A first embodiment of the invention provides a Gires-Tournois etalon (DGTE) 20 comprising a cladding mode suppressed optical fibre 22, including an etalon section 24 in which a weakly reflective optical waveguide grating 26 and a strongly reflective optical waveguide grating 28 are provided. The two gratings 26, 28 are chirped fibre Bragg gratings (FBGs). The chirped FBGs 26, 28 are arranged to together define an etalon cavity. Another aspect of the invention provides a dispersion compensator 60 comprising two DGTEs 62, 64 according to the first embodiment of the invention. The DGTEs 62, 64 have a linearly varying dispersion over a selected optical bandwidth. The first DGTE 62 has a positive dispersion slope and the second DGTE 64 has a negative dispersion slope. The magnitudes of the dispersion slopes of the two DGTEs 62, 64 are substantially equal, so that the dispersion compensator 60 has a constant dispersion across the selected bandwidth. The DGTEs 62, 64 are optically coupled to one another via a four-port optical circulator 66.