<p>The optical transmission apparatus transmits modulated optical signals between a first unit (1) and a second unit (2) where the first unit is rotatably mounted relative to the second. The apparatus has a waveguide (3) along a circular path on the first unit. A light coupler (4) is connected to the waveguide for coupling light into and out of the waveguide. A second light coupler (5) is arranged on the second unit and is moveable relative to the waveguide to couple light into and out of the waveguide. A cleaning unit for removing dirt and dust may also be provided. Independent claims also cover methods of cleaning the surface of a waveguide.</p>
