Editing Retroreflector (section)

===Phase-conjugate mirror===
A third, much less common way of producing a retroreflector is to use the [[nonlinear optics|nonlinear optical]] phenomenon of [[phase conjugation]]. This technique is used in advanced [[optics|optical]] systems such as high-power [[laser]]s and [[optical fibre|optical transmission lines]]. Phase-conjugate mirrors<ref>{{cite journal | last= Gower | first= M.C. | title= The Physics of Phase Conjugate Mirrors | journal= Progress in Quantum Electronics | publisher= Elsevier B.V. | issue= 2 | date=1984 | volume= 9 | issn= 0079-6727 | doi = 10.1016/0079-6727(84)90023-5 | pages=101–147| bibcode= 1984PQE.....9..101G }}</ref> reflect an incoming wave so that the reflected wave exactly follows the path it has previously taken, and require a comparatively expensive and complex apparatus, as well as large quantities of power (as nonlinear optical processes can be efficient only at high enough intensities). However, phase-conjugate mirrors have an inherently much greater accuracy in the direction of the retroreflection, which in passive elements is limited by the mechanical accuracy of the construction.