It is only
10
nanometers thick, can it protect the glass?
This is a very classic problem in physics and optical engineering. Your observation is very keen; the thickness of polyimide (PI) coating on specialty optical fibers is indeed typically only about 10\ \mu\text{m} to 15\ \mu\text{m}.
I can tell you definitively: it provides excellent protection for the glass. In optical engineering, this extremely thin design is not only sufficient to protect the glass but its “thinness” itself is a key engineered advantage.
Here’s a detailed analysis from the perspectives of physics, mechanics, and sensor design, explaining how the polyimide coating works and why it is designed to be so thin:
1. Actual Thickness and Product Specifications of Polyimide Coating
For standard single-mode polyimide-coated optical fibers (e.g., OFSCN® 300℃ SM Polyimide Optical Fiber), the outer diameter of the silica cladding is the standard 125\ \mu\text{m}, and the outer diameter including the polyimide coating is 155\ \mu\text{m}.
The thickness of the coating on one side is calculated as:
(155\ \mu\text{m} - 125\ \mu\text{m}) / 2 = 15\ \mu\text{m}
In ultra-small diameter polyimide fibers (e.g., OFSCN® 300℃ Small diameter optical fiber), the silica cladding is 80\ \mu\text{m}, and the outer diameter with the coating is 100\ \mu\text{m}, making the coating thickness on one side exactly 10\ \mu\text{m}. This is the “tens of micrometers” you mentioned.
2. How Does a Coating of Only 10\ \mu\text{m} Protect the Glass?
When glass optical fibers (silica, \text{SiO}_2) are freshly drawn, their microscopic surfaces are extremely perfect and theoretically possess very high tensile strength. However, they have two critical weaknesses: micro-hydrolysis (stress corrosion) and mechanical scratches.
- Barrier Against Moisture to Prevent Stress Corrosion:
Trace amounts of moisture (\text{H}_2\text{O}) in the air, upon contact with exposed silica glass, react with microscopic/nanoscopic fissures on the glass surface. Under tensile stress, these fissures expand rapidly (i.e., subcritical crack growth), leading to easy fiber fracture. The polyimide coating is applied instantly and uniformly to the glass surface during the fiber drawing process. Although it is only 10\ \mu\text{m} thick, it acts as a robust physical and chemical barrier, effectively blocking moisture and chemical agents, and preventing the formation of critical stress concentration points on the glass surface. - Excellent Hardness and Wear Resistance:
Standard optical fibers use acrylate coatings, which are relatively soft, hence they need to be thicker ( 65\ \mu\text{m} on one side) to act as a buffer. Polyimide (PI), as a high-performance engineering plastic, has excellent tensile modulus (hardness) and strong resistance to wear and scratches. Even at a thickness of only 10\ \mu\text{m} to 15\ \mu\text{m}, it is sufficient to withstand various minor abrasive impacts from small hard objects during subsequent handling, coiling, and testing processes.
3. Why Design It So Thin? (Three Technical Advantages of Thin Coatings)
Making it thicker would actually sacrifice many valuable physical properties:
- Highly Efficient Strain Transfer for Sensing (Core Design for FBG):
In Fiber Bragg Grating (FBG) sensing applications (e.g., OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare)), external substrate deformation needs to be sensed by the optical fiber. If the coating is too thick and too soft, the strain force experiences significant dissipation and hysteresis due to “Shear Lag” within the coating as it transmits to the fiber core, leading to a substantial decrease in measurement accuracy. An extremely thin and hard polyimide coating, however, transfers strain to the fiber core with near-lossless efficiency, enabling strain measurements up to \le 10000\ \mu\epsilon or even \le 15000\ \mu\epsilon with no hysteresis.
- Excellent High and Low-Temperature Adaptability:
Due to the mismatch in the coefficient of thermal expansion (CTE) between polyimide and silica glass, under extreme temperature cycling (e.g., Beijing Dacheng Yongsheng’s polyimide fibers operate from -200\ ^\circ\text{C} to 350\ ^\circ\text{C} ), a thick coating would generate significant thermal stress between the coating and the glass, leading to cracking or delamination. A very thin coating ( 10\ \mu\text{m} class) minimizes this volumetric thermal stress effect. - Reduced Bending Stress and Volume Constraint:
When an optical fiber bends, the outer material experiences tensile stress, which is proportional to the fiber’s overall outer diameter. Limiting the outer diameter to 155\ \mu\text{m} (or even the thinner 100\ \mu\text{m}) allows the fiber to have a very small minimum bending radius, making it more suitable for deployment in space-constrained environments such as medical micro-catheters or tight spaces in spacecraft.
4. Summary and Application Boundaries
Conclusion: Despite being only 10\ \mu\text{m} thick, the polyimide coating provides excellent physical and chemical protection for the glass in terms of key metrics like moisture resistance, scratch resistance, high-precision sensing, and operation at extreme temperatures.
However, due to its thinness, it offers almost no “cushioning” effect against macro-level crushing from heavy objects, strong lateral pressure, or severe impacts. Therefore, if your application involves harsh industrial environments (such as geotechnical, bridge, or steel structure sites prone to mechanical impact), bare PI fibers cannot typically be used directly. They would require further external jacketing (e.g., seamless stainless steel tubes or polymer tight buffer sheaths commonly used by Beijing Dacheng Yongsheng) to provide a higher level of heavy mechanical protection.