Pure Silica Core Multimode Fiber for Downhole DTS
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Solution Overview
Problem
Distributed Temperature Sensing (DTS) systems using optical fibers face challenges in harsh downhole environments with high temperatures, pressures, and hydrogen presence, leading to signal degradation and reduced spatial resolution due to hydrogen darkening and inter modal dispersion in conventional fibers.
Innovation Solution
A step index multi-mode optical fiber with a pure silica core and optimized cladding refractive index and diameter is designed to enhance numerical aperture, temperature resolution, and spatial resolution, featuring specific refractive index differences and core sizes that reduce modal delay and inter modal dispersion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional optical fibers with dopants are used in downhole environments, then telecommunication performance is optimized, but hydrogen darkening causes severe attenuation and degradation
Solution Approach 1:
The patent removes germanium dopants from the fiber core, extracting the harmful element that causes hydrogen darkening. The core is made of pure silica or silica with minimal doping, eliminating the chemical reaction pathway between hydrogen and dopants that causes attenuation in conventional fibers.
Solution Approach 2:
The patent changes the chemical composition parameters of the fiber core from conventional dopant-containing silica to pure silica or lightly doped silica. This parameter change fundamentally alters the fiber's interaction with hydrogen, preventing darkening while maintaining optical transmission properties.
2Illumination intensity
If single mode fibers are used for DTS, then bandwidth is maintained, but signal to noise ratio deteriorates due to small core size
Solution Approach 1:
The patent changes the core diameter parameter from single mode dimensions (around 9 μm) to larger multimode dimensions (50 μm or more), fundamentally altering the light coupling characteristics and signal collection capability while accepting the trade-off of managing modal dispersion through other means.
3Ease of operation
If multimode fibers with large core size are used, then coupling efficiency is improved, but inter modal dispersion broadens the pulse and degrades spatial resolution
Solution Approach 1:
The patent applies different refractive index profiles to different radial regions of the fiber core, creating local variations in light propagation characteristics. This graded index structure causes different modes to travel at different effective speeds, compressing the temporal spread of the pulse and improving spatial resolution while maintaining the benefits of a large core.
Data Source
AI summary
A new step-index multimode pure silica core fiber for DTS (Distributed Temperature Sensing) system particularly useful for downhole environments is disclosed and described. The new sensor system provides optimum tradeoffs between coupling power, spatial resolution, and temperature resolution.