Factors limiting second harmonic generation efficiency in optically poled fibers

We investigated the efficiency limitations of second-harmonic generation (SHG) in spliced optically poled Corning HI980 fiber segments. Although theory predicts quadratic growth with segment number, experiments show nearly linear efficiency scaling. Using a continuous wave model (CW), we demonstrate this subquadratic behavior primarily stems from random longitudinal shifts between quasi-phase-matching (QPM) regions in spliced segments. Further investigations through coupled generalized nonlinear Schrödinger equations (coupled GNLSEs) confirm fundamental frequency (FF) power depletion through Raman scattering and spectral broadening during propagation. Our numerical simulations successfully reproduced experimental spectral measurements, validating the model's accuracy. For the first time, we report the effective quadratic nonlinear coefficient induced by optical poling in this fiber: deff = 9 × 10-4 pm/V.