An optical glass slab, which is coated with two metal films with different thickness on both sides, is used to form a new type of ultrathick double metal-cladding optical waveguide with submillimeter-scale. Excitation of the ultrahigh-order modes are implemented by employing the free-space coupling technique. Based on the unusual features of the ultrahigh-order modes, wavelength-sensitive and polarization-insensitive, ultra-narrow bandwidth filtering (0.08 nm) and comb filtering (channel spacing 100 GHz) with tunable, low loss (0.5 dB) and polarization-insensitive operation capabilities are demonstrated experimentally.