a Finesse of 30k with the present mirrors :

T1=120 ppm ;A1=(2.6+1) ppm;
T2=1.5 ppm ;A2=(4.5+0.27) ppm;

corresponds to 39 ppm of additional losses for each mirror and a theoretical gain of about 11k.

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.

- Today, Ronic and I measured the finesse and FSR after optimizing the locking. FSR was adjusted to 160.27 MHz to match the pulsed laser repetition rate. The finesse was 3029.
  Note: It's now CW laser injected, SBOX's old mirrors. There are lots of dust on the old mirrors without cleaning.
- Then we cleaned the cavity inside and outside, and removed four mirrors.
- Tomorrow we will check the clean condition and install new mirrors if we can. Before installation, it may be helpful to discuss how to minimize the introduction of dust.

An Initial cleaning of the KBoX outside was done, All the optics and mechanics placed that were on the table were removed.

Only the old Mitylaser amplifier remains on the table.

Since we will be using the ThomX Celia amplifier, we will remove it and place it in a secure location (to be decided).

Tomorrow a cleaning of the inside of the cavity box is to be done.

The cleaning was done twice with half an hour interval between the two times
reading on the dust meter was taken after each time, and it was zero averaged over a 10-min

The inside of the cavity was cleaned today. It was very dirty and had big dust particles and fibers inside.

Cleaning was done for everything; wall, floor, fixed mounts, wires (still have some dust difficult to remove), edges.

The dust meter average was measured outside the cavity (2471 p/m3 av 6 min)  and inside after cleaning (1424 p/m3 av 10 min).

The cavity was left open under the clean air flow, will observe it under UV, clean and measure Dust tomorrow.

Images attached are after cleaning.

Objects inside the cavity at the moment :

• 4 main mirror mounts
• 4 mirror fixers on mounts and 3 screws each (total 12)
• 2 reflecting mirror mounts
• 2 dielectric mirrors (mounted and to be cleaned)
• two mounts for D-shaped mirrors.
• 2 D-shaped mirrors (unmounted, and they will need cleaning)
• 2 metal irises (pinholes)

After an overnight where the cavity box was left open, a measurement of the Dust meter was taken:

Outside on the optical table on direct air flux (1701 p/m3 Av 10 min) and

Inside the cavity box (998 p/m3 Av 10 min)

The dust inside the cavity has decreased by ~ 30 % for 0.3 u particles.

Note: the cavity box is still open.

Closing the series of the telescope for the beam @ 133.33 MHz

- Last week, we obtained three curves of the variation of different cavity modes with power (Fig. 1). By comparing the gain for similar cavity mode sizes, we found that the gain always drops with increasing power.

- We measured the pulse width. The pulse width of the seed laser, after CVBG, amplified at 2A was measured by UPD (rise time < 70ps). Code filtering was performed by comparing the data to reduce the effect of rise time. The final result was t= 186 ps after CVBG and t=162 ps for the amplified at 2A.

- Today we will measure finesse using CW laser.

Additional information:

The pulse duration has been performed in RF on a UPD-70-IR2-P photodiode from Alphalas GmbH by carefully deconvoluting the response function of the photodiode measured directly with the sub-picosecond laser beam.

Figure 1 shows the pulse width through the CVBG. Figure 2 is the pulse width when amplified to 10W.

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

Today with Dorian we tested three mirrors with which we previously haven't been able to obtain any resonance or optical beating :

- C16111/11 : The mirror looks normal under a microscope, apart from a few inclusions and maybe a small scratch towards the edge. We tested it with a 161186 M1 mirror and we weren't able to obtain any optical beating on the cavity's mirrors or resonance.

- 161185 (1) : The mirror looks normal with the naked eye. We tested it with a 161182 M2 mirror and we were able to notice some optical beating on the cavity's mirrors as well as small resonance peaks and a higher-order transverse mode.

- 161185 (2) : The mirror shows some damage on the substrate side (not the coated side). We tested it with a 161186 M2 mirror and we were able to notice some optical beating on the cavity's mirrors as well as huge resonance peaks and a Gaussian transverse mode.

The updated recap file is available here, as well as a few pictures.

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.

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

This afternoon we opened the cavity and put the D-shape mirrors at their correct place, close to the beam.
we checked the relative position of the mirrors to the beam using the 2nd stage of the amplifier (<1W) and with the sensitive (and cleaned) orange optical card.
with this configuration, we can see very clearly the beam inside the cavity (~ 100µW) and we can check easily if the D-shape mirrors are correctly placed.

the motors used to move the D-shape are the Newport Picomotors 8303-V
with roughly 30nm/step sensitivity and 50mm of range (~1 600 000 steps)
the 4 axis controller used ot move these motors is the Newport 8742.

For both Vertical and Horizontal D-shape mirrors:
* when you do +N steps on the controller, you retract the D-shape mirror from the beam
* when you do -N steps on the controller, you push the D-shape mirror to the beam

the 0 position on the controller corresponds to the D-shape close to the beam.

now, the FP cavity is closed and pumped to go back to vacuum.

The cavity box is vacuum pumped at 6*10^-2 mbar.

summary:

- the motors used to move the D-shape are the Newport Picomotors 8303-V
the sensitivity is roughly 30nm/step
the range is 1 600 000 steps or 50mm

- the 4 axis controller used ot move these motors is the Newport 8742.
channel 1 is for the vertical D-shape
channel 2 is for the horizontal D-shape
+N steps on the controller, you retract the D-shape mirror from the beam
-N steps on the controller, you push the D-shape mirror to the beam

the 0 position, vertically and horizontally is close to the beam.
the stand position is at ~ +200 000 steps in both directions.