# What is concentricity? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [July 29, 2026, 12:09pm UTC](https://forum.ofscn.net/t/what-is-concentricity/2434 "2026-07-29T12:09:56Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [July 29, 2026, 12:09pm UTC](https://forum.ofscn.net/t/what-is-concentricity/2434/1 "2026-07-29T12:09:56Z")
Does the fiber optic cable have to be in the exact center of the steel pipe? What happens if it is off-center? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [July 29, 2026, 12:10pm UTC](https://forum.ofscn.net/t/what-is-concentricity/2434/2 "2026-07-29T12:10:44Z")
In optical engineering and fiber optic sensor design, whether the optical fiber must be located at the geometric center of the stainless steel protective tube depends on the **encapsulation type (loose tube vs. tight buffer)** and the **specific application environment (temperature, strain, multi-dimensional force sensing, etc.)**. In many high-precision applications, the **concentricity** of the optical fiber relative to the protective steel tube has extremely critical physical and engineering significance. * * * ### I. Physical Analysis: What Happens if the Optical Fiber is Eccentric? If the optical fiber is not located at the center of the steel tube, it typically leads to the following physical effects and engineering problems: #### 1. “Cross-Sensitivity” of Strain Induced by Bending - **Geometric Center as Neutral Axis** : Any cylindrical or symmetrical structure of a metal tube has a geometric plane during bending that neither stretches nor compresses, known as the **neutral plane/neutral axis**. - **Perfectly Concentric Structure** : If the optical fiber is at the absolute center of the steel tube, during bending, the fiber lies precisely on the neutral axis. In this case, even if the steel tube undergoes large-angle bending, the optical fiber will not experience axial tensile or compressive stress, thus avoiding interference from bending strain on the sensor. - **Effect of Eccentric Structure** : If the optical fiber deviates from the geometric center of the steel tube (with an eccentricity of y ), when the steel tube bends, the fiber will deviate from the neutral axis. According to the principles of mechanics of materials, the fiber will bear additional axial strain caused by bending: \epsilon = \frac{y}{\rho} (where \epsilon is the axial bending strain experienced by the fiber, y is the eccentric distance of the fiber from the neutral axis, and \rho is the bending radius). This eccentricity leads to severe **cross-sensitivity** between the bending stress and temperature or external axial tension, causing measurement data distortion. #### 2. Asymmetric Transverse Shear Stress and Stress Birefringence When the optical fiber is tightly buffered within the steel tube by solidified adhesive, significant differences in the **coefficient of thermal expansion (CTE)** between the metal tube (stainless steel), the solidified adhesive, and the silica optical fiber generate mechanical stress with environmental temperature changes: - **Eccentricity** leads to uneven thickness of the adhesive layer around the fiber. When heated or cooled, the radial pressure acting on the fiber loses its isotropy, transforming into **asymmetric transverse shear stress**. This asymmetric stress will destroy the circular geometric symmetry of the fiber core, introducing **stress birefringence** within the core. This, in turn, causes changes in the polarization state of light, increasing polarization-dependent loss (PDL) or polarization mode dispersion (PMD), and affecting the measurement accuracy of interference-based or polarization-sensitive sensors. #### 3. Asymmetric Thermal Response Latency In environments with rapid temperature changes, an eccentric fiber has asymmetric thermal conduction paths because one side is close to the tube wall while the other is far from it. This causes the sensor to exhibit directional thermal response latency errors to rapid temperature fluctuations from different directions. #### 4. Reduced Mechanical Tensile Protection The primary role of seamless stainless steel tubing is to provide compressive and mechanical protection for the fragile optical fiber (e.g., the glass cladding of standard single-mode fiber has a \text{Cladding Diameter} = 125\ \mu\text{m} ). - If the optical fiber is severely eccentric and tightly adheres to the tube wall, when the steel tube experiences localized lateral squeezing, external pressure can more easily act directly on the fiber, bypassing the buffering medium (such as adhesive or filling compound). This can cause micro-bending loss in the fiber, and in extreme cases, can lead to fiber fatigue and fracture. * * * ### II. Practical Engineering Design: Loose Tube Fiber Optic Cable vs. Tight Buffered Sensors In industrial practice, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) adopts different concentricity design strategies for different sensing application scenarios: #### 1. Distributed Sensing and General Communication (Loose Tube Design) For distributed temperature (DTS), strain (DSS), or acoustic (DAS) detection, loose tube seamless steel tube fiber optic cables are commonly used: - [OFSCN® 85°C Seamless Steel Tube Optical Cable](https://www.ofscn.org/dofs/85-c-sst-optical-cable.html) - [OFSCN® 200°C Seamless Steel Tube Optical Cable](https://www.ofscn.org/dofs/200-c-sst-optical-cable.html) - [OFSCN® 300°C Seamless Steel Tube Optical Cable](https://www.ofscn.org/dofs/300-c-sst-optical-cable.html) - [OFSCN® 650/700°C Seamless Steel Tube Optical Cable](https://www.ofscn.org/dofs/700-c-sst-optical-cable.html) In these products, to incorporate **Excess Fiber Length (EFL)** to counteract thermal stress from temperature variations between the stainless steel tube and the optical fiber at extreme high or low temperatures, the fiber exhibits natural wavy micro-bending within the steel tube. **At this point, the fiber is not physically in the strict center position** , but loosely adheres to the inner wall of the steel tube. In such loose tube and unfilled designs, the steel tube primarily provides mechanical protection and heat transfer, and slight eccentricity has no significant negative impact on sensing. #### 2. Fiber Bragg Grating (FBG) Sensors and Mechanical Measurement (Tight Buffer Design) If Fiber Bragg Gratings (FBGs) are encapsulated within a steel tube and used as strain gauges, pressure sensors, displacement sensors, or bending sensors: - Special high-rigidity injection curing or all-metal welding encapsulation is required. - During the processing of such products, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) implements **precise concentricity control** through tooling molds and curing processes. This ensures the optical fiber is located at the geometric center (neutral axis) of the stainless steel tube, thereby eliminating false tensile or compressive strain introduced by installation bending and reducing asymmetric transverse shear caused by temperature drift, ensuring the sensor has high repeatability and accuracy. * * * The following are the structures of high-precision seamless steel tube optical cables produced by Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®): ![OFSCN® Seamless Steel Tube Optical Cable](https://forum.ofscn.net/uploads/default/original/1X/4bc856cdafec927803a4e69b1cb132d968f5ba25.jpeg) ![OFSCN® High-Temperature Seamless Steel Tube Optical Cable 200℃](https://forum.ofscn.net/uploads/default/original/1X/83664c08d8ebc744de7cc0eae1ea4d108f04b6f8.jpeg) In summary, **loose tube fiber optic cables used for protection and transmission allow for non-central positioning** ; however, for **tightly buffered sensors used for precision mechanical and anti-interference temperature measurements, the optical fiber must maintain high concentricity as much as possible**. Otherwise, eccentricity will introduce bending strain interference, asymmetric thermal response, and anomalous multi-axial stress.