New measurements later.
The finesse has slightly increased but so did the standard deviation: 30933+/-1420. It is to be noted that Ronic has been operating on the breadboard before and between the measurements.
| Filename |
Timestamp |
Linewidth [s] |
Linewidth [Hz] |
Finesse |
| RefCurve_2026-11-13_21_155802 |
15:58:02 |
0,54 |
5403,19 |
29668 |
| RefCurve_2026-11-13_20_155732 |
15:57:32 |
0,524 |
5240,75 |
30587 |
| RefCurve_2026-11-13_19_155703 |
15:57:03 |
0,469 |
4692,11 |
34163 |
| RefCurve_2026-11-13_18_155623 |
15:56:23 |
0,507 |
5073,13 |
31598 |
| RefCurve_2026-11-13_17_155513 |
15:55:13 |
0,513 |
5133,84 |
31224 |
| RefCurve_2026-11-13_16_155411 |
15:54:11 |
0,51 |
5101,46 |
31422 |
| RefCurve_2026-11-13_15_155226 |
15:52:26 |
0,538 |
5377,59 |
29809 |
| RefCurve_2026-11-13_14_155134 |
15:51:34 |
0,538 |
5383,13 |
29778 |
| RefCurve_2026-11-13_13_155029 |
15:50:29 |
0,532 |
5317,79 |
30144 |
| Average |
30933 |
| Standard deviation |
1420,3 |
| Alice Renaux wrote: |
|
The OEC is now under vacuum. We reoptimized the alignment.
The finesse is 30446+/-1141.
| Filename |
Timestamp |
Linewidth [s] |
Linewidth [Hz] |
Finesse |
| RefCurve_2026-11-13_10_150041 |
15:00:41 |
0,53 |
5296,38 |
30266 |
| RefCurve_2026-11-13_9_150021 |
15:00:21 |
0,518 |
5183,49 |
30925 |
| RefCurve_2026-11-13_8_150001 |
15:00:01 |
0,557 |
5565,98 |
28800 |
| RefCurve_2026-11-13_7_145942 |
14:59:42 |
0,544 |
5443,76 |
29446 |
| RefCurve_2026-11-13_6_145923 |
14:59:23 |
0,514 |
5145,31 |
31156 |
| RefCurve_2026-11-13_5_145918 |
14:59:18 |
0,53 |
5296,8 |
30264 |
| RefCurve_2026-11-13_11_150101 |
15:01:01 |
0,497 |
4968,71 |
32262 |
| Average |
30446 |
| Standard deviation |
1140,6
|
| Alice Renaux wrote: |
|
We broke the vacuum but kept the vacuum chamber closed.
The oscillator repetition rate is 160.3MHz in the air, so the target OEC repetition rate is 160.3MHz in the vacuum, which corresponds to 160.257MHz in the air.
We moved one of the mirrors (motor 2) by -2mm, increasing the OEC length by 4mm. The beam size downstream M2 slightly decreased (x-direction : 1.6-1.8mmmm-->1.6-1.7mm, y-direction : 1.6mm-->1.59mm), which makes it plausible that motor 2 corresponds to M3. The OEC frequency is now 160.67MHz in the air.
We optimized the alignement.
We moved the other mirror (motor 1) by +2mm, increasing the OEC length by 4mm. The beam size downstream M2 did not change, which seems to further confirm that motor 1 corresponds to M2 and motor 2 to M3. The OEC frequency is now 160.33MHz in the air.
We further moved motor 1 (M2 mirror) by +0.45mm. The OEC frequency is now 160.257MHz in the air.
We did not reoptimize the alignment.
We measured the finesse :
- Linewidth : 0.541 s <-> 5406 Hz
- Finesse : 29652
The data and the fit are avaliable in attachement RefCurve_2026-11-13_3_113050.png.
We reoptimized the alignment but barely gained anything. We pumped the vacuum chamber.
| Alice Renaux wrote: |
|
This morning, we measured the Menhir 160MHz repetition rate. It is measured to be 160.3MHz thanks to a photodiode. The measurement is available in the attached picture.
