In this paper we report a new method for double phase conjugation particularly suited to the tungsten bronze crystal strontium barium niobate. It has also been observed to produce conjugate waves in BaTiO3 and BSKNN. This new arrangement is called the bridge conjugator because the two beams enter opposing  crystal faces and fan together to form a bridge without reflection off a crystal face. Our measurements indicate that the bridge conjugator is competitive with previously reported double phase conjugate mirrors in reflectivity, response time, ease of alignment, and fidelity.
Edward J. Sharp, William W. Clark III, Mary J. Miller, Gary L. Wood, Brian Monson, Gregory J. Salamo, and Ratnakar R. Neurgaonkar, "Double phase conjugation in tungsten bronze crystals," Appl. Opt. 29, 743-749 (1990)