Optical absorption and scattering will cause obvious signal loss in optical fiber, which will affect the reliability of optical transmission network. So how can we know the loss value of optical fiber link? This article will teach you how to calculate the loss in the fiber link and how to judge the performance of the fiber link.
Types 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. There are many reasons for fiber loss, such as absorption / scattering of light energy by fiber material, bending loss, connector loss and so on.
In a word, there are two main reasons for fiber loss: internal factors (i.e. inherent characteristics of fiber) and external factors (i.e. improper operation of fiber). Therefore, fiber loss can be divided into intrinsic fiber loss and extrinsic fiber loss. Intrinsic fiber loss is a kind of inherent loss of fiber materials, which mainly includes absorption loss, dispersion loss and scattering loss caused by structural defects; while non intrinsic fiber loss mainly includes fusion loss, connector loss and bending loss.
Standard of optical fiber loss
The telecommunications industry alliance (TIA) and the electronic industry alliance (EIA) have jointly formulated 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 defines that the maximum attenuation is one of the most important parameters in optical fiber loss measurement. In fact, the maximum attenuation is the attenuation coefficient of the optical cable, in dB / km. The figure below shows the maximum attenuation of different types of optical cables in EIA / TIA-568 standard.
Cable type | Wavelength（nm） | Maximum attenuation（dB / km） | Minimum bandwidth（Mhz * km） |
50 / 125 μ m multimode | 850 | 3.5 | 500 |
1300 | 1.5 | 500 | |
62.5/125 μ m multimode | 850 | 3.5 | 160 |
1300 | 1.5 | 500 | |
Indoor single mode optical cable | 1310 | 1.0 | —— |
1550 | 1.0 | —— | |
Outdoor single mode optical cable | 1310 | 0.5 | —— |
1550 | 0.5 | —— |
How to calculate fiber loss?
If you want to detect whether the optical fiber link can operate normally, you need to calculate the optical fiber loss, power budget and power margin. The calculation method is as follows.
Calculation of optical fiber loss
In optical fiber routing, it is often necessary to calculate the maximum loss on a certain length of line. Calculation formula of optical fiber loss:
Total link loss (LL) = optical cable attenuation + connector attenuation + fusion attenuation [Note: if there are other components (such as attenuators), their attenuation values can be superimposed]
Attenuation of optical cable (DB) = maximum attenuation coefficient of optical fiber (dB / km) × length (km)
Connector attenuation (DB) = number of connector pairs × connector loss (DB)
Welding attenuation (DB) = welding number × welding loss (DB)
As shown in the above formula, the total link loss is the maximum sum of the worst variables in a section of fiber. It should be noted that the total link loss calculated in this way is only an assumed value, because it assumes the possible value of component loss, that is, the actual loss of optical fiber depends on various factors, and the loss value may be higher or lower.
Calculation of power budget
What is the impact of the link loss mentioned above on the transmission of the whole link? Here we have to mention another parameter closely related to it power budget. The parameter value is mainly used to compare the calculated link loss value to ensure the correct installation of equipment. Only when the link loss value is within the power budget, the link can operate normally. The power budget (PB) is the difference between the sensitivity (PR) of the receiver and the power (PT) of the transmitter coupled into the fiber, that is, PB = PT-PR. Assuming that the average optical power of the transmitter is - 15dBm and the sensitivity of the receiver is - 28dbm, the power budget is - 15dB - (- 28dB) = 13dB.
Calculation of power margin
After calculating the link loss and power budget, we need to calculate the power margin (PM), which refers to the available power after removing the link loss from the power budget, that is, PM = PB-LL.
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