Abstract: The paper presents principle of operation and construction of prototype of interference loop of optical fibre sensor applied for vibration detection. The Sagnac interferometer with passive demodulator on 3x3 coupler was used for the sensor design. The prototype performed in co-operation with the Military University of Technology and the Military Electronic Plants in Zielonka has been examined mechanically in various weather conditions. The prototype fulfils military requirements for the instruments operating in the closed space within the range of temperatures -40^oC to +50^oC.