Phase conjugation is an intriguing phenomenon. This process operates somewhat like holography, but it is a dynamic hologram, whose “holographic plate” is defined by interfering wave fronts in a nonlinear optical medium, rather than etched as a static pattern on a glass plate.
Phase conjugation is originally and classically understood in the optics literature. One can take pairs of laser beams precisely approaching in opposite directions. If they meet (and conjugate’) perfectly, at the center, they then Phase Conjugate. The key issue is in the meeting, with “opposing pairs” precision. Getting opposing exactly phase locked laser beams to align this way is difficult because alignment accuracy down to angstrom levels is needed.
Phase Conjugaton and Time Reversal
Phase conjugation can be distinguished from Time Reversal Signal Processing by the fact that phase conjugation uses an holographic or parametric pumping whereas time reversal records and re-emits the signal using transducers. (1) Holographic pumping makes the incident wave interact with a pump wave of the same frequency and records its amplitude-phase distribution. Then, a second pump wave reads the recorded signal and produces the conjugate wave. All those waves have the same frequency. In parametric pumping, the parameters of the medium are modulated by the pump wave at double frequency. The interaction of this perturbation with the incident wave will produce the conjugate wave. Both techniques allow an amplification of the conjugate wave compared to the incident wave. (Ibid)
As in time reversal, the wave re-emitted by a phase conjugation mirror will auto-compensate the phase distortion and auto-focus itself on its initial source, which can be a moving object. ((Ibid). Time reversal has a remarkable property of reflected wave fields. (3) Phase conjugation of wave field means the inversion of linear momentum and angular momentum of light. (3) Phase conjugation methods exist in two main domains: Acoustic phase conjugation. Optical phase conjugation
The example of Phase Conjugate Mirror
A phase conjugate mirror is like a mirror, in that it reflects incident light back towards where it came from, but it does so in a different way than a regular mirror. In a regular mirror, light that strikes the mirror normal to its surface, is reflected straight back the way it came. This is also true of a phase conjugate mirror. When the light strikes a normal mirror at an angle, it reflects back in the opposite direction, such that the angle of incidence is equal to the angle of reflection.
(2). Okulov, A Yu (2008). “Angular momentum of photons and phase conjugation”. Journal of Physics B: Atomic, Molecular and Optical Physics. 41 (10): 101001. arXiv:0801.2675. Bibcode:2008JPhB…41j1001O. doi:10.1088/0953-4075/41/10/101001. ISSN 0953-4075
(3). A.Yu. Okulov, “Optical and Sound Helical structures in a Mandelstam-Brillouin mirror“. JETP Lett, v.88, n.8, pp. 561-566 (2008)]