time constant T of the exponential curve is 12.5 minutes and the stable region starts at 5T ~ 1h.

the small drop at the end of the curve ( ~ @10h30) could come from the external temperature which started to drift before (see fig.2)

today, to match the ring frequency at 500.25MHz (15th laser harmonic), we did a frequency shift on the laser using the Smaract motors (not too fast, ~2µm/s, to prevent laser modelock loss)
and we "followed" this shift on the FP-cavity using the FP-cavity motors.

we did several steps during the operation, to control the alignment, coupling and transmission.

finally, we locked again the laser and FP-cavity at 17kW for 30% of input power.
the coupling was quite low ~10%

this morning with Manar, we did a cavity mode axis change.

we checked the cavity mode centroid position on the beam profiler (placed behind P4) and changed it by roughly 1mm on the X-axis and by 500µm on the Y-axis.

for doing that, we played only on the S2 mirror while the cavity is locked and we slightly changed X and Y motors, step by step, and we realigned the laser beam with external injection mirrors when needed.

in the end, we were able to improve the transmission power by roughly 25% (810mV to 1020mV on the Transmission photodiode on the scope).

we did several Finesse measurements after that: 4950, 5190, 5100, 5120, 5200 => ~ 5100

compare to the previous Finesse value, around 4200, this is also roughly 25% better, in agreement with the transmission power increase.

conclusion: it seems the Finesse, thus the losses are almost the same "everywhere" on the mirrors => we need to clean them or replace them.

Note the direction of injection is     M1 - M2 - M3 - M4

                                                       P1 - S2 - S3 - P4

The injected beam is aligned at the center of the irises placed at the windows mounts of mirror M1 (Injection) and M2(spherical)

At Transmission of M2 :  in addition to centered beam, we observe diffraction which interferes in observing the beating at M2 output

(could be diffracted beam from the metal pipes inside or from the D-shaped mirror installed inside)

At Transmission of M3 : we observe a beam output could be part of TM00 mode (the shape is distorted !!)

suspicious reasons :

•  when we have a frequency sweep on the CW(Koheras) piezo; we observe it beating (when increasing the drive it is increasing in intensity)
• when we adjust the alignment mirrors; the beam doesn't change position and only its intensity changes

  continuation with the alignment and try to eliminate the diffraction and find the shape of the beam.

A manual change in the D-shaped mirror position to remove any possible effects from it.