Light and technology: how does optical fiber transmit light?

 

Since the first quartz filament was pulled out in 1930, we have truly entered the era of optical fiber. So far, we already have various types of optical fibers, such as multi-mode optical fiber G.651, single-mode optical fiber G.652, G.653, G.654, G.655 and so on. These fibers form the basis of our entire optical communications.

 

So how is light transmitted in an optical fiber?

 

Say we want to shine a flashlight beam down a long, straight hallway at the other end, just point the beam straight down the hallway, the light travels in a straight line, and we know it's fine. What if the corridor has a bend? At this time, a mirror can be placed at the corner to reflect the beam at the corner. What if the hallway is very winding, with multiple bends? It is also possible to line the walls with mirrors and angle the beam of light so that it bounces from side to side along the hallway. This is exactly what happens when light travels through an optical fiber, but the light is totally reflected in the fiber.

Traditional pure bare fiber consists of two layers, inner and outer. To achieve the condition of total reflection, the refractive index N1 of the inner layer is greater than the refractive index N2 of the outer layer, and N1 greater than N2 is a necessary condition for the transmission of optical signals in the optical fiber.

 

However, when the light is transmitted in the optical fiber, when the optical fiber is bent, the number of back and forth reflections increases, and the angle of reflection also increases. As long as the bending is not particularly severe, the light will not leak out of the fiber, so we usually have certain restrictions on the bending of the fiber. In addition, we know that ordinary optical fibers are divided into single-mode and multi-mode optical fibers. What is the difference in their transmission methods, the following figure can be very intuitive display.

 

Here is a brief mention of a new type of optical fiber that has become popular in recent years: hollow-core optical fiber.

 

Unlike traditional optical fibers, hollow-core fibers do not guide the transmission of light by total reflection, but by guiding light through the hollow region between the fiber media, whose cross-section looks like a honeycomb.

The principle of light transmission in a hollow-core fiber is more like using a multi-layer mirror to achieve the effect of total reflection through the in-phase reflection of very many dielectric surfaces.