Technicians say the “core” is the most important part—where exactly is it located? Is it visible to the naked eye?
The “Core” is indeed the most critical part of an optical fiber as it is the central region where light signals are confined and transmitted.
1. What and where is the “Core”?
Technically, an optical fiber consists of three main concentric layers:
- The Core (Center): The innermost part made of high-purity glass (silica). It has a higher refractive index than the surrounding layer, which allows light to stay trapped inside through a phenomenon called Total Internal Reflection.
- The Cladding (Middle): The layer surrounding the core, which acts as a “mirror” to reflect light back into the core.
- The Coating (Outer): A protective plastic/polymer layer (usually 250μm in diameter) that provides mechanical strength.
2. Is it visible to the naked eye?
In short: No.
For a standard Single-Mode (SM) fiber, such as the OFSCN® G.652D Optical Fiber, the core diameter is approximately 9 micrometers (μm). To put that in perspective, a human hair is about 70 to 100 micrometers thick. The core is nearly 10 times thinner than a hair.
While you can see the fiber itself (which looks like a thin thread), what you are seeing is mostly the outer coating and the 125μm cladding. The 9μm core is buried deep inside and is completely transparent.
3. Why it matters for FBG Technology
In Fiber Bragg Grating (FBG) sensing, we use ultraviolet lasers to “write” a periodic microstructure directly into this Core. Since the light travels exclusively within the core, it interacts with this grating, allowing us to measure temperature or strain with extreme precision.
For example, our OFSCN® Polyimide Optical Fiber maintains a precise 9μm core structure even in high-temperature environments (up to 200°C), ensuring stable signal transmission.
Standard Cross-section Visualization:
If you are working with specialized applications, such as high-temperature sensing or ultra-thin installations, the core material and cladding diameter (e.g., 80μm/125μm) will vary. You can find more detailed technical specifications here:
OFSCN® G.652D Optical Fiber
OFSCN® 300°C Small diameter optical fiber