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.

Today we managed to observe the fundamental mode and stabilize the scan on it until we improved the alignment enough.

We see some coupling, but it is very week < 5% , We improved the alignment and the polarization, but there is still no explanation to why it is very low.

The mode shape is circular with radius = 0.89 mm at transmission point, ~ 40 cm from circular mirror.

One EOM was installed along the injection into the amplifier, we saw a drop in the power measured by the first stage monitor from ~ 8.2 mW to 7.2 mW

we improved the injection up to 7.6 mW, but it still fluctuates a lot. We need to be careful about it.

The error signal looks clean, but it is very week which is due to the weak coupling.

Today, we played on the CEP but using the I-tune input of the Menhir Laser (+/- 5V maximal range).

unfortunately, one only saw a very weak improvement of the transmission by 10-20%... and the coupling improvement is almost zero.

the best improvement was for the maximal I-tune range (+5V) which maybe means that we could improve more the effect if were able to get a full range of 2pi for the CEP (instead of the present pi/2 range).

We played on the CEP using the USB command "id0=xxxxx" of the Menhir.

we put id0=48650 and we improved A LOT the transmission and coupling (~ 60%)

here is an image of the first attempts to lock... but the locking is quite difficult.
yellow : transmission
green : reflection
amplifier is still with 2nd stage only (Pout=~1W)

we are adding an AOM in the loop...

We managed to lock the cavity by adding the AOM , but the lock is still difficult to stabilize.

There is some high frequency compensated by the AOM at ~ 170 kHz (yet to understand from where it comes from)

We wanted to increase the power of the amplifier to measure the transmission output at M2,

but locking the cavity again was very, very difficult.

We will try again tomorrow.

We locked the cavity, and it is stable using the Transmission,

the high frequency that we thought could have a reason for instability, is due to the high power on the photo diode of the PDH box which can cause non-linearity effects in the signal.

We also closed the fan of the third stage and there was no significant change on the error signal and the piezo+AOM compensation signal.  

image of the lock with Coupling of ~ 60%