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.

Pyrophosphate doped potassium dihydrogen phosphate (KDP) crystal was grown from aqueous solution by the temperature lowering method. Light scatter in KDP crystal was detected with the ultramicroscopic method. The light scatter in KDP crystal was aggravated when pyrophosphate was doped into the growth solution, which was distributed ununiformly in prism and pyramidal sectors of KDP crystal. Different effects of pyrophosphate on prism and pyramidal sectors of KDP crystal can explain this case. The transmission in this crystal was measured, showing that pyrophosphate affects the transmission evidently.

The gain of forward two-wave mixing (FTWM).and time response of a Cu-doped barium strontium potassium sodium niobate KNSBN.crystal are investigated at 514.5nm in the temperature range 20-1208C. It is found that the gain reaches its maximum value at about 908C. The effective photorefractive charge density decreases with temperature. The response and erasure rates increase with increasing temperature. Moreover, the erasure process can be described by a diexponential function. A band diagram of the two-center charge transport model is presented. The phenomena are explained qualitatively.

The characteristics of intracavity second-harmonic generation (SHG) at 1.06μm in GdCa4O (BO3) 3 (GdCOB) crystals cut for different type I phase-matching (PM) directions of (θ, φ)=(66.8°, 132.3°); (19.4°, 0°); (90°, 46°) have been investigated. It was found that the intracavity SHG was significantly efficient in the PM direction of (66.8°, 132.3°), and that the intrinsic lower effective nonlinear coefficient (deff) was responsible for the less-efficient SHG in the other two directions. A maximum CW SHG output power of 2.81W was obtained with an optical conversion efficiency of 18.7%, while the corresponding effective intracavity SHG efficiency was determined to be 41.3%. The intracavity SHG efficiency of GdCOB has been found to reach two-thirds of that obtained with type II phase-matching KTiOPO4 (KTP).

Potassium dihydrogen phosphate, KH2PO4 (KDP) crystals with different concentrations of quaternary ammonium salt in solution were grown. In this paper, unusual transmittance ratio properties of KDP are reported. The transmittance ratio is smaller than unity at some locations. These results indicate that the plane wave effect and optical activity are highly displaced in KDP crystals, and the effect is varies in three-dimensional positions.