Optical Phase Conjugation in Pure and Fe Doped $LiNbO_3$

Light induced refractive index modulations in photorefractive crystals can be used to advantage to obtain optical phase conjugation (OFT) through Degenerate Four Wave Mixing (DFWM). The applications of OPC in various adaptive optical systems are well known. Among the of photorefractive crystals, $LiNBO_3$ has been chosen for the present studies on account of its greater spontaneous polarization and higher refractive index modulation constant.In this paper, we report the studies in pure $LiNb0_3$ and Fe (0.02%) doped sample, using 488 nm, which is towards the absorption edge, whereas the earlier reports were with 514.5 nm. The formation and erasure times of the phase grating and the phase conjugate signal are measured using 488 nm wavelength with the pump beams of power 8 mW and probe beam of power 1.2 mW. In the Fe doped crystal, the formation and decay of the phase grating follow a near exponential pattern. The phase conjugate signal also behaves similarly with a formation time of 840sec and a decay time of 1200 sec. These results are in agreement with the previously predicted values by other workers. In pure $LiNbo_3$ crystal, though the decay of the phase grating follows the exponential form, with a decay time of 2000 sec, the formation of the phase grating is observed to follow a rather different pattern. After a gradual increase in the diffraction efficiency in the first 600 sec, there is a fall during the next 200 sec and an increase thereafter and subsequent fall. The reasons for this type of behaviour is explained on the basis of a dynamic approach to the volume grating that is formed