Second-harmonic (SH) generation in optical fibers was observed1 and has been extensively studied recently.2-4 A model was proposed and a test experiment was performed as described by tylizrabhi, Hibino, and Stegeman.5 We observed the same grating in bulk glasses. A cw mode-locked, Q-switched Nd:YAG laser had a 100-ps-pulse duration and 6-kHz repetition. IR radiation was partly transformed to SH in KTP crystal. Both beams were focused into the glass sample by a 9-mm lens. The preparation stage took only a few minutes. To read the grating by IR radiation, the SH was stopped by a filter.
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