= Digital optical phase conjugation
<Digital optical phase conjugation> uses a <wavefront sensor> and a <spatial light modulator> to measure an incident field and launch a beam with the opposite spatial <wave phase>. A phase-only <spatial light modulator> illuminated uniformly generally does not reproduce the incident <wave amplitude> as well; exact $\gamma U^*$ requires the appropriate <wave amplitude> and <polarization of an electromagnetic wave> too. The experimentally useful question is how much of the original optical mode can be recovered with the measured and controlled field components.
In 2010, <Meng Cui> and <Changhuei Yang> demonstrated <digital optical phase conjugation> through a prepared <multiple scattering> sample. They dried mixed-size <polystyrene> microspheres on cover glass, added immersion oil, and sealed them with another cover glass. A 532 nm <laser> supplied signal and reference beams. The signal crossed the sample between two objectives; the reference interfered with it on a <charge-coupled device>. Phase-stepped <wave interference> recovered the spatial <wave phase>; a calibrated <spatial light modulator> imposed its negative on the reference-derived return beam. The reference supplied playback power, without nonlinear mixing pumps. An <optical camera> read out the recovered signal. With the same sample and a zero-phase <spatial light modulator> control, the return instead produced a <scattering of light> pattern. This comparison supports mode recovery by spatial <wave phase> control, not merely increased illumination. A separate ordinary-mirror control tested source displacements. Finite sampling and collection limited recovery; the system controlled <wave phase> rather than the complete vector field. These restrictions distinguish demonstrated reconstruction from exact reversal of every scattered component.<Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation by Meng Cui and Changhuei Yang>[Cui and Yang, 2010, §§2–3, Figs. 1–4; §4.2, Fig. 6(e–f); §§5.1–5.2, Fig. 7(a–c), printed pp. 3446–3453]{ref}
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