Reports until 16:55, Thursday 29 January 2026
keita.kawabe@LIGO.ORG -
posted 16:55, Thursday 29 January 2026 - (88963)
JAC EOM Thursday (Jennie, Elenna, Jason, Masayuki, Betsy, Keita)
- EOM was put in chamber and moved to its final location. One temporary iris, a cable bracket for the picomotors and one dog clamp for the last lens for JAC needed to be relocated as they interfered with the EOM pivot base.
- In-vac cables were identified and were connected to the EOM. Minor annoyance about the cable strain relief.
- EOM S11 coefficients were measured and were acceptable though the dip frequencies were not the same as in the lab, so we haven't retuned it.
- EOM was connected to the RF sources (now we're modulating!).
- The light goes into and comes out of the EOM.
- We forgot that the EOM wedge has a big effect to deflect the beam. We started working on the alignment downstream of the EOM and this will continue tomorrow.
Details to follow.
EOM tuning results
It's worth noting that we haven't done any tuning after EOM was transported from the lab to HAM1. Frequencies changed a bit but nothing to worry about.
Below is a table of the dip frequencies and S11 coefficients when the EOM was placed in its final location. They aren't bad, 9 and 45 are as good as anybody can do.
(We also measured them when EOM was placed at the edge of the table, there were measurable differences but nothing disastrous.)
at nominal / center frequency
Connection
FieldFox network analyzer was connected to the modulation patch panel at the bottom of PSL rack (which is kind of hard to find, see the first picture). Back of that panel is connected to the in-air cable that runs all the way to the vacuum feedthrough on HAM1.
By connecting 50 Ohm terminator to each in-vac cable, we confirmed from FieldFox that ISC_RF5-B1, B2, B3 and B4 correspond to 9, 45, 118 and 24MHz as specified in D1900511-v12 page 43 and 24.
Before connecting the cables to the EOM, we performed S11 calibration by connecting a short plug, 50 Ohm plug and nothing to the SMA connector of in-vac cables via class B SMA elbow.
After the EOM was relocated to its final location, RF sources were connected to the front of the patch panel.
RF levels of the signals directly coming from the 118MHz and 24MHz RF patch panel were: 10.76dBm for 118MHz, 14.2dBm for 24MHz according to FieldFox.
45MHz and 9MHz come from the EOM driver and I didn't bother to measure the power.
Other things
In three_persons_untangling.jpg, Jennie, Jason and Elena (left to right) are untangling JAC EOM cables together.
About the cable strain relief posts:
The four SMA cables were connected to the SMA elbow connectors on the EOM. The strain relief posts appear to be designed such that each cable is sandwiched between the two viton pieces, with one screw above and one screw below. I was a bit confused about how exactly to use the screws with hex nuts attached. I instead used the third bottom screw to loosen to slide the cable in, and then tighten. It was helpful to have someone else pinch the viton at the top while I tightened the screw. This is how it looks.
The two cables on the right hand side of the EOM mount feed straight through the strain relief into the SMA connection, this is fine. However, the placement of the post on the left hand side is a bit short of where the SMA connections are, so the cable bends a bit to make it through the strain relief post and go to the connector. This is tricky because the SMA cable is also very stiff. I don't know if this is a problem. I took several pictures of how this looks just in case. You can see how one side is straight and the other side is "S" shaped through here.
There are two more angled shots of the EOM in its place on the table.
About EOM placement:
Jason and I lifted and placed the EOM carefully together while Jennie held the cables. We roughly dog-clamped the cables to the table for now. Jason fine-tuned the EOM alignment by lining up the mechanical mount to holes on the table.
Overall, not bad for placement. We opened the light pipe after placement, locked the JAC and saw the beam go in to the EOM and then come out of the EOM on the other side!
To confirm, we used the thorlabs power meter to check. We measured about 82-84 mW before the EOM, and then 79-80 mW coming out of the EOM.
Further alignment work:
We proceeded to try to realign the beam from JM2 to JM3, given the significant deflection of the beam due to the EOM crystal. There are two irises between the mirrors to guide the alignment. Jason found it very tricky to align the beam to both. During the process, JM2 was knocked over and now has some scratching and cracks near the edge of the mirror. It is fine to use it for alignment now, but the mirror will need to be replaced. The team is working on finding a spare now.