Waveguides, nonlinear interactions

In the case of SHG in waveguide nonlinear crystals, we describe a theoretical model which accounts for the temporal behavior of the interacting pulses and the possible z-dependence of the phasematching condition. The model also describes the observed saturation and subsequent decrease in SHG conversion efficiency in the waveguide samples, as a result of two-photon absorption (TPA) of the second harmonic (SH) wave. The results of this model are later compared with experimental data from SHG experiments using femtosecond pulses in the waveguide nonlinear crystals of periodically-poled potassium titanyl phosphate (ppKTP) and appKTP. This model is presented in section 2.3. 

We will later use Eq. (13) for analyzing the experiments of SHG in ppKTP and appKTP waveguides (described in section 7). For now, we compare the analytical solution of Eq. (13) with the direct numerical solution of the differential equations in Eq. (14), which account for the temporal behavior of the interacting pulses, the nonlinear losses of the SH wave, and the possible z-dependence of the phasematching condition ... 

All of our natural experience with optics occurs in the linear domain. In order to apply nonlinear optics in practice, light must first interact with the NLO material. In our laboratories, free space interconnections are usually employed for this purpose. That is, a laser beam is aimed at the material under examination. In any practical use of NLO, such simplistic solutions will not be possible, for reasons both of safety and rugged construction of the device. Light will need to be moved around in space within the device. In many second order devices, whether they are color-specific lasers, such as doubled diode or YAG lasers, or EO modulators such as spatial light modulators (SLM s) waveguide or fiber optic connections will be used. Aspects of these materials will not be reviewed. 

It is also necessary that there be an effective nonlinear coefiScient that couples the two modes. As I indicated earlier, poled polymer films and X- or Z-oriented LB films should have large fxx values that are optimum for interactions involving TM modes which contain components. By using multilayer waveguides, it is possible to devise approaches that avoid the spatial concentration of field distributions between low-order modes. One such approach is illustrated in Figure 6.18, with X- or Z-deposited LB films. In this figure, the polar orientation is reversed at the node of the TM mode. 

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