polarization state of the cavity at higher power : 20kW, 30kW  and 33kW (slight CEP and alignment optimization) :
the polarization state changes only a little to ~ 87° and is almost linear.

the telescope matchs the cold cavity beam, so it is normal to have a power decrease on the transmission photodiode when the cavity is heating at high power.
we can try to adjust the telescope by moving lens, one by one, to increase the cavity power.

The polarimeter was giving a strange 50% of DOP of the light coming from the cavity.
we had to calibrate (LONG calibration process with care) the polarimeter to get a proper 100% of DOP !
the polarimeter needs also a good alignment with 2 mirrors, a colimated beam and a max power on photodiode between 0.7 and 0.8 (use electronic gain to adapt the level)

at low power (1.5kW inside cavity), the cavity is almost vertically polarized (89°).

We placed a PBS + 2 photodiodes (PhD1, PhD2) at the output of the amplifier to check how the polarization of the amplifier changes with power.

example with 2nd stage @ 6A :
PhD1 = 24.7 mV
PhD2 = 8.9 mV
PhD1/PhD2 = 2.78

and with 3rd stage @ 2A :
PhD1 = 353 mV
PhD2 = 82.8 mV
PhD1/PhD2 = 4.26

Conclusion : we must adapt the quarter and half waveplates for each input power to be always matched with cavity polarization !!!
One could also study how the amplifier polarization changes during time and temperature.

2 pictures :

typical beam with HOM

typical beam after moving the D-shape motor : no more HOM

Beam diameter behind M2 :

- 2nd stage @ 6A - 1kW inside cavity
sx = 2120 µm
sy = 2150 µm

- 3rd stage @ 3A - 30kW inside cavity

sx = 2260 µm
sy = 2475 µm

Today, at 3A on the 3rd stage, we saw some HOM effects.
the transmissions is about 100mW which corresponds to 30kW inside cavity.
we tried to play with D shape motors but without success.

on the plot below, a mix between Thermal effects andHOM effects (the trans step at 13s is done without any external action)

-yellow : transmission
- orange : coupling
- blue : PZT correction

the camera video does not correspond exactly to the scope plot.
it is just an example of HOM effect.

2nd stage output power was going down. We checked the pump diode technical data sheet and the operating temperature is [25°C:35°C].
We increased the chiller temperature setpoint from 19°C to 23°C. 
Then the output power increased (93mW on 2nd stage photodiode).

Last time, we switched ON directly the 2nd stage at 6A without increasing/decrinsing slowly the current.
today, we switched ON the chiller, switched ON the 1st stage, switch ON the power supply of the 2nd stage at 0A and then we increased slowly the current until 6A... and the problem disappeared.

the 2nd stage amplifier needed several hours (4-5h) to reach its nominal power (we look at photodiode level on a scope), instead of the awaited 30 minutes.

could it come from the probable spectrum shifting of the OneFive laser ?
(the power coming from the CVBG, coupled to the fiber, is lower than expected).

Yesterday the cavity has been aligned and locked with the CW Koheras laser.

the FSR has been measured by modulation technique at 133.344MHz at 1mbar pressure in the cavity.

the polarization has to be optimized for the Finesse measurement otherwise, some "shoulders" appear beside the Airy peak and reduce Finesse fit.

once it is done, 3 consecutive measurements give an average Finesse of 20800.

The initial 400kW SBOX mirrors which have been cleaned ont 28th of november have been installed this morning on the SBOX.

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.

AFM+ InfraRed spectroscopy (IR spectro) has been performed on 400kW S-BOX mirrors.

Seems that XPS made M3 and M4 dirty, but M1 have also ome dust. M2 seems clean, further AFM experiment should show that it is as clean as M1.

I just had a phone call with Jérome and he told me 2 things :

* Be carefull ! the MightyLaser amplifier is not designed to work with 33MHz laser : the streching level is not sufficient !
One could worsen the phase noise by self-modulation due to peak power or even distroy the amplifier !
One should use it only at low power !!!

* He thinks we should more or less find back the same DC levels than before even with lower seeding power and lower repetition rate.
He thinks we should look at the optical spectrum to check if we don't have some ASE in the 1st stage and 2nd stage signal !
We can send him plots or call him to discuss these points.

Onefive output power is 24mW now, and 2.41mW(after EOM) injected into fiber,

a injection power monitor added, 99% (2.06mW)  injected into amplifier, 1% (16.1uW) monitored with photodiode DET36A/M, which gives ~500mV DC signal on oscilloscope with 1Mohm impedanc;

First stage amplifier works good, monitoring phtodiode gives more than 200mV DC signal with 50ohm impedance on oscilloscope (as attached photo);

Second stage, the old monitoring photodiode is broken, a new monitoring photodiode is connected, which we don't have reference data for it,

on the optical output port of the monitoring signal, it's written 150mW, but at where we measured 40mW.

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.

29/02/19 - AFM has shown that spots on mirror's surfaces are bumps and not holes.

All the mirrors show impacts on there surface (some of them do not show deposit). Does it come from experiments or fabrication ? Are these holes or bumps ?

Cleaning on dirty surface shows something is deposited on the surface. Cleaning displaces and removes part of the deposit.