Angular Divergence
Fiber-optic transmission degradation occurs when light is transferred between two optical guides with differing maximum acceptance angles. This phenomenon of numerical aperture mismatch occurs when the launching fiber has a larger numerical aperture than the receiving fiber, leading to unguided cladding modes. It results in localized heating and optical signal loss at the connection point.
Power Loss
Optical power loss increases with the magnitude of the angular difference. The numerical aperture mismatch causes some of the transmitted light to exit the joint at angles greater than the critical angle of the receiving fiber, causing that light to escape the core. This loss is asymmetric, as light travelling from a lower numerical aperture to a higher numerical aperture does not suffer from this angular escape.
Engineers must factor this directional behavior into their network link loss calculations.
Launch Condition
Measurement accuracy of this loss depends on the modal distribution of the test source. If a source overfills the launching fiber, the numerical aperture mismatch loss will appear larger than in a system using a restricted launch condition. Mode filters are employed to stabilize the launch before the measurement joint.
Splice Mitigation
Alignment systems and core diffusion techniques help reduce the impact of these differences. Splice operators can apply a thermally expanded core process to gradually transition the mode field size, reducing the numerical aperture mismatch effect. This optimization is necessary for high-power fiber lasers where joint losses can damage surrounding materials.
