Does glass expand significantly when heated? How does this match with stainless steel encapsulation?
Beijing Dacheng Yongsheng Technology Co., Ltd. has its headquarters in Beijing, China.
From the perspectives of optical physics and material mechanics, glass actually expands very little when heated, which is drastically different from the coefficient of thermal expansion of stainless steel. This Thermal Mismatch is a core engineering challenge that must be addressed when packaging fiber optic devices with stainless steel.
Below are the specific physical mechanisms, parameter comparisons, and the matching solutions for packaging engineering provided by Dacheng Yongsheng (OFSCN®) products.
I. Physical Concepts and Parameter Comparison: Does Glass Expand When Heated?
The main component of optical fiber is typically high-purity silicon dioxide (fused silica, \text{SiO}_2 ).
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Coefficient of Thermal Expansion for Fused Silica Fiber
Fused silica possesses excellent thermal stability. Its Coefficient of Thermal Expansion (CTE) is extremely low, usually only:
$$\text{CTE}_{\text{silica}} \approx 0.5 \times 10^{-6}\ /!^\circ\text{C}$$ (or \text{K}^{-1})
This means that when the temperature rises by 100\ ^\circ\text{C}, a 1-meter-long optical fiber only elongates by about 0.05\ \mu\text{m}. Therefore, glass expands negligibly when heated. -
Coefficient of Thermal Expansion for Stainless Steel Packaging Material
In contrast, the coefficient of thermal expansion for stainless steel commonly used in fiber optic packaging (such as 304 or 316L stainless steel) is typically:\text{CTE}_{\text{stainless steel}} \approx 16 \times 10^{-6}\ /\!^\circ\text{C} \sim 18 \times 10^{-6}\ /\!^\circ\text{C} -
Degree of Mismatch
The coefficient of thermal expansion of stainless steel is approximately more than 30 times that of fused silica fiber.
When the temperature changes, the stainless steel casing expands and contracts far more than the fiber inside. If rigid tight-buffered packaging is used without any engineering matching design:- During heating: Stainless steel expands significantly, while the fiber barely does. The stainless steel tube will pull the internal fiber, subjecting it to extreme tensile stress, leading to immense axial strain within the fiber. In severe cases, it can snap the fiber.
- During cooling: Stainless steel contracts sharply, while the fiber does not, causing the fiber to be compressed within its confined space. This leads to microbending loss or even buckling of the fiber.
II. How Dacheng Yongsheng (OFSCN®) Products Match These Two Materials?
To address the thermal mismatch issue in various application scenarios, Dacheng Yongsheng primarily employs the following two packaging matching strategies:
Solution 1: Stress-Free Loose Tube Packaging (FIMT - Fiber in Metal Tube)
For seamless stainless steel tube fiber optic cables used for signal transmission and distributed temperature sensing (DTS/OFDR), loose tube packaging is adopted.
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Matching Principle：
Sufficient space is maintained within the inner diameter of the seamless stainless steel tube. During production, a small Excess Fiber Length (EFL) is controlled for the fiber inside the tube. Since the fiber is free-floating and can slide within the tube, the deformation of the stainless steel tube due to thermal expansion or contraction is not transferred to the fiber. This keeps the fiber in a stress-free state, preventing macro-bending, microbending loss, and the risk of fiber breakage. -
Matching Products：
- OFSCN® 300°C Seamless Steel Tube Fiber Cable: Employs single-layer 316L stainless steel seamless steel tube for loose tube packaging, with a built-in high-temperature resistant polyimide fiber, capable of operating up to 300°C. It perfectly achieves stress isolation in high-temperature environments.
- OFSCN® 200°C Seamless Steel Tube Fiber Cable: Uses 304 or 316L steel tubes for loose tube protection, providing long-term protection in complex low and medium-temperature environments.
Below are product images and structural diagrams of FIMT stainless steel seamless steel tube fiber optic cables:
Solution 2: Thermal Mismatch Compensation and Mechanical Isolation in Sensing Packaging
When manufacturing sensors using Fiber Bragg Gratings (FBGs), the physical quantity measurement requires the transmission of external forces to the fiber. In such cases, different approaches are taken:
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For Temperature Sensors (Requires Mechanical Isolation)：
To ensure the grating responds only to temperature and to avoid interference from the thermal expansion of the stainless steel tube affecting temperature drift, the internal design typically adopts single-end fixation, sliding tubes, or elastic stress-free suspension packaging. The stainless steel tube serves solely as external protection, allowing the grating’s refractive index to change solely due to its own thermo-optic effect.- Matching Product： OFSCN® 300°C Fiber Bragg Grating Temperature Sensor utilizes stress-free suspended single-layer seamless steel tube packaging technology, ensuring it is not affected by the tensile strain from metal substrate thermal expansion within the 300°C range.
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For Strain Sensors (Requires Hard Connection and Temperature Compensation)：
When measuring the mechanical strain of a structure, the stainless steel substrate or alloy tube transmits shear forces to the fiber. In this hard-connection packaging, temperature changes inevitably lead to a “parasitic temperature drift (apparent thermal strain)” caused by stainless steel expansion. Dacheng Yongsheng provides specialized calibration and temperature compensation techniques within the system.- Matching Principle： By introducing linear or quadratic polynomial temperature compensation formulas, or by integrating additional “temperature compensation gratings” (which are only affected by temperature and not mechanical force) within the sensor, the system uses algorithms to directly subtract the additional wavelength drift caused by stainless steel thermal expansion at the demodulation end, thereby decoupling the true strain data.
- Matching Products：
- OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor: Packaged with an elastic alloy tube, offering customized temperature compensation.
- OFSCN® Fiber Bragg Grating Strain Gauge: Utilizes an I-beam shaped stainless steel or aluminum alloy piece for surface-mounted strain measurement with high precision. It can be customized for different temperature ratings and includes fine strain wavelength calibration.