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Fighting modal dispersion

It can cause major limitations on the system's operating speeds over long distances. Double the distance and you double the dispersion effect.


May 1, 2004  


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In today’s complex networking environments, network technicians must expand beyond traditional signal loss to include testing and troubleshooting methods specific to fiber optic technology.

While it was once sufficient to test for signal loss during installation and/or troubleshooting, there are a number of additional factors that should be examined in order to ensure the integrity of the signals, including the presence of a phenomenon called modal dispersion.

Network delays and in the worst case, complete loss of communication can be caused by an increased presence of modal dispersion on a network due to progressive distortion of the quality of the original signal.

Modal dispersion is normal and is inherent with any type of communication over fiber optics. Generally speaking, it presents little problems if a new installation is performed following current guidelines and standards, using high quality fiber and equipment.

Even at that, it is not uncommon to find modal dispersion of varying degrees on a fiber optic network. The good news is, new generation fiber optic test tools are well equipped to help technicians manage these added troubleshooting complexities.

Normal occurrence

As mentioned, modal dispersion is inherent with any type of fiber optic communication. In fiber optics, light rays comprising each pulse travel in many different paths within a multimode fiber. Each mode travels at different angles as they zigzag down the fiber and as a result, they arrive at the receiver end at different times.

This arrival time variance results in distorted and overlapping pulses at the receiver end. Also, because the detector cannot tell where one pulse ends and the next begins, the spreading of the light pulses limits the frequency that can be transmitted.

Although modal dispersion is a normal occurrence, it can present problems depending on the quality of the original signal. It tends to have a greater impact in gigabit networks, since as transmission speeds increase, light pulses are pushed closer together over the same amount of time.

As a result, the receiver end has less spacing between pulses to interpret the original shape of the data signal. To help lessen the amount of modal dispersion over multimode fibers, VCSEL (vertical cavity surface-emitting lasers) sources are now used for gigabit devices, versus the traditional LED sources.

In recent years, the Telecommunication Industry Association (TIA) Task Group on Modal Dependence of Bandwidth has been working on setting a list of appropriate source and fiber selection criteria.

These include restrictions on the launch power distribution of the laser force, as well as on the bandwidth properties of the multimode fiber.

It maintains that launch conditions must be restricted so fewer modes (i.e. paths) are used in order to reduce modal dispersion and therefore increase modal bandwidth.

When the shape of a light pulse is distorted beyond specified limits, system bandwidth can be limited — much the same as when optical power loss reduces signal performance.

While the time difference between the fastest and the slowest mode of light entering the fiber at the same time and traveling over a kilometre may be only one to three nanoseconds, this modal dispersion can cause major limitations on the system’s operating speeds over long distances.

Double the distance and you double the dispersion effect.

In general, there are three main types of tools that can be used for testing fiber optic networks. An OLTS (Optical Loss Test Set) will determine how much light is received at the opposite end of the fiber.

A Visual Fault Locator is effective in indicating where faults such as macrobends, microbends and severs in glass occur. An OTDR (Optical Time Domain Reflectometer) can be used to perform in-depth analysis of insertion loss, fiber connectivity and fiber length measurement among others.

Centre line dip

Before looking at the types of modal dispersion, it is important to understand key issues related to the methods of manufacturing optical fiber. The ideal index profile of a multimode fiber is a parabola.

Previous methods of manufacturing optical fiber included having different layers of glass added to what the fiber is drawn from (known as a “perform”) to create a graded-index fiber. The perform was then heated to fuse the layers.

Incomplete fusion or impurities in the centre of the perform will cause slight dips in the index profile — commonly known as a “centre line dip” in the fiber.

It is important to remember that the light launched into a multimode fiber is different between that of an LED and VCSEL.

LEDs cause many modes of light to become excited. The manufacture of a graded-index fiber allows the longer paths of light to get to the end of the fiber at about the same time as the light taking the shorter path.

The launching of a light by an LED into the fiber is commonly known as an overfilled launch bandwidth (OFL BW) and is not as affected by a centre line dip because of the many modes of light being launched into the fiber. However, OFL BW is not a good indicator of functional performance.

A VCSEL on the other hand, launches light into a much smaller area. This restricted launch means fewer modes in the centre of the fiber. Because lasers concentrate power in the fiber’s centre, even small errors in the index profile at or near the fiber’s centre can dramatically reduce transmission performance.

An offset launch patch (i.e. a cable that changes the alignment of the launching of the light source into the fiber) can be adjusted to avoid the “centre line dip” in these situations in order to avoid the very centre of the fiber core.

Modal dispersion can come in many forms and create different problems on a fiber optic network. Some examples include:

Polarized Modal Dispersion: This distortion that occurs in fiber optic cable is caused by irregularities in the shape of the fiber optic cable and the core. The condition is amplified by temperature and splicing where one irregular piece of fiber is spliced to another. When light rays transmitted down the centre of the core travel faster than those that travel closer to the edge, information is distorted when it reaches the end of the cable. As data rates increase, PMD can become more of an issue.

Spectral, Material or Chromatic Dispersion: Because different wavelengths travel at different velocities through fiber, it is not unusual to encounter these types of dispersion problems — these often occur simultaneously. Spectral dispersion occurs when white light decomposes into a rainbow of colours by a glass prism, leading to unequal bending of the rays associated with each colour.

This may lead to chromatic dispersion, which happens when wavelengths of light travel at a different speed. This phenomenon will cause the pulse to spread out as it travels down the fiber, resulting in a deterioration of signal quality. Material dispersion refers to the actual amount of spectral or chromatic dispersion in the fiber.

If the fiber system’s spectral source emits a single frequency of light, this spectral dispersion can be eliminated. However, an LED light source has a spectral range of about 20 times that of a laser, and therefore tends to experience much greater spectral dispersion. Anomalies will appear on an OTDR — although the amount of dispersion will not be indicated.

An OLTS and an OTDR are complementary tools for testing and certifying optical fiber. Combining the best capabilities of both testers is crucial for designers, installers and ultimately the end user.

While modal dispersion is often difficult to determine in field test sets, these types of fiber test tools can be used to detect those anomalies, which could be contributors to the problem.

The diagnostic challenges for fiber optic networks will continue to grow, as we witness the increasing prevalence of cost-effective solutions for high-speed data networks, and the continuing drop in the price of hardware and fiber sources.

While this opens the
doors to more and more networking opportunities, at the same time, this growth in capabilities and performance has also led to an equally significant increase in technical challenges, including modal dispersion — a phenomenon that can lead to signal distortion, delays and at worst, loss of communication.

With proper understanding of the materials, installation methods and testing procedures however, these troubleshooting challenges can be overcome relatively quickly and cost-effectively.

Ron Groulx is a Product Specialist for Fluke Networks Canada. A graduate of Computer Science and a member of the IEEE Computer Society, he can be reached at ron.groulx@fluke.com


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