  In this article, we present a deflection measurement setup for Atomic Force
Microscopy (AFM). It is based on a quadrature phase differential
interferometer: we measure the optical path difference between a laser beam
reflecting above the cantilever tip and a reference beam reflecting on the
static base of the sensor. A design with very low environmental susceptibility
and another allowing calibrated measurements on a wide spectral range are
described. Both enable a very high resolution (down to
$\SI{2.5E-15}{m/\sqrt{Hz}}$), illustrated by thermal noise measurements on AFM
cantilevers. They present an excellent long-term stability, and a constant
sensitivity independent of the optical phase of the interferometer. A quick
review shows that our precision is equaling or out-performing the best results
reported in the literature, but for a much larger deflection range, up to a few
$\SI{}{\micro m}$.
