This morning with Ronic , in preparation for the mirror installation, we removed additional mechanics and cables from the table. 

We organized and cleaned the permanently installed cables on the table away from the area of the cavity opening. 

We opened the two top vents and left the shutters open to circulate clean air , (note the shutters surrounding the laser and amplifier are half closed for safety)

Note: before installing the mirrors, preferably the shutters should be opened a day before to have clean air in the surrounding of the cavity openings

and avoid any cleaning or air disturbance.

Today we tested a mirror from one patch

image is for the front face of the mirror

PowerPoint has more details about the images

As the general local network for the FP-cavity devices is 192.168.xxx.xxx, I changed the IP adress of the smaract laser motors (Frep and CEP) controller.

For that, I used the embedded web server as described in the manual in attached file.

the new adresses are :
IP :             192.168.1.10
Gateaway : 192.168.1.1
Subnet :      255.255.255.0
Port :          5000

the last tries to lock the 33MHz + amplifier to the 30k Finesse FP-cavity were unsuccessful.

during a laser Frep scan using the Laselock, one observes that the main cavity resonance is not able to stay inside the PZT scan range from one scan to another (500ms-1s period)
is it the effect of a large and slow phase noise ?

some informations:

- The 33MHz laser came back at lab from repair on March 2018.
- it has been sent to Alphanov in May 2020.
- it failled and has been sent to NKT/OneFive for repair in September 2021
- it came back to lab from repair in June 2022.
- on post #92 (Feb. 2020), it seems that we already locked the 33MHz laser + CELIA amplifier to the ThomX FP-cavity.

- The PZT sensitivity for the 33MHz laser is given to 0,3Hz/V for Frep <=> 2.6MHz/V for optical frequency.
=> 10V on PZT is equivalent to 26MHz of optical frequency shift which is less than FSR !

- by comparison, the PZT sensitivity for the 133MHz laser is given to 3.9Hz/V for Frep <=> 8.5MHz/V for optical frequency.

- by comparison, the PZT sensitivity for the NKT CW laser is given 10pm/100V for Wavelength <=> 30MHz/V for optical frequency

- by comparison, the PZT sensitivity for the ThomX FP cavity (Z20H38x40C) is 4nm/V for length expansion => 8nm/V for round-trip expansion <=> 0.03Hz/V for FSR expansion <=> 260kHz/V for optical frequency !!!
the PZT expansion estimation is in attached file.

finding the right modulation/demodulation PDH phase is very difficult on the main resonance because the we get non stationnary signals with a lot of oscillations.
changing the phase, in this condition, does not really change the error signal.
Then, we moved on the first secundary resonance with less gain and less coupling.
Thus, the error signal is more similar to the theoretical PDH signal => one can adjust the modulation/demodulation PDH phase to get the maximum error signal.

then, we locked pretty easily on this first secondary resonance, with a coupling around some % when we adjust the CEP motor.

we tried to lock on the main resonance but it is too noisy and unstable.
it seems we really need high BW feedback.

I tried to add a fast analog loop on the laser intra-cavity EOM but without a clear effect.
the problem is the gain of this loop : it is difficult to produce a "high voltage" (above 10Vpp) on this EOM.
I put "my" amplifier but the voltage output is limited... commercial amplifiers will have the same issue.
we can add HV amplifiers but it takes place and it will add some noise on the signal.

A loop with an AOM could be easier to install and manage... but at the price of a loss of power before the laser amplifier...
 

today with Daniele, we locked easily (but with a noisy lock) on the secundary resonance and we tried to lock on the main resonance (with very low coupling ~10% which mean a CEP ~Pi)
the lock was possible but was very noisy.

I installed a fast loop using my small DC amplifier based on OP37 (max gain=100) modified to be AC coupled to avoid to amplify the PDH box offset.
the output votage swing of the OP37 is only ~10V. Thus, the effect of this fast loop on the lock stability is not visible !

