1. Multimode fiber
When the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of light (about 1 µm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in time delay and broadening of optical pulses after long-distance transmission. This phenomenon is called modal dispersion (also known as intermodal dispersion) of theoptical fiber.
Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity fiber optic communications.
The refractive index distribution of multimode fiber is mainly a parabolic distribution, that is, a graded refractive index distribution. The diameter of its core is approximately 50 µm.
2. Singlemode fiber
When the geometric size of the fiber (mainly the core diameter) can be close to the wavelength of light, for example, the core diameter d1 is in the range of 5-10 µm, the fiber only allows one mode (fundamental mode HE11) to propagate in it, and the rest of the higher order modes are cut off, such fibers are called single-mode fibers.
Since it has a single mode of propagation and avoids the problem of mode dispersion, single-mode fiber has extremely wide bandwidth and is especially suitable for high-capacity fiber optic communications. Therefore, to achieve single-mode transmission, the fiber parameters must meet certain conditions. It is calculated by the formula that for a fiber with NA=0.12 to achieve single-mode transmission above λ=1.3 µm, The radius of the fiber core must be ≤4.2 µm, that is, its core diameter d1≤8.4 µm.
Since the core diameter of single-mode optical fiber is very small, stricter requirements are imposed on its manufacturing process.
Post time: Jan-10-2023
