An optical device for monitoring a rotatable shaft is disclosed. The optical device has an optical waveguide arranged on the rotatable shaft and an optical sensor. The optical device further has a transmitting unit transmit the light signal, a transfer device to transmit the light signal between the transmitting unit and the optical waveguide and an evaluation unit for determining a physical variable from a light signal coming from the optical sensor and transferred by the transfer device, the evaluation unit being associated with the transmitting unit. The transfer device has an optical ‘multimode’ waveguide associated with a coupling device, the waveguide being associated with the transmitting unit and a further optical ‘multimode’ waveguide, with an associated coupling device associated with the optical waveguide and arranged on the rotatable shaft eccentrically to its axis.