The advent of multiwavelength lasers has stimulated considerable interest in wavelength selection, i.e., methods of selecting for various applications one or more of the available wavelengths from a given laser. This paper describes a technique for parallel wavelength separation or recombination of laser beams external to laser cavities. The device used is based on the optical activity of quartz crystals and on the polarization dependent reflection of birefringent calcite crystals.
T. J. Harris and T. Y. Tao, "Laser Wavelength Selection," Appl. Opt. 7, 2226-2231 (1968)