- Today Daniele and I cleaned the spherical mirror by wiping it with alcohol, and the finesse increased to 47k in air.

- After vacuuming, the final finesse is about 45k. The enhancement factor is expected to be 23k.

- Then we tuned the cavity length, FSR = 216.666 MHz. Aurélien helped us to install the menhir laser of 216 MHz.

- Tomorrow we will optimize the optical path and inject the laser into the fiber.

These days, Ronic, Aurélien and I use OEwaves CW laser to measure the finesse of SBOX. We made 5 measurements at 100kHz / 4s sweeps.

The finesse is around 35k (see Figure 1), corresponding to an enhancement factor of 14k.

In our experiments, we only saw up to 9k gain with 70% coupling, corresponding to an enhancement factor of 12.8k.

It could be because of the additional losses introduced by the high power, or the mirror became cleaner after the experiment......

Additionally, we found that the output of the OEwaves CW laser was not a perfect circle, with a depression at the edge of the circle.

Today, with Ronic, we measured the finesse of the 2-mirror cavity witht the NKT CW laser.

We were able to perfrom the measurement only once, and the results of the measurement are attached to this note. We added sidebands to the laser spectrum peak thanks to an EOM, and we sweeped the modulation frequency on a 1MHz span around an estimated FSR of 216.63MHz in 10s. We found a 82kHz linewidth, hence a finesse of 2651.

(Finesse 2651 is consistent with that obtained from the mirrors' transmission coefficients, which is about 3100.)

Today, Daniele and I injected the laser into the fiber, installed the telescope, connected the second stage of the amplifier, and obtained resonances.
- The output power of the menhir laser @ 216MHz is 150mW, after CVBG is 28mW , 9.6mW injected into the fiber, and 1.6mW via AOM and EOM. This is not far from the minimum 1mW seed power required by the amplifier.
- The spectrum after CVBG is shown in Figure 1.
- The waist of this 2-mirror cavity is 0.583 mm, and the position is on the M1. A set of telescopes is designed and installed as in Figure 2.
- We injected the second stage of the amplifier into the cavity and obtained fundamental mode. Aurélien and I are trying to lock it.

The FSR of the 2 mirror (plan-spherical) Cavity was adjusted from 210 MHz to reach 216.643 MHz

it was done by having two reference irises, one at the injection point and one at the reflection 

then changing the position of injection plan mirror to slightly closer distance and monitoring the reflection on the oscilloscope to be max.

The cavity modes were still seen, and we had to only improve the injection alignment after.

Me and Ronic locked in air and measured the Finesse, which was bigger by ~ 20%

average Finesse = 30208.53614

FWHM (KHz) = 7.0179
Finesse = 30869.9522

FWHM (KHz) = 7.1257
Finesse = 30403.005

FWHM (KHz) = 7.1287
Finesse = 30390.4014

FWHM (KHz) = 7.2884
Finesse = 29724.5531

FWHM (KHz) = 7.3055
Finesse = 29654.769

Yesterday evening the cavity was Vacuum pumped up to pressure of 5.5*10^-2 and locked 

changed FSR to be 216.662 MHz and alignment a little and measured the Finesse

in Vacuum we have average Finesse = 30341.6265

FWHM (KHz) = 7.0592
Finesse = 30692.1961

FWHM (KHz) = 7.2186
Finesse = 30014.556

FWHM (KHz) = 7.1051
Finesse = 30493.7635

FWHM (KHz) = 7.1079
Finesse = 30481.9812

FWHM (KHz) = 7.1413
Finesse = 30339.2695

FWHM (KHz) = 7.1624
Finesse = 30249.776

FWHM (KHz) = 7.0239
Finesse = 30846.2719

FWHM (KHz) = 7.2614
Finesse = 29837.6477

FWHM (KHz) = 7.1935
Finesse = 30119.1768

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.