Yesterday, we changed the M1 mirror to a 161185 Gamma Factory mirror of transmission 460ppm, the cavity finesse is now 13360.

We managed to lock it today.

The 2-mirrors cavity has 460ppm of transmission for M1 and 10ppm for M2 which should exhibit a Finesse around 13400.

Today, we managed to lock the NKT laser (with an AOM for fast feedback) onto the cavity, and we made 5 Finesse measurements with the modulation technic : 14151, 13847, 13968, 14604, 13892 with an average around 14000 => LW = 216MHz/F ~ 16kHz.

On the plot (Frequency span 1MHz <=> Time span 10s)
blue curve : raw data
black curve : cleaned data
red curve : fitted data

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.)

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).

Today, with Ronic, we managed to get a successful lock with the NKT laser by setting up a new PDH box from scratch (photodiode + amplifer + mixer).

Me measured the injected power (5.4mW) and the transmitted power (33µW) after a wedge (92% transmission) and a 7ppm transmission mirror, so the intracavity power was 5.1W. We have a 950 enhancement factor for a 3100 finesse cavity, so a nominal enhancement factor of 2500. The lock was very stable, as shown in the attached picture (yellow signal is the transmitted power, orange signal is the error signal and green signal is the voltage sent from the Laselock module to the pizeoelectric actuator of the NKT laser cavity).

Then, we added a second EOM in order to perform a finesse measurement, but we weren't able to inject more than 3mW at full laser power in the cavity or to lock the laser onto the cavity.

In June, we encountered some problems regarding the transmission and error signals (see images here), looking as if the laser was switched off before the cavity was filled.

Aurélien, Ronic and I discussed this on 08/27, resulting in a list of tests to perform. We :

- checked the mirrors' thickness (maybe there was some mechanical stress if they were too thick). The mirrors' thickness is 6.35mm (1/4 inch) and is consistent with the mounts size ;

- checked the mirrors' mounts' screws' tightening (maybe the mirrors were either moving if the screws were not tight enough or some mechanical stress if they were too tight). We tightened the mirrors' mounts' upper screws.

Next thing we did with Ronic was check if the error signal depended on the modulation/demodulation relative phase, which was not the case, but it should have.

Ronic added a quarter waveplate before the half waveplate in the injection system.

Today, on 08/29, the succession of the higher-order transverse modes when scanning the seeder laser's piezoelectric actuator's voltage to scan the seeder laser's optical frequency seemed a bit strange, so we checked the resonance frequencies of several occurrences of the same transverse mode :

- (0, 0) : @ -0.4V and 6.0V ;

- (1, 0) and (0, 1) : @0.5V and 6.9V.

In both cases, several occurrences were separated by about 6.4V, which corresponds to the voltage difference to scan a full FSR. The spacing between (0, 0) and (1, 0)/(0, 1) is then about 0.14*FSR.

I checked this by writing a small piece of Python code to calculate the cavity's fundamental transverse mode and its Rayleigh length, which is displayed in Figure 1, and then by calculating the resonance frequencies wrt the (0, 0) resonance frequency for the (1, 0), (0, 1) and (1, 1) transverse modes, which is displayed in Figure 2, with the following formula: $\nu_{p, n, m}=(p+\frac{(n+\frac{1}{2})\arctan(\frac{2L_{\text{cav}}}{z_{\text{R}}})+(m+\frac{1}{2})\arctan(\frac{2L_{\text{cav}}}{z_{\text{R}}})}{2\pi})\times\text{FSR}$ for a $p$ longitudinal and ($n$, $m$) transverse mode. Here, $p=1$. The spacing betwen (0, 0) and (1, 0)/(0, 1) is about 0.11*FSR, making the previous observation consistent with the calculation. Everything seems normal.

Ronic also increased the EOM modulation voltage, increasing the modulation depth for the generation of the error signal (from 100mV RMS to 300mV RMS), making the error signal depend on the modulation/demodulation relative phase, as it should. He managed to lock the laser onto the cavity for about 1s at a time.

Next steps are to optimize the PID parameters and to add a low-pass filter/AOM to cut the higher frequencies off and improve the feedback loop.

