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Entry   Installation of a new cavity: the ABOX!!, posted by Alice Renaux at Optical room about lasers and optics Capture_d'écran_2026-11-04_160149.pngCapture_d'écran_2026-11-04_160210.png
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Message ID: 266     Entry time: Fri Nov 6 16:30:36 2026     In reply to: 265     Reply to this: 267
 Author: Alice Renaux 
 Status: Under Process 
 Type: report 
 Category: lasers and optics 
 Location: Optical room 
 Title: Installation of a new cavity: the ABOX!! 

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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