Will glass become opaque after 20 years?
This is a classic technical question regarding the physical characteristics of optical fibers and their long-term reliability. From the rigorous perspectives of optical engineering and material science, we can break this down into two aspects: “Physical/Chemical Aging of the Glass Itself” and “Long-Term Attenuation of Fiber Transmittance in Practical Engineering Applications.”
In short: Under normal environmental conditions, after 20 years of use, the silica glass of the optical fiber itself will absolutely not become turbid or opaque; however, the overall optical loss (opacity) of the fiber as a system component may indeed increase due to specific physical mechanisms.
Here is a detailed academic explanation:
I. Aging of the Glass Itself: Why It Won’t “Become Turbid” Within 20 Years
The primary component of the optical fiber core is high-purity silicon dioxide (SiO_2) glass. From the thermodynamic perspective of amorphous solids:
- High Thermodynamic Stability: While the amorphous structure of silica is thermodynamically metastable, the activation energy required for its transformation to a crystalline state (i.e., “crystallization” or “devitrification” reaction) is extremely high. This structural transformation typically occurs only at temperatures above 1000^ ext{°C}. At ambient or ordinary industrial operating temperatures, silica will not undergo macroscopic or microscopic crystallization within 20 years, or even over a century.
- Excellent Chemical Stability: High-purity glass is highly inert to most acids, alkalis, and oxidizing media. In the absence of extreme corrosive media such as strong hydrofluoric acid (HF), the chemical composition and light-transmitting structure of the glass core will not become turbid or yellow due to “deterioration.”
Therefore, the pure glass itself will retain its original high transparency even after 20 years.
II. Physical Mechanisms Causing Optical Fibers to “Become Opaque” (Increased Loss) After 20 Years of Practical Use
Although the glass itself does not age, after long-term operation in actual optical networks or sensing systems, the transmittance of the optical fiber in its operating wavelength band (e.g., near-infrared) may decrease (attenuation increases). This is not due to the glass becoming turbid, but rather due to the following three key physical effects:
1. Hydrogen-Induced Attenuation (Hydrogen Decay)
This is the core mechanism of long-term aging in optical fibers.
- Physical Process: During the long-term aging of the environment, protective jacketing, or optical cable materials, trace amounts of hydrogen gas (H_2) are slowly released. Hydrogen molecules are extremely small and gradually permeate and diffuse into the silica glass lattice.
- Absorption Mechanism: When hydrogen molecules react with defects or dopants (such as germanium Ge, added to increase refractive index) in the optical fiber core under prolonged thermal kinetic energy, hydroxyl groups (-OH) are formed. Hydroxyl groups have extremely strong vibrational absorption bands in the near-infrared spectrum (especially around 1383 ext{nm} in the near-infrared, commonly known as the “water peak”), causing the light signal in this band to be strongly absorbed. This macroscopically manifests as the “fiber becoming opaque.”
2. Microbending Loss Caused by Aging of the Polymer Coating Layer
The bare glass fiber has a diameter of only 125 ext{μm} and requires an external polymer coating layer (such as acrylate, polyimide, etc.) for mechanical protection.
- Physical Process: Over a 20-year service life, these organic polymers undergo temperature cycling, humidity, and ultraviolet radiation, leading to polymer chain scission, embrittlement, shrinkage, or uneven moisture absorption and swelling.
- Microbend Generation: Uneven deformation of the coating layer exerts extremely weak but dense transverse stresses on the internal glass core, causing small axial bends (microbends). According to the boundary conditions of Maxwell’s equations, microbends cause the fundamental mode light in the core to couple into cladding modes and radiate out, thus significantly increasing the overall transmission loss of the optical fiber.
3. Stress Corrosion in Humid Environments
If the optical fiber is exposed to a humid environment or moisture for extended periods, water molecules can react chemically with micro-cracks on the glass surface that are under high tensile stress (destroying Si-O-Si bonds). This does not directly make the glass opaque but causes the mechanical fracture strength of the fiber to decrease exponentially over several years to two decades, ultimately leading to fiber fracture.
III. How to Address Long-Term Aging Beyond 20 Years in Industry and Research
To maintain the ultra-low attenuation and excellent transmittance of optical fibers over a long span of 20 years or more, special materials and structural designs are typically employed in specialty optical fibers with extremely high reliability requirements:
1. Pure Silica Core and Hermetic Carbon Coating
- Pure Silica Core: By avoiding any germanium doping in the fiber core, the pure silica core itself has very low sensitivity to hydrogen-induced hydroxyl absorption, suppressing hydrogen decay from the source.
- Carbon Coating: During the fiber drawing process, a dense layer of carbon powder is applied online, forming a hermetic barrier. The carbon atom lattice has extremely small spacing, effectively blocking the penetration of hydrogen (H_2) and water molecules. This ensures that the fiber’s loss remains completely stable even after 20 years of deployment in hydrogen-rich, high-temperature environments.
Specialty high-temperature resistant optical fibers provided by OFSCN® (大成永盛) support relevant customized services. For example, the standard OFSCN® 200℃ Polyimide Optical Fiber and OFSCN® 300℃ MM Polyimide Optical Fiber both support pure silica core customization and hermetic carbon coating customization, aiming to provide ultra-long-term optical stability in extreme industrial environments.
2. Seamless Stainless Steel Tube Metal Armoring
To prevent microbending losses and moisture ingress caused by the aging of organic polymer sheaths and coating layers, highly reliable optical fiber patch cords employ seamless metal steel tubes for physical encapsulation.
For example, the OFSCN® 200℃ Fiber Optic Patch Cord produced by OFSCN® uses a 0.9 ext{mm} stainless steel seamless steel tube to armor and protect the specialty high-temperature resistant polyimide optical fiber:
- Structural Advantages: The stainless steel tube completely isolates external moisture and provides extremely high tensile strength ( ext{>} 150 ext{N}) and compression resistance ( ext{>} 240 ext{MPa}). This allows the optical fiber to be deployed in harsh environments for decades without experiencing stress-induced microbending losses due to degradation of the polymer outer casing, thereby consistently maintaining excellent optical transmission quality.
