What is the difference between step multimode fiber and graded multimode fiber?

According to the different ways of fiber refractive index distribution, multimode fiber can be divided into step multimode fiber and graded multimode fiber. Due to the different working principles of step multimode fiber and graded multimode fiber, there are differences in their applications. By reading this article, you will fully understand the difference between the working principles and applications of the two.

The working principle and application of step multimode fiber

There is a uniformly distributed refractive index in the step-type fiber. Because the refractive index in the cladding is low, that is to say, the refractive index of the core is greater than that of the cladding, so the refractive index decreases sharply at the boundary between the core and the cladding. A step is formed. For step-type multimode fibers, according to the principle of total reflection, light propagates in a zigzag along the axis of the fiber. Among them, the optical transmission paths entering the fiber at different incident angles are different. Although the incident light is transmitted at the same speed at the input end at the same time, the time to reach the output end is different, and there is a time dispersion, which leads to serious pulse broadening. , This is the so-called intermodal dispersion.

Since digital communication uses optical pulses for signal transmission along optical fibers, inter-mode dispersion can cause the pulse to be severely broadened and spread out when the pulse and optical fiber travel together. The more modes of fiber transmission, the more pulses that travel. This also greatly limits the bandwidth of the step multimode fiber. Moreover, the inter-mode dispersion is not suitable for optical fiber communication. For digital optical fiber systems, when the dispersion is severe, it will cause the pulses to overlap each other, leading to inter-symbol interference (ISI) and increasing the bit error rate (BER). Therefore, the dispersion of the optical fiber not only affects the transmission capacity of the optical fiber, but also limits the relay distance of the optical fiber communication system. Because of these limitations, step-type optical fiber multimode is usually used in short-distance (within a few kilometers) and low-speed (under 8Mb/s) communication systems at a relatively low cost.

Working principle and application of graded multimode fiber

The refractive index of graded optical fiber changes continuously and is not uniform according to a certain rule. The refractive index of the graded fiber is the largest at the fiber axis and the smallest near the boundary of the cladding. That is to say, the refractive index of the graded fiber gradually decreases with the increase of the core radius. In graded fiber, the change of refractive index will cause refraction, but total reflection will not occur. When the light is transmitted to the boundary of the cladding (the refractive index is minimum), the fiber will be refracted back to the fiber axis.

For graded multimode fibers, light propagates forward in the form of sinusoidal oscillations. Like step multimode fiber, different light in graded multimode fiber propagates along different paths, among which the light propagation speed is different, because the speed of light changes with the refractive index of the fiber core , The farther the light is from the fiber axis, the higher the speed. That is, the smaller the refractive index, the higher the propagation rate. At the same time, the graded multimode fiber has a self-focusing effect. The light rays corresponding to different incident angles will be gathered at the same point, and the time delay of these light rays is also approximately equal. Because of this, the inter-mode dispersion can be greatly reduced, so that the bandwidth of the graded multimode fiber is higher than that of the step multimode fiber. Therefore, most multimode fibers today are graded fibers. Compared with step multimode fiber, graded multimode fiber is usually used in communication systems with medium distance (10~20㎞) and relatively high transmission rate (34~140Mb/s), and its cost is higher.

Comparison of step multimode fiber and graded multimode fiber

In summary, the main differences between step multimode fiber and graded multimode fiber are reflected in the following aspects: