Abstract: An optical monomode guided-wave device, especially an electroabsorption modulator, having an active region with a mode size small compared with that of an optical fibre is integrated with at least one passive spot size adjuster which has an optical core that tapers simultaneously from a width and height substantially equal to those of the active region adjacent to that region to a width large compared with the width of the active region and a thickness substantially smaller than the thickness of the active region at a position remote from that region. Usually—unless the device includes its own light source—there will be two spot size adjusters, respectively at the inlet and outlet of the device.