docs: 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)