Abstract: Apparatus for the measurement of physical quantities, of the type using an optical transducer to transform the variations of a physical quantity or perturbation in a variation of the state of polarization of a polarized light beam, in which: the transducer is inserted in an optical circuit, provided with polarizing beam-splitters for splitting the light beam in components having different state of polarization and to launch them along a first optical path and a second optical path, being different to each other, which are recombined in further polarizing beam-splitters arranged at the output of the optical circuit; at the output of the optical circuit a reflector element is provided that reflects the light beam back into the optical circuit with a different state of polarization.