Thus, I added the M250 Leysop HV amplifier (see attached documentation), which is able to drive an EOM with >5MHz bandwidth and ~250V swing, after my OP37 amplifier.
with this additionnal HV amplifier, now we can clearly see the effect of the EOM loop which improves the lock stability BUT, even with a poor CEP, the lock is very unstable on the main resonance.
it seems the optical phase noise is still too large and/or its BW too high to be completely compensated.

The next step is to try to remove all the possible noise sources from the optical table:
- the laptop placed on the ionic pump
- the 2 Rigol generators on the table surface
and switch off the controller of the Smaract laser cavity motors.

If it doesn't help, we can send the error signal to a spectrum analyzer to have a better view of the different harmonics involved in the residual phase noise.
could it remain some noise above the present PDH box BW (1.9MHz LP filter) ?

lastely, we can also make an optical phase noise measurement to check if the Alphanov amplifier does not add some noise.

After removing the 2 generators from the optical table, the lock is much more stable and now, it is possible to lock on the main resonance with a poor CEP but with quite good stability.
the coupling is still very low ~ 5% for that CEP but if one improves it (CEP ->0), using the laser double-wedge motor, one clearly sees an improvement of the coupling... but at the cost of the lock stability.

the reason of the poor coupling is also because the laser amplifier is used at 0%, for which we know the part of the laser signal power, compared to the total power, is low.
(a part of the beam @1030nm is not propagating in the fiber core of the amp, and then, it cannot be coupled to the FP-cavity).

the fast lock loop on the EOM has been disabled for the moment.
it has to be installed back to improve the stability at a better CEP.

at present, the FP-cavity power is estimated at ~ 90W (~270µW in transmission of ~3ppm mirrors) for ~300mW of total power coming from the laser amp.

next steps :
- in Open Loop : check what is the best coupling we can observe for CEP=0 @ P ~ 10W (laser amp at ~ 25%)
- in Closed Loop : @ P ~ 10W => measure the best transmitted power after alignement/polarization/feedback adjust => ~ 3-10kW in the cavity ?

this morning, I tested the laser+amplifier @ 30% lock on the FP cavity with and without Smaract motors.

I recorded the PDH error signal during a lock:
- blue   : with Smaract motors controller powered ON but motors are stopped
- yellow: with Smaract motors controller powered OFF

with Smaract motors controller powered ON and motors stopped, one can see a group of resonances around 10kHz (8 - 11 - 14kHz) which disappears when the controller is powered OFF.
one can see also a group of resonances around 25-30kHz for which some peaks desappear when the controller is OFF but most of them are still there... could it come from noise on the Onefive laser PZT ?
one can see also a noise reduction at low frequency with a corner frequency around 17kHz, which could be the Unity Gain Bandwidth of the feedback loop on the laser PZT (fast feedback loop on EOM was disconnected)
=> to be confirmed

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I was able to lock with a decent noise on transmission and reflection signals @ Pin=17W (30%) of input power and with a coupling ~ 20%.
I measured 31mW in transmission => Pcav ~ 10.3kW (T ~ 3ppm)
as T1=115 ppm and F=30000, the cavity gain is T1*(F/pi)^2 = 10.5k,
so, the FP cavity power should have been 17W * 20% * 10,5k = 35.7 kW !!! (maybe the formula is wrong if the coupling loss comes from the CEP detuning effect)
=> we have to check the incoming power and the formula !

so, maximum expected power in FP-cavity could be 70W * 100% * 10,5k * (10.3/35.7) = 210 kW !!! :-(

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I was able to redo the lock easily in remote in the control room (with the Smaract motors controller OFF).

I checked with the Matlab code below the CEP detuning effect (2nm sech² spectrum... not exactly the same as in ThomX)
@ CEP = 0 => coupling = 100% and Gcav = 10.5k
if all the coupling loss comes from the CEP detuning effect :
@ CEP = pi/5 => coupling = 20% and Gcav = 2.14k (~ 10.5k x 20%)
so, it does not matter if the coupling loss comes from the CEP detuning effect or from beam mismatch or misalignment.

