Multi-Reflector Optical Fiber Sensing for Extended Length
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Solution Overview
Problem
Conventional optical frequency domain reflectometry systems for distributed sensing in boreholes are limited by the sensing length due to data digitization constraints and increased susceptibility to vibration, which restricts the effective monitoring of equipment and structures deep within the earth.
Innovation Solution
The implementation of a multi-reflector optical frequency domain reflectometry system using two or more sets of fiber Bragg gratings with distinct nominal reflection wavelengths, where each set is situated between reference reflectors to separate desirable and undesirable light signals, allowing for longer sensing lengths without increased sampling requirements or vibration susceptibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the sensing length of the optical fiber is increased, then the monitoring coverage is improved, but the sampling requirements increase and vibration susceptibility increases
Solution Approach 1:
The optical fiber is segmented into multiple sections, each containing FBGs with distinct nominal reflection wavelengths. This segmentation allows the system to process signals from different segments separately, enabling longer overall sensing length while maintaining reliable vibration resistance in each segment. The segmentation of wavelength ranges (first range, second range, third range) corresponds to segmentation of the fiber into functional zones.
Solution Approach 2:
The patent transitions from a single-dimension approach (single wavelength range) to a multi-dimensional approach by introducing multiple wavelength dimensions. The first series of FBGs reflects in a first wavelength range, the second series in a second wavelength range, and autocorrelation terms appear in a third wavelength range. This dimensional expansion in wavelength space allows simultaneous long sensing length and vibration resistance.
2Length of moving object
If the sensing length of the optical fiber is increased, then the monitoring coverage is improved, but the data digitization constraints become more severe
Solution Approach 1:
The data processing is segmented by wavelength ranges. The optical interrogator processes the first wavelength range for the first series of FBGs, the second wavelength range for the second series of FBGs, and the third wavelength range for autocorrelation terms. This segmentation of data processing reduces the complexity of digitization by handling different fiber sections independently with appropriate sampling rates.
Solution Approach 2:
By expanding into multiple wavelength dimensions, the patent distributes the data processing load across different spectral domains. This allows the system to manage long sensing lengths without overwhelming the data digitization capability, as each wavelength dimension can be processed at its own optimized sampling rate.
3Measurement precision
If blank fiber length is increased to separate autocorrelation terms, then measurement accuracy is improved, but the useful sensing length is reduced
Solution Approach 1:
Instead of separating autocorrelation terms in the spatial domain (which requires blank fiber length), the patent separates them in the wavelength domain. The autocorrelation terms naturally appear in a third wavelength range that is distinct from the first and second ranges used by the FBGs. This wavelength-based separation achieves measurement precision without sacrificing useful sensing length.
Solution Approach 2:
The patent converts the potentially harmful autocorrelation terms into a beneficial separation mechanism. By designing the system so that autocorrelation terms appear in a distinct third wavelength range, what would normally be noise or interference becomes a useful indicator for identifying and filtering unwanted signals, while preserving the full length of the optical fiber for sensing.
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 enables extended sensing lengths without the need for additional downhole hardware, effectively filtering out autocorrelation terms and increasing the available wavelengths for useful measurements, thus enhancing the monitoring capabilities of the optical fiber.
Implementation Method 1
a first series of fiber Bragg gratings (FBGs) configured to measure the parameter at a portion of the distributed locations, each FBG in the first series having a first nominal reflection wavelength; a second series of FBGs configured to measure the parameter at another portion of the distributed locations, each FBG in the second series having a second nominal wavelength
Implementation Method 2
The reference reflector forms an interferometric cavity, such as a Fabry-Perot cavity in this example, with each individual FBG. As the wavelength of light from the light source is swept, an interferogram is created with a frequency for each interferometric cavity that is proportional to the length of the cavity for each FBG.
Data Source
AI summary
An apparatus for estimating a parameter at distributed locations, the apparatus including: an optical fiber having: a first series of fiber Bragg gratings (FBGs) and configured to measure the parameter at a portion of the distributed locations; a second series of FBGs and configured to measure the parameter at another portion of the distributed locations; and an optical interrogator configured to illuminate the optical fiber and to receive light signals resulting from the illumination, the light signals including first light signals from the first series of FBGs within a first range of wavelengths, second light signals from the second series of FBGs within a second range of wavelengths, and other light signals within a third range of wavelengths, the ranges of wavelengths being distinct from each other; wherein the first light signals and the second light signals are used to estimate the parameter at the distributed locations.