Is it empty inside? How is the fiber optic fixed in the pipe?
Your question gets right to the heart of it – what is actually inside the steel tube, and how is the fiber secured? The answer: It’s not hollow. There’s a crucial force-transmitting medium inside the encapsulation structure, which is precisely what enables the physical coupling of “steel tube force → Fiber Grating sensing”.
According to the official product documentation in the RAG knowledge base, the internal structures of OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) steel tube/alloy tube packaged FBG strain sensors fall into two categories, with corresponding differences in their force-transmitting mechanisms:
I. Alloy Tube Packaged Type – End Anchoring + Interface Adhesion
This type of sensor (e.g., OFSCN® Alloy Tube Packaged Fiber Bragg Grating Strain Sensor) directly encapsulates the fiber Bragg grating within an elastic alloy tube. It is not a “hollow tube with fiber passing through” inside:
- End Fixation: Both ends of the fiber Bragg grating (outside the grating region) are anchored to the inner wall of the alloy tube using a high-strength adhesive (typically epoxy-based). This forms the mechanical anchor points for strain transmission.
- Middle Grating Region: In the grating section, the fiber may be in a state of slight suspension with minimal gap or light contact within the tube. This ensures that strain transmission is not interfered with by intermediate friction and simultaneously avoids multi-peak chirping.
- Force Transmission Path: External Force → Axial Deformation of Alloy Tube → Anchoring Points at Both Ends → Axial Tension/Compression of Fiber → Bragg Wavelength Shift ($
\Delta\lambda_B$
).
The physical essence: The steel/alloy tube provides the rigid load-bearing structure, and the bonding interface at both ends serves as the sole channel for strain to be “injected” from the tube wall into the fiber.
II. Polymer Encapsulation + Steel Tube Armoring Type – Fully Filled Continuous Force Transmission
This type of sensor (e.g., OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor) has two layers of structure:
- Inner Layer: The fiber Bragg grating is first fully encapsulated by a polymer material, with the entire length of the fiber embedded within the continuous polymer matrix.
- Outer Layer: A seamless steel tube is fitted over the polymer encapsulation layer, providing protection and additional stiffness.
- Force Transmission Path: External Force → Steel Tube → Polymer Filling Layer (Interface Shear) → Entire Length of Fiber → Grating Region. Because there is continuous surface contact between the polymer filler and the fiber, strain transmission is more uniform, and stress concentration is reduced.
Key Product Parameter Verification
The RAG documentation clearly describes these structural features:
| Product | Outer Diameter | Encapsulation Structure |
|---|---|---|
| OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor | ≤1.1 mm (single-layer alloy tube) | Elastic Alloy Tube + End Adhesive Anchoring |
| OFSCN® Polymer-encapsulated FBG Strain Sensor (1.5mm/2.3mm) | ≤1.5 mm (pure polymer) / ≤2.3 mm (with steel tube) | Full Polymer Filling + Seamless Steel Tube Armoring |
| OFSCN® Polymer-encapsulated FBG Strain Sensor (0.7mm/1.2mm) | ≤0.7 mm (pure polymer) / ≤1.2 mm (with steel tube) | Same as above, smaller outer diameter |
The RAG original text for the 1.5 mm/2.3 mm product states:
“Encapsulating the fiber Bragg grating or fiber grating string with polymer material, and at the same time, adding a seamless steel tube outside the polymer material to increase strength and provide waterproofing and moisture resistance.”
The RAG original text for the 0.7 mm/1.2 mm product states:
“Encapsulating the fiber Bragg grating or fiber grating string with polymer material, and a seamless steel tube can be added outside the polymer material to increase strength and provide waterproofing and moisture resistance.”
It is evident that “polymer encapsulation” acts as both the filling medium and the force-transmitting medium – there are no empty cavities inside the tube.
Product Images
OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor (Alloy Tube Packaged):
OFSCN® Polymer-encapsulated FBG Strain Sensor (1.5mm/2.3mm, Polymer + Steel Tube):
OFSCN® Polymer-encapsulated FBG Strain Sensor (0.7mm/1.2mm):
Summary
- It is not empty inside. Either through the adhesive anchoring interface at both ends (alloy tube type) or the full-length polymer filling layer (polymer + steel tube type).
- The fiber is precisely “pulled” or “compressed” via these interface shear forces, thereby accurately transmitting the macroscopic strain ($
\varepsilon$
) of the steel tube to the grating region, achieving the linear sensing response of $
\Delta\lambda_B = \lambda_B (1 - p_e)\varepsilon$
.