The design of the QPD telescope has been finished.
The idea is to seperate the reflected beam from the cavity into two path, and put one QPD on each path, in a way that the guoy phase on the two quadrants has around 90 degree difference. So one of the quadrant could sense the shift and one could sense the tilt.
The target beam is decided by the size of the quadrant sensor :
So I chose the target beam size at QPD should be around 1~1.5mm
The lenses I used in the simulation are the green lenses I could found on Thorlabs website.
In both of the case, the origin is set at the beam extracting port of the FI.
1.
|
Convex lens(mm) |
Concave lens(mm) |
Distance between origin and the first lens(cm) |
Distance between two lenses(cm) |
Beam size at QPD(um) |
Distance between concave lens and QPD(cm) |
|
500 |
-75 |
17.5 |
43 |
1300 |
105 |
2.
|
Convex lens(mm) |
Concave lens(mm) |
Distance between origin and the first lens(cm) |
Distance between two lenses(cm) |
Beam size at QPD(um) |
Distance between concave lens and QPD(cm) |
|
400 |
-50 |
42 |
42 |
1100 |
30 |
The guoy phase difference at two QPD is around 98 degree. So I think we could go on with this design.