Summary of investigation into the vertically split beam from the EOM
First, we confirmed that the vertical beam splitting observed yesterday originates from the EOM itself. To check the possibility of multiple reflections from lenses, we inspected the back-reflection port of the JAC output mirror. Two reflected beams were observed, most likely originating from the planar and curved surfaces of the lens, and they were mainly separated in yaw. Since the space between the EOM and the lens was blocked during this test, these reflections were conclusively identified as lens reflections. No vertically split beams were observed from this source.
A knife-edge–like test was performed by slowly lowering a metal ruler from the top at both the EOM input and output. At the EOM input, the entire beam disappeared simultaneously, whereas at the output the beam disappeared gradually from the top. This behavior confirmed that the vertical splitting is generated inside the EOM.
To accurately determine the beam positions at the EOM input and output, beam positions were measured from photographs. Taking refraction and geometry into account, it was found that the beam is slightly displaced in the horizontal direction. Details of this analysis will be documented in Keita’s alog.
Based on this result, the EOM was rotated in yaw. Dog clamps were placed at the ±y projection points of the EOM input and output, in contact with the base plate. One 0.5-mm shim was inserted between the base plate and each dog clamp to rotate the EOM counterclockwise. However, the beam pattern did not change. Additional shims were tested, but no significant change was observed. The EOM was fixed with one 0.5-mm shim at each position.
Next, pitch adjustments were explored. The original shim configuration (two shims on the +y side and one on the −y side) was changed by moving shims to the +x side (one location at center) and the −x side (two locations, upper and lower). Each location initially had two shims, and by adding or removing shims, it was observed that the vertical positions of the split beams changed. When the +x (downstream) side was raised, the vertically split beams appeared in the upper part of the beam profile (approximately 5–7 beams; Keita will upload photos). Conversely, when shims were added to raise the −x side, the split beams moved toward the lower part of the beam. With the downstream side lowered by approximately 0.25 mm, about two beams were observed in the upper part and one in the lower part.
1W input power shows 6 or more beams, but 2-3 beams can be observed even with 100mW.
With the last configuration, we proceeded the IMC scan measurement after alignment. The 2nd order mode peak was the same level as we observed when we sim up the EOM first time.
In summary, the EOM shows highly questionable and nontrivial behavior. Possible causes include diffraction due to crystal defects or multiple reflections at the AR-coated surfaces. However, identifying the exact mechanism is challenging at this stage.
Horizontal beam position offset on the EOM input and output aperture on the side plates.
We realized that the nominal beam position on the EOM input and output aperture is NOT centered on the crystal cross section projected onto the side plate face, the beam is horizontally offset in +Y direction.
Look at the first cartoon (cartoon.jpg) and references therein. The beam spot offsets are 0.91mm on the input side plate and 0.54mm on the output side plate, respectively, assuming that the beam deflection angle per surface of EOM is 2.35 degrees as implied in D2500130.
0.91mm is not a small offset, it's almost 1/4 of the crystal thickness (it's 4x4x40mm).
This means that the beam should be (see nominal_sideplate.png, note that the drawing scale of the input aperture in this is twice that of the output aperture):
~3.9mm from the left (+Y) edge of the visual alignment aid notch on the input side plate,
~3.2mm from the right (again +Y) edge of the aperture hole on the output side plate.
Measurements, adjustments and measurements made the beam closer to the nominal location.
Based on the above knowledge, we took pictures of the beam position on the input/output aperture, paying attention to the errors that could arise from the parallax (which is unavoidable), i.e. the sensor card should be as close to the face of the side plate as possible and the beam spot on the sensor card should be as close to the sentor of the camera sensor as possible. This was a tougher job than you think.
Anyway, in the first round of measurements, we convinced ourselves that the beam was:
off in -Y direction by 0.7mm relative to the nominal beam position on the input plate of the EOM,
off in +Y direction by 0.5mm on the output,
give or take 0.2mm or so (the error is based on two pictures for the input beam position with random variation in parallax coming from camera position and the distance between the side plate surface and the viewer card).
We rotated the entire EOM base by using two dog clamps against the EOM base and inserting appropriate shims (EOM_rotation.png). We didn't use the YAW adjustment feature for the EOM pivot plate because there's no way to rotate it in a controlled manner.
After the first adjustment we thought that the beam coming out of the EOM looked better (which might have been false). On the second adjustment the beam looked the same or slightly worse (which might have been false) and we reverted back to the same position as the first adjustment.
Multiple beams mostly in PIT coming out of EOM (pictures and history)
1W into HAM1, otherwise it's hard to photograph these clearly.
The first picture is right after the YAW adjustment was made but before adjusting PIT. The card is held just ABOVE the main beam, you can see four blobs that look like some kind of ghost beams. (If you try to picture the main beam, it's so bright these ghosts become hard to capture.)
The second picture is after the first PIT adjustment. You can only see maybe two blobs, but later we found that the rest went below the main beam (sorry no "below" picture).
So, to recap the history of the beam quality,
Other things.
Just to make sure, we turned down the 9MHz and 45MHz RF power to 3dBm and disconnected the 118MHz and 24MHz cables and nothing changed.
We know that the crystal wedge is supposed to be horizontal and we know that the wedge orientation is correct. When we first installed the EOM in chamber, the EOM transmission was deflected horizontally in +Y direction.