An optical transmission system in a computer tomograph for transmitting modulated optical signals between a first unit and a second unit, the first unit being supported to be rotatable relative to the second unit, comprises: a light guide disposed along a circular or annular track on the first unit; at least one first light coupler connected with the light guide for coupling light into or out of the light guide; at least one second light coupler disposed on the second unit to be movable relative to the light guide for coupling light into or out of the light guide. In order to remove from the light guide surface any contaminating matter that would lead to a high signal attenuation or an interference with transmission, a cleaning unit is provided for removing dirt and dust particles. Furthermore, a sealing of the system is provided by applying, amongst other means, pressurized air or electrostatic filtering.