Interferometers for measuring changes in optical beam direction

Apparatus for measuring angular changes in the direction of travel of a light beam, said apparatus comprising:

• at least one optical assembly for introducing a relative phase shift between the components of the beam, said relative phase shift varying in accordance with an angular change in the travel of at least one of the components;
• a detector for receiving said phase shifted beam components and generating an electrical signal having a phase that varies in accordance with the changes in said relative phase shift, and
• electronic means for receiving said electrical signal, determining said relative phase therefrom, and converting said relative phase shift to the angular change in the direction of travel of the at least one component.

A beam shearing apparatus for introducing a lateral shear between the components of a light beam, said apparatus comprising a polarizing beamsplitter cube having a polarizing interface and input and output facets and two mirrors one of which is arranged at an angle generally opposite said input facet and the other of which is arranged at an angle generally opposite said output facet.