This research aims to study and design a single mode optical fiber using simulation, with a core and glass cladding, and also helps in ensuring the efficiency to use the optical fiber designed within the broading window C-Band,and that these researches and studies tries to use this window. This research also shares to set the light on the Numerical Programmes and Simulation used now in studying and designing these optical fibers. At first we used the programOptifiberto reach the confidence of Sellmeier equation for the fiber core, and this completed by using the finite element method solving the equation of propagation the electric field and finding its 3-D distribution, using the famous simulating COMSOLMultiphysics. This completed by putting the program MATLAB to reach the two equation: the total dispersion and attenuation for the designed optical fiber.