One of the problems that plague any kind of optical experiment is that generally, we're sending light through "stuff." Whether it's a fiber or free space, it's normally not a vacuum which means that it has some index of refraction that changes with it's environment. What this means for light is that it's effective path length changes and therefore the phase of the light beam from say, a laser to a detector fluctuates. For experiments which rely on interference between separate light beams it only takes a variation in optical path lengths of a wavelength or so to create a problem, or more accurately - a total disaster.
In my presentation I'll describe a situation that arose in our laboratory in which a small gust of air or a slight hum in a neighboring lab gave us enough phase fluctuation to cause an excedrin sized head-ache. Also, I'll describe how using a simple circuit and a piezo-electric mirror, we cured that head-ache and accomplished phase stability, even in the presence of path-length fluctuations. Next I'll describe how a similar problem could be addressed in a variety of different experimental situations. I'll conclude with some pretty pictures of the system in question.