- Recently we have focused on improving the coupling efficiency. Without the telescope, the original coupling efficiency was less than 3%.
- I measured the parameters of the incident CW laser using both a HASO wavefront sensor and a CCD. I designed and installed the telescope, but the coupling efficiency still did not improve.
- After discussing with Aurélien and Ronic, it was decided to replace the M1 because the original M1 has a damaged spot in the center to the left. The damaged spot may be causing the coupling efficiency to be too low.
- Today, I replaced the M1 and realigned the cavity. Fortunately, the coupling efficiency has improved.
- We'll continue to optimize the alignment, improve the coupling, and carry out tests on the cavity.

- We got 30% coupling efficiency by installing a set of telescopes, adjusting the polarization and optimizing the alignment. The diameter of the cavity mode is about 2.1mm.
- Ronic and I successfully locked the optical cavity. Tomorrow we will test the FSR and finesse.

- These days, Ronic, Fatematuj and I measured the beam parameters of the output of the third-stage amplifier.

- We used 2 wedges and reflection filters to reduce the intensity on the CCD.

- We measured multiple points at pump current of 2 A (output power ~10 W). The waist diameter of the output is w_x = 792.26 um, w_y=873.90 um.

- The next step is to design the telescope and improve the coupling efficiency.

Today, Ronic and I installed the new telescope and locked the cavity.

- We locked at the amplifier current of 1 A and obtained 32% of coupling. (see Figure 1)

- The telescope was designed for a current of 2 A (output power ~10 W). To inject this power, we need to add some filters to devices.

- For CEP tuning, when we changed the AOM frequency while cavity locking, sometimes it caused unlock and power drops. It will be dangerous in high-power cases. So it's better to optimize the AOM frequency in low power and just tune the laser current in high power. Now the current variation range of the menhir laser is 750mA to 950mA.

- Today, Ronic and I locked at the amplifier current of 2 A and obtained ~60% coupling after optimizing the CEP (see Figure 1).

- The injected power is 10 W at 2 A. We measured only 14 kW inside the cavity, which corresponds to an effective gain of 1,400 and a full gain of 2,300. The cavity finesse is 23,000 and the normal gain should be around 6,200.

- We found fluctuations in transmission, possibly because of mode degradation. Tomorrow we will use D-shape mirrors to suppress high-order modes and optimize alignment and locking.

Today Daniele, Ronic and I cleaned the mirrors and locked the cavity. However, the finesse was only 13,000 because of the not clean enough environment and not pure enough alcohol and water.

We will carefully clean the environment, clean the mirrors again with pure alcohol and water and measure the finesse when I return. If it doesn't work, we will use plasma to clean the mirror. We have gone to the lab to confirm the plasma device and then we will study the best parameter settings: polarity, time, and current.

Have a nice weekend!

- Today we cleaned the environment and put the spin coater and microscope inside the airflow.
- Tomorrow, Daniele and I will clean the mirrors one by one using pure alcohol and water, and measure the finesse each time. If it does not improve, we will clean them with plasma.

Today, Daniele and I cleaned mirrors one by one using pure water, alcohol, and the spin coater. Here are the measurements of finesse each time:

1. Initial value: 14,076

2. Clean Mirror 2: 20,606

3. Clean Mirror 4: 18,750

4. Clean Mirror 3: 18,762

5. Clean Mirror 1: 18,563

6. Reclean Mirror 4: 15,226 (unstable lock)

7. Reclean Mirror 4 again: 16,563 (unstable lock)

The finesse reached a maximum of 20,606 but finally was down. For the last two measurements, the locking state was unstable and noisy. Tomorrow we will optimize the locking status and re-measure.

- Today, Daniele and I cleaned the cavity inside, recleaned the M2 and M4 and their mounts, optimized the locking, and the finesse is now about 25,000.

- Although it's lower than the expected 30,000-40,000, we decided to move on to the next step. In addition, the mount of M4 is near the end of the tuning range and may cause instability at high power.

- We adjusted the cavity length to match the repetition rate of the pulsed laser, and the FSR in air is 160.265 MHz.

- Tomorrow, we'll turn on the vacuum and use the pulsed laser to get resonance.

The dust meter cap was cleaned using Alcohol, and using the filter white cap the dust count was (0 Av. 10 min)

After the dust counter was hand held in direct airflow, it counted (1581 p/m3 Av. 2 min).

I tested the count also in the SAS, and it counted twice the amount ~ 4500 p/m3

Note: discussion on the next steps to take for the airflow filtering !!!!!!!!

Dust measurement done today on  top of the SBox average 10min

First finesse measurement 20 266, FSR 133.351 MHz under vacuum

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%

how the cavity beam axis is moving during a lock when the cavity is hot ?
could it explain a part of the Transmission / Coupling signal decay ?

we placed 2 Basler camera, one at (30+Z) cm and the other at (85+Z) cm (Z is about 15cm) from the M3 mirror, we recorded the video during a lock and we analyzed the centroid X and Y displacement at 2A and 3A.
frames acquisition speed is a quite slow ~ 100ms => we need to acquire the frames faster !

with these data, the displacement is no more than some pixels, which means << 100µm ... it should be completely negligeable for photdiode thorlabs DET100 with ~10mm of diameter.

the last picture displays typical locking curves (before and after lock) :
- transmission : yellow
- coupling : orange
- PZT correction : blue

Start with the cavity lock images attached, taken from last week, at start we have a coupling ~ 5%  and started improving on the alignment .

The image of 00 modes attached corresponds to the coupling seen on the oscilloscope.

We have a higher coupling with the 11 mode due to the size of the injected beam in comparison to mode

lenses were added to improve it.

Current setup at SBox for locking the cavity

After spacial matching, the coupling increased to ~ 15 - 20 % 

The locking was done for the fundamental mode, tried to improve the coupling with the motors, but the lock is very unstable to be investigated. (Note: the cavity in under normal pressure)

Cavity locked today under vacuum and no airflow, it is stable over a long time

the alignment was improved and an increase of the transmission by 20 % from its original value

unfortunately,  data from the oscilloscope was saved, but it was not in the correct format (image) to show.

Attached ; the mode shape, the new motor positions. 

Closing the series for the setup

The cavity was only locked @ 133.33 MHz but the Finesse was not measured

The FSR was changed to match the new laser MENHIR 216.6625 MHz 

Before injecting into the amplifier, the pulse needs to be stretched using a CVBG, type attached.

the CVBG is to be used at a small angle, the beam shape and spectrum after the PBS is attached.

After measuring the beam profile, a mirror was placed to direct all the power for fiber injection

power before the fiber is 13 mW and the power injected is 6.19 mW

closing series