A Focus on the Critical Optical Parameter Attenuation

Our first article in this series highlighted some bandwidth demand drivers and introductory standards information. This article will focus on critical optical parameters starting with optical attenuation. Additional articles in this series focus on other optical parameters, including chromatic and polarization mode dispersion, splice loss, and an introduction to non-linear effects.

Fiber Wavelength Bands

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There are three main loss mechanisms in fibers, and we’ll briefly discuss each. Those Attenuation mechanisms are:

  • Scattering
  • Absorption
  • Bending

Scattering

The first mechanism is “Rayleigh scattering” of light in fiber. This mechanism contributes most to the baseline attenuation of fiber. A certain amount of light is scattered in the glass. In the simplest of terms, scattered light is simply light that is no longer guided through the optical fiber, but instead propagates in some other random direction (an interesting side note is that OTDRs measure loss by using the light that is scattered backwards in a fiber so the device only needs to be connected to one end of the optical fiber). Since some light doesn’t transmit forward through the glass, loss occurs. The classic attenuation curve has a relationship of attenuation proportional to 1/λ4 and is driven by the properties of Rayleigh scattering. Rayleigh scattering is the result of small fluctuations of glass density in an optical fiber and is the same mechanism responsible for the blue color of the sky, when sunlight scatters off molecules in the atmosphere.

The scattering-related attenuation properties of the glass are determined by the materials used in the glass, and are frozen in during fiber manufacturing.

Absorption Attenuation

Impurities may absorb or reflect light. This is why fiber manufacturers pay such close attention to the quality of materials used in the glass and to cleanliness during manufacturing. Particles as small as a fraction of a micron can be large enough to absorb enough light to increase loss.

Besides particles, impurities in the raw materials used in the fiber manufacturing process itself can increase loss. That’s because the hydroxyl (OH) ion is a by-product of the manufacturing process. It absorbs light in the wavelength range around 1383 nm.

The graph to the right shows the loss performance across the wavelength range with three different grades of fiber.

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Bending

Bending is a very important mechanism. The cabling process and installation in the field can affect attenuation caused by bending.

Let’s go back to Fiber 101. Fibers use the concept of total internal reflection to guide light. The refractive index profile of the core and the cladding determine how light is guided, and the term “critical angle” is used to describe when reflection turns to refraction and light is lost from the fiber. In short, when fiber is bent tightly, light can be lost.

There are two main modes of bending – macrobending and microbending.

While the end result of both types of bending is attenuation, the mechanisms and how they manifest differ.

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Other Ways Microbends and Macrobends Show Up in the Network

The concepts of micro and macro bends affect the network in ways that are not always obvious.

Bend-related loss is also sometimes experienced in cold temperature environments. For this reason, fiber and cable qualifications should always include tests to see how products perform in cold temperatures. As a network designer, it’s always a good idea to account for at least some optical margin for small potential attenuation increases in cold temperatures.

Help is On the Way

The good news is that fiber manufacturers have developed fibers that can withstand different amounts of bending while also reducing loss compared with traditional fibers meeting the ITU G.652.D Recommendation. These fibers are called bend insensitive or bend optimized fibers, and are defined by ITU Recommendation G.657.

For the network designer and installer, a thorough understanding of various attenuation mechanisms can assist with the network planning and installation processes, enabling proper loss budgeting and the use of appropriate products for the application.

In most situations, attenuation is the most important network parameter, and this article has provided enough background for you to be well on your way to fiber geekdom on this topic. However, fiber geekdom is a journey, not a destination, and there’s always more to learn. OFS has multiple decades of experience with fiber optic networks. Please contact your local OFS representative if you would like additional information regarding any of the items in this article.

