Our publication of basic background information on fiber optic technology and reliable guides on fiber optic cabling system design provides a solid foundation for novices and experts alike.
To date, fiber optic cable installations have brought high-speed network communications to companies, campuses, universities, hospitals, libraries, offices, and homes. Currently, fiber optic cabling is also becoming the connection of choice for CCTV, government network security, factory automation systems, and medical imaging networks, among others. In the future operating environment, the development and implementation of standards such as Synchronous Optical Networking, Optical Distributed Data Interface, Asynchronous Transfer Mode, and Fibre Channel are expected to accelerate the installation of fiber optic cables.
Basic Fiber Elements
Fiber optic cables include core wires, cladding and coatings. The core, made of glass or plastic, is the light-transmitting material of the optical fiber. The larger the core, the more light passes through the fiber. The cladding provides a low index of refraction at the core interface, causing reflections within the core. In this way, the light waves are confined within the fiber. Fiber coatings are typically multiple layers of plastic that maintain fiber strength, absorb shock, and protect the core and cladding.
fiber core
Fibers can be identified by the type of path or mode that light travels within the fiber core. The two basic types of optical fibers are multimode and singlemode.
Multimode fiber cores can be of the step-index or graded-index type. Step-index multimode fibers get their name from the sharply stepped difference in the core and cladding refractive indices. In ordinary graded-index multimode fibers, the light is also directed down multiple paths. However, unlike step-index fibers, graded-index cores contain many layers of glass, each with a lower index of refraction because the layers extend outward from the shaft.
The effect of the gradient is to accelerate the light in the outer layers to match the light traveling along the shorter path on the core axis. The result is a graded-index multimode fiber that equalizes the propagation times of the various modes. Thus, data can be sent over longer distances and at higher rates before the light pulses start to overlap and become indistinguishable at the receiver end. Graded-index fibers are commercially available with core diameters of 50, 62.5 and 100 microns.
Single-mode fiber allows only a single light path or mode to travel down the core. This transmission virtually eliminates any distortion due to overlapping light pulses. The core of a single-mode fiber is very small, about 5 to 10 microns in diameter. Single mode fiber has higher capacity and capability than either of the two multimode fibers. For example, a submarine telecommunications cable can carry 60,000 channels of voice over a pair of single-mode fibers.
Design Considerations for Optical Fiber Cabling
When selecting components for fiber cabling, consider three fiber factors that affect transmission performance: fiber size, bandwidth, and attenuation.
The dimensions of an optical fiber are usually expressed by the outer diameters of its core and cladding. For example, 50/125 represents an optical fiber with a core diameter of 50 microns and a cladding diameter of 125 microns. (A micron is one millionth of a meter; therefore, 25 microns equals 1/1000 of an inch.)
Fiber core diameters can range from 8 to 200 microns, but are only commercially available in certain sizes. Typically, for these fiber sizes, the cladding diameters range from 125 to 230 microns. In general, the smaller the core diameter, the better the fiber performance in terms of low attenuation and high bandwidth. The design tradeoff is that the smaller the fiber size, the less light the fiber will capture from the transmitter. This situation sometimes requires the use of a high-power transmitter.
A practical consideration is that the fiber size must be matched to the transmitter and receiver used. In some cases, transmitters and receivers may be evaluated for several different fiber sizes. In other cases, they may only be available for a specific fiber size. However, in all cases, the performance of the fiber and the transmitter/receiver must be considered together.
Bandwidth is a measure of the data-carrying capacity of an optical fiber. The larger the bandwidth value, the larger the information capacity of the fiber. Bandwidth is expressed in frequency span (MHz-kilometers). For example, a fiber rated for 200 MHz-km can move 200 MH data by 1 km, or 100 MH data by 2 km.
In addition to the physical changes in the optical pulse caused by the bandwidth-limited frequency, the level of optical power decreases as the optical pulse propagates through the fiber. At the specified wavelength, this optical power loss or attenuation is expressed in decibels per kilometer.
Fiber loss
Light is an oscillating electromagnetic wave. Short wavelengths exist in the ultraviolet spectrum. However, fiber optic transmission typically occurs in the infrared spectrum.
The wavelength is measured in nanometers - one billionth of a meter - and represents the distance between two cycles of the same wave. Optical power losses at different wavelengths occur in optical fibers due to absorption, reflection, and scattering. These losses depend on the specific fiber and its size, purity and refractive index.
optical fiber
Fibers are optimized for operation at specific wavelengths. For example, for a 50/125 micron multimode fiber operating at 1300 nm, the typical loss is less than 1 dB/km. Typically less than 3 dB/km (50% loss) for the same fiber operating at 850 nm. These two wavelengths - 850 and 1300 nm - are the most commonly specified regions for current fiber optic systems. The fiber is also optimized for single-mode fiber delivery systems at 1550 nm.
