These days, Ronic and I achieved 200kW inside the cavity and 70% coupling efficiency.

- By optimizing the telescope, the coupling reached 70% with iris fully open and maintained 60%-70% coupling at high power.

- The cavity mode went from 2.2mm,2.5mm (38kW) to finally 2.3mm,2.8mm (200kW) without changing a lot.

- Gradually raising the power while optimizing alignment, CEP, and locking, we got the following stable power:

- Next steps:

• Explain the strange drop phenomenon that occurs at high power, where both transmission and reflection drop, as in Fig. 2.
• Maintains a half-hour locking at 200kW. Now the temperature of the amplifier at 8A is over 40 degrees, which may be risky.

all the injection power in the chart have not been measured recently but during the Loic thesis period.
and these old measurements stopped at 5.5A of pump current.... so, the data at "8A" is a pure estimation.

about the last measurement :
it was made at 6A/8A/8A/8A for the 4 pump diodes of the amplifier (because 1st stage has a Peltier issue and we cannot check its temperature), so the average current is 7.5A instead of 8A.
and the linear scale between pump current and amplifier power is ~ 12W/A, then the estimated amplifier power for the last measurement is 76W instead of 87W
and the estimated gain is more 2658.
for this current, the amplifier works out of its nominal limits (temperature set at 25°C but measured at 30°C !!!)  and the fans of the crate are making noise like hell.
so the last gain estimation should be treated very cautiously.

about the transmission and reflection signals behavior, one can write :
R + T + L = 1 => energy conservation for the cavity.
dR + dT + dL = 0 => dL = - (dR + dT)

if dX = Xfinal - Xinitial, dR and dT are < 0 on the last picture, then dL > 0.
it means that this picture seems to show that some losses are increasing from the beginning of the locking process.

several possibilities :
- we saw a strange D-shape effect on the large port of the cavity.
it seems that one of the D-shape mount/mirror is touching the intra-cavity beam producing some ghost effect on this large cavity port.
some cavity axis changing during the beginning of the lock could introduce some additionnal losses.
it can be easily tested by puting the D-shapes far from the beam.

- because of cavity axis changing at the beginning of the lock, the mirror losses are different.
but it is surprising that it is still going in the same direction... more losses at the end.
could be tested by slightly changing the optical axis of the cavity.

- "prior damage" behavior with a bump in the middle of the mirror due to thermal effect which introduces some losses at the end.
=> if it's the case, it's not a good behavior !!! :-(((
can be tested by looking at the wavefront phase in transmission.

- Non linear effect is the coatings.
but the field density seems not so much to produce this kind of effect

- A thermally induced change in the refractive index of the mirrors.
Daniele mentionned a relation between real and imaginary (related to absorption) parts of this refractive index which could explain that a reflectivity change could induce an absorption change.

Today, Ronic and I achieved 272kW inside the cavity at 7.5A. The coupling maintained 60%-70%.

- Compared to yesterday's experiment, we moved the position of the D-shaped mirrors farther in two directions to make the higher-order modes just disappear.

- Possible reasons for higher gain: D-shaped mirrors position, high power and pump vacuum cleaned cavity mirrors so that improve the finesse.

- We didn't see the strange drops like yesterday (Figure 1). However, in the window behind the M3, we can see 3 spots correlating with the intracavity power, even though moving the D-shaped very far does not make them disappear, only weakens them. We don't know where they came from. When this round of experiments is over, we can open the cavity and observe the optical paths.

- Next steps:

• Repeat the experiment to ensure that the gain does not drop.
• Long-term measurement at maximum power when the amplifier temperature is safe.
• Measure the transmittance of the cavity mirrors and the amplifier power.
• Open the cavity and observe the optical paths and the mirror surface.

Today, Ronic, Daniele, Aurélien and I measured the amplifier power and mirror transmission.

For transmission measurements, we used the same new mirrors as Sbox and ThomX, and installed an iris and a 2-inch mount to block the scattering laser.

The angle of incidence during the measurement was about 0.5°. We changed the angle and the measurements remained the same.

If the mirror being used also has a transmission of 1.75 ppm, the original 270kW is actually 463kW!!! The gain is 6549 and the finesse is 28585 (70% coupling).

We will do more tests to check it.

• Redo the experiment and check the spot behind the window at high power.
• Move the power meter to the plane mirror M2 window. It was previously behind the curved mirror M4 window.
• Compare locking curves, cavity mode sizes, and coupling efficiency at different powers.
• After finishing the high-power experiments, we will measure the finesse using CW laser and the transmission of the mirrors used.

Last week, we achieved a stable intracavity average power of 500kW, limited by amplifier power. The experimental data are shown in Figure 1.

- We measured the transmitted laser with a power meter in the windows behind M2 and M4 respectively, and the results were consistent, so the measurements were credible.

- There is only one transmitted laser spot behind both M2 and M4.

- We measured 10-minute locking data at different powers (Figure 2). 480 kW data was not optimized, and we will add 500 kW locking data later.

- We compared cavity modes at different powers (Figure 3). There are fluctuations because we only saved one data at one power. More data will be collected for averaging later.

- After finishing the high-power experiments, we will measure the finesse and the transmission of the mirrors used. As well as the pulse duration, spectrum, phase noise, and repetition rate of the 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. ^_^

- 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)

- 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.

- 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.

We measured again the 100W CELIA laser amplifier with a pump current until 8A.

as the first current pump of the amplifier has a Peltier issue, we don't exceeded 6A on this stage and we compensated with the 3 other stages.

7A average current is obtained with 6A / 7.3A / 7.3A / 7.4A
7.5A average current is obtained with 6A / 8A / 8A / 8A
8A average current is obtained with 6A / 8.6A / 8.7A / 8.7A

we did the power measured either with the "big" powermeter which is able to handle 100W
and with a smaller powermeter after a wedge, in the reflection path, which is multiplied by 39 to match the big powermeter measurement.

a fit a 12W/A from the cut-off current of 2A is a good estimation until 5A.

- 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, 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.

These days, Ronic, Aurélien, Fatematuj and I have been doing some tests on polarization issue, trying to see if it is possible to obtain higher gain under other polarization conditions.

• We installed an additional half-wave plate + PBS + PD at the transmission. By rotating the waveplate of the injection laser, we can compare the resonance signals of single and full polarization. Figures 1 and 2 demonstrate this comparison. The yellow curve is full polarization and the green one is single polarization. The intensity ratio of the different polarizations is unstable in the open-loop state.
• Based on PZT scan frequency = 3.1 Hz, amplitude = 10 V, sensitivity = 3.7 Hz/V, time difference between two polarization peaks = 100 us, we can calculate △Frep = 10mHz and △v = 42kHz, which means the frequency variation between two polarizations. We see two polarizations only at the main resonance.
• By the way, we found two spots behind the M3 window (see Figure 3) and the power of both is related to the intra-cavity power. We moved the position of D-shaped mirror and the second spot became weaker and larger like mirror's edge. Maybe the D-shaped mirror is causing a part of laser to be reflected through the window, but it's unclear exactly how the optical path works.

Next steps:

• We will lock the cavity in different polarization and see if there is higher gain.
• We will move D-shaped to the maximum and see if the second spot disappears.
• We will check the coupling value and try to optimize the telescope using adjustable stages.

just to add some details :

about the S and P polarization frequency shift:
the PZT scan is 10Vpp at 3.1Hz => the slope is 62 V/s because of the triangle shape of the PZT scan.
so 100µs of separation of the 2 polarization is equivalent to 6.2mV on the PZT.
as the PZT sensitivity is 3.7Hz/V on Frep, the separation of the 2 polarization is equivalent to 23mHz on Frep.
△Frep/Frep = △v/v =>  △v = 41.8 kHz

about the possibility to separate S and P polarization states on secondary resonances:
the total spectral width is ~2nm which is equivalent to 565GHz and contain about 3500 laser harmonics at 160MHz.
the central spectral harmonic is roughly the number n0=1.82M, so with the first secondary resonance condition, the Frep/FSR detuning corresponds to n0*Frep = (n0+1) FSR
so (Frep - FSR) = FSR/n0 ~ 88Hz.
then if the central frequency, related to n0, is on an S-polarization resonance, the harmonics at (n0+475) will be on the P-polarization resonance and so on...
the conclusion is the power detected by a photodiode on secondary resonances are a mix of S and P polarizations (if the laser input beam is also a combination of S and P polarizations)
and we cannot make an observation of different peaks with different polarizations in transmission of the FP-cavity.
for that, it is mandatory to be on the main resonance with Frep = FSR (CEP~0) as condition of resonance.