=> we should have more power at 20% coupling, not 10kW but 35kW !!!
=> we have to check the real input power !

we checked yesterday morning the real input power @ 30% for the amp => it is 16W in agreement with the previously measured values

on friday 23/06 afternoon, after scanning the hexapod z-axis, we observed a cut of the power stored in the FP-cavity (with constant input power/coupling) at both ends of the scan, due to the losses of the beam-pipe aperture.
we placed the hexapod exactly in the middle of this range ~ z=-1.68 mm (which is not the middle of the maximum range of the hexapod), we opened the slits on the X-table and we found the first signal.

after scanning, shuting the laser beam and electron beam ON and OFF, we confirmed this signal was coming from X-rays.

on monday 27/06, roughly in the same condition (25kW of power stored in FP-cavity), we scanned properly the same z-axis with the hexapod and we measured the pico-ampermeter current related to the produced X-rays to get the approximated size of the laser and electron beams at the IP position : see the raw data.

Daniele did the data analysis : see the attached analyse_profil_vert_X-1.pptx file
the total rms size (sigma of the Gaussian profile) is 100µm.

this morning, we measured the FP-cavity waist size by measuring the waist size of beam at the focal distance of a lens used after the spherical mirrors.
we measured the rms size of the FP-cavity mode to be 60µm in agreement with simulations, which leads to 80µm rms for the electron beam waist size.

today, we observed on the photodiode used in reflexion of the cavity, that its voltage level stops increasing after 50% of power for the laser amplifier when the cavity is not locked (the FP-cavity is just a reflective mirror).
the photodiode itself is not saturated (low power sent after a wedge and an absorbing filter) and the reading is done on 50 ohms.
so we are investigating the reason of this "saturation".

1) we did a beam power measurement just after the 2 mirrors, right after the amplifier, with the "big" powermeter:

power ratio        Power (W)
10%        =>  0.9
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  33
60%        =>  41
70%        =>  48
80%        =>  55.5
90%        =>  62.2
100%      =>  68

plot in attached file

2) we did the same measurement after the periscope:

power ratio        Power (W)
10%        =>  0.82
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  32.6
60%        =>  39
70%        =>  44
80%        =>  47.4

we clearly see a power reduction from 50% and beyond.
with the viewer we saw 2 effects : a slight clipping in the telescope and some speckles on the periscope mirrors mainly.
the laser is also slightly shifted on the 2nd mirror after the amp.
it is possible the laser beam moved due to thermal effect => we will need to realign properly the whole injection line and be aware about mirrors and lens cleaning !

3) we did some measurements using 2 wedges with the "small' powermeter in reflection of the cavity.
we remove the small aperture half-waveplate to see the effect and we used a lens to focalize the beam:
the half-waveplate has an effect on the reflectivity of the wedges, this is the reason of the difference in the measurement.

                                                               Power (µW)
power ratio        with lambda/2           w/o lambda/2             w/o lambda/2+lens
10%        =>           12.4                           4.8                               5.1
20%        =>            132                            48                                51
30%        =>            262                            96                               101  
40%        =>            370                            137                             147   
50%        =>            420                            158                             175 
60%        =>            430                            165                             181

we observe a clear "saturation" after 50%-60% which is very similar to what we observed with the photodiode in reflection when the cavity is not locked.
the small iris used in front of the injection window of the cavity is a good "candidate" for clipping the beam at high power (we observed it was still quite hot after stopping the beam).
=> we have to redo the measurements after removing it.

Today, we locked the cavity with input power @ 30% for the amp => we got 40kW with only 30% of coupling (and a bad lock => we could have more power inside cavity).

P@30% = 16W
Coupling = 30% => 4.8W of input power => Measured Gain = 8300

on the August 30rd and 31th, a global power shutdown was forseen for ThomX.

to prevent damage on equipments, I switched off all of them (and disconnected power cables from the wall plugs).

yesterday, after the week-end, all the equipments have been restarted and the cavity as been locked properly.

the power coupled to the input fiber of the amplifier has decreased a little bit from 3.8mW to 3mW during summer.
=> we need perharps to do some alignment on the Schaftner-Krischoff mount.

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%

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, 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

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

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 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.