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Articles Fiber Optics in Harsh Environments
 

Fiber-based systems offer apparent advantages over electrical methods in large plants and factories where the harsh environment threatens data reliability and security. Noise is the most common problem. High voltage and motor feeders, motors and motor drives, and even fluorescent lighting can generate sufficient electromagnetic interference (EMI) to corrupt wire or radio-based systems.

In a distributed control system (DCS) or supervisory control and data acquisition (SCADA) installation, this problem will at least render the system unreliable. An unstable factory network can result in the uncontrolled operation of equipment and possible personnel injury or facility damage. If such an environment is unavoidable, fiber optics will effectively eliminate this problem.

Fiber optics have come a long way since 1854, when John Tyndall established that a jet of water streaming from a container could transmit light. Later nineteenth century experimenters used parabolic mirrors and internally metalized tubes to guide light. In the 1920s, scientists used the first bundles of glass tubes to transmit crude but recognizable images.

Back to Basics

The concept of refractive index is basic to understanding fiber-optic technology. To effect the most complete transfer of energy through a 1nedlure, the light energy has to be completely contained within the medium. This containment is done by making use of the medium's refractive index.

Every material capable of transmitting light has a characteristic refractive index, defined as the ratio of the velocity of light in the material to the velocity of light in a vacuum. When light encounters a boundary between materials of differing refractive indices (e.g., air and water), a part of the light is bent while a part of it is reflected.

If the angle at which the light ray (also known as a mode) striking the boundary exceeds some angle of total ray is reflected. light ray enters the other material. critical angle is the maximum angle of refraction.

Fiber-optic cable consists of three parts: the core, the cladding, and the coating. The core transmits the light and has a high refractive index. The cladding contains the light within the core because its lower refractive index causes all the light rays to reflect back into the core. This "total internal reflection" or "fiber-optic effect" is the technology's underlying principle. The coating, usually an acrylate polymer, protects the core/cladding assembly.

Optical fiber is typically made from high-purity silica glass. Plastic fiber of varying configurations is also available. but the attenuation of light energy can approach one thousand times that of glass fiber.

You identify optical fiber by its core/ cladding size. A singlemode, step-index fiber with a core diameter of 8 microns and a cladding diameter of 125 um would have a designation of 8/125. Similarly, a multimode, step-index fiber could be designated 200/230. (The term step index means that there is a sharp change in refractive index from the core to the cladding.)

The length and integrity of the transmission path and the core/cladding arrangement affect the bandwidth, or the frequency range that the optical fiber transmits. Fiber bandwidth is expressed in megahertz-kilometers (MHz-km).

Attenuation in optical fiber is due mainly to scattering and absorption within the core material. Dirty or poorly made connections will diminish the light intensity. Attenuation, or loss, is expressed in decibels per kilometer; thus, loss is introduced over distance and at connections. Any connection, regardless of quality, will introduce a loss or "insertion loss."

Core/Cladding Arrangements

Three core/cladding arrangements are in common use. Each exhibits characteristics that affect the bandwidth and, thus, the application. Basically, the refractive indices and diameters of the core and cladding are manipulated relative to each other, producing fibers that are optimized for specific applications.

The single-mode, step-index fiberoptic cable has the highest bandwidth (exceeding 25 GHz-km) and the lowest loss. A typical fiber of this type is designated 8/125. The single-mode fiber core allows one mode of transmission. This means that only one ray of light is transmitted.

The transmitted light's change in velocity over its wavelength range limits the bandwidth. This "chromatic dispersion" is an inherent property of the core material and increases with distance.

Single-mode cables are difficult to connect or splice because the small core diameter makes the alignment of the fibers critical. Attaching connectors or splicing requires specialized equipment and trained personnel. Single-mode fibers are mostly used in long-distance communications.

At the other end of the bandwidth scale is the multimode, step-index fiber. A typical designation is 200/230. The core diameter is large relative to the light rays transmitted; therefore, more than one ray, or mode, can be transmitted.

