Abstract: By applying the perturbation approximation method to dispersion equation of symmetrical metal-Cladding dielectric medium waveguide,the author finds that when the thickness of waveguide layer decreases to sub millimeter range,the effective index will approach  zero,and the ultrahigh-order modes are no longer related to polarization.Then the author defines  the sensitivity of waveguide,and gets its relationship with effective index,thickness,wavelength and dielectric constant.When the effective index approaches zero,the ultrahigh-order modes will have a super high sensitivity.This theory is of certain directive significance to biosensor and photoelectronic device.

Key words: optical waveguides；the dispersion equation；effective refractive index, sensitvity