The filtering of optical signals, and in particular the interleaving/de-interleaving of optical signals is becoming a necessary step in Dense-Wavelength Division Multiplexing (D-WDM), because of a requirement for smaller channel spacing due to higher levels of traffic. Finite Impulse Response (FIR) filters, including lattice and birefringent waveplate versions, are a particular type of optical filter used for interleavering/de-interleavering optical channels, which can be defined by their transfer function H(f). To ensure dispersion free filtering, the present invention provides a cascaded optical filter, comprising two optical filters, wherein the transfer function of the second filter is the complex conjugate of the first filter, i.e. H 2 (f)=H 1 ∗(f), or the complex conjugate of the first filter H 1 ∗(f) multiplied by the transfer function of a dispersion free optical filter G(f). In the waveplate version, the relationship between the crystal angle of the waveplates and the polarization of the input light is manipulated to ensure that the second filtering step eliminates the dispersion caused by the first. Since the polarization of the light can be altered between the first and second filters, birefringent filters can be conceived using a single stack of waveplates that are passed through twice. In lattice filters, a symmetric pulse response is an indication that the cascaded filter is dispersion free. In this case, the second filter is the inverse of the first filter, which individually would give the opposite pulse responses, but together provide a symmetric pulse response.