This type of fiber has a much lower bandwidth than the single-mode fiber, usually topping out at about 20 MHz-km. The usable bandwidth is even lower, in the vicinity of 5 MHz-km. The sharp difference in refractive indices (step index) causes some of the transmitted light rays to be reflected many times as they travel the length of the fiber.

This results in the transmitted light rays arriving at the receiving end at different times. A sharp pulse at the transmitted side, for example, would arrive at the receiving end in a "spread." This "modal dispersion" limits this type of fiber to low-end process and specialized medical applications.

Between the single-mode and multimode step-index fibers is the multimode, graded-index optical fiber. In contrast to the other two types of fiber, the difference in core/cladding refractive indices is gradual, not stepped. The core has a high index that becomes gradually lower as it approaches the outer diameter of the cladding. Instead of producing a sharp boundary between the two components. with resultant degradation in performance, light rays in a graded-index fiber are reflected to make the path lengths of all rays equal.

The gradation of refractive indices produces a focusing effect in the core. Consequently, modal dispersion does not greatly affect multimode, graded-index fibers. A common fiber of this type is designated 62.5/125 and has a bandwidth between 100 and 800 MHz-km. This type of fiber finds wide use in commercial and industrial data communications systems.

Light sources for multimode fibers are lasers or high-performance LEDs. Because of the large core size of this type of fiber, alignment when connecting is not as critical as it is with single-mode fibers.

Each type of fiber will only transmit light that enters the fiber core's "cone of acceptance." If a ray hits the core at an angle outside of this cone, the light will not be internally reflected. This parameter is the fiber's numerical aperture (NA). Larger cones have a larger NA, which also indicates the relative light-gathering power of the fiber. Thus, a 200/230 fiber would have a larger NA than a 8/125 fiber.

Cable Makeup

Depending on the application, the distance involved, and the location, several types of cable configurations and connector types are available. Optical fiber is fragile and must be protected, mostly from mechanical stresses such as bending, crushing, thermal effects, and pulling during installation.

Even though tensile strength typically exceeds 600,000 pounds per square inch, glass fiber can lose a significant amount of strength when exposed to water. Excessive bending will also cause degradation in performance. With a reasonable amount of care, you can install optical fiber easily, and it will last for about 30 years or more. The most common mode of failure in optical cable is the "backhoe syndrome." As with any underground installation, accurate drawings, close supervision, and the use of warning tapes serve to avoid costly outages.

A tight-tube (or tight-buffer) design has a PVC coating, which tightly bonds to the fiber, limiting movement (see Figure 3). This cable type can have strength members, which you pull through conduit and cable trays. This design, however, has low crush resistance and is susceptible to deformation due to thermal expansion; thus, it is recommended for indoor use only.

A loose-tube design gives a fiber free movement (see Figure 4). Each component of the cable (the sheath or outer coating, the strength member, and the buffer tubes that carry the fibers) has different thermal characteristics. By allowing the fibers and the components surrounding them free movement, deformation is avoided.

Loose-tube construction has much better crush resistance than tight tube because of the buffer-tube protection of the fibers. Loose-tube cables have a strength member, which is used as the pulling member for conduit installation. In some installations, the strength member replaces the messenger used in an aerial installation.

The strength member is made either of a high tensile strength plastic or steel. Note that adding a steel strength member renders the cable conductive. Loose-tube cables are usually filled with a gel, which surrounds the fibers and increases protection from water. This also improves crash resistance because of the gel's cushioning effect.

You mostly use this cable type for outdoor applications, but you can also use it in harsh industrial environments. A drawback to this type of cable is the difficulty in handling individual fibers. The fiber coating does not have to be as thick as in tight-tube construction; thus, attaching connectors is difficult.

Breakout kits are available for loosetube cables. These provide each fiber with a sheath that is color-coded and equipped with a strength member. The thicker sheath lets you attach any connectors.

Breakout cables are a hybrid solution (see Figure 5). In a breakout cable, each fiber is treated as a separate unit, complete with a sheath and strength member. This design eliminates the need for a breakout kit, because the sheath lets you attach connectors easily.

