After stripping off the colorful jacket, is the shiny glass inside the one that transmits light? OFSCN®
After stripping off the colored outer jacket (coating), the exposed “shiny glass” is not entirely for light transmission. This part of the glass, in terms of physical and optical structure, actually contains two layers: the Cladding and the Core. The part truly used for transmitting optical signals is only the “Core” at the very center, which is imperceptible to the naked eye.
Three Physical Layers of an Optical Fiber
From outside to inside, standard optical fiber consists of the following three layers:
- Outer Layer: Coating
This is the “colored jacket” you strip off (common materials include polyacrylate, polyimide, etc.). The coating is not involved in light transmission; its primary function is to protect the fragile glass inside from mechanical damage, abrasion, and moisture.
The outer diameter of the coating for standard communication optical fibers is typically 250\ \mu\text{m} or 255\ \mu\text{m} . - Middle Layer: Cladding
This is the main part of the “shiny glass strand” you see with the naked eye after stripping the coating. The outer diameter of the cladding for standard single-mode fiber is 125\ \mu\text{m} . The refractive index of the cladding (denoted as n_2 ) is slightly lower than that of the core. Its primary role is not direct “light transmission” but rather to confine light firmly within the core through the mechanism of Total Internal Reflection, preventing optical energy from leaking outward. - Innermost Layer: Core
An extremely fine channel located at the exact center of the entire glass strand. It has a higher refractive index (denoted as n_1 ) to satisfy the condition for total internal reflection ($ n_1
Why Can’t We See the Difference with the Naked Eye?
This is because the core and cladding are both made from molten, drawn high-purity silica (quartz glass) during manufacturing, and they are completely fused together. The reason the core has a slightly higher refractive index is due to the artificial doping of extremely small amounts of elements (such as germanium dioxide \text{GeO}_2 ) into the silica. This doping is completely transparent and uniform in appearance, making it impossible for the naked eye to distinguish the 9\ \mu\text{m} core within the 125\ \mu\text{m} glass strand.
Combining with Standard Product Specifications
Taking the standard fiber optic specifications provided by Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) as an example, we can clearly see the geometric distribution of these three layers:
- OFSCN® G.652D Optical Fiber
- Core Diameter: 9\ \mu\text{m}
- Cladding Diameter: 125\ \mu\text{m}
- Coating Diameter: 255\ \mu\text{m} (the stripped colored jacket layer)
- OFSCN® G.657 Optical Fiber (Bend-Insensitive Single-Mode Fiber)
- Core Diameter: 9\ \mu\text{m}
- Cladding Diameter: 125\ \mu\text{m}
- Coating Diameter: 255\ \mu\text{m}
Summary
When you strip off the colored jacket:
- The 125\ \mu\text{m} glass strand presented to your eyes is primarily the Cladding, which serves to confine and reflect light.
- The part truly used for light transmission is only the ultra-fine Core at the center of this glass strand, with a cross-sectional area accounting for only about 0.5\% of the total (with a diameter of just 9\ \mu\text{m} ).