With Alice, we installed the EOWaves oscillator to be locked on the FP-cavity (T1 ~ 2500ppm, T2 ~ 10ppm => F ~ 2500)
FSR = 216MHz => LW = 86kHz.

We installed the PDH box, and we got some error signal, but the shape of the transmission signal and error signal is a bit strange...
It grows smoothly, and when the power is large enough, one can see a sudden and fast drop.
Could it be some mechanical problem with the mirrors' mounts ?
I opened the SBOX this morning to do some inspection, and the mirrors seem properly installed in the mounts.

In some rare cases (last picture), the "instability" effect is not dominant, and we are able to maintain a quasi-lock during some 1- 2ms.
But it is still impossible to lock the cavity.

(We did a test with a vacuum in the SBOX at ~2mbar, but the problem is the same.)

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.

Today with Ronic, we changed the FP cavity's mirrors to Layertek (Gamma Factory) mirrors :

- M1 : n°161186 --> fused silica, unknown absorption, unknown diffusion, T1=2500ppm transmission planar mirror ;
- M2 : n°161182 --> fused silica, unknown absorption, unknown diffusion, T2=10ppm transmission, 5m ROC mirror.

RTL (round-trip losses) ~ 2500 + 10 ppm (we forget the unknown parameters for absorption and diffusion).
The maximal finesse we could expect is thus F=π([(1-T1)(1-T2)]^(1/4))/(1-[(1-T1)(1-2)]^(1/2)) ~  2*pi /RTL ≈ 2500 assuming no absorption and no scattering.

The FP cavity's FSR is ≈ 216 MHz given its length.

We managed to see some optical beating on the FP cavity's mirrors and to reach the fundamental transverse mode of the FP cavity by adjusting the injection mirrors, but when scanning the laser's wavelength, some higher-order modes appear and the fundamental mode is reached when the voltage applied to the piezoelectric actuator of the laser's cavity is ≈0V. The actuator is not meant to work with negative voltages, so we translated one of the FP cavity's mirrors so that the fundamental transverse mode's resonance frequency is in the middle of the voltage range.

We also removed the D-shaped mirrors, as they are only useful when working with high power.

Yesterday and today, I replaced the OEWaves CW laser with the NKT CW laser. Its screen does not display anything, so it has to be operated through the GraphiK software.

I then re-aligned the cavity with a new adjustment adaptation tool between the mirror mounts and the irises.

The motor positions are :

- 1 : 3.354420 mm

- 3 : 2.128850 mm

- 4 : 3.468480 mm

- 5 : 3.157300 mm

I then connected the LaseLock module to scan the NKT laser wavelength on a roughly 0-10 V range at a 2Hz rate, so that it could match the cavity's resonance frequency.

Without optimizing the injection, I monitored the transmitted power with a photodiode paired with an amplifier. The pictures are available through this link : https://box.in2p3.fr/s/TGgwkKgYik7MyqW, and an example picture is attached. Their timestamp is in their filenames, and it seems that the transmission varies quite a lot on a ≈10 s scale, and these variations seem to be periodic on a ≈1 minute scale. The peaks seem weirdly wide, almost up to 100-200 MHz (≈FSR).

The NKT laser PZT sensitivity is ~0.09pm/V of wavelength variation, which is equivalent to ~ 25MHz/V
So, the full range of the scan is roughly 250MHz (more than a FSR) for 10V.

It seems impossible to get such large resonances unless the Finesse is very low => let's try to change M1 by a spare GammaFactory plan or 10m ROC mirror.

At the beginning of the week, I installed a half-wave plate, a beamsplitter cube and a quarter-wave plate, as well as the end-of-cavity mirror, which I unsuccessfully tried to align.

I removed the end-of-cavity mirror and started the laser injection over again with the two motorized mirrors placed in front of the vacuum chamber (but left Ronic's setup as it was).

Today, I aligned the laser by putting an iris on the cavity mirrors' mounts. I tested the repeatability of such a setup, both by removing and putting back the same iris and by using a different iris. The result is in the attached pictures. Is it good enough ?

