We demonstrate the promising configurations that permit very narrow spectral filtering. The waveguide filter consists of a glass slab with an anomalous thickness of about 500 μm, which is sandwiched between two pre-coated metal films. By using the free-space coupling technique, a polarization-insensitive and tunable notch filter with a spectral bandwidth as narrow as 0. 08nm has been obtained due to the excitation of ultrahigh-order modes of the symmetrical metal-cladding optical waveguide.