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: |
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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: |
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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: |
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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: |
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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: |
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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: |
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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.
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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: |
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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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