Threshold analysis for transverse instabilities in photorefractive cou
nterpropagating two-wave mixing through reflection gratings is perform
ed. A numerical algorithm for the treatment of wave equations in this
geometry is developed, displaying the emergence of running transverse
waves. They appear above a threshold in the applied electric field, an
d their transverse wave number and oscillation frequency agree well wi
th the values predicted by stability analysis. (C) 1991 Optical Societ
y of America.