Standards and calculation methods for optical fiber loss
In the rapid development of modern communication technology, optical fiber is the "highway" for information transmission, and its importance is self-evident. Optical fiber communication has become an ideal solution for long-distance and large-capacity data transmission with its unique advantages such as high bandwidth, low attenuation and anti-electromagnetic interference. However, optical fiber is not lossless during transmission. As an important indicator for measuring the quality of optical fiber transmission, optical fiber loss has a vital impact on the performance of optical fiber communication systems. This article will introduce in detail the standards and calculation methods of optical fiber loss.
Standards for optical fiber loss
Optical fiber loss mainly comes from three aspects: transmission loss, additional loss and joint loss.
1. Transmission loss: Transmission loss is the phenomenon that the optical signal gradually weakens along the length direction of the optical fiber, which is mainly caused by physical mechanisms such as absorption, scattering and Rayleigh scattering of optical fiber materials. Different types of optical fibers have different transmission loss standards. The transmission loss of common single-mode optical fiber in the 1550nm band is usually between 0.2 and 0.3dB/km, while some specially made low-loss optical fibers, such as ultra-low-loss optical fiber (ULLF), can have a transmission loss as low as 0.15dB/km or even lower. This low-loss feature enables optical fiber to support longer transmission distances and higher transmission rates, which is crucial for building long-distance backbone networks.
2. Additional Loss: Additional losses mainly include bending loss, inhomogeneity loss and material scattering loss. These losses are mainly caused by the physical structure of the optical fiber. Bending loss is caused by the scattering of light signals in the optical fiber or leakage into the cladding when the optical fiber is bent by external force. Inhomogeneity loss is caused by the uneven physical properties of the density and refractive index of the optical fiber core or cladding material. Material scattering loss is the light scattering phenomenon caused by impurities, defects or bubbles in the optical fiber material. Although these additional losses are relatively small, they still need to be strictly controlled in application scenarios with long optical fiber links or high quality requirements.
3. Joint Loss: Joint loss is the light loss caused by mismatch, contamination, misalignment and other reasons during the connection of optical fibers. Different brands, different moduli and different refractive indices are suitable for different types of standard joints, resulting in different joint losses. At present, the main types of optical fiber connectors on the market are FC, PC, ST and other types. Different types of connectors are suitable for different application scenarios. The size of the connector loss directly affects the total loss of the optical fiber link. Therefore, in practical applications, it is necessary to select appropriate connector types and connection methods to reduce connector losses.
Calculation method of optical fiber loss
The calculation method of optical fiber loss is mainly based on the accumulation of loss values of each component in the optical fiber link. In optical fiber wiring, it is often necessary to calculate the maximum loss on a line of a certain length to ensure that the link can operate normally. The optical fiber loss calculation formula usually includes three parts: cable attenuation, connector attenuation and fusion attenuation.
1. Cable attenuation: Cable attenuation refers to the attenuation of optical fiber per unit length, usually expressed in dB/km. The calculation formula of optical cable attenuation is: Cable attenuation (dB) = maximum optical fiber attenuation coefficient (dB/km) × optical fiber length (km). For example, for an outdoor single-mode optical cable with a wavelength of 1310nm, its maximum attenuation coefficient is 0.5dB/km. If the cable length is 10km, the cable attenuation value is 0.5dB/km×10km=5dB.
2. Connector attenuation: Connector attenuation refers to the additional loss introduced by the optical fiber connector. The size of connector attenuation depends on factors such as the type and quality of the connector and the connection method. The calculation formula for connector attenuation is: Connector attenuation (dB) = number of connector pairs × connector loss (dB). Usually, the insertion loss of the connector can be obtained by referring to the specification sheet provided by the supplier or by actual testing. When calculating the total link loss, the attenuation values of all connectors should be added together. For example, if two ST connectors are used and the maximum loss of each ST connector is 0.75dB, the connector attenuation is 0.75dB × 2 = 1.5dB.
3. Splicing attenuation: Splicing attenuation refers to the loss of optical fiber during the splicing process due to poor splicing quality, mode mismatch and other reasons. The size of splicing attenuation is related to factors such as the performance of the splicing machine, the skill level of the operator and the splicing environment. The calculation formula for splicing attenuation is: splicing attenuation (dB) = number of splices × splicing loss (dB). The TIA/EIA standard specifies that the maximum loss of splicing is 0.3dB, so if the number of splices is 1, the splicing attenuation is 0.3dB × 1 = 0.3dB.
Combining the loss values of the above three parts, the total loss of the optical fiber link can be obtained. Total link loss (LL) = optical cable attenuation + connector attenuation + splicing attenuation. For example, a single-mode optical fiber is installed between two buildings with a transmission distance of 10km and a wavelength of 1310nm. At the same time, the optical fiber has 2 ST connectors and 1 splicing head. According to the above calculation method, the attenuation of the optical cable is 5dB, the attenuation of the connector is 1.5dB, and the attenuation of the fusion splice is 0.3dB. Therefore, the total loss of the optical fiber link is 5dB+1.5dB+0.3dB=6.8dB.
By calculating the total link loss, it is possible to evaluate whether the transmission performance of the optical fiber link meets the design requirements. In practical applications, in order to ensure that the system can operate stably, it is usually necessary to reserve a certain power margin to cope with the possible increase in loss in the future.
As one of the important indicators for measuring the quality of optical fiber transmission, the standard and calculation method of optical fiber loss are of great significance to the design, installation and maintenance of optical fiber communication systems. In the future, with the continuous development and innovation of optical fiber communication technology, we have reason to believe that optical fiber loss will be further reduced and transmission performance will be further improved.