Yuefan and Yuhang

As planned last week, today we measured the reflection beam again after the 500mm lens with cavity locked. The fitting shows in the attached figure. 

Same as last time, the position of the waist in two axis are more or less the same, but they both moved around 8cm from 59cm from the lens to 51cm to the lens this time.

The waist size is very different. In last measurement the waist was 64um and 92um, this time is 60um and 119um.  This gives the reflection beam waist size and position.

1. 711um. 0.863m before the lens, which should be between the periscope and first mirror.

2. 1274um, 5 m before the lens.

The difference in these two measurements, we guess is caused by the alignment condition. Because when we installed the mirrors and lens last week, they were well centered, but today when we aligned the cavity, the beam looks obviously lower than the center of the lens, which will induce some more astigmatism. But since the lens has a focal length of 500mm, it didn't affect so much, but in the design the second lens has a focal length of -50mm, if everytime we realign the cavity, the beam is off center on that lens, it could be a problem.

The solution we were thinking about is to put one or two iris on the reflection beam path when we centered all the optics, then they will act as reference for future alignment.

Anyway, apparently the beam has astigmatism, which may bring some problem to the automatic alignment system. So tomorrow if it is possible, we will move the last lens on the green input path because it is really bad algined now.