Step-type: fiber core refractive index higher than the cladding refractive index, the entry of a packet of light energy in the core continue to produce total reflection at the interface layer and move forward. The fiber core refractive index is uniform, slightly lower refractive index cladding. Fiber center core to the cladding refractive index of glass is mutation, only one step, so called step-index multimode fiber, referred to as step-index fiber, optical fiber, also known as mutations. A lot of this fiber transmission mode, various modes of transmission path is not the same, the time to reach the finish line after the transmission is not the same, resulting in delay difference, by broadening the optical pulse. Therefore, this fiber mode dispersion is high, not wide band transmission, the transmission rate can not be too high, not ideal for communication, only for short-distance low-speed communications, such as: industrial control. However, single-mode fiber as mode dispersion is very small, so the single-mode fiber are used mutant. This is a kind of optical fiber research and development earlier, is now gradually being phased out.
Graded-index optical fiber: Fiber step to solve the drawbacks, it has developed, the development of graded-index multimode fiber, short fiber gradient. Center core to the glass fiber cladding refractive index is gradually smaller, make high-order mode propagation of light by sinusoidal form, which can reduce the mode dispersion, improving fiber-optic bandwidth, increased transmission distance, but the cost is higher now than mostly graded multimode fiber optical fiber. Fiber cladding refractive index gradient distribution of step-index fiber, as for the uniform. Gradient refractive index of the center's core optical fiber, the direction along the core radius decreases. Due to the high-order mode and low order modes of light at the interface of different refractive index layer according to the law of refraction refraction, to go into the low refractive index layer, so the direction of travel of light and optical axis angle formed by the gradual change small. The same process occurs constantly, until the light refractive index layer in a total reflection, the light to change direction, toward the center of a high refractive index layer of the road. At this time, direction of travel of light and optical axis angle posed in the refractive index of refraction of each layer once its value increases again, and finally to the refractive index of the largest local center. In this later. And the same process repeats, thus achieving a light transmission. It can be seen, the light fiber in the gradient will be consciously adjusted to the final destination, this is called self-focusing.