The CFBG has been removed successfully and the amplifier is closed now.

The soldering machine had a calibration trouble and was sent to the company for review during holidays.

The output power has been checked and is the same as before (tested until 30W ouput power).

Amplifier placed next to the pulsed laser on the table under the airflow

The output is in free space with height from the table of about 10 cm, note the injection height into the cavity is between 14 - 15 cm.

a power cable is placed but not plugged.

Connection to the computer is made using USB B on amplifier to USB A  (note need a longer cable, the available on is too short)

The cooling, to be connected to an outside chiller, will use the bottom one shown in the image attached.

The two water tubes, from chiller to amplifier,  has been marked. The size of the tubes from the amplifier fit inside the tubes from the chiller.

The software to control the CELIA amplifier is on the laptop that was placed in ThomX bunker.

I have placed it in the PLIC room next to the SBox table.

it is called 'Alphanov Control Software'

Closing series

The amplifier was installed on the optical table next to SBox table.

The CVBG that will be used for stretching before amplification needs cleaning (they are very fragile, 'ask victor for best method')

The software to drive the diodes of the amplifier we have is not the compatible one.

this morning with Alice, we sent the Menhir 160MHz injected in a fiber (with 6mW at the end of a long fiber) into the laser amplifier, to look for leakage or damage in the first stages of the amplifier (the amplifier is totally off).

first of all, we checked for light scattering around the laser crate with a sensitive optical card => nothing

and then, we checked for light scattering inside the laser crate with an optical viewer => we just saw 1 or 2 small spots located at the end of an optical element at the 2nd stage level.
but it's difficult to understand the optical path and know the different elements with the 1st stage still in place.

we think it is mandatory to open the top of the crate and lift the 1st stage to have a better look inside the optical parts which are at the 2nd and 3rd stage levels:
we could remove the front side of the crate without any damage to any fibers in the crate.

we just saw 1 fiber, glued to an optical element on the 2nd stage, and going to the 3rd stage.
the 1st stage is just an electronics parts stage which seems easy to be removed.

... to be discussed...

this afternoon, we saw some electric cables badly connected to their power supply.
we fixed it by soldering them together and screwing the result to the power supply.
(see 1st image)

we lift the plate of the 1st stage and we check for optical leakage in the fibers (see 2nd image + picture of the top part of the cassette).
(Aurélien took several images)

without the 1st stage amplification, we saw some lealage only in the bottom part of the "optical cassette".
light was scattered mostly from one side (2 spots) and we saw also very weak scattering in the other directions.

with the 1st stage amplification, we clearly saw the losses from the bent fibers inside the top part of the cassette => it's a good sign.
but after the 5%-10% coupler (the one used for the diagnostic of the power to allow the use of the 2nd stage), we don't see any losses, which means there is no light in this part !
the fiber break could be in between...
Aurélien should send the images to Jérome to get a diagnostic.
the old schematic is attached but it has been modified in the Loic Thesis (p. 165)

we identified the black optics components as 2 isolators (AFW-PISO-30-1W-FB) and 1 circulator (AFW-CIR-PM-30) from AFW technologies.

friday morning, we add a zoom call with Jerome Lhermite about the amplifier repair.
he approximately confirmed the amplifier scheme from the Loic thesis.
he suggested to:

1) identify the circulator ports.
they have some tapes with text written on them.
the goal is to understand if it is still used in the present setup and if a CFBG could still be connected to it (and from which one end could be the fiber seen "broken").

2) use the 5% output tap of the amplifier to check if some light is outed if the input or circulator fibers are injected with 1st stage switched ON or OFF.

3) follow the "broken" fiber to check to which element it is connected to => we should need to unroll the fibers in the bottom "fiber cassette"... :-(

Measurement done on 18/10/18.

At high power, the shape of the 0.0 mode does not change. The D-shape only generate losses in the cavity. Then the power stored in the cavity decrease. As with this configuration, the cavity beam size decrease when power increase, the beam size decreased.

Measurements done @4A on 3rd stage.

Then we get the D-shape away from the beam to not cut it and decreased the amplifier power to validate the beam size at a known value. So the power as been decreased to 2A (= 125 mW in trans) and the beam size was x=2079 y=2255, similar to the 125 mW with D-shape mirrors values.

