The optical frequency comb generation schemes mainly include mode-locked laser, electro-optic modulation comb, nonlinear Kerr micro-resonator comb and nonlinear supercontinuum comb. For the nonlinear supercontinuum comb scheme, the silica-based high nonlinear fiber with near-zero flattened normal dispersion is required. However the fiber dispersion has variation along the fiber owing to the fabrication inaccuracy. Furthermore, nonlinear supercontinuum comb generation based on the nonlinear fiber has not been systematically studied. In this paper, an optimal design of four-clad flat normal dispersion high nonlinear silica fiber with a triangular core refractive index distribution for the flat optical frequency comb generation is carried out. The effects of the fiber cladding width and refractive index on the fiber dispersion characteristics and cut-off wavelength were studied through using the finite element method mode solver. The optimally designed fiber can obtain relatively flat near-zero normal dispersion in the wavelength range of 1400~1700 nm, the dispersion range is -3~0<i>ps</i>/(<i>km·nm</i>), and the dispersion slope is close to 0 near 1550 nm. The effective mode field area of the nonlinear silica fiber is about 11<i>μm</i><sup>2</sup>, and the nonlinear coefficient can reach 12.8<i>W</i><sup>-1</sup>·<i>km</i><sup>-1</sup>.Based on the electro-optic modulation pulse pumping the flat normal dispersion high nonlinear silica fiber, the flat optical frequency comb generation is systematically simulated with the generalized nonlinear Schrödinger equation. The time-frequency evolutions of a hyperbolic secant pulse, a Gaussian pulse and a super Gaussian pulse are simulated using the X-Frog technology. The time-frequency spectrograms connect the time and frequency domains of the pulse, which clearly shows the change of pulse chirp during the propagation. The effect of various parameters on the optical frequency comb are studied, such as the fiber length, second-order dispersion, third-order dispersion, pulse peak power, pulse half width, pulse initial chirp, and pulse shape. An optical frequency comb with 3dB flatness and bandwidth of about 40nm can be achieved based on hyperbolic secant pulse or Gaussian pulse pumping. Comparing with the hyperbolic secant pulse and Gaussian pulse, the super Gaussian pulse can produce a flatter optical frequency comb. An optical frequency comb with 2dB flatness and bandwidth of about 92nm can be achieved based on the super Gaussian pulse pumping. Therefore, based on the proposed normal dispersion high nonlinear fiber, it is possible to realize an optical frequency comb with repetition rate above 10GHz, power flatness within 3dB, and spectral bandwidth of about 40~90 nm. The simulation results are beneficial to promote the localization of normal dispersion high nonlinear silica fiber and its application in flat optical frequency comb.
