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UV Poling 531 nonlinearity by optical poling and that the resultant dc field is proportional to the internal field. The dominant mechanism behind optical poling was described by the coher- ent photovoltaic effect introduced in reference [14]. Spatially periodic charge ejection results from the photoionization of defects caused by both two photons of fundamental and one photon of second harmonic light, which create an asymmetric photocurrent. The charges released drift in the gradient of the opti- cal field until they are trapped. In fibers, trapping takes place preferentially at the core­cladding interface. The dark conductivity of the glass limits charge accumulation, and the preparation process saturates. A large number of subsequent studies confirmed the original description of the coherent photovol- taic mechanism. One of the issues resolved was whether the required periodic field was created by aligned dipoles or by periodic charge accumulation. The latter was found to be dominant [15]. A frequency doubling conversion efficiency of a few percentage points has been achieved in Ge-doped silica fibers from 1.064 mm to 0.532 mm [4,13], but is only possible at high pump intensities. Phase mismatch at lower powers and opti- cal erasure at high intensities set a limit to the efficiency of the process. The recorded electric field amounts to $10 5 V/m, and the nonlinear coefficient, w (2) induced is limited to $10 À3 pm/V. Although optical poling has been extended to other centrosymmetric materials (e.g., plastics), few practical applications of