  A shearing interferometer is presented which uses polarization control to
shear the wavefront and to modulate the interference pattern. The shear is
generated by spatial walk-off in a birefringent crystal. By adjusting the
orientation of the birefringent crystal, the components of the wavefront
gradient can be independently measured to allow determination of the full
wavefront vector gradient as well as reconstruction of the wavefront. Further,
the monolithic nature of the crystal used for shearing allows the
interferometer to be setup without need for precise alignment of any
components. An algorithm incorporating homodyne detection is presented which
analyzes the modulated interferograms to determine the components of the
wavefront gradient, from which the wavefront is reconstructed. The thermal
deformation of a mirror subject to heating from absorption of a Gaussian pump
beam was accurately observed with a sensitivity better than \lambda/160. We
show that this sensitivity is scale invariant, and present a method to account
for the non-uniform spatial frequency response of the interferometer.