Today, Ronic and I lock the cavity on the other polarization and achieved 50kW with 22W injection.

- After we optimize and lock the cavity in vertical polarization, we rotate the waveplate at the transmission to minimize the signal, then rotate the waveplate at the injection laser to maximize the signal. We lock the cavity in this horizontal polarization and optimize CEP and alignment. The results are: when injected at 10W, the circulating power in vertical polarization is 21.4kW and in horizontal polarization is 23.3kW. The coupling are both ~30%.

- The cavity reflection signal obtained in horizontal polarization is weaker than that in vertical polarization (1/10), so we usually lock on vertical polarization first.

- We then increase the power to see the change in coupling. From current 2A to 3A, coupling change from 30% to 40%. Finally, we obtained a circulating power of 50kW with 22W injection (3A current). In the initial stage of locking, high-order modes appear, but in stable locking, there is only fundamental mode and no mode degeneration. Although there are many fluctuations in transmission and reflection.

- Tomorrow, we will optimize the coupling and add removable stages under the telescope.

1st version of the document.
if some information is not correct or missing, give any comment by replying to this post.

In the last two days, Ronic and I connected the amplifier and locked the cavity.

- We installed an iris on the output to remove a part of the pump.

- We turned on the second stage of the amplifier. When locking, the injected power is 220 mW and the transmitted power after M2 is 26 uW.

- Low gain and coupling efficiency due to bad mode matching and CEP.

Next steps:

- Turn on the third stage of the amplifier, measure the beam parameters, and adjust the telescope.

- Check the adjustment range of AOM frequency that enables the amplifier to operate safely.

- Measure consecutive fundamental mode resonances to determine the direction of AOM frequency tuning.

These days, Ronic, Daniele and I achieved stable cavity locking with the menhir pulsed laser.

- After vacuuming, the current cavity finesse is now about 23,000. The diameter of the cavity mode is w_x=2.2mm, w_y=2.7mm.

- We had to compensate for frequency drift by manually adjusting the cavity length to keep locking.

Now the problem is that CEP's compensation range is not enough. The laser CEP is drifting from day to day. We adjusted the CEP by tuning the pump current of the menhir laser, but the adjustment range was not enough.

- The laser pump current is varied in the locking state and the variation of repetition rate is recorded. The current range is 850mA to 950mA and the repetition rate changes by 24 Hz. The calculation process is shown in Figure 3.

- By calculation, the variation of CEP caused by the variation of laser current is only π/2, which we hope is 2π.

- For Gamma Factory, the target FSR is 40 MHz, so the 4-pulse stack provides 4 times CEP tuning range to meet the requirements. But for our experiment, it seems not enough now.

The next step is to evaluate the gap to the maximum gain and draw the curve of CEP. Then we will discuss solutions.

Here is a simulation of the relative FP-cavity gain vs the CEP for a Finesse of 23000 and taking into account the Menhir laser optical spectrum and several CVBG parameters.

I added the commented Matlab code to produce this plot.

Last week, Ronic and I focused on CEP measurements of the menhir laser.

1. Measurements without Cavity Locking:
   • Direct measurement of repetition rate (Frep) with a spectrum analyzer. Altering the laser pump current from 950mA to 850mA, Frep changed by +28Hz.
   • Measurement of the variation of carrier-envelope frequency (Fceo) by beating with CW laser. Altering the laser pump current from 950mA to 850mA resulted in a beating frequency of n0*dFrep + dFceo = +/-2.4MHz, so dFceo ~ 50MHz.
2. Measurement with Cavity Locking:
   • Maintaining cavity locking, we changed the laser pump current and AOM frequency to record the transmitted power of 5 consecutive fundamental mode (TEM00) resonances.
   • The pump currents were set to 850 mA, 900 mA and 950 mA, and the AOM frequency were set to 210 MHz and 250 MHz. We then plotted the measured transmission amplitude values against the theoretical gain curve (see Figure 1).
   • By adjusting the CEP, we reach the top point on the curve, which is the maximum gain. At this point, the coupling frequency increases from 10% to 50% (see Figure 2).
   • We observe that a 100mA change in pump current adjusts the CEP for pi/2, while changing the AOM frequency by +/-40MHz adjusts the CEP for pi. In summary, our CEP tuning range is about 3pi/2 (130 MHz) - not the full 2pi, but still probably giving us maximum gain.
3. Next Steps:
   • Investigate factors associated with changes in CEP, such as laser temperature or pressure.
   • Discuss with Menhir the feasibility of expanding the laser pump current adjustment range (now limited to 100mA).
   • Optimize AOM frequency and locking status, connect the amplifier.

here is the code to get this last curve

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.

