Why can some optical fibers be bent around a finger like an electric wire, while others lose their signal when bent?
The physical essence of signal transmission in optical fibers relies on the Total Internal Reflection mechanism that occurs at the interface between the fiber core (Core) and the cladding (Cladding). When the angle of incidence of the light signal exceeds the total internal reflection critical angle \theta_c, the electromagnetic field energy is confined losslessy within the waveguide structure (the core) and propagates forward.
The difference in signal performance of various optical fibers when bent is determined by their Macrobending Loss and internal Refractive Index Profile design:
1. Why do ordinary single-mode fibers lose signal when bent?
For standard G.652D single-mode fibers:
- Structural Characteristics: The refractive index difference between its core and cladding is relatively small, providing a gentler confinement of the optical field.
- Loss Mechanism (Macrobending Loss): When the fiber is bent, the geometric position of the waveguide at the bending interface shifts, causing the equivalent angle of incidence of the light wave in the curved section to decrease. When the bending radius is small (e.g., less than 30\text{ mm}, or even in the 10\text{ mm} range when wrapped around a finger), the equivalent angle of incidence will be less than the total internal reflection critical angle \theta_c.
- Result: Light energy that should have been totally internally reflected and propagated within the core penetrates the core interface, leaks into the cladding, and dissipates as radiation modes. This is why the signal in ordinary fibers instantly interrupts due to macrobending loss when bent.