Jennie W, Jason O, Keita K.
As reported in this alog (#89073) from Masayuki and Keita, after we turned the power in HAM1 up to 1W we found a series of vertically spread ghost beams aroubnd the main beam after the EOM and before JM3.
These could not be removed by translating, yawing or pitching the EOM position relative to the beam. It was decided in a larger meeting with EOM design personnel that we would first check if the crystal was cracked or damaged anywhere in case this is the cause.
First photo shows the EOM from above, using a green torch to illuminate the beam path. I can't see any scatter from defects or cracks in the crystal.
Second photo shows possibly a chip at the corner, but this should not affect the beam as its right at the edge.
Third and fourth show side view with illumination from the top at an angle.
In summary we did not see any 'smoking gun' to cause these ghost beams.
Very rough power estimate for the ghost beam(s) is ~O(1%)
Jennie and Jason set up another temporary iris between JM2 and JM3, centered it with 1W into HAM1 to carefully block the ghost beams without blocking the main beam, then changed the power to 100mW (for safety) and measured the power at various places. Measurement accuracy cannot be great (Jennie and Jason says the numbers were jumping around as it was difficult to hold the power meter head at a fixed position mid-air) but I would say the power in the ghost beams is ~O(1%).
| JAC out | ~105mW |
| Between JM2 and the iris (includes wrong-pol beam) | 104~105mW |
| After the iris (wrong pol as well as ghosts blocked) | 99~100mW |
| Wrong-pol beam | 1~3mW |
| Background light (no beam) | 1~2uW |
Where do they go?
After opening the temporary iris that we just put in all the way, the iris just downstream of JM3 was already blocking some of the ghost beams as well as the wrong polarization beam (JM3iris.jpg). Vertical beams don't look vertical because the iris is not a flat plane and we have a large parallax here. Anyway, it seems that we can block further if we want to from the top and the bottom.
The picture of the last iris on HAM1 shows that something is blocked on the left (+Y) side (outputiris.jpg). Looks like the iris is clipping something on the right but the camera couldn't be positioned to have a good view for both sides.
The last picture (after_last_iris.jpg) shows the beam right after the last iris on HAM1. You can see that some ghost beams are still coming through.
With this beam injected into HAM2 and misaligning MC2, we looked into IOT2L to see the MC REFL beam. We weren't able to find ghost beams there, though Jason and I felt that the beam is not super clean.
One question Jason had was whether or not the diverging beams that originate from the EOM location are supposed to keep diverging after lenses.
The beam after the second lens is actually not diverging. According to this plot, we suppose to be able to find the splitted beams in the IOT2 table.