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About the Author:

Mark Boxer - Technical Manager, Solutions and Applications Engineering for OFS

Mark Boxer is Technical Manager, Solutions and Applications Engineering for OFS. In this role, he assists customers deploying fiber in a wide variety of network design scenarios around the world and analyzes trends in telecommunications markets that drive new product innovation. Mark has a BME degree from Georgia Tech, and has spent his 30+ year career in the fiber industry. His experience includes varied roles in manufacturing and applications engineering for fiber-based products and markets. Other activities include inventor of six US Patents, member and past Secretary of the IEEE Power Engineering Society Fiber Optic Working Group, contributing member to the Fiber Broadband Association (FBA) (formerly FTTH Council) Technology Committee and Board of Directors member of the FBA.

Placing Fiber Optic Cable in Underground Plant

Placing Fiber Optic Cable in Underground Plant

1. Overview

This document covers cable placing in conduit, innerduct, handholes, and manhole structures. The innerduct may be direct buried or placed in larger diameter conduits. In some applications, the innerduct may be lashed to an aerial strand.

This document covers conventional cable placing techniques that are used to pull or blow (cable jetting) the cable into the conduit or innerduct.

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2. General Rules

2.1 Route Survey and Inspection

It is recommended that an outside plant engineer conduct a route survey and inspection prior to cable installation. Manholes and ducts should be inspected to determine the optimum splice locations and duct assignments. A detailed installation plan, including cable pulling or blowing locations, intermediate assist points, and cable feed locations should be developed based on the route survey.

2.2 Maximum Rated Cable Load

The maximum rated cable load (MRCL) for most OFS outside plant fiber optic cables is 600 lb; however, the cable documentation should always be checked because lower values of MRCL may apply for some cables. When using pulling equipment to install cable, measures should be taken to ensure that the MRCL is not exceeded. This includes the use of breakaway swivels, hydraulic pressure relief valves, and electronic tension control systems.

2.3 Minimum Bend Diameter

Fiber Optic Diameter Bending During Installation

The minimum bend diameters for OFS cables are defined for both dynamic and static conditions. The dynamic condition applies during installation when a cable may be exposed to the MRCL, e.g., while pulling the cable around a sheave or capstan.

2.4 Temperature Limits

Storage and installation of OFS fiber optic cable is limited to the temperature ranges. Be aware that solar heating due to sunlight exposure can increase the cable temperature well above the ambient temperature.

3. Underground Optical Cable Precautions

Before starting any underground cable placing operations, all personnel must be thoroughly familiar with local company safety practices. Practices covering the following procedures should be given special emphasis:

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4. Innerduct

Fiber optic cable is most often placed in a small-diameter innerduct rather than a large-diameter conduit. For existing conduit structures, multiple innerducts can be placed in a single conduit to provide multiple cable paths in the duct. Innerduct is also recommended because it provides a clean continuous path for the installation of the fiber optic cable.

4.1 Diameter Ratio and Area Ratio

Diameter and Innerduct area ratios.

Diameter ratio and/or area ratio are used to determine the optimal cable OD that should be installed in an innerduct. Either ratio can be used, but consistently using one or the other is important to avoid confusion.

4.2 Direct Buried Applications

Studies have shown that vertical undulations in direct buried innerduct can greatly increase the required cable installation forces.

5. Cable Lubricant

Cable lubricant should be used when placing fiber optic cables. Recommended cable lubricants include Polywater4, Hydralube5, and similar cable lubricants that are compatible with polyethylene cable jackets. Both the winch line (or pulling rope) and the cable should be lubricated.

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6. Cable Placing Methods

6.1 Backfeed Technique

The backfeed technique is a common installation method that is used to divide the cable installation into two separate pulls. The backfeed technique may also be used near equipment offices when one end of the cable must be pulled by hand into the building, or at manhole locations where the cable route changes direction.

6.2 Forward-Feed Technique

In the forward-feed technique, the leading end of the cable and excess cable length are pulled out of the innerduct at an intermediate manhole and stored on the ground in a figure-eight. This technique can be used multiple times during a cable installation to greatly increase the distance between cable splices.

6.3 Figure-Eight Installation Techniques

If figure-eight techniques are used during cable installation, the cable should be handled manually and stored on the ground. Place the cable on tarps to prevent damage from gravel, rocks, or other abrasive surfaces.

