Transmission Principle of Multimode Fiber

Multimode fibers transmit light through multiple propagation paths, or modes, confined within a large core by total internal reflection, allowing simultaneous transmission of several light beams.Core ConceptMultimode fibers (MMFs) have a relatively large core diameter, typically 50 or 62.5 micrometers, which allows multiple light modes to propagate simultaneously . The core is surrounded by a cladding with a slightly lower refractive index, creating a refractive index contrast that confines light within the core through total internal reflection . This principle ensures that light entering the fiber at angles below the critical angle is guided along the fiber without escaping into the cladding.Modes of PropagationEach mode represents a distinct path that light can take through the fiber. In multimode fibers, light can travel along different trajectories, including straight paths and zigzag reflections off the core-cladding interface . The number of supported modes depends on the core diameter, numerical aperture (NA), and wavelength of the light. Step-index fibers have a uniform core refractive index, while graded-index fibers gradually reduce the refractive index from the center outward, which helps minimize modal dispersion .Modal Dispersion and Signal TransmissionBecause different modes travel slightly different distances, they arrive at the fiber end at different times, causing modal dispersion. This effect limits the bandwidth and transmission distance of multimode fibers, making them suitable for short- to medium-range applications such as data centers, LANs, and imaging systems . Proper launch conditions—ensuring light enters the core without significant power in the cladding and within the NA limit—are critical to reduce signal distortion and optimize transmission efficiency .Advanced ConsiderationsMultimode fibers can also support principal modes that are resistant to bending and environmental perturbations, which is important for applications like mode-division multiplexing and endoscopy . The output beam profile is sensitive to launch conditions, fiber bending, and temperature, which can affect the interference between modes and the overall signal quality .ApplicationsMultimode fibers are widely used in telecommunications, sensing, imaging, and industrial or medical applications due to their ease of light coupling, cost-effectiveness, and ability to transmit multiple modes simultaneously . They are particularly advantageous when using LEDs or laser diodes with poor spatial coherence, as these sources can efficiently launch light into the large core of multimode fibers .