Akutsu, K. Tanaka
continued to 17883
We took BS3 but the strange beam shape is not changed. so he is innocent. Next, we suspect lens L2.
### What we did
- When we looked again today at the BS I was having problems with yesterday, the beam did not seem to be clipped much at the BS. So we removed the BS while monitoring the transmitted light of the BS with a beam profiler. At this time, there was no change in the shape of the beam. In other words, beam clipping was not occurring at the BS. Figure 1 shows the BS removed. Figure 2 shows the shape of the beam at this time. In other words, BS is innocent. In particular, the clip does not seem to have happened anywhere.
- Then, the next thing that caught my attention was the lens L2 in front of the BS. According to the optical layout on MCE table (JGW-D1910144) and lens model number, the lens used as L2 is a biconcave lens with a focal length of -50mm (Thorlabs). First, to check the accuracy of the lens, we rotated the lens to see if the shape of the beam changed. As a result, the shape remained the same, so it was not a problem of fabrication accuracy.
- One possibility is that the beam transmitted from the MCE is an ellipse, close to a perfect circle, and the -50 mm lens accentuates the eccentricity.
- We brought lenses with different focal lengths to check various things. First of all, when I put in a lens with a focal length of -100mm, the shape of the beam basically became an ellipse. (Figure 3, 4)
- We measured the beam profile with this -100mm lens in place. Figure 5 shows the measurement and fitting results. We also measured the beam profile with the lens removed. Figure 6 shows the result and fitting. At any rate, as far as I can tell from Figures 5 and 6, the size of the beam is almost the same at the lens.
- Next time, I will see if the transmitted light from the MCE is really like this.