to measure the Finesse, instead of having 2x EOM for the PDH and for the modulation sweep, I simply used 2x generators coupled with DC-blocks to a T connector (SMA) screwed directly on the EOM input.

as the transmission signal is fluctuating, it is not easy to have a good fit of the Airy peak.

if I measure the width at half of the maximum of the peak, I found roughly 10kHz instead of the awaited 2kHz... :-(

one needs a better evaluation with a more stable transmission signal and also to be sure that the L-shape metal piece (used to remove high order modes) does not introduce some losses and then reduce the Finesse...

After changing the way of injecting modulation for PDH and modulation for FSR scanning (we split the modulations to 2 different EOMs), the locking is very stable and we can measure the Finesse.

FSR = 33.34MHz

hereafter, the transmission power during an FSR scan and its fit (sweep of 300kHz of FSR in 2s)

we took 5 acquisitions which give a Finesse of :

4236
4254
4400
4045
4177

=> roughly Finesse = 4200 ! far from the 17000 previously obtained........

one thing which could reduce the Finesse is the L-shape metal piece if it is slightly inserted in the FP-cavity mode path.
understanding where this L-shape effectively is is not easy... some pictures are attached.

This morning, while trying Guillaume solution of disconnecting the 24 V necessary to the second and third stage and raise the threshold of Temp preamp 1 (to only operate the ampli with first stage only)

Ronic disconnected the 24 volt connection, turned on amplifier LAL software but no MMD3 error or a big temp on Temp preamp 1 , software worked fine with no issue. 

we connected the 24 V again and the software worked with no issues.

Still not sure what happened for it to work correctly !!!!!!!!!!

• We used the amplifier at 0% all day with no errors (to test , I turned it for 10 minutes @ 10% with no error too)
• We changed the lase lock box with ThomX one (having Ethernet connection), it was connected to computer and works ok
• Ronic installed a signal amplifier for the PDH error signal. (yet to be fully optimized)
• we see a better coupling than before in the cavity.
• The dominant mode is 00 mode, but we see many higher order modes, need to alight better.

Yesterday, with Ronic we locked ThomX cavity on the second resonance

Laser : 33.36 MHz , Amplifier : 0 % (worked with no errors all day)

adding image for the lock on the second resonance (locked), 00 Mode image, fit, Beam measurements

and image of the first resonance signal (not locked)

Info : when locking on the secondary resonance, we aligned better and adjusted on the oscillator CEP and increased the transmission signal 

Controllers connected to computer

through Ethernet :

• Cavity Mirror motors
• Lase lock
• oscillator CEP and Frequency motors (easier to follow the drift with this controller, and we can see it moving)

by USB :

• Amplifier controller

Today, I installed a power meter at the transmission point of the 1st spherical mirror , transmission 3 ppm (direct from cavity window beam profiler, no filter)

We locked the cavity at 2 different resonances of the fundamental mode, the lock in both cases was stable for a round 1 minute.

at different transmitted power of 33.85 uW ( 11 W average inside cavity) and then locked again at 82.65 uW (27.5 W average inside cavity) transmitted power 

Note : the coupling is almost zero for both of the resonances locked !!!!

Oscillator : 33.33 MHz ( 33.356 MHz , frequency read on LAL software)

power injected into amplifier (after injecting into fiber and an EOM) : 3.886 mW (on LAL software)

Amplifier power : 0% (injected power into cavity ~ 300 mW)

Both images of oscilloscope have same voltage scales, only difference timescale and the color code is : 

yellow : transmission

blue    : reflection

green : piezo voltage

red     : error signal

Today with Ronic,

starting with major events that happened (Water circulation + ThomX valves )  

• Early in the morning a company worked on the water network, it seemed it was not restarted properly so there was no water circulation in the main ThomX pipes.
  • There was an error on the ThomX amplifier chiller , which was noticed at the end of the morning work , where we couldn't find the cavity resonance , could be due to the temperature increase of the amplifer.
  • Solved :  the issue was solved by restarting the water network and the amplifier chiller was restarted an no error found and temp. Stabilized around 25°
• Around 12h15 ThomX there was a power cut off for less than a second (micro cutt), which cased all the valves to close, the valve air compressor did not restart after the power cut off, we restarted it with Daniele in the evening with the help of Marie and the valves open around 5-6 bar , you will see then all the controllers green and the valves will open.

Results of the day :

• All the power supplies and function generators are under the table, nothing on the table (reduced noise on the signal)
• EOM of Oscillator Off (for now, might turn on if needed)
• Amplifier on 0% , output ~ 300 mW
• Locked on the first resonance of the cavity,  lase lock parameters to be optimized better
• Transmitted power increased to ~ 267.2 uW
• Average power inside cavity = 89. 06 W , peak power = 0.2 kW (pulse width = 12.6 ps , frep = 33.36 MHz )
• current effective cavity gain > 300 

Oscilloscope :

•  error signal, not shown as it was too noisy to have in the image
• blue : reflection (low bypass filter ), yellow : transmission (resistance of 100 k ohm added), green : piezo signal
• 1st image : showing lock and scan regions.
• 2nd image: time zoom on the locked signal 

oscillators controller / smartAct:

• Frequency and CEP control , the parameters in the photo attached are the best for the moment to see the smooth change of the resonance peaks when operating the motors, and we can stay FSR range and compensate the variation easily. 
• even though we stayed at closed loop (for easier adjustment), we still managed to lock , will try to switch off later to see if reduces a lot of noise. 

