Tips on How to Calculate Fiber Loss in a Network

Fiber optic networks have gained popularity over the years. These networks use fiber optic devices and are known to offer benefits such as low electromagnetic noise, high data transmission rates, increased security, and so on. Nowadays, legacy copper networks are being slowly replaced by fiber optic networks in various industrial applications. Although you may get these fiber optic devices easily, designing a network may not be easy. There are several factors that you must address to ensure the overall integrity and performance of the network. Fiber loss in network is one such factor that is often overlooked, while laying the fiber optic network. This mainly happens due to ignorance of engineers involved. Fiber loss is a term for signal loss, which affects the reliability of the transmission. Thus, calculating fiber loss and taking appropriates steps is important. This post offers insights on calculating the fiber loss and tips on how to reduce fiber losses in the network.  

Important Factors to Consider While Calculating Fiber Optic Losses

Fiber optic system design is not an easy task, rather a balancing act. It should address all system factors that may lead to losses. Fiber losses are caused by a set of intrinsic and extrinsic factors. Fiber loss or signal loss, also known as fiber attenuation, is caused by intrinsic and extrinsic properties of single mode and multimode fiber. If you are eager to know how to calculate fiber loss in networks, perhaps you need to address these factors. The following are a few factors that you need to consider while calculating fiber loss budget.

• Intrinsic Attenuation Losses: This term addresses three types of losses – absorption losses, scattering losses, and dispersion losses.
  • Scattering Losses: They are caused due to small variations in compositional fluctuations, material density, manufacturing defects, and structural inhomogeneity. Often, these variations can be observed at a microscopic level.
  • Dispersion Losses: At times, optical signals may distort while traveling through the fiber. This results in dispersion losses. There are two types of dispersion losses – intra-modal and intermodal. The intra-modal dispersion is mainly caused due to the spreading of pulse in a single mode fiber. This pulse spreading happens due to variation of the propagation constant or the refractive index along the length. On the contrary, intermodal dispersion is the pulse broadening, which occurs due to the delay in propagation between multimode fibers modes.
  • Absorption Losses: These are addressed as one of the major reasons for optical losses in the fiber optic cables. The photon interact with different glass components, metal ions or electron during the transmission. The light is absorbed during this interaction and transferred into other energy forms such as heat, wavelength impurities, and molecular resonance.

Intrinsic attenuation for multimode fibers of 50/125 µm and 6.25/125 µm is 3.5dB/km@ 850nm and for single mode fibers of 9 µm is 0.4 dB/Km @1310nm and 0.3 dB/Km@1550nm.

• Insertion Losses: Also known as connector losses, these are the losses in light power, which occur when the device is inserted in an optical fiber or transmission line. Generally, factory assembled single mode connectors have losses in the range of 0.1-0.2 dB, and field terminated connectors may have losses as high as 0.2-1.0dB. Multimode connectors have losses of losses of 0.2-0.5 dB. 
• Transmitters: LASER transmitters and LED transmitters are two important types of transmitters used in most fiber optic networks. LASER transmitters can be availed in three types – low, medium, and high, which are alternatively known as short reach, medium reach, and high reach transmitters. LED transmitters are available as standard and high power LED transmitters. The right choice of transmitters will depend on the type of fiber used. You need to focus on a factor light output at the connector while choosing these transmitters. For instance, -5dB is the most common light output. A transmitter is also known as an emitter.
• Fiber Type Used: Most networks use either single mode or multimode fibers or both. Most multimode fibers possess a loss factor of 2.5 (@850nm) and 0.8 (@1300 nm) dB/km. Against this, the single mode fibers possess a loss factor of 0.25 (@1550nm) and 0.35 (@1310nm) dB/km. The single mode fibers are compatible with LASER transmitters, which are available as short reach and long reach types. Multimode fibers are often integrated with LED transmitters, as they do not travel more than 1 km due to lack of power. High power LED transmitters are used with single mode fibers.
• Fiber Loss Factor: The loss factor is usually defined by the manufacturer in dB per kilometer. The calculation of fiber loss factor can be simply made as the loss factor x the distance. This distance is the total length of the fiber optic cable, and not just the distance of the network.
• Margin: It is a broad term encompassing several factors such as aging of receiver and transmitter components, the fiber aging, twisting and flexing of the fiber optic cable, scope of addition of devices in the future, splices added to repair breaks in cables. Generally, the fiber optic loss budget margin is kept between 3 and 10 dB.
• Fiber Optic Splicing: The splicing helps join two ends of optical fibers. It is performed to ensure the light passing through the cable is as strong as a single fiber. Mechanical and fused are the two important types of splices used in fiber optic cable networks. Of these, mechanical splices use connector sets on the fiber ends, while in the fused type, the fiber ends are directly mated. The loss in mechanical splices is usually considered in the range of 0.1-1.5 dB per connector, and in fusion splices it is 0.1-0.5dB per splice. Fused splices are most commonly preferred due to their low loss factor.
• Bending: A small section of fiber optic losses is initiated by bending. This bending may be caused due to improper handling of cables. There are two types of bending – macro bending and micro bending. Macro bending refers to the large bending in the cable, while micro bending refers to small bending in the cable.

