a better image of the damaged spot, image taken with the arrow for the reflective surface facing the other direction (image shows position)

The image of M1 for ThomX reflective surface was taken at min zoom (full image scale 13 mm) and max zoom (full image scale 2 mm) on microscope

The spot appears to be not close to the center of the mirror, at max zoom in the center we do not see the spot it is just out of the image 

the last image has the mirror position adjusted to center the damaged spot for a better image of it.

ThomX injection mirror has been cleaned and placed again inside the optical cavity.

This time to avoid the damaged spot I have displaced the mirror mount horizontally to have a distance between center of the beam and the spot ~ 2.5 - 3 mm.

The alignment was affected slightly but recovered by adjusting the mirror mount nobs, (00 mode observed in air)

The cavity was closed is being pumped with vacuum.

To be done: adjust the cavity length and find the resonance, improve the outer alignment, lock the cavity

Yesterday , we locked the cavity and we see a sign of a high finesse on the transmission signal, but no measurement of Finesse was done.

we have a coupling of ~ 45%, which is a loss of 20% from the previous coupling of 60%

an estimate done by Ronic MATLAB simulation for the coupling drop where we have 200 pp additional losses and gain of 2.6 k we should get a transmission of 1.1 mW for injected power of ~ 300 mW

which is consistent with the power measured after a 50% beam splitter on transmission we got 0.51 mW (total would be 1.02mW)

in addition, there is a beam that is next to the mode of the cavity , confirmed it was not a reflection from the beam splitter or the optics.

it could be that we are still close to the damaged spot ?

To compare between the 2 images of the cavity mode:

• the mode by itself has an integration time of 0.06 ms, position (x, y) = (1142.969, -53.932) um on the beam profiler 
• the mode saturated with the spot next to it almost at max intensity has an integration time of 50 ms, position (-3700, -2000) um

comparing the positions of both spots, they have difference (4842.969, 2053.932) um

------ > total difference on the beam profiler ~ 5.3 mm , the distance from the spherical mirror to the beam profiler is ~ 40 cm

On Wednesday 21st , I opened the cavity did an additional 2 mm shift of the injection mirror and put it under vacuum again.

Locked the cavity, and observed the transmitted beam.

The second spot is still visible on the beam profiler , the distance difference between the 2 spots is ~ 5.2 mm (the same as before )

no difference in distance, decreases the likelihood that it is from the damage (to be investigated more)

in addition, we have locked at the reflection from the cavity to confirm the spot next to the beam.

We took two images when the laser was locked with the cavity and when it was not.

We clearly see that the spot is indeed related to the mode of the cavity. And probably the damaged spot.

(Difference is size on the reflection image is due to the distance is larger than the transmission + the spherical mirror effect is not there)

In the morning, Vacuum broken and rotated M1 ThomX 90 degrees clockwise, locked the cavity in air and we observe a degeneracy close to the fundamental mode.

In the afternoon, I cleaned M1 from Gamma factory using pure ethanol and pure water with the spincoater

then placed it as the coupling mirror, aligned and locked in air. we observed similar degeneracy to before next to the fundamental mode.

During the process, M2 spherical from ThomX installed in the cavity was not changed. There could be damage on it, will investigate tomorrow.

Today, M2 Spherical-3 from ThomX that was installed inside the SBox was removed, there was one big dust on the surface of the mirror, mirror was cleaned

using pure ethanol and pure water with spincoater. (images taken with arrow far from us)

M1 from Gamma factory, fixed with the addition of the ring with 3 screws.

The mode was immediately seen after, did not have to align. After locking the cavity, we do not see the degeneracy resonance we saw yesterday.

But we still see the spot on the bottom left of the mode in transmission. The integration time for both centers maximums were 0.34 for mode and 200 for spot.

After optimizing the polarization and the CEP, we managed to get a coupling of ~ 25%

The lab purchased the Menhir laser @ 216MHz.

it has been sent back to Menhir photonics for inspection, and then is now at lab.
it has been reinstalled to the SBOX setup with injection in a CVBG for pulse stretching before amplification.

the power after CVBG is 24mV.
the power coupled to the fiber is only 6.4mW => to be optimized.

the spectrum has been mesured after CVBG and seems correct : picture is attached.

the 2 files describe the specfications for the 16 mirrors ordered (4 for ThomX + spare, 4 for SBOX + spare) and the measurements made by the LMA.

I add also a 3rd file in which all the "special' mirrors are referenced.

from the begining of the week, Xinyi, Aurélien, Viktor and myself started to install a new setup for the Menhir 160MHz oscillator.
the goal is to rich a record power in the FP-cavity.

- the 160MHz Menhir oscillator has been injected in a fiber.
we reached ~ 25mV on 50ohms which is equivalent to 0.5mA in a DET10 photodiode.
=> ~1mW coupled in the fiber => it is not enough to put an EOM and an AOM before the amplifier.
=> one needs to improve the fiber injection.
in fact, I checked the power in the fiber with a powermeter, and it is ~16mW !
at this level of power, one needs to add some optical density before connecting to a photodiode, or it can be saturated.

- we calculated the mirrors position in the SBOX vessel to obtain a 160MHz FSR FP-cavity.
see in attached files, the calculations and scheme in the PPTX file and the Matlab code to get some results.

- we cleaned the optical table and verified with the dust counter that the SBOX environnement is clean.
the 2nd airflow box (from the entrance) seems more dusty (measureed directly close to the top) than the others.
we also opened the vessel and cleaned it.
see the dust measurement inside the SBOX.

- we checked the motors inside the vessel :
=> spherical and plan mirrors translation stage control with the ESP300.
the translation stage have been placed on the middle of their range.
=> the 2 D-shape mirrors translation stage control with PICOMOTORS controller Newport 8742.

