Theoretical solution of second-harmonic generation in periodically poled lithium niobate and chirped periodically poled lithium niobate thin film via quasi-phase-matching

We present an effective nonlinear coupled mode coefficient model to evaluate and analyze the second harmonic generation (SHG) process under the quasi-phase-matching (QPM) scheme in periodically poled lithium niobate (PPLN) thin film. The theoretical model derives directly from the nonlinear coupled mode equations and simplifies the QPM process in PPLN thin film. The simulated results agree excellently with the rigorous numerical calculations in a variety of situations. We also propose using the chirped PPLN (CPPLN) thin film to achieve the SHG of ultrashort laser pulse and develop a broadband nonlinear coupled mode theory to evaluate this complex nonlinear coupling interaction process. All the frequency components in fundamental wave (FW) and second harmonic wave (SHW) have been considered in the theory, and the spectral evolution of FW and SHW can be obtained by solving a series of nonlinear coupled mode equations. CPPLN thin film supports broadband up-conversion in telecommunications bands and can generate broadband entangled photons in infrared region, which shows great potential in optical communication and quantum optics. The developed theories provide an efficient tool to design and optimize the PPLN and CPPLN thin film for various applications based on nonlinear frequency conversion and help to achieve high-efficiency on-chip SHG of the ultrashort laser pulse.