Fiber Optic Cable Underground Installation

When figure-eighting heavy cables (264 fibers or more), the cable stack should be offset to prevent sheath dents and cable damage. Although sheath dents do not typically damage fibers, this type of cosmetic damage is undesirable. When utilizing the offset method, each crossover point of the cable stack should be offset about 2 inches instead of being stacked directly on top of each other.

Standard vs Offset Figure Eight

6.4 Handholes

Handholes are frequently used to provide access to cable splices and slack storage coils. On long cable pulls, handholes may be used to facilitate intermediate-assist placing operations. The intermediate-assist handholes are typically installed near obstacles or at a predetermined spacing that coincides with the maximum expected cable installation length.

7. Pulling Fiber Optic Cable

The following instructions assume general familiarity with outside plant cable placing procedures. They also assume that the innerduct is in place and a lubricated pulling tape or rope has been installed in the innerduct.

7.1 Feed Manhole

Breakaway Swivel Connector

Mount the cable reel on the reel carrier so that the cable feeds off the top of the reel. Position the cable reel adjacent to the manhole and in-line with the innerduct. The cable reel should be positioned close enough to the manhole so that excessive cable length is not dragging on the ground, but far enough away to maintain slack cable in the event of a sudden start or stop during the pulling operation. A distance of 10 – 15 feet is generally sufficient. Attach the pull line to the fiber optic cable using a cable grip and swivel connector. Caution: A breakaway swivel connector is required if a tension limited winch is not used to pull the cable.

7.2 Intermediate Manholes

The innerduct in intermediate manholes may be continuous through the manhole or it may be interrupted. In either case, the innerduct should be positioned in a straight path from entry duct to exit duct. If the innerduct is continuous and has been racked, remove the innerduct ties and straighten the innerduct through the manhole. If necessary, slack innerduct can be cut out using an innerduct cutter. Secure the innerduct in the manhole to prevent it from creeping into the main duct during cable placing operations.

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7.3 Pull Manhole

OFS recommends the use of tension-limited winches for pulling fiber optic cable. The tension control may be accomplished by electrical, mechanical, or hydraulic methods. In any case, the tension-limiting device must be routinely calibrated as recommended by the equipment manufacturer. Cable winches that display cable tension but do not have automatic cutoff are not sufficient to protect the cable. If a tension-limited winch is not used, a breakaway swivel must be used to connect the fiber optic cable to the pulling line.

7.4 Capstan Winches

7.4.1 General

Breakaway swivels do not protect the fiber-optic cable after the cable pulling-eye passes the intermediate capstan winch; therefore, intermediate-assist capstan winches must be tension-limited. The capstan must also meet the minimum cable bend-diameters.

7.4.2 Set-up

The capstan winches should be positioned along the cable route where the expected pulling tension will be 600 pounds or less. Proper positioning of the capstans prior to the start of the pull will eliminate construction delays caused when an unplanned intermediate capstan assist must be added to the placing operation.

7.4.3 Slack Cable Loop

During the pulling operation, a slack loop of cable must be maintained on the pull-off side of the capstan as shown in Figure.

Intermediate Manhole Set-up

7.4.4 Adding Intermediate Capstans

 If an intermediate capstan is added during the cable pull and the cable pulling eye has already passed through the manhole, a loop of slack cable must be pulled to the intermediate manhole before cable is wrapped on the capstan.

Adding an Intermediate Assist Capsta

7.4.5 Removing Cable from Intermediate Capstan Assist Winch

At the conclusion of the pull, the cable on the capstan is twist free. However, one twist per wrap will be generated in the cable if it is removed from the capstan and straightened.

8. Blown Optical Cable Installation

Cable blowing systems use high-pressure, high-velocity airflow combined with a pushing force to install the cable. A hydraulic or pneumatic powered drive wheel or drive belt is used to push the cable into the innerduct at the feed manhole. Controls and gauges on the cable blowing system allow the operator to monitor and adjust the air flow and push force that is exerted on the cable. Some cable jetting systems use a plug at the cable end to capture the compressed air and generate a small pulling force on the end of the cable.

