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

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

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....

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

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 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 were able to lock the laser on the FP-cavity and the FP-cavity on the Ring reference oscillator in a same time from the control room.

the procedure when laser, cavity and reference oscillators are far in frequency, is :
1) start the amplifier to get the laser signal in the reference photodiode used to measure the 33MHz.
2) look at the beating frequency with the reference oscillator and quickly cancel it using the Smaract motors.
one need to remember the direction and distance corrected by the Smaract motors.
3) compensate the motion in the FP-cavity using these rules :
         - 600nm with the Smaract motors <=> 100 steps on the FP-cavity motors
         - decreasing the value on the Smaract motors <=> increasing the value on the FP-cavity motors
4) start the lock between the laser and FP-cavity
5) adjust both cavity length step by step until reaching < 5Hz of beating
6) start the lock between FP-cavity and reference oscillator

we still see the 20Hz noise both in the PDH error signal and in the laser PZT feedback signal.
it could mean :
    either the FP cavity is stable but the Laser cavity is oscillating (noise source) and badly compensated by its own PZT
    or the FP cavity is oscillating (noise source) and the Laser cavity is oscillating too (due to feedback) but badly compensated with the laser PZT
the bad compensation in both case could come from a mechanical resonance which produces a phase jump that the PID is not able to properly compensate, even with a high gain (integration).
=> to be checked with a behavorial simulation
the noise source in the laser cavity could be the Smaract motors.
=> we need to switch them off to compare the noise with and without

this afternoon, I firstly connected the HV output of the Laselock to the FP-cavity PZT : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/248

locking the FP-cavity on the local synthesizer works better with the improved HV output range.
the ~20Hz oscillation in the beating signal of the FP-cavity/synthesizer is still there.
one can observe exactly the same oscillation on the Laser PZT signal.
I think the FP-cavity is oscillating at this frequency and the laser PZT correction signal is compensating it.
this oscillation produces also an error signal with the beating of the laser harmonic and the synthesizer.
an other possibility for this low frequency oscillation could be the hexapod stability ?

then, I switched to the 500MHz Ring Synthesizer instead of the local one.
we were able to lock all the elements, FP-cavity and Laser, in a same time.
we have Vp=3.25V of signal amplitude when not locked and dVrms = 140mVrms of rms noise once locked => rms jitter = dVrms / (2pi F Vp) ~ 13.7ps rms
with F=500MHz.

reaching a state where both loops are locked is not simple as :
- the FP-cavity motors are noisy when they move, and one direction has some backlash compensation which produces a long unlock.
- the laser motors are less noisy but too noisy to be moved w/o unlock
- moving the FP-cavity motor changes the laser PZT position and can put the system out of the locking range for the laser PZT.

the correct strategy is :
1) lock the laser on the FP-cavity using the laser motors (no lock of the FP-cavity compare to the synthesizer).
2) wait to have a quite good thermal stabilization => this can take time especially if it is the first lock of the day when the cavity is cold.
3) measure the frequency beating between FP-cavity and synthesizer.
4) pre-compensate the direction of the laser PZT with the laser motor when you will play on the FP-cavity motor
ex : a step on the right on P4 motor will move the laser PZT to the top, then you need to pre-compensate this move by placing the laser PZT voltage lower with the laser motor (Smaract).
5) do the move on P4 motor
6) check that the frequency beating between FP-cavity and synthesizer is reduced
7) redo 4) 5) 6) until the frequency beating is < 5Hz and stable
8) start the lock on the FP-cavity PZT.

due to the power drifts inducing frequency drifts, it will be difficult to work continuously more than a couple of minutes.
the procedure has to be done continously from 4) to 8)

having HV output on the laser PZT should help to improve both lockings range and inscrease the locking duration => to be checked w/o fast feedback loop (FFL) or with FFL if needed.