In the morning, we locked on the first resonance. With an increase on the amplifier power @ 20-25% and 30%, which reduced the noise

we manage to get a coupling when improving the CEP up to 55 - 60 % coupling : put there is still a drift on the CEP

The controller for the oscillator frep and CEP produces a lot of noise, even if it is disconnected. we need to switch it off to remove its effect (Kevin will order a new one)

The image attached is of the oscilloscope lock on the first resonance @30% amplifier power ~ 10 W injected into cavity,

transmitted power ~ 36 mW (cavity average power 12 kW)

cavity effective gain > 1200

in the afternoon, we installed and aligned a gentec power meter we can monitor from the computer. (will need to buy a permeant one for ThomX)  

there was an issue with the laselock USB connection, yet to be solved.

and the control of the computer.

The computer is possible to connect from the control room,

we are able to run remotely the amplifier, cavity motors, oscillator motors, lase lock (note: there is an issue with the keyboard, we are not able to use it with remote access!!!!!!!!)

Note on Amplifier: On Friday, Daniele and Ronic faced again the temperature issue that came up before. it is due to the fluctuation of the diode temperature.

It was fixed today by Ronic and Daniele. it seems there are three diodes for monitoring the temperature, only one was connected, and it had issues, with some directions from alphanov. 

Ronic just removed the defective diode and soldered (connected) a different one. The amplifier should work without this issue. 

now we start searching for X-rays after locking the cavity remotely  

Last week, with Ronic and Victor we aligned the cavity using CW laser (Koheras)

• we have also installed 4 mirrors in the cavity with the best theoretical qualities (serial number and images included)
  • The Plan ULE mirror P4 thickness was not compatible with the piezoelectric mount. We had to install a metal spacer behind the mirror to be able to fix it. the metal spacers in from ThorLabs made from INOX, It is 1 inch diameter (25.4 mm), we drilled a hole in the middle of 15 mm in the Workshop in bat 200. Then cleaned it using an ultrasound bath in pure water, using a small Ultrasound bath machine in Maverics platform Vide and Surfaces next to ThomX eglo.
  •  
  • This gives some difficulties in imaging the transmission at P4. To help with the imaging, we used the plastic #D printed irises that Yann made long time ago
  • we took images of the mode inside the cavity at P4 transmission.

• We also observed some oxidization on the metal bars of the controllers, more on the side of S2 and P4. we are yet to test the motion of the motors in the z-axis.
• Friday evening we closed the windows using a new copper rings (last one we have, Ronic placed a new order).
• The pump was turned on by Bruno and Eric yesterday afternoon. The current vacuum level is 6-7*10^-6 mbar

the pre and the turbo pimps are on so still too noisy to work, we have access all day tomorrow.

Today, we prepared lenses for a telescope for the CW laser injection into the cavity, yet to be installed

Image of the transmission at P4 attached, an estimation of the beam size of the mode see is ~ 5.5 mm 

The vacuum in the 2 optical cavities is decreasing but slowly. today it is 2.6 - 2.4 * 10^-7 mbar

This morning with Manar, in order to prepare the installation of the news FP cavity mirrors,

- we installed the Koheras below the optical table
- we put a fiber prolongator at the end of the Koheras fiber and we put a mounted collimator at the end of the fiber.
we added a mirror at 45° to inject the beam to injection axis of the table.
- as the beam was still slightly divergent, we added a 750mm lens ~20cm after the collimator to improve the beam collimation.
now the collimated beam is going approximatively through the middle all of the iris in the optical path.

we finished by ~2h of cleaning of the optical table and elements, which were very dirty.
it is still a bit dirty. see the attached file to get the numbers provided by the particle counter after the cleaning.

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.

Yesterday morning we locked the cavity on a strange mode in the cavity (we call it mode, but not sure what it is ??)

we also managed to lock on a higher order mode 02

we get an estimated 30% coupling (expected value), and note that we have not installed a telescope yet.

This morning with Daniele and Viktor,

- we confirmed the "potato" mode was well centered on the injected beam and was not due to some misalignment of the injected beam compared to the cavity axis.
we look at the reflected beam during a lock. we can see the direct reflected beam superposed with the cavity mode (cf attached image).

- then we assumed the "potato" mode problem could come from the too short distance between spherical mirrors which introduces ellipticity and finally can lead to some instability.
we manage to move further spherical mirrors S2 and S3 by moving them from 1 350 000 to 800 000 steps.
=> we increased the distance by 2x 550 000 steps (with 6µm for 1000 steps) => we increased the distance between spherical mirrors by 6.6 mm
the new mode shrinked a lot and at the end was spherical at 73%.
the mode size is now about 4400 x 6000 µm but with the major and minor axis still at 45° !
 

