Yuefan and Yuhang

This work is inspired by astigmatism we had for green reflection from the filter cavity. Astigmatism can be generated by misalignment or refractive error.

If it is because of misalignment, this can be improved by doing the alignment. If it is because of refractive error, we can move a bit beam on the lens to correct the refractive error.

The filter cavity reflection always has the same direction astigmatism no matter what is the alignment condition. So we think most of astigmatism comes from the refractive error.

We always check the beam shape on the first and second iris(along the vacuum tube), but we should notice that the beam shape is difficult to tell. The reason is that we look at the iris while the beam hits on the iris with angle. Also, we checked the beam we see on the filter cavity transmission camera when we misalign the input mirror. Always this filter cavity transmission has astigmatism(like filter cavity reflection). So we think the astigmatism is mainly because of refractive error.

To solve this problem, we tried to center beam on every lens. We have five lenses between SHG and FC. However, we found most of them are not centered in the lens. But this is based on a clear phenomenon that SHG reflection was seen to have some fringes if we reflect it far and large enough to see clearly. The elongation is the diffraction fringes at one side of the SHG hole and caused by the clipping of the SHG hole side. We know that the beam direction of SHG reflection is defined by the axis of SHG crystal and the SHG in-coupling mirror. We need to adjust the crystal and in-coupling mirror to have a better axis.

Although we centered beam on each lens, the filter cavity reflection still has astigmatism. Maybe astigmatism comes from SHG, and we should assemble it again. Maybe astigmatism happens inside the vacuum chamber, and we need to consider how to improve this situation.