Understanding and Calculating Fiber Optic Loss: Types and Standards
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1. Preface
Accurate measurement and calculation of fiber optic links are crucial for verifying network integrity and ensuring performance. Significant signal loss (i.e., fiber optic loss) occurs within the fiber due to light absorption and scattering, affecting the reliability of optical transmission networks. So, how can we know the loss value on the fiber optic link? This article will teach you how to calculate the loss in the fiber optic link and how to judge the performance of the fiber optic link.
2. Types of Fiber Optic Loss
Fiber optic loss, also known as optical attenuation, refers to the light loss between the transmitter and receiver. There are various causes of fiber optic loss, such as absorption/scattering of light energy by fiber material, bending loss, connector loss, etc. In summary, fiber optic loss is mainly caused by two factors: intrinsic factors (i.e., inherent characteristics of the fiber) and extrinsic factors (i.e., improper operation of the fiber), thus fiber optic loss can be divided into intrinsic fiber loss and extrinsic fiber loss. Intrinsic fiber loss is an inherent loss of the fiber material, mainly including absorption loss, dispersion loss, and scattering loss caused by structural defects; while extrinsic fiber loss mainly includes fusion loss, connector loss, and bending loss.
3. Standards of Fiber Optic Loss
The Telecommunications Industry Association (TIA) and Electronic Industries Alliance (EIA) jointly developed the EIA/TIA standards, which define the performance and transmission requirements for optical cables and connectors. These standards are widely accepted and used in the fiber optic industry today. The EIA/TIA standards clearly state that maximum attenuation is one of the most important parameters in measuring fiber optic loss. In fact, maximum attenuation is the attenuation coefficient of the optical cable, measured in dB/km. The table below shows the maximum attenuation of different types of optical cables according to the EIA/TIA-568 standard.
4. How to Calculate Fiber Optic Loss?
To ensure a fiber optic link operates correctly, you need to calculate its loss, power budget, and power margin. The calculation methods are as follows. In fiber optic cabling, it is often necessary to calculate the maximum loss over a certain length of line. Fiber optic loss calculation formula: Total link loss (LL) = Cable attenuation + Connector attenuation + Fusion attenuation [Note: If there are other components (such as attenuators), their attenuation values can be added].
Cable attenuation in decibels (dB) is calculated by multiplying the maximum fiber attenuation coefficient (in dB/km) by the length of the cable (in km).
Connector attenuation in decibels (dB) is the number of connectors multiplied by the connector loss (in dB).
Fusion attenuation in decibels (dB) is the number of splices multiplied by the fusion loss (in dB).
As the formula shows, total link loss is the sum of the worst-case variables in a fiber section. It is important to note that the total link loss calculated in this way is only an assumed value, as it assumes the possible values of component loss. That is, the actual loss of the fiber depends on various factors, and the loss value may be higher or lower. Below is an actual case to demonstrate how to calculate fiber optic loss. As shown in the figure, a single-mode fiber is installed between two buildings with a transmission distance of 10km and a wavelength of 1310nm. The fiber also has 2 ST connectors and 1 splice.
Fiber Cable attenuation - According to the standard table above, the maximum attenuation value for outdoor single-mode fiber at a wavelength of 1310nm is 0.5dB / km, so the cable attenuation value is 0.5dB / km × 10km = 5dB.
Connector attenuation - Since 2 ST connectors are used, and each ST connector has a maximum loss of 0.75dB, the connector attenuation is 0.75dB × 2 = 1.5dB. In actual calculations, the insertion loss of connectors can refer to the specifications provided by the supplier.
Fusion attenuation - According to the TIA/EIA standard, the maximum loss of fusion is 0.3dB, so the fusion attenuation is 0.3dB × 1 = 0.3dB.
Thus, the total loss of the fiber optic link is 5dB + 1.5dB + 0.3dB = 6.8dB. Note that the above calculation method is only an assumed value. To obtain the most accurate loss value, an Optical Time Domain Reflectometer (OTDR) can be used for measurement.
5. Calculation of Power Budget
What impact does the mentioned link loss value have on the entire link transmission? Here, we must mention another closely related parameter - power budget. This parameter value is mainly used to compare with the calculated link loss value to ensure the correct installation of equipment. Only when the link loss value is within the power budget can the link operate normally. Power budget (PB) is the difference between the receiver's sensitivity (PR) and the transmitter's output power (PT), calculated as 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.
6. Calculation of Power Margin
After calculating the link loss and power budget, determine the power margin (PM) by subtracting the link loss from the power budget: PM = PB - LL.
Similarly, taking the 10km indoor single-mode fiber as an example, as calculated above, its power budget is 13dB, and the link loss is 6.8dB, so the power margin is 13dB - 6.8dB = 6.2dB. A power margin greater than zero indicates the link has ample transmission power.
