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 u2018 multimode u2019 waveguide associated with a coupling device the waveguide being associated with the transmitting unit and a further optical u2018 multimode u2019 waveguide with an associated coupling device associated with the optical waveguide and arranged on the rotatable shaft eccentrically to its axis application_number US 31072507 A description parts which correspond to one another are provided with the same reference symbols in fig1 to 7 fig1 shows an optical device for monitoring a shaft 10 especially a shaft connected to a machine or a generator and supported to allow it to rotate the shaft 10 is able to be rotated around its oriented axis 13 here the end face side of the shaft 10 is labeled 12 arranged on and or in the outer surface 11 of shaft 10 is an optical waveguide 20 with optical sensors 21 for detecting temperature and or expansion of the shaft 10 as shown in fig1 the sensors 21 are fbg sensors incorporated into single mode optical waveguides 20 with a number of fbg sensors 21 each individual sensor 21 can.