Over the years, optical networks have become increasingly popular. These networks use optical fiber equipment and have the advantages of low electromagnetic noise, high data transmission rate, and improved security. Nowadays, in various industrial applications, the traditional copper network is slowly being replaced by the optical network. Although you may have easy access to these fiber optic devices, designing networks may not be easy. To ensure your network’s overall integrity and performance, you must address several factors. The loss of optical fiber in the network is often ignored when laying an optical fiber network. This is mainly due to the ignorance of the relevant engineers. Optical fiber loss is a term for signal loss affecting transmission reliability. Therefore, it is very important to calculate the fiber loss and take appropriate steps. This article provides insights into calculating fiber loss and tips on reducing fiber loss in a network.
Type of Fiber Loss
Optical fiber loss is also called optical attenuation, which refers to the amount of optical loss between the transmitting end and the receiving end of the optical fiber. There are many reasons for optical fiber loss, such as optical fiber material’s absorption/scattering of light energy, bending loss, connector loss, etc.
There are two main reasons for optical fiber loss: internal factors (i.e., inherent characteristics of optical fiber) and external factors (i.e., caused by improper operation of optical fiber). Therefore, optical fiber loss can be divided into intrinsic optical fiber loss and extrinsic optical fiber loss. Intrinsic fiber loss is an inherent loss of optical fiber materials, mainly including absorption loss, dispersion loss, and scattering loss caused by structural defects; extrinsic fiber loss mainly includes fusion loss, connector loss, and bending loss.
Standards for Optical Fiber Loss
The Telecommunications Industry Alliance (TIA) and the Electronics Industry Alliance (EIA) jointly developed the EIA/TIA standard, which specifies the performance and transmission requirements of optical cables and connectors, and is now widely accepted and used in the optical fiber industry. The EIA/TIA standard specifies that maximum attenuation is one of the most important parameters in fiber loss measurement. The maximum attenuation is the attenuation coefficient of the optical cable in dB/km. The following figure shows the maximum attenuation of different types of optical cables in EIA/TIA-568 specifications and standards.
How to calculate the fiber loss?
In optical fiber cabling, it is necessary to calculate the maximum loss on a certain length of the line. Calculation formula of optical fiber loss:
The Total Link Loss = Cable Attenuation + Connector Loss + Splice Loss
Cable Attenuation (dB) = Maximum Cable Attenuation Coefficient (dB/km) × Length (km)
Connector Loss (dB) = Number of Connector Pairs × Connector Loss Allowance (dB)
Splice Loss (dB) = Number of Splices × Splice Loss Allowance (dB)
As shown in the above formula, the total link loss is the maximum sum of the worst variables in the optical fiber section. It should be noted that the actual link loss calculated in this way is only a hypothetical value because it assumes the possible value of the component loss; that is, the loss of optical fiber depends on various factors, and the loss value may be higher or lower.
The following is an example of how to calculate fiber loss. As shown in the figure below, single-mode optical fiber is installed between the two buildings, with a transmission distance of 10km and wavelength of 1310nm. At the same time, the optical fiber has 2 ST connectors and 1 fusion joint.
Optical cable attenuation – according to the above standard table, the maximum attenuation value of outdoor single-mode optical cable with a wavelength of 1310nm is 0.5dB/km, so the optical cable attenuation value is 0.5dB/km × 10km=5dB。
Connector attenuation – because 2 ST connectors are used, and the maximum loss of each ST connector is 0.75 dB, the connector attenuation is 0.75 dB × 2=1.5dB.
In the actual calculation, the connector insertion loss can refer to the specification value provided by the supplier.
Fusion attenuation – It is specified in TIA/EIA standard that the maximum loss of fusion is 0.3dB, so the fusion attenuation is 0.3dBx1=0.3dB.
It can be concluded that the total loss of the optical fiber link is 5dB+1.5dB+0.3dB=6.8dB. Please note that the above calculation method is only a hypothetical value. To obtain the most accurate loss value, you can use OTDR optical time domain reflectometer to measure.
To detect whether the optical fiber link can operate normally, in addition to calculating the optical fiber loss value, the calculation of the power budget and power margin value is equally important. The calculation methods of both are as follows:
Calculation of Power Budget
How does the link loss value mentioned above affect the transmission of the whole link? Here we have to mention another parameter closely related to it – power budget. This parameter value is mainly used to compare the calculated link loss value to ensure the correct installation of equipment. The link can operate normally only when the link loss value is within the power budget. The power budget (PB) is the difference between the receiver sensitivity (PR) and the transmitter power coupled into the optical fiber (PT), that is, PB=PT-PR. Assuming the average optical power of the transmitter is – 15dBm and the receiver’s sensitivity is – 28dBm, the power budget is – 15dB – (- 28dB) =13dB.
Calculation of Power Margin
After calculating the link loss and power budget, you need to calculate the power margin (PM), which refers to the available power after removing the link loss from the power budget, PM=PB-LL.
Taking the 10km indoor single-mode optical cable as an example, its power budget is 13dB, and the link loss is 6.8dB, so the power margin is 13dB-6.8dB=6.2dB. The calculated value is greater than zero, indicating that the link still has enough transmission power.
Tips for Reducing Losses
- Minimize tight bends that cause light to refract through the fiber cladding. If you need to coil the fiber, keep the radius as large as possible.
- Clean the connector ferrules a little often, especially before and after testing, and always use the correct tools and consumables.
- Decide which is higher – your “power” budget or your cable inventory budget. Buying cheap fibers will further increase costs.
- Avoid any excessive stress on the fiber, especially during installation. Push when possible, and if the cable needs to be pulled, do not exceed the maximum tensile load of the cable.
- Minimize the number of splices or connections in the network. If this means better planning or more innovative lead-in cables, then this investment is probably worth it.
It is important to solve the loss of optical fiber links when designing and establishing the optical fiber network. However, several factors need to be considered to address this loss. This paper discusses these considerations in detail and how to calculate the fiber loss and minimize the fiber loss in the network. Although these calculations will help you take all necessary measures to prevent the loss of optical fiber in the network, installing high-quality equipment and cables is equally important. To ensure quality, you can purchase equipment and cables from trustworthy manufacturers such as HOLIGHT.