How to Calculate Fiber Loss / Fiber Loss Calculation Formula

The formula for calculating fiber loss is:

Total Fiber Loss (dB)=Fiber Attenuation (dB/km)×Fiber Length (km)+Connector Loss (dB)+Splice Loss (dB)

• Splice Loss (dB) = Splice Loss Allowance (dB) x Number of Splices
• Cable Attenuation (dB) = Maximum Cable Attenuation Coefficient (dB/km) × Length (km)
• Connector Loss (dB) = Connector Loss Allowance (dB) x Number of Connector Pairs

The total loss is the sum of several variables in the fiber segment. Although you learn how to calculate fiber loss, remember it is just an estimation. The actual values may be high or low based on different factors, so you need to maintain a balance.

For instance, if you consider a 40 km single mode link with 5 splices and 2 connector pairs at 1310nm, the calculation would be.

• Cable Attenuation (dB) = 40km x 0.4 dB/km
• Splice Loss = 0.1dB/km x 5
• Connector Loss = 0.75dB x2
• Safety Margin = 3.0dB

Link Loss: 40km x 0.4 dB/km + 0.1dB/km x5 + 0.75dBx + 3.0dB

The calculation shows that at least 21dB power is needed to transmit across the fiber optic link. You need to measure and verify the actual link loss after setting the network. This will help you identify performance issues if any.

Tips to Reduce Fiber Loss in a Network

You need to leave enough margin to accommodate the performance degradation over time. This consideration will help ensure the power output of light is within the sensitivity of the receiver. Here are a few changes that you can make in the fiber link design and installation to minimize fiber loss in a network.

• Ensure you use high-quality cables throughout the network. The cables must have similar properties.
• Try to use qualified connectors wherever possible. Always ensure to keep insertion loss less than 0.3dB, and any additional losses less than 0.2dB.
• Try to follow the environment and splicing requirements properly while splicing.
• Ensure to use clean connectors.
• Choose the best methods while laying fiber optic cables.
• Ensure to use the entire disc for configuration. You can use a single disc for more than 500 meters. This will help minimize the number of joints.
• You need to install protection from environmental elements such as electrical, lightening, anti-mechanical, and anti-corrosion protection.
• Try to use all components including cables, connectors, transmitters, media converters, switches of high quality. This will help you ensure high performance and minimize fiber loss in the network.

Conclusion

Addressing fiber optic link loss is very important while designing and setting up the fiber optic network. However, addressing this loss also involves several considerations. These considerations and how to calculate fiber loss and tips to minimize fiber loss in network is discussed here in detail. Although these calculations will help you take all the necessary actions to prevent fiber optic loss in a network, installing quality devices and cables is equally important. You can be assured of quality by sourcing your devices and cables from trusted manufacturers like VERSITRON. The company provides fiber optic devices such as fiber optic network switches, high definition transmitters and receivers, fiber optic media converters, video distribution amplifiers, and CCTV fiber optic video kits. For more information, please visit, www.versitron.com.