Fig. 3. Wavelength modulation for high sensitivity with small (A–C) and large (D–F) cantilever deflection. (A and D) Interferometer- (black) and wavelength-modulation amplitude from the lock-in amplifier (red) show constant visibility over large cantilever deflections with a π/2 phase-shift. (B and E) Scaled interferometer and lock-in signals give a direct relation to cantilever deflection as the angle through the circle given by interferometer and lock-in signal (phase in rad). The unwrapped angle shows a linear relation to the deflection with high sensitivity over small (C) and large (F) deflections.