def V_d2bV_by_V_Approx(V):
    """
    Approximate V*d^2(bV)/dV^2 for single mode fiber.

    This value is needed to determine the waveguide dispersion.  This
    approximation is for the fundamental mode in the fiber and is good
    to 1% when 1.4<V<2.4.  Approximation by Marcuse (1979)

    Args:
        V:      V-parameter of the fiber     [--]
    Returns:
        V*d^2(bV)/dV^2                       [--]
    """
    return 0.080 + 0.549 * (2.834 - V)**2