A spectral polarization separator (SPS) designed to separate two wavelength-separated signals applied to the SPS with the same state of polarisation (SOP) into signals with orthogonal SOP's comprises two or more birefringent fibre elements optically in series. The differential delays and relative orientations of these elements are chosen so that polarisation separation is effected without introducing any polarisation mode dispersion (PMD). This contrasts with a prior art single birefringent element SPS whose operation necessarily does introduce PMD. A particular application for the SPS is in a dual wavelength transmitter designed for overcoming polarization hole-burning effects in a long-haul optically amplified transmission system.