An improved optical polarizer has been proposed for use with a high-energy pulsed laser. This polarizer would separate the p- and s-polarized components (the components polarized parallel and perpendicular, respectively, to a designated plane or axis) of an incoming beam of light into two outgoing beams propagating at different angles. This device would offer low loss (<1 percent), high extinction ratio (between 104 and 105), and a high threshold for laser-induced damage - a combination of features that, heretofore, has not been available in a single polarizer.
The polarizer would comprise two sapphire prisms fabricated and arranged as shown in the figure to exploit birefringence to effect angular separation of the two outgoing beams. The optical axis of the left prism would be perpendicular to the plane of the figure, while that of the right prism would lie in the plane of the figure and nearly perpendicular to the direction of the incoming beam. As a result of the birefringence of sapphire and the different orientations of the optical axes of the two prisms, the different polarization components of the incident light beam would be refracted to different angles at the diagonal interface between the prisms.
In a typical application, it is required to have one of the outgoing beams undeviated (that is parallel to the incoming beam). To satisfy this requirement, the prisms would be wedged: that is, the entrance and exit faces would not be parallel to each other and, instead, would be slanted by a small angle chosen so that, for one polarization component, the angular deviation caused by refraction at these faces would cancel the angular deviation caused by the birefringence. The angular deviation caused by refraction at these faces would be only slightly dependent on wavelength: that is, the device would be nearly achromatic. By eliminating surfaces orthogonal to the incoming beam, the wedging of the prism would also minimize optical feedback to the laser.
In other, similar prism assemblies, it is common practice to minimize reflection losses at the mating surfaces of the two prisms by use of optical cement. Because optical cement is susceptible to damage by high-power laser beams, it would not be used here: Instead, optical contacting - essentially, molecular adhesion of ultraclean, highly polished surfaces - would be used to minimize reflection losses and mechanically join the two prisms. Reflection losses would be reduced further by antireflection coating of the entrance and exit faces.
This work was done by Norman P. Barnes of Langley Research Center. For further information, contact the Langley Commercial Technology Office at (757) 864-6005.