Numerical analysis of nonlinear optical fiber couplers with a gaussian mode field distribution

Nonlinear coupling characteristics taking account of evanescent field overlap were evaluated numerically by assuming a Gaussian field distribution for the mode field in nonlinear directional couplers (NLDC). The nonlinear coefficients Q3 to Q5 were found to be Q3 = 3.7 × 10−12 cm−1, Q4 = Q5 = 0 for the plane wave approximation used in Jensen's paper [1], with a core radius of 4 μm, a core spacing of 5 μm, a core refractive index of 1.455, and a nonlinear refractive index of 1.1 × 10−12 esu; and the wavelength of the incident light was 1.06 μm. On the other hand, in the case of the Gaussian field approximation, Q3 = 2.5 × 10−12 cm−1, Q4 = 2.2 × 10−14 cm−1, and Q5 = 7.3 × 10−14 cm−1 were obtained, indicating that these values cannot be ignored. The full coupling period is increased by increasing the core spacing, and the full coupling period is decreased by using a longer wavelength. Further, from the dependence of nonlinear coupling characteristics on optical wavelength, it is confirmed that wavelength selective characteristics with a sharp bandpass filtering can be realized in NLDC.