Remember that tight-tube construction does not allow for free movement and provides low protection against mechanical stress. It does, however, have a thick coating for ease of handling. Loose-tube construction, on the other hand, allows free movement and provides a good degree of protection.

Breakout cables let fiber subunits move freely, and they protect each fiber by virtue of their thicker coating/strength member arrangement. Each fiber subunit is configured as a tight tube. Breakout cables also come equipped with a separate strength member just like the loose-tube design.

Know Your Connections

Several connector types, such as the generic connector shown in the sidebar "Kinds of Connectors," are available. As the technology matures, the cost and difficulty of making a connection will decrease. At present, attaching connectors requires specialized equipment and trained personnel. Attaching connectors is time consuming, and even the best connectors introduce an optical, or insertion, loss.

Connectors are classified by the shape and size of the alignment ferrule and by the means of mating the housings. The ferrule is a sleeve that holds the fiber in position inside the connector assembly. The fiber-to-fiber alignment inside the connector coupling assembly is critical for the most complete and efficient transfer of light energy.

In a single-mode fiber with a core of only 8 um, any misalignment results in a significant or complete loss of signal. In a 200/230 fiber, even as much as a 15% misalignment can be tolerated. If the connector's mechanical tolerances can be tightly controlled, then proper installation of fiber into ferrule is the only variable.

Each connector requires a different technique for proper installation. You should consult the manufacturer's literature for the latest information on these connectors.

Connect for Life

When a permanent connection between fibers is desired, optical fibers can be physically spliced (see Figure 7). Splicing, while limiting flexibility, is cheaper and results in a stronger, permanent connection. The following two methods are used:

Mechanical splice: This splice requires treatment and precise alignment of the fiber ends. It is a low-cost, long-term alternative to a connector that introduces less loss in most cases. This type requires a significant investment in equipment, and some training is required.

Fusion splice: This is the most reliable and low-loss splice. Trained personnel fuse together the fiber ends using specialized equipment. This type of splice will stand up to a high degree of physical stress and introduces the lowest optical loss.

Patch panels are used as distribution points, storage locations for spare fiber, and splice cases. Strategically located patch panels will allow easy configuration and reconfiguration of the network.

As you add and remove devices, the patch panel serves as a convenient point at which fibers can be terminated, tagged, and made available for use. You can store unterminated, spare fibers for future use. Panels also provide protection from environmental conditions, and many include splicing bays.

Cable Testing

There are several methods for testing fiber-optic cables. The most basic is the pocket flashlight. This is a quick and dirty method for checking continuity or for "ringing out" a single fiber. A note of caution: If you are unaware of what is happening at the other end of the fiber, never look into the end of a fiber-optic cable! One or more fibers may be carrying potentially harmful levels of light energy. Point the cable at the wall while testing.

Somewhat more sophisticated is the optical time domain reflectometer (OTDR).The OTDR measures backscatter, reflections caused by a break or some other fault in the optical fiber. A laser pulse is injected into the cable and, when a fault is detected, a portion of the pulse is reflected back to the OTDR. On well-documented systems, the fault can pinpoint the location of any problems with the cable.

For periodic checkouts, the optical-loss test set is used. This is basically a light source of specific or selectable frequency and power output and a receiver that indicates optical power. The test set is inexpensive compared with the OTDR.

It is good practice to test optical cables before and after installation. Testing before will spot any manufacturing or shipping problems and eliminate having to rip out the installation later. It also provides a baseline for later comparison. You should always test the cable after installation. This also provides a baseline and can spot a problem developed during installation.

Transmit and Receive

An optical modem converts electrical energy into light energy. The modem receives voltage levels from the data terminal. It then translates these voltages into light pulses of varying duration arranged to conform to one of several data communications protocols.

