def MFR(V):
    """
    Approximate the mode field radius for a step-index single mode fiber.

    The approximation is fairly accurate for V>1. In the multimode range
    (V > 2.405), it applies to the fundamental mode.

    D. Marcuse, "Loss analysis of single-mode fiber splices", Bell Syst.
    Tech. J., 56, 703 (1977)

    Args:
        V:      V-parameter of the fiber                            [--]
    Returns:
        approximate mode field radius normalized by the core radius [--]
    """
    return 0.65 + 1.619 * V ** -1.5 + 2.879 * V ** -6
