Fiber Optic Attenuation (Loss), Causes of Attenuation and Improvements

What is fiber optic attenuation (fiber optic loss)

Fiber optic attenuation (fiber optic loss) refers to the phenomenon that the intensity of light gradually decreases due to various factors as the transmission distance increases when the optical signal is transmitted in an optical fiber.
Attenuation is one of the main transmission characteristics of optical fiber.
Usually the terms fiber optic attenuation and fiber optic loss are used interchangeably.

 

Attenuation (loss) factor

We know that to complete the process of communication, in addition to transmitting the optical signal, the optical signal at the receiving end must be able to be properly received and accurately demodulated.

Fiber attenuation causes optical signals to gradually lose energy during transmission, thus affecting the quality and distance of the signal. In order to ensure that the optical signal is transmitted efficiently, the loss of the optical fiber needs to be reduced as much as possible.

Our goal is to reduce fiber optic loss, so how low is low?

How to measure fiber attenuation (loss)?

Fiber optic loss characteristics can be measured by the loss (attenuation) coefficient, which is the attenuation per unit length of the fiber, in dB/km.

α = 10*log(Pin / Pout) / L

Where L is the length of the fiber usually km as the basic length unit, Pin and Pout are the input and output optical power, respectively.

An example.

The length of the fiber is 100km, the input optical power of the fiber is 100mW, the output optical power is 1mW, calculate the attenuation coefficient of the fiber.

Bring the formula: α = 10*log(100/1) / 100, can get the fiber attenuation coefficient is: 0.2dB/km.

 

Causes of attenuation (loss) and ways to improve it

Optical signal transmission in the optical fiber, because of scattering, absorption, defects and other factors produce energy loss, causing attenuation.

 

First, absorption loss

Absorption loss is caused by fiber optic materials and impurities on the absorption of light energy, the optical energy in the form of heat energy consumed in the fiber, is an important loss in the optical fiber loss, absorption loss includes the following:

1, intrinsic absorption loss

Intrinsic absorption loss is a loss inherent in the fiber material, is unavoidable, it determines the loss limit of the fiber. Intrinsic absorption is inherent in the quartz material itself, including infrared absorption and ultraviolet absorption.

 

 

UV Absorption: In the short wavelength range, the electrons of fiber optic materials absorb the incident light energy to jump to a higher energy level, causing the energy loss of the incident light. Ultraviolet absorption is caused by the electron jump, which has an impact on fiber optic communications in the 700 to 1100nm wavelength region.
Infrared absorption: in the long wavelength range, the light wave interacts with the fiber lattice, and part of the light wave energy is transferred to the lattice, which intensifies its vibration, thus causing the loss. Infrared absorption is caused by molecular vibration, which has an effect on fiber optic communication in the wavelength region of 1500-1700nm.

 

2, Impurity absorption loss
Impurity absorption loss is the fiber optic material contains iron, copper, chromium and other impurity ions, metal ions, the more content, the greater the loss caused by them, they have a great impact on the short wavelengths, long wavelengths of the impact of the smaller.
The use of high-purity fiber optic materials, strict control of the content of these metal ions, can make them caused by the loss of rapid decline.

The other is OH ions, OH ions have a greater impact on the loss of optical fiber. In the 950nm, 1240nm, 1390nm wavelength near the absorption loss peak, to 1390nm on the absorption of the most serious, which is often heard of "water peak" absorption.

 

 

Second, scattering loss

Scattering loss is due to the optical fiber material density is not uniform or fiber waveguide structural defects such as optical power leakage out of the core caused by the loss.

Intrinsic scattering is the most important scattering in material scattering, the loss power is linearly related to the power of the propagation mode. It is due to the material atoms or molecules and the material structure of the inhomogeneity of the refractive index of the material, making the material to produce microscopic inhomogeneity caused by the scattering of transmitted light waves. This scattering is inherent in the material, can not be eliminated, is the lowest limit of optical fiber loss, Rayleigh scattering that belongs to this category.

 

Third, radiation loss

Radiation loss is due to the existence of small structural fluctuations at the interface of the core cladding, the loss caused by the uneven structure of the waveguide inside the fiber.

Part of the transmitted energy will be radiated out of the fiber core when the fiber structure is irregular and become a radiation mode, so that the loss increases.

This loss can be reduced by improving the manufacturing technology.

 

Fourth, bending loss

Bending loss is the loss caused by bending of the fiber optic axis.

Any naked eye visible fiber axis for a straight line offset is called bending or macro-bending.

Fiber bending will cause coupling between the modes within the fiber, when the propagation mode of energy coupled into the radiation mode or leakage mode, bending loss will occur. This loss increases exponentially as the radius of curvature decreases.

Another type of loss is the fiber axis to produce a random micron-level lateral displacement state into, called micro-bending loss. The reason for micro-bending is the optical fiber in the coating, cable, extrusion sheath, installation and other processes, the fiber is subjected to excessive uneven lateral pressure or longitudinal stress, or optical fiber manufacturing due to the coating layer or jacket temperature expansion coefficient of the optical fiber is not the same, etc. caused.

For bending loss, fiber optic lines can use low bending loss cable, or strictly follow the industry standard specifications for cable laying, reasonable control of the bending radius of the fiber optic cable, as far as possible to reduce the bending loss.

 

We have specialized in optical fiber cables business for nearly 10 years and gained many well-known clients all over the world with long term partnership. If you are interested in our fibers, please contact me.

 

 

Contact: