text: Tunable examples of a type of these devices include the Berek compensator and the Soleil-Babinet compensator.
full_question: Tunable examples of a type of these devices include the Berek compensator and the Soleil-Babinet compensator. Spatial hole burning in continuous wave lasers can be avoided by placing two of these devices on either side of the gain medium, a process called a "twisted-mode technique." An alternative to one type of these devices is the Fresnel rhomb. The amount of retardation is a multiple of two pi larger in a "multiple-order" one of these devices than in a "zero-order" one of these devices. In one type of these devices, the waves propagating parallel to the "fast axis" acquire a phase shift of pi over 2 with respect to waves propagating parallel to the "slow axis," because the index of refraction is split into "ordinary" and "extraordinary" components. A birefringent crystal which converts linearly polarized light to circularly polarized light is an example of, for 10 points, what type of optical device which comes in "half" and "quarter" types?
first_sentence: Tunable examples of a type of these devices include the Berek compensator and the Soleil-Babinet compensator.
raw_answer: waveplates [or quarter-wave plates; or half-wave plates; or phase retarders or retardation plates until "retardation" is read; prompt on "polarizers" until "polarized" is read; prompt on "birefringent crystals" until it is read]