The prismatic phase shifter is a novel device that can be used to control the phase of a coherent beam of light. Phase shifting is becoming an integral function of most modern interferometric systems. Phase shifting can be accomplished by use of any of a variety of older devices including moving mirrors, rotating quarter-wave plates, moving gratings, and Bragg cells, all of which have disadvantages. Surface losses and/or changes in polarization are induced by these various devices. In the case of a moving mirror, subwavelength translation can be difficult. Some of the other older devices produce lateral shifts of the beam.

The prismatic phase shifter introduces an easily controlled change of phase to a beam of light without the disadvantages of the older devices. This device consists of a prism mounted on a translation stage. The prism is placed in the beam oriented at the angle of minimum deviation. The orientation of the stage is chosen so that translation causes the prism to move along its plane of symmetry.

Phase Shift Is Introduced into the beam of light when the prism is moved along its plane of symmetry.

The figure shows the prism before (dashed) and after (solid) translation. Because of the symmetry of the configuration, the beam is not deviated or translated, and a phase shift is introduced to the beam. For decreasing apex angle, increased translation is needed to obtain a given phase shift. This effect is termed "optical leverage" and makes it possible to obtain small phase changes with relatively coarse mechanical adjustments.

The change in effective optical-path distance, ΔOPD, is given by

ΔOPD = 2 (nℓNL)

where n and N are the indices of refraction of the prism and the surrounding medium, respectively; is half the difference between the old and new distances along the optical path inside the prism; and L is half the difference between the new and old distances along the optical path outside the prism. By the law of sines,

where Y is the translation distance, α is the apex angle of the prism, and θ is the angle of incidence for minimum deviation. This angle is given by

Furthermore, is given by

Therefore, ΔOPD is given by

When this device is operated at Brewster's angle, it imposes no surface losses. The only requirement the prism material must satisfy is that it be transparent at the wavelength of interest and be of reasonable optical quality. Because translation of the prism to produce change of phase does not cause deviation or translation of the beam, the device can be used in a system in which strict alignment must be maintained.

This work was done by Brooks A. Childers of Langley Research Center. LAR-14637