9. Optical Cable Coiling

9.1 Coils Stored at Intermediate Holes

Many end users require that coils of slack cable be stored in intermediate manholes along the cable route. These slack storage coils are used for future branch splices or route rearrangements. It is important that the coiling method accommodates the proper coil diameter and does not introduce kinking or excessive twist into the cable.

9.2 Fold-Over Method

The Fold-Over Method is recommended for storing moderate lengths of slack cable. Form a cable bight and then twist the bight to form the first cable coil. Fold the coil over to form the second cable coil.

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9.3 Teardrop Method

The teardrop coiling method is recommended for storing longer lengths of cable since it is easier to roll the cable than perform repetitive folding operations. The cable is stored twist free by rolling the cable bight in a manner similar to that used on the cable end.

9.4 Garden Hose Method

The garden hose method is recommended for large diameter cables because only one turn of cable is handled at a time. The storage coil can be formed directly on the manhole racking as each additional loop is added. Each loop can be taped in place as it is added to the storage coil. This method can be used to store any length of slack cable.

9.5 Coils Stored at Splice Locations

Slack cable must be stored at splice locations to allow for splicing. Typically, a cable length of 50 to 100 feet is required for splicing purposes; however, the actual cable length may vary depending on the accessibility of the manhole.

10. Racking Fiber Optic Cable and Innerduct

Cable racking normally begins in intermediate manholes and proceeds manhole by manhole toward each end of the cable. Slack for racking the fiber optic cable may come from either the feed or the pull manhole depending on which end is closer and the amount of excess cable that is available. The preferred method of obtaining racking slack is pulling by hand. If the cable cannot be moved by hand, a split cable grip can be attached to the cable and the cable can be pulled using a cable winch or a chain hoist. Do not exceed the maximum tension rating or violate the minimum bending diameter of the cable while pulling the slack.

Cable coils should be racked in a location where they will not be damaged, preferably on the manhole wall behind in-place cables. Do not decrease the diameter of the cable coils. If slack cable must be removed from the coil for racking purposes, remove one or more loops from the coil and then enlarge the coil to absorb excess slack. Tie the coil securely in place with plastic ties.

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Advances in Laser Lithotripsy for Kidney Stones

Stone-retrieval Basket with IntegratedOptical Fiber for Laser Lithotripsy

This article originally appeared in Medizin & Technik  

A German manufacturer of stone-retrieval baskets works with a U.S. manufacturer of specialty optical fiber. The result is a basket that, thanks to its coaxially integrated optical fiber, can simplify and shorten minimally invasive urological surgery. 

The treatment of kidney stones has changed dramatically over the years. Instead of open surgery, today minimally invasive endoscopic-based procedures can be used. Once a stone is found, it can usually be removed using a nitinol basket. If the stone is too far up the urinary tract, fragmentation using laser energy is used to pulverize it. Pulverization is achieved by the introduction of an optical fiber to deliver the laser energy. This procedure is called intracorporeal lithotripsy.  

Pulverization using laser energy may vary. Combining a long pulse duration with low pulse energy and high pulse frequency will blast the stone into dust. The small dust particles are eliminated, naturally. But the high pulse energy will cause the ambient temperature to rise and may cause damage to surrounding tissue. An alternative to pulverization is fragmentation. Fragmentation uses laser energy with a short pulse duration, high pulse energy and low pulse frequency. The resulting fragments can then be captured using a stone-retrieval basket.  

Usually the stone is fragmented prior to the pieces being captured by the basket. But sometimes, depending on the location of the stone, the reverse order is necessary. In these cases, where the stone is captured and then fragmented, there is the risk of the laser energy damaging the stone-retrieval basket as well as the surrounding tissue.  

