Abstract: By applying the perturbation approximation method to the dispersion equation of Double Metal-cladding medium waveguide,the characteristics that the ultra-high order guided mode under N→0 is unrelated to its polarization can be obtained when the thickness of wave-guided layer is of the order of sub-millimeter.Then the sensitivity of waveguide was defined,and got its relationship with effective index,thickness,wavelength and dielectric constant.When the effective index approaches to zero,the ultra...