Remote Optical Fiber Sensing with Transport Fiber and Raman Gain
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Solution Overview
Problem
Existing optical fiber DAS and DTS systems are limited to a range of around 35km due to attenuation and backscatter, which is insufficient for applications like pipeline security and area security, and extending the sensing length reduces sensing bandwidth.
Innovation Solution
A long range optical fiber sensor system with a remote interrogator and separate transport fibers for forward and return paths, using high power fiber, ultra low loss fiber, optical amplifiers, and Raman pump sources to maintain pulse repetition rate and signal power, and incorporating wavelength selective reflectors to prevent connector damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the sensing fiber length is extended beyond 35km, then the sensing range is improved, but the signal-to-noise ratio deteriorates due to fiber attenuation
Solution Approach 1:
The patent introduces transport fiber as an intermediary component between the interrogator and sensing fiber. The transport fiber carries sensing pulses from the interrogator to the sensing fiber and returns backscatter signals, allowing the sensing fiber to be positioned remotely while maintaining signal integrity through dedicated forward and return path fibers.
Solution Approach 2:
The system is segmented into distinct functional components: interrogator, transport fiber (forward and return paths), and sensing fiber. This segmentation allows optimization of each component for its specific function, with transport fiber optimized for low loss transmission and sensing fiber optimized for sensing performance.
2Length of stationary object
If multiple independent DAS systems are deployed to extend monitoring range, then the sensing range is improved, but the system complexity and deployment cost increase
Solution Approach 1:
The transport fiber infrastructure serves multiple functions: it provides the optical path for sensing pulses, carries backscatter signals back to the interrogator, and enables remote positioning of the sensing fiber. This multi-functionality eliminates the need for separate systems at multiple locations.
3Length of stationary object
If the sensing pulse power is increased to extend range, then the sensing range is improved, but the connector reliability deteriorates due to damage risk
Solution Approach 1:
The transport fiber acts as a mediator that allows high power pulses to be generated by the interrogator and delivered to the sensing fiber without exposing intermediate connectors to damaging power levels. The system can use connectorless splices or protected connector interfaces in the transport fiber path.
4Length of stationary object
If optical amplifiers are added to maintain signal power over long distances, then the sensing range is improved, but the device complexity increases
Solution Approach 1:
The system uses the sensing pulse itself to pump Raman amplification in the transport fiber. The high power sensing pulse generates Raman gain that amplifies both the forward propagating pulse and the returning backscatter signals, eliminating the need for separate pump lasers or electrical amplifiers.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables long-range sensing with maintained bandwidth and signal-to-noise ratio, allowing monitoring at significant distances without reducing sensing bandwidth.
Implementation Method 1
at least one Raman pump source providing optical energy to the transport fiber
Implementation Method 2
with at least one optical amplifier in the forward path and/or return path
Implementation Method 3
due to attenuation in the fiber of both of the outward sensing pulse, and the resulting backscatter along the fiber
Implementation Method 4
DAS and DTS systems use backscatter or reflections from along the fiber to sense acoustic energy incident on the fiber, or ambient temperature around the fiber
Data Source
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AI summary
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. In further embodiments fiber amplifiers such as erbium doped fiber amplifiers may be included in line in the transport fiber path, typically located just before the sensing fiber in the direction of pulse travel from the interrogator, to help restore the pulse signal power before it enters the sensing fiber. In yet further embodiments at least one Raman pump source can be provided to inject light pulses on to the transport fiber at a Raman pump wavelength to stimulate the generation of signal photons at the interrogator pulse wavelength via the stimulated Raman scattering (SRS) phenomenon. This helps to maintain the forward pulses and backscatter/reflection signals as they traverse the transport fiber path to and from the sensing fiber.