According to the different modes of refractive index distribution, multimode fibers can be divided into step-type multimode fibers and gradual-type multimode fibers. Because of their different working principles, there are differences in their applications.
Working principle and application of step type multimode fiber
There is a uniform distribution of refractive index in step type fiber. Since the refractive index in cladding is low, that is to say, the core refractive index is higher than that of cladding, so the refractive index at the boundary between the core and cladding decreases sharply, thus forming a step. For step type multimode fiber, light propagates along the fiber axis according to the principle of total reflection. Among them, the light transmission paths into the optical fiber with different incident angles are different. Although the incident light transmits at the same speed at the input end, it takes different time to reach the output end, resulting in the time dispersion, which leads to the serious broadening of the pulse, which is the so-called inter-mode dispersion.
Because digital communications use light pulses to signal along optical fibers, modal dispersion causes the pulses to be severely widened and spread as they travel with the fiber. The more modes the fiber travels the more pulses it has to walk. This also greatly limits the bandwidth of step - mode multimode fiber. Moreover, inter-mode dispersion is not suitable for optical fiber communication. For digital optical fiber system, when the dispersion is serious, it will lead to overlapping of pulses, which will lead to ISI and increase BER. Therefore, the dispersion of optical fiber not only affects the transmission ability of optical fiber, but also limits the relay distance of optical fiber communication system. Because of these limitations, step fiber multi-mode are often used at relatively low cost in communication systems over short distances (within a few kilometers) and at low speeds (below 8Mb/s).
Working principle and application of tapered multimode fiber
The refractive index of tapered fiber varies continuously according to a certain law and is not uniform. The refractive index of the tapered fiber is the largest at the fiber axis and the smallest near the cladding boundary. In other words, the refractive index of the tapered fiber decreases with the increase of the core radius. In a gradient fiber, a change in the refractive index causes refraction, but not total reflection, and when the light is transmitted to the cladding boundary (with the minimum refractive index), the fiber will be refracted back to the fiber axis.
For the tapered multimode fiber, the light travels forward in the form of sinusoidal oscillation. Just like the step type multimode fiber, the different light propagates along different paths in the graded multimode fiber. Among them, the light propagation rate is different, because the light rate will change with the refractive index of the fiber core. The farther the light is from the fiber axis, the higher the speed will be. In other words, the smaller the refractive index, the higher the propagation rate. At the same time, the tapered multimode fiber has self-focusing effect, and the corresponding rays at different incident angles will concentrate at the same point, and the time delay of these rays is approximately equal. Because of this, the inter-mode dispersion can be greatly reduced, so that the bandwidth of tapered multimode fiber is higher than that of step multimode fiber. As a result, most multimode fibers today are tapered. Compared with step multimode fibers, tapering multimode fibers are usually used in communication systems with medium distance (10~20 tabs) and relatively high transmission rate (34~140Mb/s), resulting in higher cost.
