A simple method to dramatically enhance the optical magnetoelectric (ME) effect is proposed and demonstrated for a polar ferrimagnet GaFeO₃. We patterned a simple grating with a period of 4 µm on a surface of GaFeO₃ crystal and used the diffracted light as a probe. The optical ME modulation signal for the Bragg spot of the order n=1 reaches 1-2% of the bare diffracted light intensity in a magnetic field of 500 Oe, which is amplified by more than 3 orders of magnitude compared to that for the reflection of bulk GaFeO₃.