Optical Fiber Termination Structure for Low Return Loss
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Solution Overview
Problem
Optical fiber terminations face challenges in achieving low return loss and minimizing cross-talk in small volume applications, especially when embedded in close proximity to unknown surfaces or with multiple cores not centered in the fiber structure, due to significant reflections and diffuse reflections from materials used for absorption.
Innovation Solution
A small volume optical fiber termination structure with a diffusion region formed by heating the fiber end and incorporating an absorptive material, such as borosilicate glass or metal tubes, to absorb light and reduce reflections to a return loss of −70 dB or less, while allowing light to expand and interact with the absorptive material at a grazing angle for efficient absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a standard cleaved or polished fiber end is used, then the termination is simple and small, but the reflection coefficient is about 3.5% yielding a return loss of only −14.5 dB
Solution Approach 1:
An absorptive material is introduced as an intermediary between the optical fiber core and the external environment. This material has an index of refraction intermediate between the core (n1) and air (n2), creating a gradual index transition that reduces reflection. The absorptive material also converts any reflected light into other forms of energy, preventing it from returning to the fiber core.
Solution Approach 2:
The index of refraction parameter is changed gradually from the core index (n1) through the absorptive material to air (n2). This gradual parameter change reduces the abrupt index step that causes reflection. Additionally, the absorptive material's ability to convert light energy to other forms changes the energy parameter, eliminating reflected light.
2Loss of energy
If an absorptive material is placed at the fiber end to reduce reflections, then return loss improves, but the termination volume increases beyond the allowed small volume constraint
Solution Approach 1:
The absorptive material is applied locally only at the fiber end face where reflection occurs, rather than extending along a long distance. The material is confined to a small volume that fits within the allowed termination space, concentrating the anti-reflection function at the critical interface between the fiber core and external environment.
Solution Approach 2:
The absorptive material is nested within or integrated with the fiber coating structure. The termination structure contains the absorptive material within a compact volume that fits within the allowed termination space, with the absorptive material positioned strategically at the fiber end face.
3Measurement precision
If the fiber is embedded close to a surface to sense as close as possible, then sensing precision improves, but uncontrolled light emerging from the termination is reflected back into the fiber core
Solution Approach 1:
The absorptive material converts the harmful reflected light into other forms of energy (such as heat), preventing it from returning to the fiber core. This conversion of harmful reflected energy into a benign form eliminates the interference problem while allowing the fiber to be positioned close to external surfaces for high-precision sensing.
Solution Approach 2:
The absorptive material is positioned at the fiber end face to preemptively absorb or redirect light before it can emerge and be reflected by external surfaces. This preliminary anti-action prevents the harmful reflection from occurring in the first place, allowing the fiber to be embedded close to surfaces without suffering from reflection interference.
4Adaptability or versatility
If multiple cores are used for position and shape sensing, then sensing capability improves, but cross-talk between cores increases due to reflections and diffuse reflections
Solution Approach 1:
The absorptive material serves as an intermediary that prevents light from one core from reflecting and entering adjacent cores. By absorbing or redirecting light at the fiber end face, the material eliminates the cross-talk pathway between cores, allowing multiple cores to be used for position and shape sensing without interference.
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
The solution effectively achieves exceptionally low return loss and cross-talk in small volume optical fiber terminations, ensuring minimal light reflection back into the fiber core, even when proximate to varying surfaces, by utilizing a highly absorbing material with a gradual index transition and controlled light interaction.
Implementation Method 1
an absorptive material... that absorbs light emitted from a core of the optical fiber to reduce reflections into the core
Implementation Method 2
The optical fiber is associated with a first index of refraction, and the absorptive region is associated with a second index of refraction. The diffusion region gradually transitions between the first index of refraction and the second index of refraction to reduce reflections that occur from the mismatch
Implementation Method 3
a diffusion region formed from heating the end of the optical fiber together with a proximal portion of the absorptive region
Data Source
AI summary
A method and structure for terminating an optical fiber are disclosed that provide an optical fiber termination structure with a small volume and very low return loss, even when the termination is in close proximity to reflective surfaces. In one example embodiment, the optical fiber termination reduces reflections into the one or more cores to a return loss of −70 dB or less regardless of the presence of surfaces proximate the optical fiber termination. At the same time, a length of the optical fiber termination is less than 5 mm and a largest transverse dimension of the optical fiber termination is less than 325 um. The optical fiber termination is useful in fiber sensing applications in general and is particularly effective for terminating a multi-core fiber used in a distributed shape sensing application.