How much do you know about fiber loss? Do you know how to calculate it?

In optical fiber installation, accurate measurement and calculation of optical fiber links are very important steps to verify network integrity and ensure network performance. The optical fiber will cause obvious signal loss (ie, fiber loss) due to light absorption and scattering, which affects the reliability of the optical transmission network. So how can we know the loss value on the fiber link? This article will teach you how to calculate the loss in an optical fiber link and how to judge the performance of an optical fiber link.

Type of fiber loss

Optical fiber loss is also referred to as 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 fiber loss, such as absorption/scattering of light energy by fiber material, bending loss, connector loss, etc.

All in all, there are two main reasons for fiber loss: internal factors (that is, inherent characteristics of the fiber) and external factors (that is, caused by improper operation of the fiber). Therefore, fiber loss can be divided into intrinsic fiber loss and extrinsic fiber loss. Intrinsic fiber loss is an inherent loss of fiber material, which mainly includes absorption loss, dispersion loss and scattering loss caused by structural defects; non-intrinsic fiber loss mainly includes splicing loss, connector loss and bending loss. To know how to reduce these fiber losses, you can visit "How to reduce various types of losses in optical fibers?" ".

Optical fiber loss standard

The Telecommunications Industry Alliance (TIA) and the Electronics Industry Alliance (EIA) 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 clarifies that the maximum attenuation is one of the most important parameters for 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 the EIA/TIA-568 standard.

How to calculate fiber loss?

If you want to check whether the optical fiber link can operate normally, then you need to calculate the optical fiber loss, power budget, and power margin. The calculation method is as follows.

Calculation of fiber loss

In fiber optic wiring, it is often necessary to calculate the maximum loss on a certain length of line. Optical fiber loss calculation formula:

Total link loss (LL) = optical cable attenuation + connector attenuation + splice attenuation [Note: If there are other components (such as attenuators), their attenuation values ​​can be superimposed]

• Optical cable attenuation (dB) = maximum optical fiber attenuation coefficient (dB / km) × length (km)

• Connector attenuation (dB) = number of connector pairs × connector loss (dB)

• Splice attenuation (dB) = number of splices × splice loss (dB)

As shown in the above formula, the total link loss is the maximum sum of the worst variables in a piece of fiber. It should be noted that the total link loss calculated in this way is only a hypothetical value, because it assumes the possible value of component loss, that is to say, the actual loss of the fiber depends on various factors, and the loss value may be more Higher or lower.

Let's take a practical case as an example to demonstrate how to calculate fiber loss. As shown in the figure below, a single-mode fiber is installed between the two buildings with a transmission distance of 10km and a wavelength of 1310nm. At the same time, this fiber has 2 ST connectors and 1 fusion splice.

• Optical cable attenuation-according to the above standard table, the maximum attenuation value of an 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 0.75dB, the connector attenuation is 0.75dB×2=1.5dB. In the actual calculation, the insertion loss of the connector can refer to the specification value provided by the supplier.

• Splicing attenuation-specified in the TIA/EIA standard, the maximum loss of splicing is 0.3dB, so the splicing 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. Note that the above calculation method is only a hypothetical value, if you want to get the most accurate value of the loss, using optical time-domain reflectometer OTDR measurements. For the time domain reflectometer OTDR For more information, visit "optical time domain reflectometry The working principle and usage method of OTDR, and matters needing attention .

Calculation of power budget

What effect does the aforementioned link loss value have on the transmission of the entire 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 that the equipment is installed correctly. 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, you need to calculate the power margin (PM), which refers to the available power after link loss is removed from the power budget, that is, PM=PB-LL.

Also take a 10km indoor single-mode fiber optic cable as an example. According to the above calculation, 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 has enough transmission power.