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, 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.

- 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.

Over the last two days, Viktor, Ronic and I have started to install the mirror mounts and try to align the cavity.
- We used the Menhir laser @ 160MHz for alignment.
- To make it easier to operate, we removed some lenses and waveplates, and kept only a few necessary reflective mirrors.
- We measured the distance with rulers and placed the mounts in designed positions.
- We installed Iris on the mirror mounts, used a CCD camera to determine if the beam was in the center, and optimized the two reflective mirrors outside the cavity.
- There were some problems with the controller of the injection mirrors (Newport™) in front of the cavity, and Ronic has fixed them successfully.
- Next week, we will continue to align the cavity, measure the cavity mode, and design the telescope. We will install the old SBX mirrors for alignment first, and then replace them with the final good mirrors.

Over the past few days, Viktor, Ronic and I have continued to align the cavity. We installed 4 mirrors and monitored the transmitted laser with a CCD and photodiode. By adjusting the motors of the cavity mirror stages and the injector mirrors, we obtained resonances and less symmetric TEM20-like patterns. Possible reasons for this are a mismatch between the beam sizes of the laser and the cavity mode, and the mounts are installed in rough positions.
Tomorrow, we plan to use the CW laser to realign the optical cavity and position the mounts more precisely.

- Over the last few days, Viktor, Ronic and I have reinstalled the mounts and realigned the cavity with CW laser and old mirrors. By optimizing the injector mirrors, we got the fundamental mode at the transmission. We measured the beam size in the M2 window with a diameter of 2.5 mm.

- The current coupling efficiency is low. There is a need to increase the coupling in order to lock the cavity and measure FSR and finesse.

- The next step is to measure the incident light parameters and design the telescope to improve the coupling efficiency.

from the begining of the week, Xinyi, Aurélien, Viktor and myself started to install a new setup for the Menhir 160MHz oscillator.
the goal is to rich a record power in the FP-cavity.

- the 160MHz Menhir oscillator has been injected in a fiber.
we reached ~ 25mV on 50ohms which is equivalent to 0.5mA in a DET10 photodiode.
=> ~1mW coupled in the fiber => it is not enough to put an EOM and an AOM before the amplifier.
=> one needs to improve the fiber injection.
in fact, I checked the power in the fiber with a powermeter, and it is ~16mW !
at this level of power, one needs to add some optical density before connecting to a photodiode, or it can be saturated.

- we calculated the mirrors position in the SBOX vessel to obtain a 160MHz FSR FP-cavity.
see in attached files, the calculations and scheme in the PPTX file and the Matlab code to get some results.

- we cleaned the optical table and verified with the dust counter that the SBOX environnement is clean.
the 2nd airflow box (from the entrance) seems more dusty (measureed directly close to the top) than the others.
we also opened the vessel and cleaned it.
see the dust measurement inside the SBOX.

- we checked the motors inside the vessel :
=> spherical and plan mirrors translation stage control with the ESP300.
the translation stage have been placed on the middle of their range.
=> the 2 D-shape mirrors translation stage control with PICOMOTORS controller Newport 8742.

- today, Viktor and Xinyi should start the installation of the mirrors mount and make some test to check if the beam is properly propagated inside the FP-cavity, before installing the final mirrors.
the mirror mounts are the Newport SU100TW-F2K zero-drift low waveform distortion : https://www.newport.com/p/SU100TW-F2K
they can accept mirrors with 6-6.35mm thickness => normally the SBOX mirrors from the LMA have a 6.35mm thickness.
see 1st file from this post : https://elog.lal.in2p3.fr/FPC/SBOX+commissioning/174

the 2 files describe the specfications for the 16 mirrors ordered (4 for ThomX + spare, 4 for SBOX + spare) and the measurements made by the LMA.

I add also a 3rd file in which all the "special' mirrors are referenced.

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.

