However, during the investigation we also looked at the photodiode signal with spectrum analyser and noticed that the 56MHz-USB (being at 23.56 MHz because of the 80 MHz heterodyne) does have nearby peaks in the power spectrum (see first picture), even though these did not cause the fringe-like problem. The nearest peak is at a good 10 kHz distance. This is less than the frequency resolution of the phase camera (freq. resolution is poor because points on the image are acquired sequentially and hence the measurement time per point is only 33 us). Hence the 56 MHz USB might/will be affected by the power in this nearby peak. The 56 MHz LSB (2nd picture) does not have extra peaks in the spectrum, nor do the 6 MHz sidebands and carrier.
The 56 MHz USB of the B1p photodiode also shows the same nearby peaks and hence it is not a specific problem of the B4 phase camera, but something that seems to be present in the beam. At this moment the cause of the extra peaks is unclear.
This evening we also checked the alignment of the phase camera B1p beams and found that the overlap of the two beams is close to perfect. Hence we did not touch the alignment.
Between 23 and 24 MHz the spectrum analyser always shows some peaks, but they are maximal near 23.5 MHz. So we were extremely unlucky that our signal of interest (56 MHz USB, which is at 23.56 MHz) was exactly in the range where the spectrum analyser does a bad job.
Thanks Matthieu!
Given that the alignment of the B1p phase camera is good, it seems that the alignment is not what is causing the fringe pattern.
There is a hypothesis that it is caused by spurious beams, and it was suggested that the issue oculd be on the EDB bench leading up to the phase camera where some lensing effect is also occuring, since the beam is now larger than before. Another suggestion was to check the B1p cam in SDB2 to see if the pattern/spurious beam could also come from the new parking position of the PR mirror.
Looking at one of the latest single bounce occurences (Fig. 1) the fringe pattern is clear on the B1p phase camera, but the B1p cam on SDB2 sees a gaussian beam. This suggests that the spurious beam causing the fringing should be on the optics leading to the phase camera, and that the PR parking position shouldn't be the problem. However, there seems to be a small beam on B1p cam on the right of the main beam, not sure if that could be some spurius beam as well.
Still about the fringe patter on B1p phase camera, I checked the images from November 1st single bounce,and the second beam on the B1p cam is quite clear (Fig. 1). To check if this second beam on the B1p cam is related to the fringing on the B1p phase camera, I went and had a look at a previous time where there was no fringing pattern and also there is no second beam on the SDB2 B1p cam (Fig. 2). So it seems there is a spurious beam before both the SDB2 B1p cam and the phase camera, that could be the cause of the fringing pattern at the B1p camera.