For single-mode fiber, used mostly in 1ong-d'lstance communications, a semiconductor laser is used to transmit light pulses in the 1,300 to 1,550 nm range. These devices produce high-quality, coherent light and can support transmission rates of over 2 Gbps at distances in excess of 30 km. For multimode fibers, high-performance LED modems in the 850 to 1,300 nm range are generally used in transmission rates over low to medium distances. This type of modem is used in most LAN applications. Typical bandwidths for 1,300-nm LEDs would be about 100 Mbps/10 km. IEEE 802.3J (Ethernet) will support 10 Mbps/4 km, while fiber distributed data interface (FDDI) will run at 100 Mbps/2.2 kin.

Cost differences between laser and LED modems are significant. LEDs are used for the majority of LAN or factory applications. Lasers are simply unnecessary. Adequate bandwidth, reliability, and lower cost make any of the high-quality LED-based modems suitable for most communications applications.

Detectors come in two flavors: the PIN photodiode and the avalanche photodiode (APD). The PIN photodiode (named for its semiconductor layers) generates charge carriers when exposed to light. These charge carriers, or photocarriers, form a photo current that is then translated into usable data. PIN devices require low bias voltages and are abundantly available matched to the common LED source ranges.

The APD also generates photocarriers when struck by light. These charges cause an avalanche of charge carriers and are, therefore, capable of forming a much larger output current. APDs are much more sensitive than PINs. APDs are used primarily for laser sources over long-distance links. But APDs are more costly, require bias voltages in excess of 100 volts, and are sensitive to temperature variations.

Optical Budgeting

With a good understanding of the equipment, the next step is to lay out the network. Fundamental to proper operation of optical links is the need for transmitted power to reach the receiver. This sounds obvious, but it is often overlooked.

Every component in an optical communications system introduces loss. Before specifying modems, the designer must have a thorough understanding of the proposed system's present and future performance goals. The first step is to calculate the optical budget and the total system loss.

Optical fiber has a characteristic loss per unit distance. This is usually expressed in dB/km and is dependent on the wavelength of the light source. For a typical 8/125 general-purpose fiber, the attenuation is 0.8 dB/km @ 1,550 nm. Higher quality fiber of the same size can approach 0.4 dB/km. A good attenuation for a 100/140 fiber is 3.5 dB/km @ 1,300 nm.

Each connector introduces a loss (the insertion loss) into the link. This is expressed in dB/connector or simply dB. Connector insertion losses range from approximately 1.5 dB for an SMA-905 to 0.3 dB for an SC connector.

Splices also introduce an insertion loss, which varies widely depending on type and quality. For the sake of calculating the system loss, you should use a figure of 0.10 dB/splice unless you've tested the splice and know its insertion loss.

Assume that we have a link with four connectors, two splices, and a total of 6 km of fiber. We are using a 62.5/125 fiber with a loss of 1.0 dB/km. Transmitter output power into the fiber is -12 dBm (decibels as referenced to 1 milliwatt). Our receiver's sensitivity is -27 dBm. Subtracting the transmitter's output power from the receiver's sensitivity gives us an optical budget of 15 dB.

Assuming a source of 1,300 nm, we can now summarize the system or link losses:

6 km fiber @ 1.0 dB/km = 6.0 dB
4 connectors = 1.2 dB
(ceramic ST) @ 0.3 dB
2 splices @ 0.10 dB = 0.2 dB
Total system loss = 7.4 dB

Subtracting the system loss from the optical budget gives us a margin of 7.6 dB. This is a comfortable margin, which will allow future expansion of the link. If no expansion is planned, we can use a less powerful and less costly modem.

We should also consider changes in fiber size over the length of the link or a bypassed node. Furthermore, a 1-dB loss should be allowed for single-mode fiber links to allow for dispersion and absorption. In all cases, a margin of at least 3 dB should be maintained. A margin of 1 dB is not acceptable, considering that LEDs and other components degrade with time, which could render such a link susceptible to erratic performance or complete failure.

Fiber-optics technology is still maturing. A good grounding in the basics is a necessity for anyone who wishes to design, implement, or even effectively use a fiber-based system. There are numerous short courses and literally tons of documentation available for those who wish to learn more about the technology described in this article. For a quick review of basic networking technology, see the December 1995 issue of Industrial Computing.

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