Aim Laser on Center of Photodetector (Adjust Mirror 2)
1)
XEP software should already be open.
2) Check (A+B) voltage value (measures total intensity of light hitting the photodetector), which
should be ~3-3.5 V if using ACTA tips provided by the lab.
- The correct value will be different if you use your own tips.
- If the (A+B) sum is significantly less than 3V, this indicates that either the ‘mirror 2’
alignment is way off, or the laser spot on the cantilever may actually be the reflected spot
as mentioned above. Adjust the ‘mirror 2’ knobs, first turning one knob until you find a
local maximum of (A+B) voltage, then turning the other knob until you find the local
maximum again. Repeat as needed until the sum is at the appropriate value.
- If after several tries, you still can’t get the sum high enough, remove the head and check
mirror 2’s position visually – it should be roughly parallel to the base. Adjust mirror 2
knobs as needed and then replace head
3) Adjust ‘Mirror 2’ knobs to position laser spot on center of photodetector, reducing the (A-B)
and (C-D) voltages to less than +/-0.3V. The XEP software will indicate the position of the laser
as a red dot once it is reasonably near the center of the photodetector.
Option A: Adjust the knobs while watching the laser position in the software and turning
in the correct direction to move toward the center of the diagram/minimize the (A-B) and
(C-D) voltages.
Option B: Use the diagram below as a guide for which knob to turn and in which
direction depending upon where the laser is shining.
1
2
T = tighten
L = loosen
T1, L2
L1, L2
L1
T1
L2
T2,L1
T2, T1
T2
Mirror 2 Knobs
Photodetector
2
1