The next logical development in intracorporeal lithotripsy is an instrument that coaxially integrates optical fiber with the stone-retrieval basket. This improved instrument enables positioning of the basked and the optical fiber at the same time. The stone is safely trapped and fragmented without damaging surrounding tissue or the basket. Surgery time is shortened since only one instrument is needed.  

This new device was developed by Endosmart GmbH in Stutensee, Germany together with OFS, a U.S. designer and manufacturer of specialty optical fiber.  

A typical laser system for lithotripsy is based on Ho:YAG (Holmium:Yttrium-AluminumGarnet) laser which uses at a wavelength of 2123 nm with an average power of 30 W. Pulse duration, peak power and frequency are adjusted according to the individual treatment. For example, the laser pulse could be up to 18 kW peak power or 3.5 J pulse energy. To enable orientation of the instrument, the system delivers a visible red or green pilot light.  
 

Light is guided even under extreme bending 

The step-index multimode optical fiber used to guide the laser can have a pure silica core and a fluorine-doped glass cladding or a Germanium-doped core with a pure silica cladding. The different refractive indices of core and cladding enable the laser to propagate longitudinally in the fiber core. For guiding the light under extreme bending, an additional UV cured fluoroacrylate coating is applied. The fluoroacrylate coating has a lower refractive index than either of the glass claddings and acts as a secondary cladding for guiding the light. The optical fiber that is used with the nitinol basket described above has a core diameter of 272 µm and a silica cladding diameter of 299 µm. Around that, a 330 µm UV cured fluoropolymer coating is applied acting as a second optical cladding and finally, an ETFE buffer of 400 µm is applied.  

Glass fibers are also used for medical diagnostics. Current developments are focused on simultaneous diagnosis and treatment. 

5 Things You Should Know About IEEE Std 802.3cm™-2020

What is IEEE Std 802.3cm-2020, 400 Gb/s over Multimode Fiber?

The work of the IEEE Std P802.3cm Task Force was approved as a new standard by the IEEE-SA Standards Board on 30 January 2020, creating the latest 400 Gb/s Ethernet standard using multimode fiber. 400 Gb/s is the highest Ethernet speed, and 400 Gb/s optical modules are needed in hyperscale (Google, Microsoft, Alibaba, and others) and very large-scale enterprise datacenters. 802.3cm defines 400 Gb/s solutions over both 4-pair (400GBASE-SR4.2) and 8-pair (400GBASE-SR8) multimode links. The IEEE P802.3cm Task Force was chaired by Robert Lingle, Jr., Senior Director of Market Strategy at OFS.

400GBASE-SR4.2 is the first multimode standard to use two wavelengths (850nm and 910nm), enabling 100 Gb/s transmission over a single fiber pair. It takes advantage of the multi-wavelength capabilities of OFS LaserWave® WideBand (OM5) fiber with 150 meter link distances, while supporting 100 meter links over LaserWave FLEX 550 (OM4) fiber and 70 meter links over LaserWave FLEX 300 (OM3) fiber. This builds on well-established 40 and 100G BiDi and SWDM technology that has been offered by switch and transceiver suppliers over the past decade. A key motivation for the 400GBASE-SR4.2 transceiver type is support of the installed base of multimode fiber cabling, designed around 100 meter reach over OM4 MMF, as well as extended reach over OM5 MMF, especially in large enterprise datacenters. 400GBASE-SR8 uses eight pairs of multimode fiber, with each pair supporting 50 Gb/s transmission. It operates over a single wavelength (850nm). OM4 and OM5 will support 100 meter links, while OM3 can support up to 70 meters. A key motivation for 400GBASE-SR8 is support of new cabling architectures in hyperscale datacenters.

What applications will use these links?

