Harmonic conversion efficiency conditions

Figure 6.17. Harmonic conversion efficiency in a waveguide under phase-matched conditions versus path length for various values of ot a 10-mW input power and for the optimum value of the overlap integral as explained...

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. 

Three features of this laser source merit further discussion. First, in a typical kinetic experiment, the 1/e chemical lifetime of the photolytically produced radicals varies between 0.2 and 25 ms, a representative mean being ti/e = 2 ms. For statistical reasons, one desires to collect a minimum of 20 concentration versus time data points per 1/e concentration decay period. For multichannel scaling detection, these typical kinetic conditions imply a maximum dwell period per channel of 100 ys. The ultraviolet laser source described above emits 2.5 x 10 pulses per 100 ys interval thus, relative to chemical decays, this rapidly pulsed source is viewed by the experiment as a cw excitation probe. Second, given that a pulsed initiation/cw detection kinetics configuration is desired, one may ask why a cw laser source is not used. The rationale here is that the visible-to-ultraviolet conversion efficiency is much higher when the quasi-cw source rather than a cw source is used. Frequency doubling efficiency varies in proportion to the fundamental peak power density present in the second harmonic generation crystal,... 

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