Fig. 3.

Explanation of the working principle of the autofocus. d₁ to d₄ are distinct axial distances, indicating a family of QPD(V) vs current curves. Panels show how the state of the system (QPD(V), current) changes in response to an axial displacement from an in-focus condition d₂ (Panel A) to a position d₃. Panel (C) illustrates how, in order to bring back the QPD(V) to its original set-point, a current is applied. In Panel (D) the adaptive set-point changes along the in-focus dashed curve, while the system state moves along the d₃ QPD(V)-current curve trying to reach the new set-point. The process iterates in small steps (Panel E) until the sample reaches again the in-focus condition where the set-point and system states overlap (Panel F).