Matlab code for size vs position and power : 

clear all
close all

x = [1820 1820 1925 1936 2117 2260];
y = [2013 2013 2029 2090 2249 2392];
Position = [0 0.2 0.4 0.6 0.8 1];
Trans = [337 330 306 245 128 17]

hold on
[ax,h1,h2] = plotyy(Position,x,Position,Trans)
set(get(ax(1), 'Ylabel'), 'String', 'Beam diameter (um)');
set(get(ax(2), 'Ylabel'), 'String', 'Transmitted power (mW)');
xlabel('Position of the D-shape (mm)')
plot(Position,y,'g')
hold off

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

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.

Yesterday, we tried to better adapt positions of the telescope lenses, dynamically, during the lock, to improve the matching between input beam and cavity mode.
it is a difficult task because it is quite sensitive to the alignment. we need to realign very often... and it is a long process.
at the end, we concluded that we need to move to much the lenses to be feasible, then we stopped.

then we tried also to change the cavity mode by moving the spherical mirrors inside the cavity but again, the telescope is too far from its expected parameters.
we need to make a cavity mode smaller at high power and we need to move too far the spherical mirrors, then we stopped also this trial.

the conclusion is we need to better measure the cavity mode and make a telescope better adapted to the "hot" cavity.
it is still strange to measure a tranmsission signal AND a coupling signal with a "thermal" decay at the beginning of the lock for both and we expect that they complementary and should vary in contrary direction.
very strange as we use very large PhD which should net be sensitive to misalignments.

Today, Ronic, Daniele, Aurélien and I measured the amplifier power and mirror transmission.

For transmission measurements, we used the same new mirrors as Sbox and ThomX, and installed an iris and a 2-inch mount to block the scattering laser.

The angle of incidence during the measurement was about 0.5°. We changed the angle and the measurements remained the same.

If the mirror being used also has a transmission of 1.75 ppm, the original 270kW is actually 463kW!!! The gain is 6549 and the finesse is 28585 (70% coupling).

We will do more tests to check it.

• Redo the experiment and check the spot behind the window at high power.
• Move the power meter to the plane mirror M2 window. It was previously behind the curved mirror M4 window.
• Compare locking curves, cavity mode sizes, and coupling efficiency at different powers.
• After finishing the high-power experiments, we will measure the finesse using CW laser and the transmission of the mirrors used.

In preparation to measure the output beam profile from the amplifier at high power, I have placed two wedge mirrors just before the beam dump to be able to extract the beam.

One of the wedges was taken from ThomX bunker, also the HR and AR mirrors were taken to plic room in case we might need them

an updated setup is attached  

Beam divergence after amplifier 4.46 mrad

In the last two days, Ronic and I connected the amplifier and locked the cavity.

- We installed an iris on the output to remove a part of the pump.

- We turned on the second stage of the amplifier. When locking, the injected power is 220 mW and the transmitted power after M2 is 26 uW.

- Low gain and coupling efficiency due to bad mode matching and CEP.

Next steps:

- Turn on the third stage of the amplifier, measure the beam parameters, and adjust the telescope.

- Check the adjustment range of AOM frequency that enables the amplifier to operate safely.

- Measure consecutive fundamental mode resonances to determine the direction of AOM frequency tuning.

today with Alice,

- we measured the height of the SBOX windows center : 140mm from the optical table.
- we set the laser fiber colimator exactly at this height.
- we placed 2 mirrors to align the future telescope path at exactly 140mm height along the whole possible travel of the lenses in order to keep them aligned.
the horizontal position is also aligned on this path and a ruler has been placed along this path to help to move the future telescope lenses without misalignment.
- we aligned 2 iris on this path to keep this path axis in case of misalignment.
- we placed 2 iris at the center of the input and output FP-cavity windows.
- we precisely aligned the laser beam on these iris.

in the next days, we need to align the mounts in the SBOX and align also the 2 FP-cavity mirrors.
the output mirror will be a "bad" ThomX 2.24m ROC ULE mirror and the input mirror will be a plan 460ppm Gamma Factory mirror.

The current setup of the optical table attached.

The cavity is aligned and the lock of the fundamental mode has been attempted, but the mode is drifting too quickly to be able to follow.

I have placed the cavity under vacuum for a better stability.

a simple telescope for the CW was adjusted to having 2 lenses of 300 mm placed ~ 250 mm away from the colimator and 200 mm lens placed after it with 510 mm.

closing series