Why does the encapsulation process account for more than half of the sensor’s price?
In optical sensor engineering, the packaging process often accounts for more than 50% to 80% of the total sensor manufacturing cost. This phenomenon is jointly determined by the physical characteristics of fiber optic sensors, the need for engineering protection, and the stringent requirements for high-precision physical quantity transmission.
I. Core Physical Contradiction: Extreme Fragility vs. Rigorous Industrial Application Environments
The core sensitive element of Fiber Bragg Gratings (FBGs) is essentially a fragile silica glass fiber:
- The core diameter is typically only about 9\ \mu\text{m} , with a cladding outer diameter of approximately 125\ \mu\text{m} .
- Unprotected bare fiber (Bare Fiber) has a microscopic surface that is highly susceptible to fracture or severe optical loss due to lateral shear forces, micro-bending effects, moisture intrusion, and environmental corrosion.
To transform a fragile glass fiber core into an industrial-grade sensor that can operate stably in field conditions (such as high and low temperatures, high humidity, underwater environments, mechanical shock, etc.) for extended periods, complex mechanical and material packaging engineering is essential to provide physical protection.
II. Key Technical Reasons for High Packaging Process Costs
1. Precision Materials and Special Machining Costs
Packaging materials are not ordinary protective sleeves; they require special materials with high tolerance requirements, selected based on the sensor’s function:
- Metal and Alloy Micro-tubes: Such as miniature seamless stainless steel tubes or elastic alloy tubes, typically with outer diameters controlled between 0.5\text{ mm} and 2.3\text{ mm} . The cost of micron-level processing for fine tube diameters, internal wall cleaning, and annealing treatment is significantly higher than that of the fiber raw material itself.
- Polymer Materials and Anti-corrosion Coatings: The manufacturing costs of specialty polymers or ceramic-based cladding materials resistant to high and low temperatures, acids, and alkalis are relatively high.
2. Stress Transfer and Thermal Expansion Matching Control (High Technical Barrier)
The purpose of packaging is not just “protection” but also “precise transmission of physical quantities”:
- Strain Sensors: It is crucial to ensure that the protective tube/filling medium and the internal fiber Bragg grating form a slipless and creep-free matrix coupling, allowing even small external strains (e.g., \ge 3000\ \mu\varepsilon or even greater) to be transmitted to the fiber grating region with 100\% fidelity.
- Temperature Sensors: “Stress-free packaging” must be achieved (preventing the protective structure’s thermal expansion mismatch from inducing additional mechanical stress on the internal fiber, which could lead to temperature-strain cross-sensitivity and wavelength drift).
3. Process Complexity and Low Fault Tolerance
Sensor packaging involves a series of high-precision operations:
- Bare fiber stripping and recoating (e.g., polyimide recoating)
- Precision micro-cutting and splicing of fiber end faces
- Assembly and grinding of fiber connectors (e.g., FC/APC, ST, etc.)
- Fixed-length micro-gripping, sealing, or micro-welding
Any deviation at the sub-micron level in any of the above steps can lead to the destruction of the entire sensor or degradation of its spectrum.
4. High-Standard Temperature/Strain Factory Calibration and Heat Treatment
After packaging, sensors must undergo complete physical calibration and thermal cycling annealing:
- Full-range wavelength calibration is required in a standard temperature-controlled chamber or on a tensile testing platform.
- First- or second-order polynomial calibration coefficients are derived (in units of \text{^\circ C/pm} or \mu\varepsilon/\text{pm} ).
The depreciation of testing equipment, labor hours, and energy consumption associated with this calibration process are also significant components of the packaging cost.
III. Official Packaged Product Examples
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) has developed various micro-packaged fiber Bragg grating sensor structures for different application scenarios:
1. Seamless Steel Tube Packaged Temperature Sensor
OFSCN® 100°C Fiber Bragg Grating Temperature Sensor utilizes miniature single-layer seamless steel tube packaging technology to protect the fiber Bragg grating from moisture and pressure. The standard product has an outer diameter of 0.9\text{ mm} (customizable down to 0.6\text{ mm} ).
2. Elastic Alloy Tube Packaged Strain Sensor
OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor employs a miniature elastic alloy tube structure for packaging, achieving efficient strain transfer and protection. The default product outer diameter is \le 1.1\text{ mm} .
