A distributed sensing apparatus based on Optical Time Domain Reflectometry, OTDR, comprising: an optical source ; an optical splitter in optical communication with the optical source, the optical splitter having first and second outputs; a sensing fibre in optical communication with the first output of the optical splitter; a combining unit arranged to combine a reference signal derived from the second output of the optical splitter with a backscattered signal derived from the sensing fibre, wherein the combining unit comprises one or more fused fibre couplers having at least three outputs; and a signal processing unit configured for processing information derived from the combining unit so as to provide distributed sensing data, characterized in that the combining unit comprises a polarization diversity arrangement comprising one or more polarization sensitive elements, configured to align, in the combining unit: the polarization of a first signal derived from the backscattered signal with that of a first signal derived from the reference signal; and the polarization of a second signal derived from the backscattered signal with that of a second signal derived from the reference signal, and in that the signal processing unit is configured to perform demodulation based on a demodulation scheme which requires three signals that are shifted in phase by 120° with respect to one another. an optical source ; an optical splitter in optical communication with the optical source, the optical splitter having first and second outputs; a sensing fibre in optical communication with the first output of the optical splitter; a combining unit arranged to combine a reference signal derived from the second output of the optical splitter with a backscattered signal derived from the sensing fibre, wherein the combining unit comprises one or more fused fibre couplers having at least three outputs; and a signal processing unit configured for processing information derived from the combining unit so as to provide distributed sensing data, characterized in that the combining unit comprises a polarization diversity arrangement comprising one or more polarization sensitive elements, configured to align, in the combining unit: the polarization of a first signal derived from the backscattered signal with that of a first signal derived from the reference signal; and the polarization of a second signal derived from the backscattered signal with that of a second signal derived from the reference signal, and in that the signal processing unit is configured to perform demodulation based on a demodulation scheme which requires three signals that are shifted in phase by 120° with respect to one another. the polarization of a first signal derived from the backscattered signal with that of a first signal derived from the reference signal; and the polarization of a second signal derived from the backscattered signal with that of a second signal derived from the reference signal, and in that the signal processing unit is configured to perform demodulation based on a demodulation scheme which requires three signals that are shifted in phase by 120° with respect to one another. <EOS>