In optical fibers, core and cladding are the two main components that enable light transmission through the fiber.
The core is the central, inner part of the fiber where light travels. It is made from glass or plastic with a high refractive index, allowing light to be efficiently guided through it. The core size and material determine the fiber's performance, particularly in single-mode and multi-mode fibers. Single-mode fiber has a small core, typically around 9 micrometers in diameter, allowing only one mode of light to propagate. It’s ideal for long-distance and high-bandwidth transmission. Multi-mode fiber has a larger core, around 50 to 62.5 micrometers, allowing multiple modes of light. It's more suitable for shorter distances, like within buildings or data centers.
Cladding is located surrounding the core. It has a slightly lower refractive index than the core, which helps keep the light confined within the core through a phenomenon called total internal reflection. This difference in refractive indices between the core and cladding causes light to bounce within the core as it travels, maintaining the signal strength over distances.
Zemax models core and cladding with mixed mode, as below. The nonsequential component is to integrate multiple layers of material for core and cladding. Light propagation is described in sequential part.
The three fibers above demonstrate laser beams with different numerical apertures (NA):
ncore and nclad are the refractive index of the fiber's core and cladding. θ is the half-angle of the cone within which light must enter to be propagated within the fiber.
Here in the model, three fibers of different NAs are listed compared to the system NA. The blue, green and red represent lower, equal and greater system NA than fiber NA.
The overall system is described in the sequential surface editor below, with 3 configurations. The mixed mode inserts nonsequential component into sequential surface list.
The two components in the nonsequential component list represent cladding (commented as Outer) and core (commented as Inner). The material of cladding is defined as FK3, with a refractive index of 1.4645 and abbe number of 65.769.
The material of core is defined as K5, with a refractive index of 1.5225 and abbe number of 59.483. Both materials can be found in the material catalog of SCHOTT.
While the three layouts of NAs demonstrate light coupling into fiber. The three scenarios can lead to two coupling results.
The first is light NA ≤ fiber NA (blue and green), which is the ideal scenario. If the light source’s NA is equal to or smaller than the fiber's NA, nearly all emitted light can enter the fiber and propagate through the core. This results in high coupling efficiency.
The second is light NA > fiber NA (red). In this situation, only the portion of the light that falls within the fiber’s NA acceptance cone will enter. Light rays beyond the fiber’s acceptance angle will not be guided and will scatter or be lost. This leads to lower coupling efficiency, as a significant amount of light cannot enter the fiber.
