Long range optical fiber sensing systems
A long range optical fiber sensor such as a distributed acoustic sensor has a sensing fiber located remotely from the interrogator, with a length of transport fiber path connecting the two. Because no sensing is performed on the transport fiber then the pulse repetition rate from the interrogator can be high enough such that the pulse repetition rate and pulse power are optimised according to the sensing fiber length and hence sensing frequency response and sensitivity are also optimised according to the sensing fiber length.
1. 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
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;
the system being characterised by:
at least one transport fiber 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;
at least one optical fiber amplifier arranged in series with the transport fiber between the transport fiber and the sensing optical fiber, and;
an optical fiber amplifier pump source arranged to provide pump light to the optical fiber amplifier, the optical fiber amplifier 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.
2. A long-range optical fiber sensor according to claim 1 , wherein the transport fiber comprises a forward transport fiber arranged to convey optical sensing pulses from the optical source to the sensing optical fiber, and a return transport fiber arranged to convey back scatter and/or reflections from the sensing optical fiber back to the interrogator.
3. A long range optical fiber sensor according to claim 2 , wherein a separate optical fiber amplifier is provided on each of the forward and return transport fibers.
4. A long range optical fiber sensor according to claim 3 , wherein a respective optical fiber amplifier pump source is provided for the respective optical fiber amplifiers, wherein the pump source for the optical fiber amplifier on the return transport fiber provides a continuous wave pump signal, whereas the pump source for the optical fiber amplifier on the forward transport fiber provides a pulsed pump signal that co-propagates with the forward optical sensing pulses.