Following the storage of ~ 50 kW inside the cavity and a sudden drop in transmitted power from the cavity 

damage to the mirror surface was suspected.

We broke vacuum and took images of the surface of the 2 mirrors in the cavity, the spherical and the planar mirror 

image 1 , spherical reflective surface (no visible damage with the UV light, and no visible damage under the microscope)

image 2 , planar coupler mirror reflective surface (no visible damage under UV light, but under the microscope there is a damaged spot close to the center)

image 4 is the planar surface reflective surface at zoom 8 on the microscope.

Tomorrow will try to shift the injection mirror to avoid hitting the damaged spot.

After discussing, we have decided against shifting the mirror to avoid the time lost.

We changed the injection mirror to a different mirror from Mighty Laser set, Transmission of mirror 80 ppm. (no visible damage at the center of the mirror, only a small scratch on the back)

mirror cleaned using pure ethanol and pure water with spin coater, also the spherical mirror was cleaned again.

After changing the mirror M1 and cleaning the 2 mirrors

They were placed in the cavity box (avoiding touching the mounts to not affect the alignment and placing the mirrors as close to the previous position)

we saw horizontal higher order modes immediately after injecting power,  horizontal misalignment!!.

We aligned the injected beam, adjusted the cavity length and saw the modes 01 (high transition) and 00 (ok but still much lower than 01)

The image attached shows some coupling < 5% , we adjusted the CEP, but it was the max coupling.

we will need to align better tomorrow to increase 00 modes.

The cavity box is closed and placed under vacuum again.

The cavity was locked under vacuum, coupling is approximately < 10%

The lock was relatively easy and the usual sign (oscillations on the transmission tail) of high finesse on the transmission signal was not seen.

We improved the lock using the alignment, polarization and CEP , the maximum we got was ~ 12% coupling

only the second stage is on !

Decision : this is the maximum we can obtain using the M1 from mighty laser

tomorrow we break the vacuum and try with shifting M1 from ThomX position to avoid the damaged spot

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%

Measurments of Finesse with the 2 polarization states, let's call them H (higher) and L (lower): 24500 for the H and 23500 for the L.

We checked the polarization states in transmission of the FP cavity after a PBS. The H was stronger in PBS trans and the L stronger in PBS ref.

We measured the power in reflection of the PBS and added a half WP that we aligned with the PBS polarization. Then, to get the maximum power we had to tilt the half WP of 22° for the H and 18° for the L.

Finally we checked the extinction through half WP and PBS for H and L. 

- For H : max 75 mW min 5 mW. Ratio 6.66%

- For L : max 70 mW min 4 mW. Ratio 5.7%

Right after Koheras : max 3.5 mW min 47 uW. Ratio 1.3%

2 measurement.

First one locked, in transmission of M2 with 2nd stage 0A.Total 89 cm from the big waist (planar mirrors). 2 wedges used.

Second one at input beam with the same power. Datas taken at the same equivalent position than the first one. 3 wedges used.

* Loic has to fix the number of files (3) regarding the number of measurements (2)

* splitted Intensity and phase HASO files

* image for each file

with just intensity, the coupling is 98%, and with phase, x direction coupling is 95%, y direction 97%, so the telescope is good.

Optimization of the polarization has been made the 03/10/18. Checked in reflection of the cavity in reflection&transmission of a PBS, locked and unlocked. Only with 2nd stage.

Ratio values are reflection of PBS divided by transmission or the opposite.

Measurement made on 03/10/18 (nothing has been done since there).

Stable power in the cavity of 225 kW.

Motors have been installed on 16/10/18. No problem with them.

Effect of the motors tested on 17/10/18. No improvement. But they give the possibility to perfectly cut HOM or let them go through as show the following picture of a 2.2 mode at ~340 mW in trans and 70% coupling @4A.