XPS has been proceeded on the 400kW SBOX mirrors M3 and M4 (the initial cavity spherical mirrors) in frebruary 2019. Deposited a lot of particles on these mirrors.
All the mirrors received a Infrared spectroscopy the 12th of november 2019. Deposited glue on the non-reflective face (was used to hold them).
15th of november (2019): The four 400kW SBOX mirror's have been cleaned with aceton and isopropanol.
28th of november (2019): The four 400kW SBOX mirror's have been cleaned with spin coater.

Summary:

Aceton and isopropanol removed most of the particles and all the glue. But it let some traces on the mirror surface on all the mirrors (so there is some kind of grease on the surfaces).
Spin coater removes all the traces.

See pictures. On all the first images, we also see the dust which is on the non reflective face through the mirror. On M3 and M4 there is still the "glue" on the non reflective face on their frst images + refletive faces very dirty because of XPS.

Migthylaser amplifier has been moved from the SBox table to the PLIC table.

The mirrors went in the cavity the 28th of november (We did several power up to 30kW stored and only one to 40kW then the power went down to 2kW during the run).

Microscope study shows that mirrors get some dust during the handling [travel from microscope to SBOX --> installation --> in SBOX for +1month and power up --> travel to microscope].

Almost all of these dusts can be removed with cleaning.

There is only one important difference between 28th of november and today, a large spot on M1. 

These days, Ronic, Daniele and I achieved stable cavity locking with the menhir pulsed laser.

- After vacuuming, the current cavity finesse is now about 23,000. The diameter of the cavity mode is w_x=2.2mm, w_y=2.7mm.

- We had to compensate for frequency drift by manually adjusting the cavity length to keep locking.

Now the problem is that CEP's compensation range is not enough. The laser CEP is drifting from day to day. We adjusted the CEP by tuning the pump current of the menhir laser, but the adjustment range was not enough.

- The laser pump current is varied in the locking state and the variation of repetition rate is recorded. The current range is 850mA to 950mA and the repetition rate changes by 24 Hz. The calculation process is shown in Figure 3.

- By calculation, the variation of CEP caused by the variation of laser current is only π/2, which we hope is 2π.

- For Gamma Factory, the target FSR is 40 MHz, so the 4-pulse stack provides 4 times CEP tuning range to meet the requirements. But for our experiment, it seems not enough now.

The next step is to evaluate the gap to the maximum gain and draw the curve of CEP. Then we will discuss solutions.

Here is a simulation of the relative FP-cavity gain vs the CEP for a Finesse of 23000 and taking into account the Menhir laser optical spectrum and several CVBG parameters.

I added the commented Matlab code to produce this plot.

Last week, Ronic and I focused on CEP measurements of the menhir laser.

1. Measurements without Cavity Locking:
   • Direct measurement of repetition rate (Frep) with a spectrum analyzer. Altering the laser pump current from 950mA to 850mA, Frep changed by +28Hz.
   • Measurement of the variation of carrier-envelope frequency (Fceo) by beating with CW laser. Altering the laser pump current from 950mA to 850mA resulted in a beating frequency of n0*dFrep + dFceo = +/-2.4MHz, so dFceo ~ 50MHz.
2. Measurement with Cavity Locking:
   • Maintaining cavity locking, we changed the laser pump current and AOM frequency to record the transmitted power of 5 consecutive fundamental mode (TEM00) resonances.
   • The pump currents were set to 850 mA, 900 mA and 950 mA, and the AOM frequency were set to 210 MHz and 250 MHz. We then plotted the measured transmission amplitude values against the theoretical gain curve (see Figure 1).
   • By adjusting the CEP, we reach the top point on the curve, which is the maximum gain. At this point, the coupling frequency increases from 10% to 50% (see Figure 2).
   • We observe that a 100mA change in pump current adjusts the CEP for pi/2, while changing the AOM frequency by +/-40MHz adjusts the CEP for pi. In summary, our CEP tuning range is about 3pi/2 (130 MHz) - not the full 2pi, but still probably giving us maximum gain.
3. Next Steps:
   • Investigate factors associated with changes in CEP, such as laser temperature or pressure.
   • Discuss with Menhir the feasibility of expanding the laser pump current adjustment range (now limited to 100mA).
   • Optimize AOM frequency and locking status, connect the amplifier.

here is the code to get this last curve

The SBox Length (FSR), was changed to match the repetition rate of the Menhir pulsed laser Frep = 216.6625 MHz

It was changed by removing the 2 D-shaped mirrors placed between M1 and M2, distance between M1-M2 ~= 169 mm 

it was still needed to be adjusted with M1 motor to reach the correct FSR.

The pulsed laser is placed on the able with injection into a fiber

The Finesse of the cavity has progressed from ~2500  to ~ 18 000

** Note: PowerPoint attached shows the progress over the last two weeks on the laser and 

The Menhir 216MHz laser has been put back in place on the cavity table. Its output power is measured to be 160mW with an attenuator. Its spectrum is available in "spectre.xlsx" and "spectre.png". The main wavelength is a bit shorter (1028.75nm) and the spectrum a bit narrower (4.73nm) than expected.

The Menhir 160MHz has been put back in place on the CELIA amplifier setup. Its output power is measured to be 150-160mW with an attenuator as expected. Its spectrum is available in "spectre_avant_cvbg.xlsx" and "spectre_avant_cvbg.png".

The pulses are stretched by means of a CVBG. Their spectrum is available in "spectre_apres_cvbg.xlsx" and "spectre_apres_cvbg.png".

The laser is coupled into an optical fiber with an output power of 11.5mW for a 32mW input.

30/01/19 - The following powerpoint shows the results discussed with LMA people.

What came out from the discussion is:

- Their cleaning method uses demineralized water drop on a spinner. It is probably the explanation of the circular traces on the mirror's surfaces but we still don't know what is this deposit (XPS is running out on 2 of this mirrors at this time).

-  According to their point of view, the spots could come from the coating deposition technique and are "normal". No real explaination, should not come from the substrat which is ultra-polished but can come from some clustering in the coating.

We gave them 2 of the mirrors so they can check if it is possible to clean them. They'll also do a measurement of the mirror's topology.

The kept transmission for M4 is 2.17 ppm. Which is the value measured after alignement (center of the mirror).

These measurements have been taken before alignement.

High dependence against the position. Few measurement several mm away from the center each one at different position give 194, 193, 208, 200 ppm.

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%

Center of M1:

At different positions, transmission give 194, 193, 208, 200 ppm?

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.