Repeater design for distributed acoustic sensing on multispan fiber links
Aspects of the present disclosure are directed to alternative repeater design(s) that advantageously improve signal-to-noise of distributed acoustic sensing (DAS) systems using coherent detection of Rayleigh backscatter in multi-span links including inline amplification that may be employed—for example—in undersea submarine systems. The repeater designs incorporate Rayleigh combine units (RCU) and Rayleigh drop units (RDU) to reduce Rayleigh backscatter loss as Rayleigh signal(s) is/are routed to a link that propagates the backscatter signal in an opposite direction relative to interrogation pulse(s).
1. A distributed optical fiber sensing/distributed acoustic sensing system comprising:
a length of bi-directional optical fiber cable having multiple spans of fibers with inline optical amplifiers positioned after each span, said cable configured such that at least one fiber propagates light in a first direction and at least one fiber propagates light in a second direction, said second direction counter-propagating opposite to the first direction;
a distributed optical fiber sensing/distributed acoustic sensing interrogator in optical communication with the length of bi-directional optical fiber cable including a fiber that propagates light in the first direction and a fiber that propagates light in the second direction;
an intelligent analyzer configured to analyze DFOS sensing data received by the DFOS interrogator system; and
one or more repeaters located at point(s) along the length of the optical fiber cable, said repeaters including a plurality of Rayleigh combine units (RCUs) and a plurality of Rayleigh drop units (RDUs).
2. The sensing system of claim 1 wherein the RCUs and RDUs are configured to operate in combination to selectively route Rayleigh backscatter to an optical fiber propagating in an opposite direction.
3. The sensing system of claim 2 wherein the RDUs are configured to pass probe pulses, communications signals and Rayleigh backscatter of a counterpropagating fiber, while dropping Rayleigh backscatter resulting from the probe pulses.
4. The sensing system of claim 3 wherein the RDUs are configured to provide a switching pulse to an RCU on a counterpropagating fiber.
5. The sensing system of claim 4 wherein the RCU is configured to pass the probe pulses and communications signals and receive Rayleigh backscatter dropped by the RDU on the counterpropagating fiber, said RCU configured to respond to the switching pulse provided by the RDU to combine the Rayleigh backscatter in an adjacent span with Rayleigh backscatter generated in upstream spans of the same counterpropagating fiber.
6. The sensing system of claim 5 wherein the RDU includes an optical circulator to route optical signals and an optical bandpass filter to extract Rayleigh backscatter corresponding to probe pulse(s).
7. The sensing system of claim 6 wherein the RDU includes an optical amplifier that amplifies Rayleigh backscatter before dropping it to the RCU.
8. The sensing system of claim 7 wherein the RCU includes wavelength multiplexers and demultiplexers to extract Rayleigh backscatter of the counterpropagating fiber from probe pulse(s) and communications signals.
9. The sensing system of claim 8 wherein the RDU and RCU are positioned at a point of the cable after an inline optical amplifier.
10. The sensing system of claim 7 wherein the RCU is positioned at a point of the cable before an inline optical amplifier while the RDU is positioned at a point of the cable after the inline optical amplifier.