What’s the difference between 1310nm and 1550nm?

In optical fiber transmission, the most important loss has three aspects: Asorption of optical medium, Rayleigh scattering, and Bending loss.

Among the above factors, the loss caused by the medium asorption of the optical fiber it  self mainly occurs above 1700nm. Compared with the Windows of our fiber optic communication, 850nm, 1310nm, 1550nm are not the main factors that we consider.

As shown in the figure below, in the above Windows, fiber loss is mainly caused by Rayleigh scattering caused by impurities in the fiber.

So what's the difference between 1310nm and 1550nm wavelengths transmission in fiber?

 Rayleigh scattering refers to that when the particle scale in the medium is much smaller than the wavelength of the incident light (less than one tenth of the wavelength), the intensity of the scattered light is different from each other, and the intensity is inversely proportional to the fourth power of the wavelength the wavelength of the incident light.

The derivation of Rayleigh scattering formula can be concluded as follows:

That is, the shorter the wavelength of light, the more energy Rayleigh scattering loses. Therefore, the fiber loss at 1310nm is greater than the loss at 1550nm. So we also often say that when measuring fiber loss, as long as the power loss of 1310nm meets the requirements, then 1550nm is not a problem.

For bending loss, when the fiber is bent, the incident Angle of the optical signal is still greater than the critical Angle within the allowable bending radius, and total reflection still occurs in the fiber. However, when the optical fiber is excessively bent, the incident Angle of light is less than the critical Angle, and no total reflection will occur at the bending point, resulting in light spillover into the cladding, resulting in bending loss.

So, is 1310nm or 1550nm better for bending resistance in optical fibers? Let's take a look at the relationship between the critical curvature radius of fiber bending and the size of light wavelength, as shown in the following formula:

That is to say, the smaller the critical radius, the stronger the ability to bend loss, can bend enough. When the fiber is bent to a certain extent, as long as 1550nm light passes through, 1310nm light is definitely ok. Finally, do you know what is the minimum bending radius allowed for common optical fibers in the market?