An apparatus is described for controlling the optical path length in an optical
device, e.g. an interferometer, and more importantly to maintaining the optical
path length difference in an interferometer. The apparatus may include an adjustable
plate optically coupled with a beamsplitter. The plate may be rotated such that
its surface receives light propagated from the beamsplitter at a non-zero incident
angle. In one embodiment, temperature sensitivity is addressed by ensuring that
the refractive index of the plate is greater than the refractive index of the beamsplitter.
In another embodiment, the apparatus includes combination spacers having a component
selected in dependence upon a composition, thickness, and orientation of the adjustable plate.
