Return a pupil-plane Gaussian mode with 1/e diameter given by 
    *modeDiameter*, normalised so that integral power over the mode is unity

def FibreMode(gridSize,modeDiameter):
    """
    Return a pupil-plane Gaussian mode with 1/e diameter given by 
    *modeDiameter*, normalised so that integral power over the mode is unity
    """
    rmode=modeDiameter/2
    return np.exp(-(RadiusGrid(gridSize)/rmode)**2)/(np.sqrt(np.pi/2)*rmode)