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, I added a Thorlabs lowpass filter at 1kHz after the mixer and a resistor at the Laselock output to the FP-cavity PZT.
I set the PID with P=10 and I=D~0

in attachement, a plot of the 500MHz mixer output, before and during the lock.
during the lock, one gets a residual oscillation around 20Hz... where does it come from ???
could it be some mechanical resonance, as we already had before (it was ~30Hz) : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/84

when unlocked, one gets : A*sin(dPHI) with A=3V => 6Vpp signal
when locked, one gets a 400mVpp error signal => A*dPHIpp = 0.4pp => dPHIpp = 133 mrad

dPHI = 2*pi*F500M*dt => peak-peak jitter dtpp = 42 ps => rms jitter = 15ps

this value can be measured directly on the 500MHz signals coming from the laser and the reference synthesizer.

this afternoon, I tried to lock the FP-cavity on a local 500MHz synthesizer as a frequency reference
(it's faster to change the frequency from the sythesizer than from the FP cavity !!!)

I found some parameters on the PID of the Laselock to phase-lock the reference and the FP-cavity
but the locking quality is poor => we produce some oscillations which are copied by the FP-cavity/laser lock on the laser PZT.

the error signal (FP-cavity/reference) is quite noisy, then maybe one can try to do some analog filtering at a lower frequency ?
=> one can use Thorlabs LPF at 10kHz or 1KHz.
one can try also to filter the signal going to the PZT to reduce the excitation of the PZT resonance.
=> use a variable resistor in serie with the PZT capacitance to make and RC filter (Cpzt ~ 70nF)

for reminding, the previously working PID parameters, found with the 133MHz laser/533MHz reference lock, were:
P=10, I=0.01, D=0 for mid SR, and no filtering.

once we will found good PID parameters, we will be able to switch to the Ring oscillator at 500MHz as a reference,
then work with HV from the Laselock to improve the locking range.

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.

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

this afternoon, I installed the frequency detection scheme in the bunker :

- I changed the large reflected photodiode DET100 by a fast small DET10.

- a 50ohms splitter is connected to the photodiode :

* one cable is going directly to the scope for monitoring
* one cable is going to the 500MHz sharp bandpass filter to select only this harmonic.
despite the small BW of the filter, one gets 3 harmonics : 500MHz + (0 +/- 33) MHz with a bit less power on sidebands.
power on 500MHz : ~ -30dBm if the laser amplifier is at 30%

- after the BPF, one goes directly to the RF amplifier which is 2x Minicircuit amp ZX60-33LN in cascade to get ~ +3dBm

- then one goes to the level 17 mixer on the RF port.
the LO port is feeded with the 500MHz coming from the Ring reference oscillator.

in attachement is a picture of the output signal : ~ 1.5Vpp
I think the width of the signal is coming from the 33MHz sidebands which are not perfectly removed by the 20MHz BW of the scope.
one finds a frequency shift of 250kHz.
as the laser frequency has been set at 500.25MHz (on the 15th harmonic), would it mean the Ring reference oscillator was at 500MHz sharp ? => to be checked with Kevin.

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.

this morning we locked the laser and the FP-cavity with ~ 40kW => some HOM appeared "randomly" depending on the power.

we tried to play manually on the L-shape arm :
- first, we moved horizontally to put the L-shape in the beam axis
at some point, we saw the power level divided by ~2, we stopped and came back to restore the full power.
- then, we tried to move vertically.
whatever the direction, we were not able to see a clear cut of the beam.
BUT "strangely", at some point, the PZT drift followed the direction of the motion and was not really depending on the "cooling" or "heating" process when the lock stops or restart.
"strangely" again, at the endpoint, the power dropped by ~20-30% after a delock but it was impossible to restore the power when we put the arm at the initial position.
=> we had to adjust the FP-cavity alignment to restore the power !!!
=> we had the feeling that moving the arm could have misalign the cavity axis !!! :-(
=> we have to discuss with Yann and get the CAD files of the cavity to have a better understanding...

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.

today, during an attempt in improving the CEP with the Smaract controller CH2, we lost the laser modelock.

at this point, we decided to reference the Smaract CH2 for CEP :
now, 0 mm is roughly the middle of the motor range.
the endpoints of the motor are : +/- 5.9 mm
once we found back the laser modelock, we searched for the region where the modelock was still effective : +/- 1.8 mm

previously, we were using a spare Smaract MCS controller to drive the Onefive linear stages (without -LV -low vibration- option and using Ethernet),
during the repair of the initial MCS controller with -LV option.

this morning, it has been remplaced by the initial OEM Smaract  MCS controller integrated by Onefive (with -LV option and using USB).

it worked fine with the Precision Tool Commander 2 software !

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.