“…Assuming the reference beam and the readout beam are nearly counterpropagating, as the object image is spatially shifted inside the object plane, the new gratings written by the object and the reference waves are always nearly satisfying the Bragg conditions for the fixed readout beam. Rela tively large shift invariance is expected in the object plane, which has been experimentally demonstrated by Nicholson et al 7 For the reference plane, how ever, the shift invariance becomes much more strin gent. This is because, when the reference image is shifted horizontally inside the reference plane, the newly formed gratings tilt away from the fixed read out beam, introducing a phase mismatch in the diffraction process.…”