Microbending loss
Without protection, the fiber is prone to microbending, which can lead to loss of optical power within the core. Microbends are tiny fiber deflections caused by transverse mechanical forces. To overcome this problem, two basic types of fiber protection are used: loose buffer and tight buffer.
In a loose buffer configuration, the fiber is placed in a plastic tube that has an inner diameter much larger than the fiber itself. Then, the inside of the plastic tube is usually filled with gel. The loose tube isolates the fiber from external mechanical forces acting on the cable. For multi-fiber cables, several tubes, each containing one or more fibers, are combined with strength members to unstress the fibers and minimize cable elongation and contraction.
By varying the number of fibers in the tube during fabrication, the degree of microbending due to temperature changes can be controlled. In this way, the increase in attenuation over the temperature range is minimized.
In a tight buffer structure, direct extrusion of plastic is used to cover the base fiber coating. The tight buffer structure enables a compact and lightweight design similar to fiber optic configurations and often results in flexible, crush-resistant fiber optic cables. However, this structure also results in reduced isolation between the fiber and temperature-varying stress. And, while relatively flexible than loose buffers, sharply bent or twisted fiber optic cables due to macrobends can produce optical losses that can exceed nominal specifications.
An improved form of the tight-buffered structure is a drop cable in which the tightly-buffered optical fibers are surrounded by aramid yarn and a jacket, usually made of polyvinyl chloride. These single fiber subunit elements are then covered with a common jacket to form a drop cable. This "cable-in-cable" structure has the advantage of direct, easy connection and installation of connectors.
Each type of fiber optic cable construction has its inherent advantages and limitations. Typically, loosely buffered fiber optic cables are used for outdoor installations, while tightly buffered fiber optic cables are used for indoor installations. However, once a loosely buffered structure or a tightly buffered structure has been chosen, the system designer has already decided on the trade-off between microbending loss and flexibility to achieve optical manipulation goals.
For installing fiber optic cables, mechanical properties such as tensile strength, impact resistance, flexing and bending are important system design considerations. Before installation, system designers must also consider environmental factors, such as the required resistance to moisture, chemicals, and other types of atmospheric or subsurface conditions.
Normal cable loads experienced during installation can eventually put the fiber in a state of tensile stress. These stress levels can lead to microbending losses, resulting in increased attenuation and possible fatigue effects.
To help transfer stress loads during short-term installations and long-term applications, various internal strength members have been added to the cable construction. The addition of strength members provides tensile load characteristics similar to electronic fiber optic cables, keeping the fiber unstressed by minimizing elongation. In some cases, the strength member may also serve as a temperature stabilization element.
Because the fiber has little stretch before breaking, the strength member must have low elongation under the expected tensile load. When selecting a strength member, other mechanical factors such as impact resistance, flexing and bending must be investigated.
Common strength members used in fiber optic cable manufacturing include aramid yarn, glass fiber optic epoxy rods, and steel wire. Pound for pound, aramid is five times stronger than steel. When an all-dielectric structure is required, aramid and glass fiber optic epoxy rods are often chosen. When extremely cold temperature performance is required, steel or glass fiber optic epoxies are generally recommended because of their better temperature stability. For better protection in outdoor applications, fiber optic cables can be specified with single or double jackets, or armored fiber optic cables can be used for aerial, underground pipeline and direct buried installations.
Fiber count
To specify the number of fibers used in a fiber optic plant, system designers must carefully evaluate potential future network requirements. This step is critical due to rapid changes in fiber optic technology.
Depending on the number and type of applications in the network and the level of redundancy required, the number of fibers in the backbone or each wiring closet can range from 2 to over 100. Due to expensive multiplexing equipment, separate dedicated fibers are usually used for each application. Even though some systems clearly state the number of fibers required, there are no current rules for the number of fibers.
For example, if two buildings were to be networked with an FDDI backbone, four fibers would be required in the fiber optic cable between the buildings—two fibers for transmission and two fibers for reception. However, to accommodate future needs, at least twice as many fibers should be installed in the system backbone. Not only is the number of fibers installed to meet current needs, but spare fibers can also be installed as well as other fibers for future expansion, ensuring a flexible, scalable fiber optic cable plant.
Given the number of variables, designing solutions for fiber optic networks is not an easy task to meet today's needs while being ready to adapt to rapidly emerging communication technologies. But system designers who take the time to master and spend the resources "future-proofing" today's cabling designs will be better positioned to easily and quickly upgrade their systems for future applications.
