A long range optical fiber distributed sensor system, comprising: an optical source arranged in use to produce optical sensing pulses; a sensing optical fiber deployable in use in an environment to be sensed and arranged in use to receive the optical sensing pulses; and a sensing apparatus arranged in use to detect light from the optical sensing pulses reflected and/or backscattered back along the sensing optical fiber and to determine any one or more of an acoustic, vibration, temperature or other parameter that perturbs the path length of the sensing optical fiber in dependence on the reflected and/or backscattered light; at least one transport fiber (24, 26) arranged between the sensing optical fiber and the optical source to transport the optical sensing pulses from the optical source to the sensing fiber and to transport backscatter and/or reflections from along the sensing fiber back to the sensing apparatus; the system being characterized by: at least one optical fiber amplifier (57, 59) arranged in series with the transport fiber (24, 26), and; an optical fiber amplifier pump source (52, 54) arranged to provide pump light to the optical fiber amplifier (57, 59), the optical fiber amplifier (57, 59) arranged in use to amplify at least one of: i) the forward optical sensing pulses; or ii) the forward optical sensing pulses and the returning backscatter and/or reflections. an optical source arranged in use to produce optical sensing pulses; a sensing optical fiber deployable in use in an environment to be sensed and arranged in use to receive the optical sensing pulses; and a sensing apparatus arranged in use to detect light from the optical sensing pulses reflected and/or backscattered back along the sensing optical fiber and to determine any one or more of an acoustic, vibration, temperature or other parameter that perturbs the path length of the sensing optical fiber in dependence on the reflected and/or backscattered light; at least one transport fiber (24, 26) arranged between the sensing optical fiber and the optical source to transport the optical sensing pulses from the optical source to the sensing fiber and to transport backscatter and/or reflections from along the sensing fiber back to the sensing apparatus; the system being characterized by: at least one optical fiber amplifier (57, 59) arranged in series with the transport fiber (24, 26), and; an optical fiber amplifier pump source (52, 54) arranged to provide pump light to the optical fiber amplifier (57, 59), the optical fiber amplifier (57, 59) arranged in use to amplify at least one of: i) the forward optical sensing pulses; or ii) the forward optical sensing pulses and the returning backscatter and/or reflections. at least one optical fiber amplifier (57, 59) arranged in series with the transport fiber (24, 26), and; an optical fiber amplifier pump source (52, 54) arranged to provide pump light to the optical fiber amplifier (57, 59), the optical fiber amplifier (57, 59) arranged in use to amplify at least one of: i) the forward optical sensing pulses; or ii) the forward optical sensing pulses and the returning backscatter and/or reflections. i) the forward optical sensing pulses; or ii) the forward optical sensing pulses and the returning backscatter and/or reflections.