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

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%

We start of power output from amplifier of 1W with only the first and second stage on then we start with the third stage power increase,

Note: when changing the current on the third stage 4 diodes, better to do it step by step for each one with a step of ~ 0.5 A

• we placed at the transmission point a splitter the transmitted power before was 35 mW after placing it we had 26 mW
  • 97 % transmission , 3% reflected which goes to the beam profiler

• We see degeneracy modes at the last step of 42.16 mW , so we started to test the D-shaped mirrors
  • the full range of the motors is 50.8 mm
  • each step =  30 nm , 30 000 steps = 1 mm

At transmission power of 42 mW, coupling 60%, we see fundamental mode with a degeneracy

The D-shape motors were moved to a position 2 000 000 steps (in theory they should be the max position)

but no change appeared on the 00 mode or the higher order mode.

We will break the vacuum and check the position of the D-shaped mirrors.

Power increased in the amplifier

reflective filters added at the transmission point to be able to have signal on the Beam Profiler, Transmission diode and power meter

at max stage we reached at 2.5 A on the diodes we have

8000 gain , coupling ~ 60% , with power inside the cavity 50 kW

The higer order mode that was observed today,

it was also observed on Friday  along the other higher order mode recorded on the logbook.

after cutting it with the D-shaped mirror we recorded the shape (image attached)

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

Today we managed to observe the fundamental mode and stabilize the scan on it until we improved the alignment enough.

We see some coupling, but it is very week < 5% , We improved the alignment and the polarization, but there is still no explanation to why it is very low.

The mode shape is circular with radius = 0.89 mm at transmission point, ~ 40 cm from circular mirror.

One EOM was installed along the injection into the amplifier, we saw a drop in the power measured by the first stage monitor from ~ 8.2 mW to 7.2 mW

we improved the injection up to 7.6 mW, but it still fluctuates a lot. We need to be careful about it.

The error signal looks clean, but it is very week which is due to the weak coupling.

Today, we played on the CEP but using the I-tune input of the Menhir Laser (+/- 5V maximal range).

unfortunately, one only saw a very weak improvement of the transmission by 10-20%... and the coupling improvement is almost zero.

the best improvement was for the maximal I-tune range (+5V) which maybe means that we could improve more the effect if were able to get a full range of 2pi for the CEP (instead of the present pi/2 range).

We played on the CEP using the USB command "id0=xxxxx" of the Menhir.

we put id0=48650 and we improved A LOT the transmission and coupling (~ 60%)

here is an image of the first attempts to lock... but the locking is quite difficult.
yellow : transmission
green : reflection
amplifier is still with 2nd stage only (Pout=~1W)

we are adding an AOM in the loop...

We managed to lock the cavity by adding the AOM , but the lock is still difficult to stabilize.

There is some high frequency compensated by the AOM at ~ 170 kHz (yet to understand from where it comes from)

We wanted to increase the power of the amplifier to measure the transmission output at M2,

but locking the cavity again was very, very difficult.

We will try again tomorrow.

We locked the cavity, and it is stable using the Transmission,

the high frequency that we thought could have a reason for instability, is due to the high power on the photo diode of the PDH box which can cause non-linearity effects in the signal.

We also closed the fan of the third stage and there was no significant change on the error signal and the piezo+AOM compensation signal.  

image of the lock with Coupling of ~ 60%

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 previous Sbox telescope was dismantled and the mechanical components cleaned.

its lenses are still in the mounts, it looks that two of them are spherical and two are cylindrical

2 are -100 mm and 2 are +150 mm, there is also a box containing fused silica lenses that could be used.

Note: at high power use only fused silica lenses not BK7 type

Beam divergence was measured using a method called  "Focal Length Divergence Measurement Method"

Where a lens of a known focal length is placed on the beam path and the beam waist is measured at the focal distance using a beam profiler.

We ramped the power up to 10 W

for a focal length = 400 mm,

we measured a FWHM = 2.1 mm,

corresponding to a divergence = 4.45 mrad  (edit : wrong software use)

for comparison, we measured the FWHM 8.1 mm  @ 1.55 m and extracted the divergence directly 4.46 mrad  (edit : this measurement is wrong - wrong use of the software)

Note: better to use a lens of a focal lens higher than 100 mm (to reduce the error in the distance measured)

I placed a periscope to adjust the high of the beam from the amplifier output from ~ 10 cm from the table to ~ 15 cm

a dichroic mirror placed after it to reject the pump laser, all the mirrors on the path to the cavity were replaced with dielectric mirrors BB01-E03

the length of the path from the amplifier output to the cavity coupling mirror ~ 2 meters

setup defines the different optics placed in the path

Note: the beam goes all the way to the cavity, put it is not yet optimized to the irises.

