Based on the quasi - phase matching theory (QPM), when the nonlinear optical crystal lithium niobate (LN) is fabricated into a periodically reversed domain structure, it is called a periodically poled lithium niobate crystal (PPLN). This structure can compensate for the phase mismatch caused by dispersion, thereby maximizing the utilization of the effective nonlinear coefficient of the nonlinear optical crystal and significantly enhancing the frequency conversion characteristics of the nonlinear crystal.
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