Abstract

Photoinduced χ⁽²⁾ gratings can be written in optical fibers by simultaneous irradiation with beams at co and 2ω. Although this phenomenon is not understood in detail, the experimental results show that the presence of optically active centers in the fiber is essential for the observation of this effect. We have studied photoinduced SHG in rare earth doped aluminosilicate fibers. In addition to measurements at 1064 /532 nm with a mode-locked Nd–YAG laser, we have investigated the dependence of the induced second harmonic intensity on the wavelengths of the writing beams by using a mode-locked Ti–sapphire laser. In a Tm-doped fiber we find a ≈10³ increase in the induced SH intensity when the wavelength is tuned to the maximum of the ³H₄ → ³F₄ absorption of Tm³⁺ at 785 nm. These results show the importance of resonance enhancement of the nonlinear optical process that is responsible for χ⁽²⁾ grating formation. We will also present results for other rare earth ions and their implications for understanding the origin of SHG in optical fibers.

© 1992 Optical Society of America