Certified Fiber Optics Practice Exam

What happens to the number of modes in an optical fiber as the wavelength increases?

1. It increases

2. It decreases

3. It remains the same

4. It becomes unpredictable

The correct answer is: It decreases

As the wavelength of light propagating through an optical fiber increases, the number of modes that the fiber can support generally decreases. This phenomenon can be understood in the context of how modes in a fiber are defined. Optical fibers operate by guiding light through a core that has a higher refractive index than its cladding. The number of modes is influenced by the core's diameter and the wavelength of light in relation to the fiber's structure. Specifically, longer wavelengths result in lower spatial frequencies of the light waves, which in turn can lead to fewer modes being able to fit within the same fiber. Additionally, as the wavelength increases, the acceptance angle for light entering the fiber narrows, further reducing the number of modes that can propagate. When the mode number decreases, only lower order modes are preserved, which affects signal propagation and bandwidth characteristics. This relationship is critical for understanding fiber optic design, as it influences choices about fiber type depending on the application and the wavelength used. In contrast, shorter wavelengths typically allow for a greater number of modes due to their smaller size, enabling more efficient transmission capabilities.