Certified Fiber Optics Practice Exam

Which parameter decreases as the core diameter of an optical fiber decreases?

1. Bandwidth

2. Modal dispersion

3. Number of modes

4. Numerical aperture

The correct answer is: Number of modes

The correct choice highlights that as the core diameter of an optical fiber decreases, the number of modes that the fiber can support also decreases. In single-mode fibers, which have a smaller core diameter (typically around 8 to 10 microns), only one mode of light can propagate. This is in contrast to multimode fibers, which have larger core diameters (such as 50 or 62.5 microns) and can support multiple modes of light propagation. This effect is crucial because it directly influences how light travels through the fiber. A smaller core diameter limits the number of light paths, thereby reducing modal dispersion. Modal dispersion occurs in multimode fibers due to different modes traveling at different speeds, which can lead to signal distortion over longer distances. Therefore, a decrease in the core diameter effectively supports fewer propagation modes, which can help maintain signal integrity in systems that are distance-sensitive. Understanding this principle is important for fiber optic design and selection, especially in telecommunications where bandwidth and transmission distance are critical factors.