Alignment was improved to ~ 1.2 V on the photodiode.

Only the second stage was on with ~ 1 W output from amplifier

we measured the transmission from S2 to be 2.3 mW

The lab purchased the Menhir laser @ 216MHz.

it has been sent back to Menhir photonics for inspection, and then is now at lab.
it has been reinstalled to the SBOX setup with injection in a CVBG for pulse stretching before amplification.

the power after CVBG is 24mV.
the power coupled to the fiber is only 6.4mW => to be optimized.

the spectrum has been mesured after CVBG and seems correct : picture is attached.

- We re-measured the gain before moving the mirror. Gain ~9000 was achieved at 3A, but as the power increased, the gain dropped and was difficult to optimize. In fact, we found that each day the gain was a little higher than the previous day.

- We then moved the M3 spherical mirror 1.7mm to make the beam size larger and measured the variation in cavity mode size at different powers. (Figure 1, red is the original result and blue is the result for a larger cavity mode). It is clear that the larger the cavity mode, the larger the slope. The new slope of w_y is 7.9mm/MW. Tomorrow we will make the cavity mode smaller (like in Carstens' paper) and compare the three curves.

- It is not simple to compare the gain variations of different cavity modes because it takes more time to optimize the telescope and alignment. Ronic suggested that we could compensate for the cavity mode variation by moving the spherical mirror to see how the gain changes at different powers while keeping the cavity mode unchanged.

- In addition, we measured the spectrum of the menhir laser, after cvgb, amplifier output at 3A (Figure 2). We found that the peak changed from 1031 nm to 1032 nm after CVBG, probably because of the imperfect alignment of CVBG.

I locked the Koheras CW on the FP cavity but the lock was pretty noisy and very difficult to acquire.

I tried to produce some modulation sidebands close to 216MHz to measure the Finesse but the power loss was very small so, the signal to extract the Finesse would be unreadable !
(generator voltage was at maximum Vout = 0.5Vrms on 50 ohms... is it normal ? or the used 10GHz EOM suffers some problems ?)

At one moment, I lost the lock and was not able to find it again... it seems the Koheras is too noisy for this cavity (may be is it a good news for the Finesse ?).
Tomorrow, I will try to use the OEwave + amplifier to lock the cavity.

Starting with the alignment of the KBox using CW Koheras.

The size of the CW output from the collimator at 1 meter from is ~ 2.4 mm, to be adjusted with lenses to match the cavity !!! (attached image)

For the 2 alignment mirrors, I am only able to control 3 axes for now (motor issue for the fourth to be fixed !!) , Horizontal and vertical for Ma1 and Horizontal for Ma2(closes to injection window).

Started with only irises placed on both the M1 and M2. Placed a beam profiler at the window behind M2 to observe an output beam.

Motors position for the 3 axes and beam shape and position after M2 attached.

After installing the 2nd EOM, we had some trouble to be able to lock again.
One possible reason was the very low signal level in transmission, which is important to trigger the locking system (and stop it).

See the Alice post for details, but we were able to measure only once the Finesse of the cavity at around 2600.

After the Finesse measurement, we opened the box to change the M1 mirror... so the box is at ambient pressure now.

I took back the avalanche photodiode from the Minicav room and installed it on the setup to replace the FPC transmission photodiode.
Now, the transmission peaks are at the 1V level, and it's very easy to trigger on...
The system locked very easily, even without being under vacuum.
It will help if we need to inject very low power laser (e.g. OEwaves after 2x EOM and AOM).

To prepare for the amplifier. I installed the D-shapped mirrors after cleaning (using aceton and ethanol) between the cavity 2 mirrors.

Before closing, I observed the back reflection from M1 (injection mirror). both the injection and reflection lines were off, due to that there was an internal reflection hitting the walls of the cavity.

I tried to correct it by slightly adjusting on M1 without losing the mode, but unfortunately we lost it. 

Tried to go back to the original position using the reflection iris reference, with no success.

Cavity axis is lost, and we need to align again.

The D-shaped mirrors are installed properly and not cutting the path of the beam.

The cavity has been aligned again and 00 modes beating observed, and external reference points has been placed.

Today, Ronic and I installed the new mirrors and got resonance. We can see the oscillations in this high-finesse case. We haven't carefully optimized the alignment. Coupling efficiency is about 15% and the cavity can be locked.
Tomorrow we will optimize the alignment and locking, and measure the finesse.

- In the last two days, Ronic and I installed new mirrors after cleaning the environment, and locked the cavity.
- We added an AOM to feedback on the high-frequency noise, but the locking condition was still not good enough. We found out that the signal generator available for this AOM has a long delay time (3 us), which may lead to low feedback bandwidth. So tomorrow we will use another AOM and signal generator to optimize the locking.
- Under this not good enough locking, we measured the finesse. Unfortunately, the finesse was measured as 15,478, which is much lower than the expected 42,000. It means that about 260ppm of additional loss was introduced. We will measure the finesse again after optimizing the locking and coupling.

By the way, attached are the delay time results for phase modulation of different signal generators:
- RIGOL DG4162: 0.7 us (best)
- SIGLENT SDG6022X: 3 us
- SIGLENT SDG7032A: 2.9 us

- Today, Ronic and I changed the signal generator to a low-noise one (with a delay time of only 0.5 us). Then we moved the D-shaped mirrors, optimized the alignment and locking. We re-measured the finesse and it is 16,760. It improves but not much.