The Stretcher is the "D 24-02-II", it must be used in double path before the amplifier injection.

There are two compressors in single path, "D 25-14-I" and "D 25-14-II".

Eric calculations are attached below.

The goal of these measurements is to check if one can find some correlation between the CFP lock losses observed during a run and some CEM noise in the bunker.
the main issue is coming from a 20Hz noise apearing/disapearing in the laser cavity PZT signal.
is it a real cavity length noise that must be compensated or is it a pickup noise in the error signal which produce some unwanted compensation ?
the second issue (apearing much less often) is a higher frequency noise visible mainly in the transmission signal of the CFP which becomes wider and noisy during 1-2 seconds.

This morning, I installed a simple wire loop all around the table (the machine is OFF).
this wire is connected to the wire of a BNC connector at one end and to the ground of the same BNC connector at the other end to form a loop.
this BNC connector is connected to a 1kHz low pass filter to remove high frequency CEM noises and connected to a 10-1000 variable gain amplifier, plugged to the CH3 of the CFP scope (instead of the PDH/PZT-CAV signal).

I locked the CFP above 80kW after tuning the CEP.

I took several pictures to illustrate what I observed :
- on the right, the scope signal in time domain (blue = TRANS / green = PZT laser / cyan = REFLECT / pink = LOOP noise)
- on the left, the spectral analysis in frequency domain (red = PZT laser / white = LOOP noise)

in these 3 measurements, I don't do anything to the CFP, I just wait for the signals.

1) normal condition : see picture "Without Noise @ 20Hz"
the PZT signal in time domain is relatively flat and the 20Hz noise is barely visible in frequency domain.
we don't see any correlation with the LOOP noise for which one can see clear 50Hz and 100Hz peaks.

2) 20Hz noise condition : see picture "With Noise @ 20Hz"
the PZT signal in time and frequency domain exhibit a clear 20Hz oscillation.
the REFLECT and TRANS signals are also correlated to this 20Hz oscillation.
but still no correlation with the LOOP noise in time or frequency domain.

3) 20Hz noise lock loss condition : see picture "Delock @ 20Hz"
because of the lock loss the spectral analysis of the PZT signal is meaningless,
but in time domain, one can clearly see the growth of the 20Hz signal, inducing large noise on TRANS and REFLECT signal but still no correlation with the LOOP noise.

Conclusion : the 20Hz noise seems not related to any CEM noisy signal but more probably to a real mechanical noise in the CFP (not in the laser cavity because it would be seen in the phase noise measurements).

in this measurement, I tried to produce a tone around 400Hz with my voice, close to the FP cavity.
it is difficult to right volume as the cavity can easily lose the lock.

4) Voice noise condition : see picture "Voice noise @ 400Hz"
here, I changed the spectral span to 500Hz.
one can see a small bump in frequency domain around 400Hz due to the accoustic noise in the PZT signal.
the general shape of the PZT spectral signal is maybe related to the PID parameters : one can see more signal at lower frequencies.
in time domain, one can see TRANS and REFLECT signals are more noisy than before and these signals shapes seems identical to what has been observed when one has lock loss due to high frequency noise.
maybe we could put a mic in the bunker, connected to one scope to check if sometime one doesn't have much more accoustic noise...
 

This afternoon, we started almost all the parts of the machine with Vincent and Nicolas, and we didn't see any change in the Loop noise signal.
we saw only a peak comb at 10Hz on the Loop noise signal when the septum is ON. the amplitude of the peaks is then related to the voltage on the septum.
but we still don't see any noise correlation on the PZT signal.

the global conclusion about CEM noise is it is not related to laser PZT noise.

if the 20Hz oscillation is coming from a mechanical unstability in the CFP, we should be able to trigger it by moving the longitudinal motors of the CFP.
we tried also to move the MOT.03 and MOT.06 motors and we didn't see any clear correlation with the 20Hz oscillation.

the 20Hz oscillation could come from the CTA pressure variations on the housing => we can try to trigger the oscillation on the housing.

S3 and S2 @ +1 640 000 and P1 and P4 @ -825 000

Diameter in x: 4.7mm
Diameter in y: 4.7mm

FSR initiale : 33.34 MHz (S3 = -825000 et S4 = -825000)

Beam size initiale : wx=2.05mm et wy=2.15mm

Finesse initiale : 3400

FSR après éloignement des miroirs sphériques : 33.29 MHz (S3 ~= -1496000 et S4 ~= -1496000)

Beam size : wx=1.7mm et wy=1.85mm

Finesse : 3600

FSR après rapprochement des miroirs sphériques : 33.39 MHz (S3 ~= -250000 et S4 ~= -500000)

Beam size : wx = 2.1mm et wy=2.4mm

Fit de la caméra jamais au dessus de 90%. ~~80%.