Enhancement of two-wave coupling in a Ce : KNSBN crystal with optimum polarization of the writing beams

We have improved the two-wave coupling amplification and the signal-to-nois e ratio of an amplified signal with photorefractive cerium-doped potassium sodium strontium barium niobate (Ce:KNSBN) by employing optimum polarizatio n orientation of the pump beam while the signal beam retains extraordinary polarization. The optimum polarization angle of the pump beam was found exp erimentally to be 30 degrees with respect to the extraordinary polarization direction in a symmetrically incident system. Nearly 1.9-times enhancement of the gain and nearly 3-times enhancement of the signal-to-noise ratio we re achieved at a signal-to-pump beam intensity ratio of 1:40. At a beam int ensity ratio of 1:2000, a two-wave coupling gain of nearly 500 was obtained , which is much higher than the gain previously reported. Modified coupled- wave equations that involve beam fanning with certain assumptions concernin g the fanning parameters were used. The numerical solution is in good agree ment with the experimental data. The results are compared with those obtain ed with a 45 degrees -cut BaTiO3 crystal. (C) 2001 Optical Society of Ameri ca.