A design concept has been proposed for an optomechanical apparatus that would implement a variable optical delay line with a fixed angle between its input and output light beams. The apparatus would satisfy requirements that emphasize performance in interferometric applications: to contain a minimum number of optical surfaces, each used at low angle-of-incidence, and to be nominally free of shear (transverse motion of the beam) on any optical element. As an additional advantage, the apparatus would afford partial compensation of vibration disturbances associated with adjustment of the optical delay by both reducing the amount of motion required to achieve a desired optical delay and by splitting the total motion between two assemblies. As compared to prior art implementations of delay lines, the only disadvantage of the concept is that the motions of the optical elements must be well coordinated through mechanical linkages or electronic controls. The optical elements would be two flat mirrors -- M1 and M2 -- mounted on linear actuators. The actuation axes of M1 and M2 would be parallel to the incoming and outgoing light beams, respectively. M1 would be mounted on its actuator at a fixed angle required to aim the beam reflected from it to the center of M2. In turn, M2 would be mounted on its actuator at a fixed angle required to aim the outgoing beam in the desired direction. Moreover, the angles of M1 and M2 would be chosen so that the angle between M1 and the incoming beam equals the angle between M2 and the outgoing beam.
