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.

Adding the schematic of the measurements.,

the 4° Beam Deviation, AR Coating ( B coating)

The 3 data points taken of the beam profile are fitted in Gaussian Beam with taking the beam waist at the output of the compressor box.

Background is subtracted for all data points, the fit is more than 90% for all points.

a reading was observed with distance less than 200 cm before the periscope and the fit starts to degrade

reason probably be related to the bump beam !!! To be investigated by placing an iris.

This Morning with Victor the amplifier was turned on @ 20% amplification (output ~ 9 W ) for approximately 3 hours and no error appeared.

I took data of the beam profile at different points without a lens after the 2 wedges 

@ 0% (only pump beam), output ~ 300 mW (taken from previous data) , output after 2 wedges 2.2 uW  

@ 20% amplification , output ~ 9 W (taken from previous data) , output after 2 wedges 58.6 uW

fit to be added after processing the Beam Profiles data files.

Data with the lens to be taken !!

This morning, with Ronic, we opened the compressor box to observe if there is any misalignment of the mounts or the CVBG's

This morning we took data of the beam profile at 4 different points to see the evolution of

• the beam shape
• divergence

over distance for all amplification percentages

Data plotted for the distance vs radius for all and divergence in rad is extracted and plotted as a variation of amplification power.

Added is the Excel file with all the details and the PowerPoint for referance

Information about today's work is added to logBook !!

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.

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° !
 

This morning, with Ronic and Daniele, we locked the cavity again (The lock was relatively easy and stable during the reading of the finesse)

We took 2 reading of finesse

1st   : @ 33.2808 MHz (what the cavity length was at)           :  Finesse = 27 024

2nd :  @ 33.3382 MHz (adjusted to match the pulsed laser)  :  Finesse = 30 433

The difference between the 2 is the FSR, during the change we also did some alignment to counter the reduced transmitted power.

We took an image of the beam profile before and after changing FSR and there was ~ 70 um change in the beam position on the Horizontal axis of the beam.

Yesterday,

• we installed the second high power reflecting mirrors at the output of the amplifier.
• We aligned and installed a telescope that has adjustable distance between the 2 lenses of -100 mm, and + 200 mm, with approximate diameter at the injection window of  ~ 4.5 mm.
• Amplifier was turned on only at the first stage only (output power ~ 300 mW) aligned and injected into the cavity.
• Measured the repetition frequency of the One Five oscillators, Frep = 33.326239 MHz
• Changed the FSR of the cavity to match it.
• Using the Piezo scan on the oscillator, we observed 00 Mode, Fundamental. the shape is the same 
• We see transmission on the diode, but there was zero coupling observed (yet to be investigated experimentally )
  • a reason could be the CEP of the oscillator ,
  • bad matching of the beam waist position,
  • noise from the amplifier (as we were operating it at very low power)
  • additional alignment needed,

• On Monday with Daniele,
  • we did full characterization of amplifier beam @ 10% amplification, adjusted the telescope accordingly and injected into the cavity aligned and improved on the fundamental mode.
• On Tuesday with Daniele and Kevin
  •  I added a low pass filter between 1 - 1.9 MHz on the reflected signal, to reject the oscillator signal (33.33 MHz)
  • removed the external resistors on the signal, only the internal resistance of the oscilloscope used ( transmission : 1 M ohm  , Reflection : 50 ohm  )
  • We connected the motors to control the CEP and adjusted on them until we reached ~ 5 -10% coupling
• Images:
  • 1st : showing the size of the beam just before injecting into the cavity @ 10%
  • 2nd: signal with only improved alignment (not CEP adjustment yet), no low pass filter added yet
  • 3rd : zoom on the signals (reflection and error) while adjusting on the CEP
  • 4th :attempting to lock the cavity , after getting the max coupling using the CEP motors

Note : the voltage ranges are not the same between the images

Added Note: the coupling we get is at low power, only preamplifier is on (~ 300 mW)

was done at the end due to excitation of higher order modes.

Wanted to improve the fundamental first, then increase the power.

Today with Ronic and Daniele we attempted to lock the cavity, but the alphanov amplifier did not turn on because of an error in MMD3 (related to the third stage)

the new error is having the MMD3 error show on the alarm window of the LAL software, and having the Alarms on the line 3 of the alphanov software red for both EXT/CPU and Laser T Max (never been red before)

this is after a 2-week work stop, but the last time when Daniele changed the fiber connector it worked for 3 hours with no errors.

Ronic, is in contact with Guillaume from Alphanov to fix the issue !!!

Work scheduled all Wednesday, in hopes the issue is fixed by then.

presently, the voltage on the laser PZT is 0-10V, but during the several runs we did, this voltage is too low to compensate low frequency fluctuations:
we need to change often the motors position implying a cavity lock loss.

we have to check if running the laser PZT with the HV output of the Laselock is possible without too much additionnal noise.
=> it can be done simultaneously with the fast feedback loop implementation, which has to be done.

Yesterday, I checked the 2 output HV channels of the LaseLock : they work properly but the noise is bigger than the low voltage outputs
~ 1mV rms on 0-10V outputs
~ 8 mv rms on 0-100V output

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.