What is the colored plastic layer on the outside of the glass called? Can it be fused without stripping it?
This colored
plastic sheath is known as the fiber coating in optical engineering.
1. What is Fiber Coating?
Standard communication optical fibers typically consist of three concentric layers:
- Core: Located at the very center of the fiber, it is the channel through which optical signals travel.
- Cladding: Surrounds the core, confining light within the core via total internal reflection. For standard single-mode fibers, the cladding outer diameter is usually 125\ \mu\text{m}, and its material is high-purity silicon dioxide (quartz glass, i.e., \text{SiO}_2).
- Coating: Encapsulates the cladding, providing mechanical protection, shock absorption, moisture resistance, and scratch prevention. In ordinary single-mode fibers, the material is typically polyacrylate, with a coating outer diameter of about 255\ \mu\text{m}. The colored appearance is due to colored inks applied to the surface of the coating, allowing for quick identification of different fibers in multi-fiber cables.
2. Can Fibers be Spliced Without Removing the Coating?
Absolutely not. Before fiber splicing, the coating must be completely stripped from the splicing area using a fiber stripper to expose the glass cladding beneath.
Directly splicing without removing the coating will lead to the following severe consequences:
- Vast Difference in Melting Points: Quartz glass (silicon dioxide) has a melting point of approximately 1700^\circ\text{C}, while the temperature resistance limit of polymer coatings is typically only a few hundred degrees Celsius.
- Instantaneous Burning and Severe Contamination: Fiber splicers use high-voltage electric arcs generated by discharge electrodes to melt the glass. If the coating is not removed, the arc’s high temperature will cause the polymer to instantly burn, carbonize, and vaporize. This not only produces harmful black smoke but also severely contaminates the electrodes, the collimating microscope, and the precision V-grooves, leading to damage or permanent loss of accuracy in the splicing machine.
- Splicing Failure and Optical Signal Interruption: The carbonized residue and impurities from the burning will be directly mixed into the molten glass. This prevents proper core alignment, causes significant insertion loss or complete blockage of the optical signal due to impurities, and results in a splice joint with virtually zero mechanical tensile strength, breaking with the slightest touch.
The standard splicing process must be: Strip coating \rightarrow Wipe bare fiber with high-purity alcohol \rightarrow Precisely cleave end face \rightarrow Place in splicer for fusion \rightarrow Re-protect with heat-shrinkable sleeve.
3. Different Coating Technologies and Engineering Applications
In specialty optical engineering, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers a variety of specialty fibers with different coating materials to adapt to various temperatures and environments:
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Standard Polyacrylate Coating
The most common general-purpose coating, offering excellent bending performance.- OFSCN® G.652D Optical Fiber: Standard single-mode fiber with polyacrylate coating, outer diameter of 255\ \mu\text{m}.
- OFSCN® G.657 Optical Fiber: Standard bend-insensitive single-mode fiber, also with polyacrylate coating, outer diameter of 255\ \mu\text{m}.
- OFSCN® G.652D Optical Fiber: Standard single-mode fiber with polyacrylate coating, outer diameter of 255\ \mu\text{m}.
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High-Temperature Polyimide Coating
In high-temperature environments, ordinary polyacrylate fails and decomposes. In such cases, a polyimide coating with extremely high mechanical strength and excellent temperature resistance is required.- OFSCN® 300℃ SM Polyimide Optical Fiber: Features a polyimide coating with a thinner outer diameter of only 155\ \mu\text{m}, offering a wide operating temperature range from -200^\circ\text{C} to 350^\circ\text{C}.
- OFSCN® 300℃ SM Polyimide Optical Fiber: Features a polyimide coating with a thinner outer diameter of only 155\ \mu\text{m}, offering a wide operating temperature range from -200^\circ\text{C} to 350^\circ\text{C}.
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Extreme Temperature Gold Coating
In applications involving extreme temperatures, such as aerospace and metallurgy, metal coatings are used to replace polymers.- OFSCN® Gold-coated Optical Fiber: Its coating material is metallic gold, with an operating temperature range from -270^\circ\text{C} to 700^\circ\text{C}.
- OFSCN® Gold-coated Optical Fiber: Its coating material is metallic gold, with an operating temperature range from -270^\circ\text{C} to 700^\circ\text{C}.
4. Supplementary Knowledge: Coating Stripping and Non-Stripping Processes in Device Fabrication
During the manufacturing of Fiber Bragg Grating (FBG) sensors, the handling of the coating process significantly impacts device strength:
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Stripping and Re-coating Process:
For example, when writing gratings using ultraviolet (UV) lasers, such as in OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare), the polyacrylate coating must be stripped before writing because it absorbs UV light. After writing, a specialized process is required to re-apply the polyacrylate coating. -
High-Strength Femtosecond Writing Process Without Stripping:
To completely eliminate micro-cracks on the glass surface caused by stripping and wiping, OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare) utilizes an advanced femtosecond laser point-by-point writing technique. Due to the nonlinear absorption characteristics of femtosecond lasers, they can directly penetrate the external polymer coating and focus within the fiber core for writing. The fiber coating does not need to be stripped throughout the entire process, thus perfectly preserving the fiber’s native mechanical strength.
