Contradirectional two-wave mixing (CTWM) is demonstrated to be a perfect configuration for low-noise and high-gain image amplification. Compared with the image amplified by the forward two-wave mixing configuration, the signal-to-noise ratio is increased from 6 to 23104 in the experiments in Rh-doped BaTiO3 crystal. The gain of the signal beam is also enhanced effectively. We have contrasted the differences between the two experimental configurations and gave a credible explanation for the low noise and high gain that exists in the CTWM configuration.

This paper presents the experimental results of twowave mixing and self-pumped phase conjugation (SPPC) in barium titanate (BaTiO3) crystals doped with Rh or Ce. It has been shown that the main parameters of SPPC depend very strongly on the concentrations of dopants, the incident angles and the crystal cut used. High gain in two-wave mixing corresponds to low phase-conjugate reflectivity in all samples. It is impossible to maintain the highest reflectivity and gain simultaneously, because the gain will definitely be limited when we try to get the highest reflectivity, and vice versa. Some qualitative explanation of the experimental results is also given.

Potassium dihydrogen phosphate (KDP) crystals with different concentrations of H3BO3 in solution were grown. Light scatter was detected with ultramicroscopic method. The distinct characteristics of biaxial crystals caused by in-herent stress can be found by using cone-shaped interference in the samples. In this paper, abnormal transmittance ratio properties are reported in doped H3BO3 KDP crystals.

Expressions of the coupling constant for contradirectional two-wave mixing (CTWM) have been derived. The calculations show that the energy transfer in CTWM has some unique characteristics different from the forward two-wave mixing (FTWM), in which the energy transfer direction is dependent on the two beam orientations with respect to the crystal polar axis and the sign of the light-induced carriers as well as the electro-optic coefficients. As a result, the CTWM can have a reverse energy transfer direction to the FTWM under a certain condition. The experimental results in two typical crystals of Rh-doped BaTiO, and in Ce-doped strontium barium niobate (SBN: Ce) demonstrate the theoretical investigations very well.

KDP crystals with di!erent concentrations of EDTA in solution were grown. Light scatter was detected with ultramicroscopic method. It has been proven that light scatter in KDP crystal aggravates with the increasing concentration of EDTA in the growth solution. The increase with light scatter occurs since EDTA inhibits the growth of KDP crystal at high concentration, which results in the formation of mother liquid inclusions, which in turn causes light scatter. The density, size and shape of light scatter are varied within di!erent sectors of the KDP crystal. The mechanism of the e!ect is discussed.