The OEC FSR was measured to be 161.05MHz in the vacuum and 161.012MHz in the air, leaving a 8mm difference in the optical path between the OEC and the oscillator.
| Alice Renaux wrote: |
|
On Tuesday evening, we stopped the pump to avoid overheating.
The vacuum level was around 0.7 mbar.
This morning, the vacuum level was roughly the same.
We restarted the pumping and reached 0.1 mbar.
Once at this level, it was impossible to lock with the NKT oscillator despite the analog fast feedback loop with the AOM.
So, we installed the OEWaves oscillator (we saw some "instability" warnings in its status window for the first time) and, after tuning the error signal and the fast feedback loop, the system locked immediately.
Finesse measurement:
Scan speed = 20 kHz/S
Pic duration = 238.5 ms
LW = 4.77 kHz
FSR = 161 MHz
Finesse = 33750
| Alice Renaux wrote: |
|
We changed M4 from C23018/12 to C23018/9. The scattering losses of M4 go from 2.8-4ppm to 6ppm. The new expected finesse is 41,501-42,454.
The new finesse measurements are:
- Linewidth : 0.293 s <-> -5860.591 Hz
- Finesse : 27474
The data and the fit are in attachement RefCurve_2026-11-10_7_163813.png.
Another measurement:
- Linewidth : 0.300 s <-> 6007.489 Hz
- Finesse : 26803
The data and the fit are in attachement RefCurve_2026-11-10_8_164036.png.
We closed the vacuum chamber and started to pump.
| Alice Renaux wrote: |
|
We changed M3 from C23018/11 to C23018/10. The scattering losses of M3 go from 3.8-4.5ppm to 11ppm. The expected finesse is then 42,056-43,392.
The new finesse measurements are:
- Linewidth : 0.246 s <-> 4923.625 Hz
- Finesse : 32703
The data and the fit are in attachement RefCurve_2026-11-10_1_153027.png.
Another measurement:
- Linewidth : 0.265 s <-> 5293.066 Hz
- Finesse : 30420
The data and the fit are in attachement RefCurve_2026-11-10_2_154017.png.
Another measurement:
- Linewidth : 0.270 s <-> 5404.667 Hz
- Finesse : 29792
The data and the fit are in attachement RefCurve_2026-11-10_3_154254.png.
Another measurement:
- Linewidth : 0.270 s <-> 5404.667 Hz
- Finesse : 29792
The data and the fit are in attachement RefCurve_2026-11-10_4_154303.png.
We added a quarter-wave plate and a half-wave plate to manage the incident laser polarization. The new finesse measurement are:
- Linewidth : 0.303 s <-> 6069.969 Hz
- Finesse : 26527
The data and the fit are in attachement RefCurve_2026-11-10_5_155648.png.
Another measurement:
- Linewidth : 0.325 s <-> 6503.829 Hz
- Finesse : 24757
The data and the fit are in attachement RefCurve_2026-11-10_6_160153.png.
| Alice Renaux wrote: |
|
This afternoon we changed M2 from C23018/1 to C23018/3. The characteristics remain the same, apart from the scattering losses that go from 4-5.5ppm to 4.5ppm. The expected finesse is then 43,969-45,663.
The new finesse measurements are:
- Linewidth : 0.281 s <-> 5614.873 Hz
- Finesse : 28677
The data and the fit are in attachement RefCurve_2026-11-10_145752.png.
| Alice Renaux wrote: |
|
This morning, I moved the D-shape far from the beam.
The cavity was completely misaligned vertically and horizontally.
the Finesse is roughly the same as yesterday:
Scan speed = 20 kHz / s
Pic duration = 1 s
LW = 20 kHz
Finesse = 8070
| Alice Renaux wrote: |
|
Second finesse measurement after M2 had been changed:
- Linewidth : 1.125 s <-> 22.5 kHz
- Finesse : 7155
The data and the fit are in attachement RefCurve_2026-11-09_2_161811.png.
| Alice Renaux wrote: |
|
Note: We had cleaned each mirror with ultrapure water right before installing them in the vacuum chamber.