Here is a view of beam propagation in the optical software : GaussianBeam

the red filled shape is the model of the CELIA amplifier beam propagation with a divergence of 4.46 mrad
(the 2 black dots is the measurement of the beam size without any lens to change the beam propagation).

the 2 black lines have been put at the input and output cavity mirrors position relative to the CELIA amplifier position, respectively 2m and 2.7m roughly.
the cavity mode radius should be 0.55mm and 0.7mm respectively.
the cavity mode shape is represented by the 2 red lines (very close to the red filled shape which is the beam).

the most simple working telescope could be a +250 lens at 280mm from the CELIA amplifier.
it gives a beam radius of 0.53mm at the input mirror and 0.64mm at the output mirror.
the overlapping is more than 99%

the 2nd file is the GaussianBeam file.
 

Note the correct beam divergence is approximately ~ 2.3 mrad

M2 = 1.1 in this fit, but it is not yet optimized !!!!! could be reason for not accurate telescope reading.

Have tInstalled a new telescope with lenses

250 mm @ 86.8 cm from amplifier ,

-150 mm @109 cm (~ 22 cm between lenses)

the beam waist measured at a point on the reflection which is relatively the same distance to the injection mirror and the beam was much smaller than before

@ ~ 2 meters from amplifier + telescope ,  FWHM = 1.2 mm ,  waist = 0.85 * FWHM = 1.02 mm

Am adjustment on the lenses position to have a smaller waist.

+ 250 mm @ 88 cm from amplifier

-150 mm @ 111 cm from amplifier

the overlap with this placement is ~ 91%

the measured beam FWHM at the injection point M1 estimated to be ~ 0.94 mm

waist = 0.85*0.94 = 0.79 mm , it is still much larger than the needed 0.58 mm radius waist.

There is an improvement in reducing higher order modes, but the fundamental is still too weak to see, we observe higher order even modes 11 , 44 , ...

We increased the power of the amplifier up to 10 W to see if there is a change in the beam shape at the injection point or the transmission.

There was no change in the shape of both of them from the reading at 1 W (with only the 2nd stage on)

Only saw an increase in the transmission power, which is expected.

removing the reading which is not correct (wrong use of software)

redone a reading similar using a lens of focal 250 mm got a FWHM-X = 0.64 mm , FWHM-Y = 0.84 mm

using the vertical to calculate the divergence, we get divergence ~ 2.17 mrad which is closer to fit obtained for the beam profile by taking data points along the path

attached is also the amplifier beam data taken at different points and their fit using Gaussian beam software

for a focal length = 400 mm,

we measured a FWHM = 2.1 mm,

corresponding to a divergence = 4.45 mrad

we wanted to calculate the right telescope with 2 spherical lenses.

1) we have the FP cavity mode size which is 0.58mm at the input mirror and 0.7mm at the output mirror.

2) we planned to measure the laser beam at the output of the amplifier working at P=1W (2nd stage ON only).

we did several measurements at different positions from the amplifier output.
for each of these measurements, we were able to fit the intensity profile I = I0 * exp(-2 *r^2 / w^2) on x or y axis, then we have w(z).
attached files give an example of the beam image at z=40mm and an example of the beam fits for w and y.

with all the w(z) measurements, we were able to fit the divergence of the beam => 2.3 mrad
attached file show the radius measurements and the divergence fit.
with this divergence, we should find a waist bigger than 140 µm (value for M²=1).

unfortunately the smallest beam radius measured is 116 µm which would give a M²<1 that is not allowed !
then it seems the measurements have not been done correctly... :-(

we will try to do them again... maybe at P=10W or 50W ?!?

today, we measured the 2nd stage CELIA amplifier pump wavelength : 970-990 nm

today we did several test with the Dichroic shortpass mirror (Thorlabs DMSP1000) and with a 10nm optical filter around 1030nm (which works in tranmission at AOI=0).

one used the dichroic mirror in reflection: one should cut the pump @970-990nm and we should keep only the signal @1030nm.
but we still saw plenty spots around the central beam (see the image).
adding the 10nm optical filter on the camera, the image did not change !
then we confirmed the whole signal (centered beam + spots) are well @1030nm.
this spots could be the remaining high order modes of the large fiber used for the 3rd stage of the amplifier.