OFDR Multiplexer Spatially Separates Fiber Bragg Grating Sensor Arms
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Solution Overview
Problem
Current optical fiber sensing systems face challenges in multiplexing multiple parallel fiber Bragg grating (FBG) sensor-fibers to a single acquisition channel of a closed Michelson interferometer system, particularly in distinguishing each branch in the spatial domain, leading to light-starved conditions and increased complexity.
Innovation Solution
A multiplexer apparatus is used to connect each sensor arm with a known length of single mode fiber and calibrated optical fiber, allowing each sensor arm to be interrogated as part of a single acquisition channel, effectively separating them in the spatial domain while retaining FBG-reference arm beat frequency information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple parallel fiber Bragg grating sensor-fibers are multiplexed to a single acquisition channel via a fiber splitter, then the number of measurement points increases and system cost decreases, but light-starved conditions occur and it becomes difficult to distinguish each branch in the spatial domain
Solution Approach 1:
The patent divides the single acquisition channel into multiple virtual channels by introducing different optical path lengths to each sensor arm. The fiber splitter is combined with reference arms of different lengths, creating distinct optical path segments that allow each sensor arm to be distinguished in the spatial domain through Fourier transform processing, thereby resolving the light-starved condition while maintaining multiplexing capability.
2Device complexity
If multiple parallel fiber Bragg grating sensor-fibers are multiplexed to a single acquisition channel via a fiber splitter, then system cost decreases, but the complexity of distinguishing each branch in the spatial domain increases
Solution Approach 1:
The patent transforms the problem from temporal multiplexing to spatial domain multiplexing by introducing different optical path lengths as an additional dimension. Each sensor arm is assigned a unique optical path length, allowing simultaneous acquisition of all sensors in a single wavelength scan. The spatial domain distinction is achieved through Fourier transform processing that separates signals based on their unique path length characteristics.
3Measurement precision
If traditional electrically-wired networks of strain sensors are used, then measurement capability is achieved, but complex wiring layouts are required which are costly and prone to corrosion and electromagnetic interference
Solution Approach 1:
The patent replaces electrically-wired sensor networks with an all-optical fiber sensing system. Instead of using electrical wires to connect strain sensors, the system uses optical fibers with Fiber Bragg gratings that modulate light based on strain, temperature, or other physical stimuli. This substitution eliminates the need for complex electrical wiring, removes susceptibility to electromagnetic interference, and reduces corrosion risks while maintaining measurement precision.
4Measurement precision
If photogrammetry is used for structural health monitoring of aircraft wings, then wing deflection can be monitored, but a clear line of sight is required which is often impossible or impractical
Solution Approach 1:
The patent introduces optical fiber sensors as intermediaries that can be embedded within or attached to the structure being monitored. Instead of requiring external line-of-sight observation, the fiber optic sensors are integrated into the structure itself, allowing measurement of strain, temperature, and other parameters from within the structure. This intermediary approach enables monitoring of structures where external observation is impossible or impractical.
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
This approach simplifies optical, electronic, and computational complexity, enabling efficient monitoring of large and distributed structures by resolving multiple sensor arms as spatially distinct segments within a single acquisition channel, reducing the risk of data loss and system cost.
Implementation Method 1
Within the multiplexer apparatus each output channel includes a calibrated-length of optical fiber Lfiber. These output channel fibers are calibrated such that the fiber length of each successive output channel equals the calibrated length of the preceding channel Lfiber plus Lbuffer+Lsensor of the preceding sensor arm.
Implementation Method 2
With optical frequency domain reflectometry (OFDR) all the FBGs are supplied with laser light having the same central wavelength, and their positions along the fiber are detected by measuring the beat frequency of any individual grating's reflection with the reflection from a reference arm of the interferometer having a known length.
Implementation Method 3
Optical fiber sensors (OFS) are typically composed of numerous optical fibers and numerous Fiber Bragg gratings (FBGs) periodically-spaced along the length of each fiber. Each FBG creates a periodic variation of the optical refractive index in the core of its associated optical fiber, and when coupled to an interferometer it becomes possible to detect strain individually through change in its resonant wavelength
Data Source
AI summary
A method and system for multiplexing a network of parallel fiber Bragg grating (FBG) sensor-fibers to a single acquisition channel of a closed Michelson interferometer system via a fiber splitter by distinguishing each branch of fiber sensors in the spatial domain. On each branch of the splitter, the fibers have a specific pre-determined length, effectively separating each branch of fiber sensors spatially. In the spatial domain the fiber branches are seen as part of one acquisition channel on the interrogation system. However, the FBG-reference arm beat frequency information for each fiber is retained. Since the beat frequency is generated between the reference arm, the effective fiber length of each successive branch includes the entire length of the preceding branch. The multiple branches are seen as one fiber having three segments where the segments can be resolved. This greatly simplifies optical, electronic and computational complexity, and is especially suited for use in multiplexed or branched OFS networks for SHM of large and/or distributed structures which need a lot of measurement points.