The mirrors' motors positions are :

- 1 : 3.349670 mm ;

- 3 : 2.060280 mm ;

- 4 : 3.484560 mm ;

- 5 : 3.269710 mm.

today with Alice,

- we measured the height of the SBOX windows center : 140mm from the optical table.
- we set the laser fiber colimator exactly at this height.
- we placed 2 mirrors to align the future telescope path at exactly 140mm height along the whole possible travel of the lenses in order to keep them aligned.
the horizontal position is also aligned on this path and a ruler has been placed along this path to help to move the future telescope lenses without misalignment.
- we aligned 2 iris on this path to keep this path axis in case of misalignment.
- we placed 2 iris at the center of the input and output FP-cavity windows.
- we precisely aligned the laser beam on these iris.

in the next days, we need to align the mounts in the SBOX and align also the 2 FP-cavity mirrors.
the output mirror will be a "bad" ThomX 2.24m ROC ULE mirror and the input mirror will be a plan 460ppm Gamma Factory mirror.

this morning with Alice and Daniele, we removed all the optics elements and equipments from the SBOX optical table and started to clean it.
the dust meter count 0 on all particle sizes after the cleaning.

we observed a small part of the SBOX which seems to be oxydised (see picture).

the two previously used mirrors of the SBOX (C23018/7 and C23017/2) were already in their plastic boxes outside of the SBOX.
they are still on the optical table.

we have to decide which mirrors to put in the cavity:

if we don't want to use a "new" ThomX coupling mirror M1, we have to use a Gamma-factory mirror (161185) with T=460ppm for example (we don't have any other FS plan mirrors).

if we don't plan to work at high power in the SBOX for the moment, we could use an "old" ThomX M2 mirror with ROC=2.241m (C1611/11) to avoid any risk of contamination of a "new" ThomX M2 mirror.

this afternoon, we checked the dust meter which is at 0 for 1 and 5µm dust particles but ~ 2000-3000 for 0.3µm particles.

we opened the two top panels to let the air flow clean the inside of the vessel.

we observed some other minor oxydised regions (than the one taken in picture) on the external parts of the inox panels but at first sight, nothing inside the vessel.

now, it seems the table is clean enough (dust meter counts 0 particles) to install a 2 mirrors FP-cavity.

this morning with Alice, we sent the Menhir 160MHz injected in a fiber (with 6mW at the end of a long fiber) into the laser amplifier, to look for leakage or damage in the first stages of the amplifier (the amplifier is totally off).

first of all, we checked for light scattering around the laser crate with a sensitive optical card => nothing

and then, we checked for light scattering inside the laser crate with an optical viewer => we just saw 1 or 2 small spots located at the end of an optical element at the 2nd stage level.
but it's difficult to understand the optical path and know the different elements with the 1st stage still in place.

we think it is mandatory to open the top of the crate and lift the 1st stage to have a better look inside the optical parts which are at the 2nd and 3rd stage levels:
we could remove the front side of the crate without any damage to any fibers in the crate.

we just saw 1 fiber, glued to an optical element on the 2nd stage, and going to the 3rd stage.
the 1st stage is just an electronics parts stage which seems easy to be removed.

... to be discussed...

this afternoon, we saw some electric cables badly connected to their power supply.
we fixed it by soldering them together and screwing the result to the power supply.
(see 1st image)

we lift the plate of the 1st stage and we check for optical leakage in the fibers (see 2nd image + picture of the top part of the cassette).
(Aurélien took several images)

without the 1st stage amplification, we saw some lealage only in the bottom part of the "optical cassette".
light was scattered mostly from one side (2 spots) and we saw also very weak scattering in the other directions.

with the 1st stage amplification, we clearly saw the losses from the bent fibers inside the top part of the cassette => it's a good sign.
but after the 5%-10% coupler (the one used for the diagnostic of the power to allow the use of the 2nd stage), we don't see any losses, which means there is no light in this part !
the fiber break could be in between...
Aurélien should send the images to Jérome to get a diagnostic.
the old schematic is attached but it has been modified in the Loic Thesis (p. 165)

we identified the black optics components as 2 isolators (AFW-PISO-30-1W-FB) and 1 circulator (AFW-CIR-PM-30) from AFW technologies.

friday morning, we add a zoom call with Jerome Lhermite about the amplifier repair.
he approximately confirmed the amplifier scheme from the Loic thesis.
he suggested to:

1) identify the circulator ports.
they have some tapes with text written on them.
the goal is to understand if it is still used in the present setup and if a CFBG could still be connected to it (and from which one end could be the fiber seen "broken").