400 Gb/s multimode links can be used in a variety of applications. These include not only 400 Gb/s switch-to-switch (point-to-point) links, but several new applications, including 400GBASE-SR8 – 8x50GBASE-SR breakouts, or 400 Gb/s shuffles (fig. 1). The breakout application minimizes the number of ports on the Top-of-Rack (ToR) switch, providing connectivity to higher numbers of servers from a single switch. In similar fashion, the shuffle application allows a single 400 Gb/s switch port to support 100 Gb/s links to 4 different switches. 400GBASE-SR8 supports both flexibility and higher density: a 400G-SR8 OSFP/ QSFP DD transceiver can be used as 400GBASESR8, 2x200GBASE-SR4, 4x100GBASE-SR2, or 8x50GBASE-SR. 400GBASE-SR8 is already being deployed as 2x200GBASE-SR4. 5 THINGS YOU SHOULD KNOW ABOUT A Furukawa Company IEEE Std 802.3cm™-2020 For more information, visit our website at www.ofsoptics.com 1 2 Example: 400GBASE-SR8 – 8×50 Gb/s Breakout.

Shuffle Arrangement

Optical Fiber Cable Shuffle Arrangement Illustration

Breakout Arrangement

Optical Fiber Breakout Arrangement Illustration

What does this mean for hyperscale data centers?

Both 400GBASE-SR4.2 and 400GBASE-SR8 applications can be used for point to point 400 Gb/s links between switches. Additionally, new applications are being deployed in hyperscale data centers. As server speeds reach 50 and 100 Gb/s, racks will contain fewer servers, leading to a change in switch architecture away from ToR to Middle-of-Row (MoR) or End-of-Row (EoR) switches. Copper DAC links are reaching link distance and bandwidth limitations that will make it very difficult to support this change in architecture, leading to demand for a low cost, short reach optical solution. 400GBASE-SR8 provides support for eight 50 Gb/s server links from a single MoR or EoR switch port, significantly increasing bandwidth density on the switch faceplate.

Learn more about OFS and hyperscale data center fiber cabling.

400GBASE-SR4.2 Point to Point Link Distance

What does IEEE Std 802.3cm mean for enterprise data centers?

400GBASE-SR4.2 is the first 400 Gb/s standard that takes advantage of the 4-pair OM3/OM4/OM5 infrastructure many enterprises installed earlier, first for 40 Gb/s Ethernet and later, 100 Gb/s 100GBASESR4, and 200 Gb/s 200G-SR4. It provides a graceful evolution path for enterprise networks, using the same cable infrastructure through at least four Ethernet generations. Future advances point toward the ability to support even higher data rates as they become needed. It takes advantage of the latest multimode fiber technology, OM5 fiber, using multiple wavelengths to transmit 100 Gb/s over a pair of fibers over 150 meters, compared to 100 meters for OM4 and 70 meters over OM3.

400GBASE-SR8 will be used in enterprise data centers as 50 Gb/s servers are deployed. Most enterprise datacenter servers operate at lower data rates, however with 10 Gb/s server links being quite common.

What is coming next?

IEEE has already created a study group to investigate the development of 100 Gb/s per wavelength multimode solutions, known as the “100 Gb/s Wavelength Short Reach PHYs Study Group.” This will enable support of next generation 100 Gb/s server ports, expected in 2021-2022. By providing native 100 Gb/s support, no expensive “gearboxes” will be required to combine 50 Gb/s lanes, providing a low cost, power efficient optical solution. Beyond 2021-2022 timeframe, once an 800 Gb/s Ethernet MAC is standardized, using this technology with two-wavelength operation could create an 800 Gb/s, four-pair link, while a single wavelength could support an 800 Gb/s eight-pair link.

See all of the details behind the IEEE Std.802.3cm Standard.
Learn more about our LaserWave Multimode Optical Fibers

Single-Mode Optical Fiber Geometries

Understanding Optical Fiber Specifications

This article in our technical series will focus on single-mode optical fiber geometries.

If you subscribed to our Light Post Emails you know we previously covered bandwidth demand drivers and introductory standards as well as fiber optic dispersion.In this article we’ll work our way through a typical fiber specification, highlighting the importance of various single-mode optical fiber geometry specifications.

