Telescope of the mode cleaner cavity

I measured the beam transmitted by the beam splitter of the MZ, did the fitting and found the beam waist position of that beam.

The origin was set at the front surface of the beam splitter. The beam waist size is w0=51.2um and waist position is z=-9.5cm.

Then I took this as the initial value to calculate the lens we are going to use for the telescope. So the condition is, the MC input mirror is 60cm away from the origin, and the beam waist should be around the position of the input mirror and has a size of 277um.

I let the program chose the lenses with focal length we have now and got this result.

L1: f1=100mm z1=5cm
L2: f2=200mm z2=37.5cm
L3: f3=200mm z3=55cm

So with this design the final beam waist is w1=279um, z1=60cm, which is the result we want.

The other thing is before we were using a 200mm lens to focalize the green beam we distracted from the Faraday to the PD. But yesterday we found out, if we want to give power supply and the signal output, we need more space for cable. But the position of the PD before was too close to the post of the steering mirror. So today I changed the lens into a 150mm one, and moved the PD 5cm forward.