- Tomorrow, we will clean the environment, open the cavity, and use UV light to see if there is any dust on the surface of the mirrors.

Yesterday we checked the mirrors with UV light and there was some dust on the mirrors, especially M2.

Today, Daniele, Ronic and I removed M2 and observed it with a microscope. It was indeed dirty, despite we were careful in installing it before. After that we cleaned it with alcohol and mirror paper, then with a spin coater and pure water. After cleaning, we observed it again and it was much better but not perfect. Then we installed the M2 back. But we haven't succeeded in alignment and getting resonance.

Tomorrow is the newcomer's day, so we will continue with the cleaning and measurements on Friday.

Today, Viktor and I started installing the two-mirror cavity.
- Firstly, we cleaned the environment and the dust counter showed good cleanliness
- After opening the cavity we tried to determine the source of the strange spot with a laser detection card and found that the beam was very close to the front edge of the longitudinal D-shaped mirror. In addition there was nothing else strange.
- The setup of the two-mirror cavity is shown in Figure 1. We have to use the menhir laser of 216MHz. The mirrors used are shown in Figure 2.
- We have installed the M2 and will continue the installation tomorrow.

Today Viktor and I completed the installation of the two-mirror cavity and managed to lock and measure the finesse.

- The finesse is 36k now (see figure 1). For the designed value of the mirror, the expected finesse is ~50k.

- The diameter of M2 transmission is 1.67 mm,1.65 mm (see figure 2).

- The installation process took a lot of time in orienting the PBS. In addition, we found that the cavity reflected beam and the window reflected beam would interfere (see figure 3). The small spot in the lower right corner is the window reflected light.

- We need to discuss whether the next step is to clean the mirrors or vacuum and move on. 

Today, Ronic and I optimized the locking at the amplifier current of 2 A and obtained ~ 21 kW inside the cavity.

- When all the iris open, the injected power is 10 W and the coupling is ~ 40%, corresponding to an effective gain of 2,100 and a full gain of 5,250. But the coupling may not be the true value because there is a large spot around the output beam.

- We have optimized the CEP, alignment, D-shaped mirrors and locking state. We optimized alignment after leaving the iris open and the inside power went from 14kW to 21kW.

- The transmission and reflection signals both have some same fluctuations, and they seem to come from the cavity. It's possible that the over-angled mirror mount could be the cause, but not sure. We will check in different power and see the stability of the signal.

- In addition, we found that the design values of the mirror incidence angles for the SBOX (3.359°, 5.900°) are different from the mirror ratings (1.146°). This may result in parameters such as reflection and transmission being different from the datasheet. It will also change the estimated maximum finesse, gain, and power inside the cavity. It might be better if the mirror parameters could be recalculated based on the actual angle of incidence.

All the mirrors show impacts on there surface (some of them do not show deposit). Does it come from experiments or fabrication ? Are these holes or bumps ?

Under the microscope,

the spherical mirrors show the deposit of metal dots on the reflecting surface and not on the back of the mirrors

The mirror with fewer spots near the center was used in the setup.

Mirror images in the previous log showing the metal deposit of the coating of the spherical mirrors

(not sent by mail due to large size of images)

- Today we moved the position of the D-shaped mirror at 6A. When motor1 (vertical) is 0.2mm away from the spot, the power in the cavity rises from 457kW to 483kW. Gain=8407 is similar to that at low power (Gain=8511). So the D-shaped mirror lost some of the gain in the previous experiments. At 4A and 5A we did not move the D-shaped mirror. (Figure 1)

- At 8A, we got 553 kW inside the cavity for one minute (Figure 2). The pump temperature is higher than yesterday (up to 34°C).

- At 7.5A and 8A, the cavity can remain stably locked, but the power fluctuation in the cavity is so large that it is difficult to optimize the alignment. This may be due to the short time the amplifier was on, the pump temperature, amplifier pointing and power fluctuations, and thermal effects in the cavity....... The amplifier operated differently at different moments.

- We measured the spectrum of the amplified laser. (Figure 3) The peak is 1032.2 nm. We will optimize the alignment and increase the power to optimize this measurement.

- Next arrangement
   Thursday: larger laser beam size
   Friday: smaller laser beam size
   Monday: finesse measurement with CW laser (Firstly check the possibility of measuring with pulsed laser)

Yesterday, Ronic, Xing, Qili and I achieved a more stable 520kW power at 7.5A (71W injection) by optimizing the alignment and locking parameters. (Figure 1)

- The cavity can be stable locked when airflow is on. At 7.5A, the pump temperature is about 28℃. The chiller temperature didn't change, to the same 23 ℃ setting. We can try 8A later (75W injection) for a short time;

- Figure 2 demonstrates the cavity mode variation, wy/Pc ~ 1.7 mm/MW, half that of the OL paper (3.3 mm/MW). The thermal deformation of our device is much smaller.

- The experimental data are shown in Figure 3. Figure 4 shows the injection power vs circulating power.

- There are some tests that can be done at the moment. I'll update on the elog after discussing the necessity today. ^_^