Optical fiber core specifications

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Cladding (Glass) Diameter – 125.0 ± 0.7 µm

Cladding diameter is the outer diameter of the glass portion of the optical fiber. For telecommunications fibers, this diameter has been 125 microns (µm) for a very long time. On the other hand, the diameter tolerance has not always been 0.7 µm.

Core Alignment Splicer Type Specifications

During the 1980s, optical fibers had outer diameter tolerances as high as +/- 3.0 µm. As you can imagine, matching up fiber cores ranging from 122 to 128 µm in diameter could result in extremely high loss. This situation is why fusion splicing machines required additional technology to help align the fiber cores. This extra technology increased the price of the splicing units.

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Mode Field Diameter (MFD)

Mode field diameter (MFD) is another specification related to fiber geometry. In a typical G.652.D compliant single-mode optical fiber, not all of the light travels in the core; in fact, a small amount of light travels in the fiber cladding. The term MFD is a measure of the diameter of the optical power density distribution, which is the diameter in which 95% of the power resides.

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Clad Non-Circularity of d 0.7 %

Clad non-circularity measures a fiber’s deviation from perfectly round, and is measured as a percentage difference versus perfect.

Core/Clad Concentricity Error (Offset) of d 0.5 ¼m, < 0.2 ¼m typically

Core/clad concentricity error (CCCE) measures how well the core is centered in the fiber. CCCE is measured in microns and, of course, the closer the core is placed to perfect center, the better it is.

Although the difference between 200 and 250 µm is not tremendously large, smaller diameter fibers can enable twice the fiber count in the same size buffer tube, while also still preserving long-term reliability.

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Fiber Curl

What is optical fiber curl?

Fiber curl assesses the non-linearity of bare glass. In other words, fiber curl measures how straight the glass fiber is when no external stressors are present. If imbalanced stresses are frozen into a fiber during the draw process, curl can result. This curl can show up during the splicing of fiber optic ribbons or when fixed V-groove splicing machines are used.

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In closing, fiber geekdom is a journey, not a destination, and there’s always more to learn. OFS has multiple decades of experience with fiber-optic cable networks. Please contact your local OFS representative if you would like additional information regarding optical fiber geometry specifications.

Fiber Optic Dispersion and other Non-Linear Effects

This article focuses on the parameters that affect available bandwidth in optical fibers, and the dispersion mechanisms of various fiber types and non-linear effects.  Dispersion describes the process of how an input signal broadens out as it travels down the fiber. There are several types of dispersions that we will cover. We’ll also take a cursory look at other important nonlinear effects that can reduce the amount of bandwidth that is ultimately available over an optical fiber.

Dispersion

Fiber Optic Dispersion Bandwidth Illustration

Most of the traffic traveling through fiber networks takes the form of a laser pulse, where the laser is pulsed on and off, effectively forming a digital square wave comprised of “1”s and “0”s. Dispersion causes a pulse to spread out over time, effectively rounding the edges, and making it harder for the detector to determine whether a “1” or a “0” is being transmitted. When this happens, the effective bandwidth of the link is reduced. The three main types of dispersion mechanisms are modal dispersion, chromatic dispersion, and polarization mode dispersion. Because these mechanisms affect fiber networks in different ways, we’ll discuss each in some depth. Please download the full article for more information.

Modal Dispersion

In general, our article on Single-Mode Optical Fiber Selection focuses on single-mode fibers since they comprise the vast majority of fiber kilometers deployed around the world. In contrast to multimode fibers, single-mode fibers are used for all high-capacity, long-distance networks due to their low attenuation and high bandwidth. A main limiting factor of multimode fibers is modal dispersion.

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Illustration of Modal dispersion in a multimode and single optical fibers

Multimode fibers carry multiple modes of light at the same time.  While a mode of light can be thought of as a ray of light, a typical multimode fiber can have up to 17 modes of light traveling along it at once. These modes all traverse slightly different paths through the fiber, with some path lengths longer than others. Modes that take a straighter path will arrive sooner, and modes that bounce along the outer edges of the core of the fiber take a longer path and arrive later. The effect on the end pulse is called modal dispersion, since it is due to the different modes in the fiber. Multimode fibers are designed to reduce the amount of modal dispersion with precise control of the index of refraction profile, through the quantity of dopants used in the core. However, it isn’t possible to completely eliminate modal dispersion in multimode fibers.

