Doesn’t light travel in a straight line? Why doesn’t the light hit the “wall” and leak out when the fiber is curved?
Light generally travels in straight lines in a uniform medium, but it can follow the curves of an optical fiber due to a physical phenomenon called Total Internal Reflection (TIR).
How Total Internal Reflection Works
An optical fiber consists of two main parts: a Core (the center where light travels) and a Cladding (the outer layer). The core has a higher refractive index than the cladding.
- Reflection, Not Leakage: When light hits the boundary between the core and the cladding at a shallow angle (called the “critical angle”), it does not pass through the “wall.” Instead, it reflects 100% back into the core.
- Continuous Bouncing: As the fiber bends, the light continues to bounce off the internal walls, zigzagging its way through the fiber even if it is coiled or curved.
When Light Does Leak: Bending Loss
While TIR is very efficient, if a fiber is bent too sharply (exceeding its “minimum bend radius”), the angle at which the light hits the wall becomes too steep. In this case, TIR fails, and light “leaks” out of the core, leading to signal loss.
Related Technology at OFSCN
In sensing applications, we use specific types of fibers to manage these properties:
- Standard G.652D Fiber: Used in products like OFSCN® Polyacrylate Fiber Bragg Gratings (Bare), which follow standard bending rules.
- Bend-Insensitive Fiber (G.657): These fibers are designed with a special structure to keep light trapped even at much tighter bend radii.
- Small Diameter Fibers: For extremely tight spaces, we offer OFSCN® Thin-Diameter Fiber Bragg Gratings, which utilize fibers with smaller diameters to improve flexibility.
If the fiber is bent specifically to measure its shape, we use OFSCN® Fiber Bragg Grating Shape Sensors to calculate the exact degree of curvature.