2) use the 5% output tap of the amplifier to check if some light is outed if the input or circulator fibers are injected with 1st stage switched ON or OFF.

3) follow the "broken" fiber to check to which element it is connected to => we should need to unroll the fibers in the bottom "fiber cassette"... :-(

The Menhir 216MHz laser has been put back in place on the cavity table. Its output power is measured to be 160mW with an attenuator. Its spectrum is available in "spectre.xlsx" and "spectre.png". The main wavelength is a bit shorter (1028.75nm) and the spectrum a bit narrower (4.73nm) than expected.

The Menhir 160MHz has been put back in place on the CELIA amplifier setup. Its output power is measured to be 150-160mW with an attenuator as expected. Its spectrum is available in "spectre_avant_cvbg.xlsx" and "spectre_avant_cvbg.png".

The pulses are stretched by means of a CVBG. Their spectrum is available in "spectre_apres_cvbg.xlsx" and "spectre_apres_cvbg.png".

The laser is coupled into an optical fiber with an output power of 11.5mW for a 32mW input.

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.

Today, Ronic and I recorded some intracavity power and cavity mode size as shown in Fig. 1.

Coupling was calculated using the locking curve of this overcoupled cavity. Pr/Pi = 1-Cgeo*Cimp, Cimp = 1-|1-2T1/RTL|^2

We can see that the effective gain, coupling, and mode size decrease with increasing power. And the beam is constantly moving.

Tomorrow we will try to optimize the telescope for the high-power hot cavity.

Today, Ronic, Daniele and I redo the high-power 2-mirror cavity experiments, and the results are shown in the table (Figure 1 and Excel 2 ).

- The intracavity power ~500kW can be obtained at 47W injection, but we then have no increase or even a decrease in intracavity power when increasing the injection power, and the coupling is decreasing. It looks like the saturation power of the current device.

- We moved the telescope last week at 2A by moving the concave lens 0.5cm closer to the cavity but almost no change in intracavity power (195kW to 193kW). The telescopes for today's experiment are in the new locations from last week, and we didn't move them today.

- Figure 3 shows the locking curve at 500kW with some thermal effect changes.

- Figure 4 shows the de-lock and to-lock curves at 14kW.

- The current results may be due to two causes, the thermal lensing effect and the physical change in the mirror coating. It is possible that the transmission of the two mirrors changes with temperature.

- The next plan is to adjust the telescope at 4A to see if we can increase the intracavity power. Meanwhile, do some simulations about dynamic locking, coupling rate, and transmittance.

here is a Matlab code to try to optimize the telescope for a hot cavity,
taking into account the thermal lens in the coupling mirror.

from that code, one can deduce using the "Gaussian Beam" software (using the attached xml file) an optimized telescope with 100% geometrical coupling @ Pcav = 700kW and absorption in the coatings = 0.6ppm

Here's a summary of our experiment last week:

The initial telescope position: 920 mm (f=+250mm) and 1148 mm (f=-150mm) from the amplifier output.

Mon May 6: We moved the concave lens 0.5mm closer to the cavity.

Tue May 7: We moved the D-shaped mirror position at high power, and the intracavity power reached a maximum of 566 kW at 7 A (as Fig 1). The telescopes are the same as on May 6.

Mon May 13: We moved the two lenses closer to the cavity by 12 cm with the two lenses 20 cm apart. At 5A and 6A, we tried several times to move the concave lens slightly to get higher power. CEP and alignment were optimized after each movement. The best power is shown in Fig. 2 and the table.

Tue May 14: We moved the two lenses far from the cavity ((in the middle of May 13 and before). We tried several times to move the concave lens slightly to get higher power. CEP and alignment were optimized after each movement. The best power is shown in Fig. 2 and the table.

We find a small peak in the transmission at high power when the cavity is just locked (as shown in Figure 4-6 at different powers).

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.

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 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.