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What is Single Mode Fiber?

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This video explains single mode fiber in plain English. You will learn light ray theory and waveguide theory of fiber mode.

Before going further, you should know the typical structure of a single mode fiber.

The left illustration shows that single mode fiber has a core with a diameter of 8 micron, a cladding with a diameter of 125um, and a buffer with a diameter of 250um.

The core and cladding are made of fused silica, a type of glass, and the buffer is typically acrylic.

The right illustration shows how a glass fiber guides light. Lights enter into the core and is guided along the fiber.

What is a single mode fiber exactly? This picture shows a simplified light ray explanation.

The top two fibers are multi-mode fibers. Multimode fibers can have a core diameter ranging from 50 microns to 200 microns or even more.

The typically light ray explanation is that the core size is large enough, so that multiple light rays can enter into the core at different angles and are guided along the fiber. Here, different light rays at different angles are called different fiber modes. Since multimode fibers can guide many modes, that is why there are called MULTI Mode fiber.

On the other hand, the bottom fiber is a single mode fiber. The typically light ray explanation is that the core is so tiny, that only one light ray which is perpendicular at the core interface can enter into the core and is guided. Since there is only one mode, that is why it is called SINGLE Mode fiber.

But actually, the theory is not so simple. When it comes to single mode fiber, light ray theory is not enough to explain it. So here you see fiber mode calculation based on waveguide theory.

I will not go into the detail of the calculation process. Here is the result of the calculation.

V number is a key parameter that describes the mode structure of  an optical fiber. The left graph shows the V number. V number determines the number of electromagnetic modes in the fiber.

From this graph, we can see that when V number is smaller than 2.405, as shown by the red dashed line, there is only one mode, called LP01 mode, can be guided by the fiber. This is called a single mode fiber. 

When V number is larger than 2.405, several modes can be guided by the fiber. The higher the V number, the larger the number of modes. This is called multimode fiber.

V number is calculated based on the fiber's structure and the light wavelength. As shown at the bottom formula. In this formula, a is the radius of fiber core, Lamda is the light's wavelength, n1 is the refractive index of the fiber core, and n2 is the refractive index of the fiber cladding.  Buffer layer is there for mechanical protection of the fiber and is not involved in the calculation.

The previous slide shows only the calculation result. If you want to see the calculation in detail, you can go to the URL listed below.

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