METHOD AND APPARATUS OF DISTRIBUTED ACOUSTIC SENSING
There is provided a DAS acquisition device which combines both a coherent OTDR mode of operation and an un-coherent OTDR mode of operation for allowing real-time monitoring of both fiber vibrations and fiber loss variations in a hardware efficient, low-cost and compact manner. The DAS acquisition apparatus comprises a coherent light source consisting of a CW coherent light source followed by a semiconductor optical amplifier (SOA) driven by a pulse generator. When the CW coherent light source is turned on (coherent OTDR mode of operation), a coherent pulsed test light signal is produced, so as to acquire DAS traces for acoustic and vibration measurement. But when the CW coherent laser is turned off (un-coherent OTDR mode of operation), an un-coherent pulsed test light signal is produced, so as to acquire un-coherent reflectometric traces for optical fiber loss measurement.
1 . A fiber optic distributed acoustic sensing (DAS) acquisition device for performing acoustic and vibration measurements as well as optical fiber loss measurement, the DAS acquisition device comprising:
a light generating assembly to be coupled to the optical fiber link for generating and propagating in the optical fiber link a test light signal comprising test light pulses, the light generating assembly comprising:
a coherent light source to produce continuous wave coherent light;
a semiconductor optical amplifier receiving light from said coherent light source;
a pulse generator connected to the semiconductor optical amplifier to drive it to generate said test light pulses; and
a detection module for detecting corresponding return light from the optical fiber link so as to acquire a reflectometric trace representing backscattered and reflected light as a function of distance along the optical fiber link;
wherein in a coherent OTDR mode of operation, said coherent light source is turned on so that said light generating assembly produces a coherent pulsed test light signal, so as to acquire DAS traces for acoustic and vibration measurement; and
wherein in an un-coherent OTDR mode of operation, said coherent light source is turned off so that said light generating assembly produces an un-coherent pulsed test light signal, so as to acquire un-coherent reflectometric traces for optical fiber loss measurement.
2 . The fiber optic DAS acquisition device as claimed in claim 1 , wherein said DAS acquisition device comprises multiple DAS acquisition channels for performing DAS acquisitions toward multiple channels each connected to an optical fiber link, wherein said DAS acquisition channels share the same single coherent light source but each comprise a semiconductor optical amplifier, a pulse generator and a detection module.
3 . The fiber optic DAS acquisition device as claimed in claim 1 , wherein reflectometric traces are acquired in groups to provide a plurality of groups of K DAS traces and wherein the fiber optic DAS acquisition device further comprises a processing unit receiving groups of K DAS acquisitions performed toward an optical fiber link using the DAS acquisition device in coherent OTDR mode of operation, and independently processing each group of DAS traces to produce corresponding DAS signals, such that independent DAS signals are obtained using independent groups of DAS traces.
4 . The fiber optic DAS acquisition device as claimed in claim 3 , wherein the DAS acquisition device comprises a controller configured to leave a time lapse ΔT in-between groups of DAS acquisitions to prevent memory saturation and allow time for processing each group of DAS traces before DAS acquisitions of the next group.
5 . The fiber optic DAS acquisition device as claimed in claim 4 , wherein said controller is configured to drive said DAS acquisition device to not propagate said pulsed test signal in said optical fiber link during said time lapse ΔT.
6 . The fiber optic DAS acquisition device as claimed in claim 4 , wherein said controller is configured to drive said DAS acquisition device to perform DAS acquisitions during said time lapse ΔT but discard them.