Chromatic Dispersion

Chromatic dispersion describes a combination of two separate types of dispersion, namely material dispersion and waveguide dispersion. Light travels at different speeds at different wavelengths, and all laser pulses are transmitted over a wavelength range. Light also travels at different speeds through different materials. These varying speeds cause pulses to either spread out or compress as they travel down the fiber. Fiber designers can use these two points to customize the index of refraction profile to produce fibers for different applications. Chromatic dispersion isn’t always a bad thing. In fact, it can be used as a tool to help optimize network performance.

For example, the first lasers used for fiber transmission operated at 1310 nm, and many networks still use that wavelength. Fiber designers therefore developed the first single-mode fibers to have minimum or zero dispersion at this wavelength. In fact, G.652 fibers are still designed this way. In these fibers, dispersion is higher in the 1550 nm window.

Today’s networks often operate with multiple wavelengths running over them. In these networks, nonlinear effects that result from the multiple wavelengths can affect network operation. We’ll give a brief overview of some of these non-linear effects in this article. Chromatic dispersion is often used as a tool to help optimize these types of networks.

Polarization Mode Dispersion (PMD)

Light is an electromagnetic wave and is comprised of two polarizations that travel down the fiber at the same time. In a perfectly round fiber deployed with perfectly balanced external stresses, these polarizations would reach the end of the fiber at the same time. Of course, our world isn’t perfect. Even small amounts of glass ovality/non-concentricity or non-concentric stresses in the cable can cause one of the polarizations to travel faster than the other, spreading out in time as they travel along the fiber. This phenomenon is called polarization mode dispersion (PMD).

Polarization mode dispersion illustration- delay in two polarizations traveling down an optical fiber.

Cabling and installation affect PMD, and even things like vibration from trains moving down tracks or wind-induced aerial cable vibrations can affect PMD. However, the impacts of these interactions are typically smaller than the inherent PMD caused by the glass manufacturing process.

Manufacturing Caused of Polarization Mode Dispersion (PMD)

There are ways to mitigate PMD. One very effective method is to make the glass fiber as geometrically round and consistent as possible. OFS uses a special technique to accomplish this. Using a patented process called fiber “spinning”; half-twists are translated through the fiber during the draw process, reducing the non-concentricities and ovalities in the glass that are the major contributors to increased PMD.

PMD Chart

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Non-Linear Effects

There are a host of other factors that network, equipment, and fiber designers have needed to consider as network capabilities have grown over the years. These factors often result as we collectively add more and more wavelengths of traffic at greater speeds and higher power levels.

It is not the intent of this article to review each of these in depth, but instead to touch on them so the reader can have a passing familiarity. The highest profile of these factors is four-wave mixing, which led to the development of non-zero dispersion-shifted fibers (NZDF). However, other non-linear effects include self-phase modulation, cross-phase modulation, Raman and Brillouin scattering, and others. As mentioned earlier, chromatic dispersion can be used to offset the effects of four-wave mixing. For those non-linear effects related to higher power levels, increasing the effective area where the light travels down the fiber can help to reduce the impact of these other non-linear effects.

Dispersions and non-linear effects are the least understood issues in the general fiber user population, mainly because the guidelines used to match up today’s fibers and electronics typically work so that the end user doesn’t need to have a detailed background to bring up a system.

OFS has multiple decades of experience with fiber optic networks. Please contact your local OFS representative if you would like additional information regarding any of the items in this article.

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OFS is a market leader in the design and manufacture of standard and custom Dispersion Slope Compensating Modules (DSCMs) also known as Dispersion Compensating Modules (DCMs). Our fixed broadband, reconfigurable, and tunable colorless modules round out a product line that is well-suited for the major transmission fiber types.