Abstract: The fiber optical measuring device contains a fiber optical transmission line into whose input end are directed a first and a second light beam. Both light beams have wavelengths differing from each other. Associated with the output end of the transmission line is a mirror. This mirror passes the transmitted second light beam therethrough and reflects the transmitted first light beam back into the transmission line. Associated with the output end is also a sensor for measuring a physical parameter. This sensor receives the second light beam and returns a portion thereof back into the output end. This portion depends on the parameter to be measured. Arranged at the input end of the transmission line is an optical device which separates the reflected portion of the first light beam from the returned portion of the second light beam.