  We present a computer-based active interferometer stabilization method that
can be set to an arbitrary phase difference and does not rely on modulation of
the interfering beams. The scheme utilizes two orthogonal modes propagating
through the interferometer with a constant phase difference between them to
extract a common phase and generate a linear feedback signal. Switching times
of 50ms over a range of 0 to 6 pi radians at 632.8nm are experimentally
demonstrated. The phase can be stabilized up to several days to within 3
degrees.
