What is the colored “skin” that needs to be stripped off the fiber? Can light be transmitted directly if this layer isn’t removed?
The colored “skin” you are referring to is known as the Coating of the optical fiber.
In the structure of a standard optical fiber, there are three primary layers: the Core (where light travels), the Cladding (which keeps the light in the core via total internal reflection), and the Coating (the outermost protective layer).
1. What is the Coating?
The coating is typically a polymer layer (such as Polyacrylate or Polyimide) applied during the fiber drawing process. Its primary functions are:
- Mechanical Protection: It protects the glass (cladding and core) from scratches, moisture, and micro-bending that could lead to fiber breakage.
- Identification: The “color” you see is often a dye added to the coating to help engineers distinguish between different fibers in a multi-fiber cable.
2. Can light be transmitted without removing it?
Yes, light can and does travel through the fiber while the coating is intact. In fact, for data transmission and telecommunications, the coating is never removed except at the very ends where connectors or splices are made. The light is confined within the Core by the Cladding; the coating sits outside the cladding and does not interfere with the internal reflection of the light signal.
3. Why must it be stripped for FBG sensing?
In the context of Fiber Bragg Grating (FBG) technology, we often need to strip the coating for two main reasons:
- FBG Writing: To “write” a grating into the core using ultraviolet (UV) light, the coating must often be removed because standard polyacrylate coatings are opaque to UV.
- Sensing Sensitivity: For strain sensing, the coating can sometimes cause “creep” or damping. In high-accuracy applications, we use specialized coatings like Polyimide which are much thinner and harder, or we use “bare” fibers.
Related OFSCN® Products
Depending on your application environment, the type of coating is critical. For example:
- For standard environments (-40°C to 100°C): OFSCN® Polyacrylate Fiber Bragg Gratings use standard telecommunication-grade coatings.
- For high-temperature or high-vacuum environments (-200°C to 300°C): OFSCN® Polyimide Fiber Bragg Gratings use a much thinner (155μm) and more robust Polyimide coating.
If you are working on a specific project, could you clarify the temperature range or the specific measurement (strain or temperature) you are looking to perform? This will help determine which coating material is suitable for your needs.