We measured properly the finesse (sweep of 20kHz/s) by fitting the linewidth using curve_fit :
- Linewidth : 0.639494857649475 s <-> 12789.8971529895 Hz
- Finesse : 12589.46792721971
The data and fit are in attachement RefCurve_2026-11-09_0_143449.png.
We then changed M2 from C23018/4 to C23018/1. We did not clean the new mirror or changed the cavity alignment. The scattering losses of M2 are 4-5.5ppm. The expected finesse is now 43,700-45,800.
The finesse new measurements are:
- Linewidth : 1.0684238581720833 s <-> 21368.477163441665 Hz
- Finesse : 7535.305336380181
The data and the fit are in attachement RefCurve_2026-11-09_1_154254.png.
| Alice Renaux wrote: |
|
We estimated the FSR and the finesse:
- FSR: 161.037MHz
- Linewidth: 0.770ms x 20kHz/s = 15400kHz
- Finesse: FSR/linewidth = 10500
Far from the expected finesse. We will clean the mirrors and the vacuum chamber again.
| Alice Renaux wrote: |
|
ADD: We added the LaseLock to scan the NKT laser wavelength and select the transverse mode that resonates in the cavity before taking the attached picture.
**********
We installed a camera (Gentec-EO Beamage-4M) after M2 and a photodiode after M4 and at the reflection on M1 (ThorLabs DET100A2). We observed resonance peaks in transmission along with a drop in the reflected signal, which indicates some coupling (15-20% roughly ?) between the laser transverse mode and the cavity fundamental transverse eigenmode (see attachement).
We installed an EOM (Photline NIR-MPX-LM-0.1 S/N 4699-12). The attenuation coefficient is 3dB (drop of 50% in the laser power after the EOM).
We installed a PDH box to generate an error signal (N°2 ThomX).
| Alice Renaux wrote: |
|
We installed a 160MHz four-mirror OEC with two planar mirrors and two spherical mirrors. The radii of curvature of the spherical mirrrors are 0.5m.
The design distances are the following:
- Cavity length: 1.87m
- M1-M2 (planar mirrors): 411.62mm
- M3-M4 (spherical mirrors): 520.00mm
- axis: 70mm
- M1-M3 and M2-M4 (diagonals) : 471.04mm
- incidence angle : 149.16mrad
The design implementation in the vacuum chamber is the following:
- wall-M1 : 253.82mm
- wall-M3 : 199.63mm
The mirrors we used are:
- M1: C23017/1. Substrate: Fused silica. Absorption: <0.6ppm. Scattering: 2.5-3.5ppm. Transmission: 115ppm. Total losses: 117.5-119.1ppm. Radius of curvature: planar.
- M2 : C23018/4. Substrate: ULE. Absorption: <0.6ppm. Scattering: 3-7ppm. Transmission: 3ppm. Total losses: 6-10.6ppm. Radius of curvature: planar.
- M3 : C23018/11. Substrate: ULE. Absorption: <0.6ppm. Scattering: 3.8-4.5ppm. Transmission: 3ppm. Total losses: 6.8-8.1ppm. Radius of curvature: 0.5m.
- M4 : C23018/12: Substrate: ULE. Absorption: <0.6ppm. Scattering: 2.8-4ppm. Transmission: 3ppm. Total losses: 5.8-7.6ppm. Radius of curvature: 0.5m.
Calculating R_i=1-(A_i+S_i+T_i) with i=[1, 4] and R=√(R_1*R_2*R_3*R_4), the expected finesse is: F=π√R/(1−R)=43211-46164.
The laser source is : NKT Photonics Koheras Adjustik + collimator. The laser beam diameter at 1/e^2 after M2 is : 1.62mm in the x direction and 1.58mm in the y direction (see attachements).
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