Will the strength of optical fibers decrease after long-term exposure to the sun?
Yes, if the bare optical fiber (or optical fiber with only the primary polymer coating) is exposed to direct sunlight for extended periods, its mechanical strength will significantly decrease, posing a risk of breakage.
The fundamental physical and material mechanisms behind this strength degradation are as follows:
1. Quartz Glass Body vs. Polymer Coating
- **Quartz Glass Body (Core and Cladding, Outer Diameter 125\ \mu\text{m}) **: Primarily composed of high-purity silicon dioxide (\text{SiO}_2), its chemical bonds are extremely strong. The energy of natural ultraviolet (UVA, UVB) rays in the solar spectrum is insufficient to directly break the mechanical structure of the quartz lattice. Therefore, natural sunlight has a negligible direct physical impact on the mechanical strength of the quartz body.
- **Polymer Coating (Outer Diameter 255\ \mu\text{m} or 155\ \mu\text{m}) **: Standard communication fibers (like OFSCN® G.652D Optical Fiber) and sensing fibers are typically coated with a layer of UV-curable polyacrylate or polyimide over the quartz to serve as a protective layer.
2. Core Mechanisms of Strength Degradation Due to Prolonged Sunlight Exposure
- Photodegradation and Embrittlement of the Coating
UV photons from sunlight can break chemical bonds in polymer molecules like polyacrylate, initiating photo-oxidative crosslinking and chain scission reactions. Prolonged exposure causes the coating to yellow, harden, become brittle, shrink, delaminate, or even crack and peel. - Microcrack Propagation and Stress Corrosion (Primary Failure Cause)
The primary function of the coating is to protect the quartz surface from mechanical abrasion and to prevent the ingress of moisture and chemical agents. When the coating fails:- The quartz glass surface is directly exposed to the environment, making it susceptible to the formation of microscopic Griffith cracks, which are invisible to the naked eye.
- Moisture in the air reacts with the tips of these microcracks on the quartz surface (hydrolysis reaction: \text{Si-O-Si} + \text{H}_2\text{O} \rightarrow 2\text{Si-OH}). When the fiber is under bending or tensile stress, static fatigue and subcritical crack growth (Stress Corrosion) occur, leading to an exponential decrease in the fiber’s breaking strength.
3. Engineering Protection Standards
- Direct exposure of bare fibers or fibers with only a primary coating to outdoor sunlight is strictly prohibited.
- For outdoor installations, it is essential to use outer sheath materials with UV resistance (e.g., polyethylene (PE) sheaths containing carbon black, cross-linked polyolefins) or to employ physical protection such as seamless stainless steel micro-ducts to shield the fiber from light and moisture.