Certified Fiber Optics Practice Exam

What occurs to the number of modes in an optical fiber as the core diameter decreases?

1. It increases

2. It decreases

3. It stabilizes

4. It fluctuates

The correct answer is: It decreases

As the core diameter of an optical fiber decreases, the number of modes that can propagate within the fiber decreases as well. This relationship is fundamentally rooted in the principles of modal propagation in optical fibers. In multimode fibers, larger core diameters allow for multiple light rays or modes to travel through the core simultaneously. When the core diameter is large enough, it can support numerous modes because these modes can follow different paths (or angles) as they bounce off the core-cladding interface. Conversely, as the core diameter diminishes, the ability for multiple modes to exist is restricted. A smaller core allows primarily a single mode to propagate, which is characteristic of single-mode fibers. This transition from multimode to single-mode operation occurs because the smaller diameter constrains the light to travel predominantly in a single path. By limiting the number of modes to one, single-mode fibers can reduce modal dispersion, which typically improves signal integrity over long distances. Thus, as the core diameter decreases, it is clear why the number of modes decreases, leading to enhanced fiber performance in specific applications where long-distance transmission is essential.