ELOG SBOX commissioning

Alice Renaux, Under Process, report, lasers and optics, Optical room, new cavity

Wed, 10 Sep 2025 11:27:45 +0200

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

Alice Renaux wrote:

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, new cavity

Tue, 09 Sep 2025 21:23:11 +0200

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, Finesse measurement

Thu, 04 Sep 2025 17:56:50 +0200

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

Alice Renaux wrote:

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, Finesse measurement

Thu, 04 Sep 2025 17:52:03 +0200

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

Ronic Chiche, Fixed, info, lasers and optics | detectors and electronics, Optical room, Installation of the avalanche photodiode

Thu, 04 Sep 2025 17:35:50 +0200

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).

Alice Renaux, Under Process, report, lasers and optics, Optical room, Successful lock (finally !)

Wed, 03 Sep 2025 18:10:34 +0200

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, 2-mirror cavity locking

Thu, 28 Aug 2025 15:29:00 +0200

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

Ronic Chiche, Under Process, issue, mechanics | lasers and optics | detectors and electronics, Optical room, first attempt to lock

Mon, 23 Jun 2025 14:38:33 +0200

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.)

Alice Renaux, Fixed, report, lasers and optics, Optical room, Damaged mirrors test

Fri, 13 Jun 2025 20:10:00 +0200

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, 2-mirror cavity full setup

Wed, 28 May 2025 18:27:27 +0200

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.

Alice Renaux, Under Process, report, lasers and optics, Optical room, 2-mirror cavity alignment

Wed, 28 May 2025 10:55:15 +0200

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.

Alice Renaux wrote:

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).

Alice Renaux, Under Process, report, lasers and optics, Optical room, 2-mirror cavity alignment

Tue, 27 May 2025 18:12:22 +0200

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.

Alice Renaux, Under Process, report, mechanics | lasers and optics, Optical room, 2-mirror cavity alignment

Fri, 21 Mar 2025 18:27:36 +0100

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.

Ronic Chiche, Under Process, info, mechanics | lasers and optics, Optical room, Alignment of a 2-mirror FP cavity

Tue, 04 Mar 2025 18:51:57 +0100

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.

Ronic Chiche, Fixed, info, utilities, Optical room, optical table cleaning

Tue, 04 Mar 2025 18:33:26 +0100

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

Ronic Chiche wrote:

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.

Ronic Chiche wrote:

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.