Reports 1-1 of 1 Clear search Modify search
AdV-DET (Commissioning)
bonnand - 16:32 Wednesday 23 November 2022 (57873) Print this report
B4 beam shape with North Arm Locked or Unlocked

Today I observed that the B4 beam has a very different shape whether the North Arm is locked or Unlocked.

Figure 1 show the images of the beam B4 on PC, SPRB cameras and also B2 beam.

Figure 2 show the same images with the arm unlocked.

The beam looks much better when it is reflected from the North Arm locked, especially true for the two cameras on SPRB and the carrier image.

The "avocado" shapes seems to appears when the North arm is unlocked.


Figure 3 show the beam radius fit given by GxC and recorded in the rds frame.

To be noticed that I have added 4 signals called SPRB_B4_Cam1/2_zoom_FitWaistX/Y.  These signals are the fit obtained in the central area of the cameras, the goal is to avoid getting artefact in the projection due to secondary beams.

The values given by the SPRB_B4_Cam1/2_zoom_fitWaistY are smaller than the one given by SPRB_B4_Cam1/2_fitWaistY because it exclude the secondary beam or most of it.


I ran my script that gives the beam profile of B4 for NArm locked (fig. 4) and NArm unlocked (fig. 5)

I don't want to comment too much because I would like to redo this test putting the secondary beam out the cameras because I am not sure it has no impact of the data.


Anyway the astigmatism of B4 seems reduced and the beam shape much better with the North Arm locked.

So I would say that some part of the astigmatism is not coming from the SPRB telescope (including viewport).

Images attached to this report
Comments to this report:
bonnand - 13:40 Thursday 24 November 2022 (57888) Print this report

Yesterday, the PR mirror was misaligned a bit more than usual during the DET activity.

So I profited to look at the beam size on the SPRB cameras during that moment.

Figure 1 shows the beam on cameras on SPRB and phase camera on EPRB, one can see that the secondary beams are out of the camera field.

And as one can see when the PR is put back at the original parking position, the value of the fit of the beam given by GxC is get larger because of the secondary beams (even when looking only at the central region), figure 2.

So the beam size given by GxC shoulf really taken with great care.

Even without secondary beams, the way the fit is done changes the results when used to extract beam waist and beam waist location of B4 on the bench.

Here I show 3 cases :

- Beam size given by the GxC fit (projection of the beam), figure 3.

  • w0_x =  298 µm @ 66 mm from Cam1.
  • w0_y =  170 µm @ -283 mm from Cam1.
  • QD2 Gouy Phase - x = -42.3 deg - y =  52.0 deg
  • QD1 Gouy Phase - x = 45.9 deg - y =  82.1 deg
  • Gouy Phase diff - x = 88.2 deg - y =  30.1 deg

- Beam size given by the matlab fit (projection of the beam), figure 4.

  • w0_x =  304 µm @ 39 mm from Cam1.
  • w0_x =  156 µm @ -262 mm from Cam1.
  • QD2 Gouy Phase - x = -37.9 deg - y =  51.2 deg
  • QD1 Gouy Phase - x = 47.4 deg - y =  83.1 deg
  • Gouy Phase diff - x = 85.3 deg - y =  32.0 deg

- Beam size given by the matlab fit (profile of the beam in the center of the beam), figure 5.

  • w0_x =  274 µm @ 81 mm from Cam1.
  • w0_x =  223 µm @ -335 mm from Cam1.
  • QD2 Gouy Phase - x = -48.9 deg - y =  47.9 deg
  • QD1 Gouy Phase - x = 48.9 deg - y =  77.7 deg
  • Gouy Phase diff - x = 97.8 deg - y =  29.8 deg

The two first cases (projection gives quite similar results) but the fit done with matlab using a slice of the beam gives a smaller difference on the beam waist in x and y as w0_x is reduced and w0_y increases.

In any case the Gouy phase difference between the 2 quadrants extracted is between 85° and 98° in x (horizontal) and around 30° in y (vertical) depending on the method of the fit.

Given that the shape of the beam is quite disorded it is a bit difficult to assess which fit method is the best one, figure 6 and figure 7 shows the fit done by matlab on the profile in Y at the center of the beam for Camera1 and Camera2 respectively.


Images attached to this comment
Search Help