  The two output signals of quadrature phase interferometers allow to benefit
both from the high sensitivity of interferometry (working inside a fringe) and
from an extended input range (counting fringes). Their calibration to reach a
linear output is traditionally performed using Heydemann's correction, which
involves fitting one output versus the other by an ellipse. Here we present two
alternative methods based on the linear response of the measurement to a
sinusoidal input in time, which enables a direct calibration with an excellent
linearity. A ten fold improvement with respect to the usual technique is
demonstrated on an optical interferometer measuring the deflection of scanning
force microscopy cantilevers.
