The main cause of fiber loss

With the advent of the information society, optical fiber communication has been widely used in more and more fields, which also has higher requirements for the transmission characteristics of optical fibers. The loss characteristics of optical fibers are directly related to the transmission distance of optical fiber communication systems, and are one of the most important transmission characteristics of optical fibers. Reducing the loss of optical fibers as much as possible is one of the important issues in realizing optical fiber communication.

Fiber loss The so-called loss refers to the attenuation per unit length of the fiber, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between repeater stations. Therefore, it is of great practical significance to understand and reduce the loss of optical fiber for optical fiber communication.

Sources of Fiber Loss

The loss of light beam propagating in optical fiber medium is an important physical parameter in the field of optical fiber communication. The degree of its 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 the optical medium.

The loss of fiber material is wavelength dependent. The relationship between the loss caused by the absorption and scattering of the medium itself and the wavelength is shown in the following figure:

Causes of Fiber Loss

Light beam energy absorption:

The optical fiber is mainly composed of silica quartz material. The loss of quartz material is related to the wavelength of light. In the wavelength region of infrared greater than 1700 nanometers, 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 which the loss caused by the absorption of the quartz material itself is very low compared with the light scattering.

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

Rayleigh Scattering of Fiber:

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

Bending Loss: Macrobending and Microbending

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

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

Macrobending means that when the vertical bending range of the optical fiber medium is in the order of centimeters, the refraction angle of the optical signal in the optical fiber is smaller than the maximum total reflection angle, and as a result, the energy of a certain optical signal leaks from the medium core to the outside of the cladding. And produce a certain loss of optical signal transmission energy.

"Return Loss and Reflection" of Optical Signals in Optical Fiber Transmission

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

A reflection event is defined as:

Reflection coefficient = 10log (P reflection / P incident), the value is negative

Return Loss (ORL) is the ratio of total reflected power to incident power and is defined as:

ORL=10log(P incident/P total reflection), the value is positive

concluding remarks

Low loss has always been the subject of optical fiber research. As the 400G system is about to be commercialized, especially the stringent requirements of the 400G system for optical OSNR, the deployment of ultra-low-loss optical fibers is becoming a hot spot. Ultra-low-loss fiber means longer span distance and fewer optical relay spans, so that the system has better OSNR performance.

Typical optical attenuation coefficients of conventional fibers are less than 0.35dB/km at 1310nm and less than 0.20dB/km at 1550nm. Ultra-low loss fiber, its maximum loss is less than 0.31dB/km at 1310nm; less than 0.17dB/km at 1550nm.

The most common methods for fiber loss testing are light source/power meter testing methods and OTDR testing methods.