Sources of Fiber Loss

What are the sources of optical fiber loss? The loss characteristics of optical fiber are directly related to the length of transmission distance of optical fiber communication system.

Optical fiber loss The so-called loss refers to the attenuation per unit length of optical fiber, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between relay stations. Therefore, understanding and reducing the loss of optical fiber has great practical significance for optical fiber communication.

Sources of Fiber Loss

The loss of light beam propagating in the optical fiber medium is an important physical parameter in the field of optical fiber communication. The degree of loss determines the maximum distance that the optical fiber can transmit the signal. For optical fibers, the main loss comes from the following aspects: energy absorption, scattering (mainly Rayleigh scattering), reflection, and bending loss of optical signals in optical media.

Causes of Fiber Loss

Light beam energy absorption:

Optical fibers are primarily composed of silica quartz material. The loss of quartz material is related to the wavelength of light. In the infrared wavelength region above 1700 nm, the infrared absorption increases rapidly, which is mainly due to the strong absorption of the optical signal in this region by the silicon-oxygen (Si-O) base. Optical fiber communication mainly works in the communication windows of 850 nanometers, 1310 nanometers and 1550 nanometers. In these windows, the loss caused by the absorption of the quartz material itself is very low compared with the scattering of light.

The energy absorption in the 1310nm and 1550nm regions is mainly due to impurities in the fiber, especially hydroxyl (OH- ) ions. Hydroxyl ions have strong energy absorption at 950nm, 1250nm and 1383nm wavelengths.

Rayleigh Scattering of Optical Fiber:

Rayleigh scattering is named after the British physicist Rayleigh. It refers to the scattering effect on the incident light when the diameter of the medium particle is much smaller than the wavelength of the light. The intenity of Rayleigh scattered light is inversely proportional to the fourth power of the incident light wavelength λ. In other words, the shorter the wavelength of light, the greater the energy loss caused by Rayleigh scattering. This also explains why the loss of light with a wavelength of 850nm in the fiber is greater than that of light with a wavelength of 1310nm, and the loss of light with a wavelength of 1310nm is greater than that of light with a wavelength of 1550nm.

Bending Losses: Macrobends and Microbends

Bending loss is a common loss in practical applications, including two forms of microbending and macrobending:

Microbending refers to the scattering loss caused by bending similar to the geometric size of the optical fiber. The reason is the problems caused in the production process of the optical fiber, as well as the damage caused by mechanical stress such as extrusion, stretching, twisting, etc. during the construction of the project. .

Macrobend means that when the vertical bending range of the optical fiber medium is on the order of centimeters, the refraction angle of the optical signal in the optical fiber is smaller than the maximum total reflection angle. As a result, the energy of a certain optical signal leaks from the core of the medium to the outside of the cladding. Therefore, a certain loss of optical signal transmission energy is generated.

"Return loss and reflection" of optical signal in the process of optical fiber medium transmission

When the optical signal is incident into the optical fiber transmission medium, due to the different refractive index of the medium section with different properties, the "mirror" reflection phenomenon of light will occur. This phenomenon is named after the French physicist Augustin Fresnel. Nell reflex. In practical applications, some faults of optical fiber, such as: fiber breakage and contamination of the end face of the optical fiber connection, will cause relatively strong reflections. Only by collecting and analyzing the energy data of these abnormal reflection event locations can the fault location be diagnosed.

The most common methods of fiber loss testing are the light source/power meter test method and the OTDR test method.