Method and apparatus of distributed acoustic sensing
There is provided a method and an apparatus of fiber optic distributed acoustic sensing (DAS) which can use low-cost coherent laser as well as low-cost acquisition and processing electronics and which can still provide reliable monitoring results for optical fiber monitoring and troubleshooting applications in optical fiber telecommunication networks. Such low-cost solution is made possible by employing grouped data signal processing. Data is processed over independent groups of data to provide an independent DAS signal for each group. This allows measurements to be less sensitive to laser fluctuations and thereby reduces coherent laser technical specification requirements and allows the use of a low-cost coherent laser (thereby reducing the cost of the laser) as well as low-cost acquisition and processing electronics.
1 . A method for performing acoustic and vibration measurements based on fiber optic distributed acoustic sensing (DAS), the method comprising:
performing repetitive DAS acquisitions toward an optical fiber link using a DAS acquisition device comprising a coherent Optical Time Domain Reflectometer (OTDR) to provide a group of K DAS traces, wherein each DAS acquisition is performed by propagating in the optical fiber link, a pulsed test signal and detecting corresponding return light from the optical fiber link so as to obtain a DAS trace representing backscattered and reflected light as a function of distance in the optical fiber link;
processing said group of DAS traces to produce a DAS signal associated with said group; and
repeating said performing repetitive DAS acquisitions and said processing said group of DAS traces to obtain a plurality of independent groups of DAS traces and a corresponding plurality of DAS signals, such that independent DAS signals are obtained using said independent groups of DAS traces.
2 . The method as claimed in claim 1 , wherein said DAS acquisition device comprises multiple DAS acquisition channels, said DAS acquisition channels sharing a single coherent light source and wherein said method comprises performing DAS acquisitions toward multiple channels each connected to an optical fiber link.
3 . The method as claimed in claim 1 , wherein processing said group of DAS traces comprises:
for each independent group of DAS traces P (n,k) (z), calculating a Root Mean Square (RMS) difference as a function of a distance z along the optical fiber link, to obtain a corresponding DAS signal √{square root over ( ΔP (n,k) (z) 2 K )},
wherein n represents the data group index (n=1 to N) and k represents the DAS trace index within one group (k=1 to K).
4 . The method as claimed in claim 1 , wherein a time lapse ΔT is left in-between groups of DAS acquisitions to prevent memory saturation and allow time for processing said group of DAS traces before DAS acquisitions of the next group.
5 . The method as claimed in claim 4 , wherein no test signal is propagated in said optical fiber link during said time lapse ΔT.
6 . The method as claimed in claim 4 , wherein DAS acquisitions performed during said time lapse ΔT are discarded.
7 . A fiber optic distributed acoustic sensing (DAS) system for performing acoustic and vibration measurements, the DAS system comprising:
a DAS acquisition device connectable toward an optical fiber link and comprising a coherent Optical Time Domain Reflectometer (OTDR) for performing repetitive DAS acquisitions, wherein each DAS acquisition is performed by propagating in the optical fiber link, a pulsed test signal and detecting corresponding return light from the optical fiber link so as to obtain a DAS trace representing backscattered and reflected light as a function of distance in the optical fiber link, wherein DAS traces are acquired in groups to provide a plurality of groups of K DAS traces;
a memory to store said groups of DAS traces, at least one group of DAS traces at a time; and
a processing unit receiving 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 feast e-DAS-acquisition device.
8 . The DAS system as claimed in claim 7 , 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 a single coherent light source.
9 . The DAS system as claimed in claim 7 , wherein said DAS acquisition device has a coherent OTDR mode of operation for performing acoustic and vibration measurements and an un-coherent OTDR mode of operation for performing optical fiber loss measurement.
10 . The DAS system as claimed in claim 9 , wherein said DAS acquisition device comprises:
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 the 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 the 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.
11 . The DAS system as claimed in claim 7 , 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.
12 . The DAS system as claimed in claim 11 , 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.
13 . The DAS system as claimed in claim 11 , wherein said controller is configured to drive said DAS acquisition device to perform DAS acquisitions during said time lapse ΔT but discard them.