- today, Viktor and Xinyi should start the installation of the mirrors mount and make some test to check if the beam is properly propagated inside the FP-cavity, before installing the final mirrors.
the mirror mounts are the Newport SU100TW-F2K zero-drift low waveform distortion : https://www.newport.com/p/SU100TW-F2K
they can accept mirrors with 6-6.35mm thickness => normally the SBOX mirrors from the LMA have a 6.35mm thickness.
see 1st file from this post : https://elog.lal.in2p3.fr/FPC/SBOX+commissioning/174

Over the last two days, Viktor, Ronic and I have started to install the mirror mounts and try to align the cavity.
- We used the Menhir laser @ 160MHz for alignment.
- To make it easier to operate, we removed some lenses and waveplates, and kept only a few necessary reflective mirrors.
- We measured the distance with rulers and placed the mounts in designed positions.
- We installed Iris on the mirror mounts, used a CCD camera to determine if the beam was in the center, and optimized the two reflective mirrors outside the cavity.
- There were some problems with the controller of the injection mirrors (Newport™) in front of the cavity, and Ronic has fixed them successfully.
- Next week, we will continue to align the cavity, measure the cavity mode, and design the telescope. We will install the old SBX mirrors for alignment first, and then replace them with the final good mirrors.

Over the past few days, Viktor, Ronic and I have continued to align the cavity. We installed 4 mirrors and monitored the transmitted laser with a CCD and photodiode. By adjusting the motors of the cavity mirror stages and the injector mirrors, we obtained resonances and less symmetric TEM20-like patterns. Possible reasons for this are a mismatch between the beam sizes of the laser and the cavity mode, and the mounts are installed in rough positions.
Tomorrow, we plan to use the CW laser to realign the optical cavity and position the mounts more precisely.

- Over the last few days, Viktor, Ronic and I have reinstalled the mounts and realigned the cavity with CW laser and old mirrors. By optimizing the injector mirrors, we got the fundamental mode at the transmission. We measured the beam size in the M2 window with a diameter of 2.5 mm.

- The current coupling efficiency is low. There is a need to increase the coupling in order to lock the cavity and measure FSR and finesse.

- The next step is to measure the incident light parameters and design the telescope to improve the coupling efficiency.

- Recently we have focused on improving the coupling efficiency. Without the telescope, the original coupling efficiency was less than 3%.
- I measured the parameters of the incident CW laser using both a HASO wavefront sensor and a CCD. I designed and installed the telescope, but the coupling efficiency still did not improve.
- After discussing with Aurélien and Ronic, it was decided to replace the M1 because the original M1 has a damaged spot in the center to the left. The damaged spot may be causing the coupling efficiency to be too low.
- Today, I replaced the M1 and realigned the cavity. Fortunately, the coupling efficiency has improved.
- We'll continue to optimize the alignment, improve the coupling, and carry out tests on the cavity.

- We got 30% coupling efficiency by installing a set of telescopes, adjusting the polarization and optimizing the alignment. The diameter of the cavity mode is about 2.1mm.
- Ronic and I successfully locked the optical cavity. Tomorrow we will test the FSR and finesse.

- Today, Ronic and I measured the finesse and FSR after optimizing the locking. FSR was adjusted to 160.27 MHz to match the pulsed laser repetition rate. The finesse was 3029.
  Note: It's now CW laser injected, SBOX's old mirrors. There are lots of dust on the old mirrors without cleaning.
- Then we cleaned the cavity inside and outside, and removed four mirrors.
- Tomorrow we will check the clean condition and install new mirrors if we can. Before installation, it may be helpful to discuss how to minimize the introduction of dust.

Today, Ronic and I installed the new mirrors and got resonance. We can see the oscillations in this high-finesse case. We haven't carefully optimized the alignment. Coupling efficiency is about 15% and the cavity can be locked.
Tomorrow we will optimize the alignment and locking, and measure the finesse.

- In the last two days, Ronic and I installed new mirrors after cleaning the environment, and locked the cavity.
- We added an AOM to feedback on the high-frequency noise, but the locking condition was still not good enough. We found out that the signal generator available for this AOM has a long delay time (3 us), which may lead to low feedback bandwidth. So tomorrow we will use another AOM and signal generator to optimize the locking.
- Under this not good enough locking, we measured the finesse. Unfortunately, the finesse was measured as 15,478, which is much lower than the expected 42,000. It means that about 260ppm of additional loss was introduced. We will measure the finesse again after optimizing the locking and coupling.

By the way, attached are the delay time results for phase modulation of different signal generators:
- RIGOL DG4162: 0.7 us (best)
- SIGLENT SDG6022X: 3 us
- SIGLENT SDG7032A: 2.9 us

- Today, Ronic and I changed the signal generator to a low-noise one (with a delay time of only 0.5 us). Then we moved the D-shaped mirrors, optimized the alignment and locking. We re-measured the finesse and it is 16,760. It improves but not much.

- Tomorrow, we will clean the environment, open the cavity, and use UV light to see if there is any dust on the surface of the mirrors.

Yesterday we checked the mirrors with UV light and there was some dust on the mirrors, especially M2.

Today, Daniele, Ronic and I removed M2 and observed it with a microscope. It was indeed dirty, despite we were careful in installing it before. After that we cleaned it with alcohol and mirror paper, then with a spin coater and pure water. After cleaning, we observed it again and it was much better but not perfect. Then we installed the M2 back. But we haven't succeeded in alignment and getting resonance.

Tomorrow is the newcomer's day, so we will continue with the cleaning and measurements on Friday.