Applications phase conjugation

Optical phase conjugation allows the reversal of the phase of the electromagnetic wave associated with a beam of light. It has applications in real-time adaptive optics where it is used to correct aberrations, in image processing and in optical computing [10-12]. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Time-Resolved Measurements. The time-dependence of the third-order nonlinearity is an important characteristic for device applications and may also indicate the nature of the process causing the nonlinearity (24), The temporal behavior of the phase conjugate signal was monitored by delaying the probe beam with respect to the pump. 

Ferroelectric liquid crystals are becoming increasingly promising for SHG devices and Pockels modulators [117]. The second harmonic intensity may be modulated [118]. A review of advanced liquid crystal polymers is given in Dubois [119]. Potential applications also include phase conjugation and real-time holography. 

On the other hand, in the ntid-infrared regime (1-5 pm) nematics are less absorptive typically the absorption contrast is about 10 cm or less, and thus longer interaction lengths are possible. Since there are few competitive materials (in terms of the large nonlinearity and low laser power threshold requirement) in these spectral regimes, nematic liquid crystals will be the natural choice for phase conjugation related applications. 

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