  We present a dual-wavelength external holographic microscopy module for
quantitative phase imaging of 3D structures with extended thickness range. This
is done by simultaneous acquisition of two off-axis interferograms, each of
which at a different wavelength, and generation of a synthetic wavelength,
which is larger than the sample optical thickness, allowing two-wavelength
unwrapping. The simultaneous acquisition is carried out by using optical
multiplexing of the two interferograms onto the camera, where each of them has
orthogonal off-axis interference fringe direction in relation to the other one.
We used the system to quantitatively image a 7.96 um step target and 30.5 um
circular copper pillars.
