Fundamentals of fiber-optic communications
Part 1-An explanation of the basics of how fiber-optic cable works and a description of the common types of fibers, connectors and transmitters.
Many radio service providers are faced with increasingly congested sites, higher levels of RFI/EMI and co-channel interference. Many large building owners are also looking for “seamless” internal radio communications for services such as SMR, PCS and cellular. Bringing the outside antenna and the inside service user together requires a wide range of technologies, with fiber-optic-based packages gaining increasing favor. This article, the first of two parts, examines fiber optic basics, including how fiber “works”; common types of fibers, connectors and transmitter/receivers; safety and testing considerations; and Web site addresses for further study. Part 2 will focus on some specific applications and equipment, including carrier links and extended coverage.
Fiber-optic cable is the application of the physics of light in a medium, in this case a coaxial fiber made of transparent material. How is this cable able to carry light energy great distances with little, if any loss? Have you ever been outside on a sunny day and looked into a swimming pool? Do you recall that part of the floor of the pool was visible and the light reflecting from the surface obscured the remainder of the floor? This is because light striking the pool surface had changed media-from air to water. A more technically correct explanation would be that the density of the media through which the light was traveling changed. The light striking the water at or below the critical angle was reflected to your eyes. Light striking at or above the critical angle was refracted to impinge on the pool floor. Moving around the pool, you can see a change in the amount of floor visible to you as the angle of the light changes relative to your position. Fiber optic cable works on similar principle of reflections of light caused by changes in media density.
Commercial, long-haul, fiber-optic cable is made of ultra-pure glass. Manufacturing pulls this glass into a hair-thin core strand that is then clad with an additional layer of glass. The outside layer, or cladding, has a different density or refractive index. As shown in Photo 1 at the right, the inner core of the glass fiber cable carries the photonic energy, and the cladding provides the difference in density that allows the light to reflect further down the cable. Light must strike at an angle within a defined numerical aperture (NA), sometimes called a “cone of acceptance,” to propagate to the distant end. For all practical purposes, this angle has been engineered into the cable, cable connector, transmitter or receiving device used. (See the sidebar below for the math used to calculate the NA.)
Fiber-optic cable comes in several “sizes” and types, as shown in Photo 2 at the right. The most efficient cable is “single-mode” fiber. The core of single-mode fiber is about 5mm to 8mm in diameter, and the cladding is 125mm in diameter. This yields excellent characteristics: a small NA, wide bandwidth and low attenuation. Fed by a laser, this cable typically operates at wavelengths of 1,300nm or 1,550nm. The cable covering is usually yellow, a holdover from when the Bell system was the primary user of this type of cable.
Multimode fiber has a core diameter of 50mm to 62.5mm and a cladding diameter of 125mm. This yields a larger NA, narrower bandwidth and higher losses within the cable. Fed by LED devices, this cable typically operates at wavelengths of 850nm or 1,300nm. It is usually orange. Finally, the lowest grade of fiber has a core diameter of 62.5mm to 100mm and a cladding diameter of 125mm. It is usually gray. This type of cable is suitable only for jumper cables or short runs because it has high loss and narrow bandwidth. The fiber core and cladding are engineered to the application and can be glass-over-glass, glass covered by plastic cladding or even plastic-over-plastic.
These glass fibers, coated with plastic for protection, are then placed into a buffer tube. The buffer tubes are laced into different types of cable. A “come-along,” or pull member, of metal wire or stranded Kevlar is usually added. An outside PVC coating, colored for identification, completes the cable. These cables have at least two fibers, but some can have as many as 256 fibers, such as the larger cables used in CATV or data service applications. Fiber-optic cable can have many terminations or connectors.
The application or device supported by the fiber-optic cable will determine the type of connector used. The most common type of connectors a technician will see are based on a “ferrule,” as shown in Photo 3, above. This component of the connector is the tip, or front, which holds the fiber in place and aids in the alignment of the fiber. Usually cylindrical in shape, the ferrule has a precision hole through the center. Type designations can be two-letter alpha designations, such as “ST” or “FC/PC,” but they can also be longer, such as “SMC” or “biconic.” The termination of the cable with a specific connector requires specialized equipment, training and a fair amount of craftsmanship. To avoid expensive training for technicians, and to assure quality and to increase production, many companies purchase finished connectors that have a 6-inch to 12-inch “pigtail.” This connector is then fusion-spliced onto the cable, ensuring constant splice-to-splice or splice-to-connector losses. Modern fusion-splicing machines are small, lightweight and semi-automated. Gone are the days of heat guns, UV lamps, smelly adhesives, portable microscopes and hand-lapping of connectors. For the technician in the field, this means increased productivity and fewer call backs related to “bad” connectors.
Now that the cable is completed, what devices are attached? Transmitters are either LED or laser diodes. First, and probably most common, are the LED-based transmitters. Modulated directly by a signal, they offer low power and a broad spectral dispersion, which leads to low coupling of light energy into cable. On the plus side, they are uncomplicated, inexpensive, require no cooling and use little drive power. LED transmitters offer no hazard to the technician or user. I know of no LED-based amplifier systems.
Las er diode-based systems, modulated by several schemes, offer excellent (very low) spectral dispersion, and they couple large amounts of energy into a fiber cable. Laser systems are more complicated, may require cooling of the laser device and may pose a hazard to the technician or user. The advantage here is extreme long-distance propagation of signal, about 80km to 100km, before amplification is required. Several laser-pumped, erbium-doped amplifiers are on the market. Although these amplifiers are not something that the average LMR/SMR technician will use, they are exciting as a demonstration of applied advanced physics.
Safety considerations Safety in fiber-optic systems is straightforward. The laser devices used by most communication products are classified under government standards as Class 1 devices because they are considered “an enclosed working device”; that is, the laser energy is contained within the fiber. The equipment should be labeled with any and all required warning placards, which clearly indicate where any laser energy may be present anytime the system is energized.
Safe maintenance practices on these systems are usually covered in the equipment technical manual. Safety rules covered in any manual are important, such as: o technicians shall not re-energize any equipment with laser transmitter(s) until they have ensured all work on system is complete and all fiber cables are properly secured to equipment.
o technicians shall not look into fiber cables, equipment apertures or connectors unless the system or cable is known to be de-energized. (See Photo 1 above.)
o fiber-optic connectors and cable ends must be properly cleaned before attachment to system equipment.
o antistatic protective wrist straps must be worn when handling circuit packs or boards.
Certain other government and industry standards exist for worker protection. The sidebar references on page 42 are for employees who, while they may not perform direct maintenance on laser-based units, may be covered by federal “Right to Know” or “Hazard Communication” regulations.
Maintenance and testing Testing in fiber-optic systems is limited to continuity checks and loss measurements. These tests are performed by using an optical time-domain reflectometer (OTDR) or a calibrated source and an optical power meter. Both are used to define losses in a fiber system caused by connectors, splices, cable defects and improper bends. The source/power meter is the least expensive means of testing. Several vendors, such as the John Fluke Co., make optical power sensor attachments for their meter lines. OTDR testing systems start at $20,000 or more, which is usually out of the reach of anyone not doing fiber installation or splicing as a primary business activity. For troubleshooting on larger systems, an OTDR can be rented or leased from several vendors. If you have a CATV system operator in your area, see if they use fiber-optic cable in their system. If so, they may be a source of low-cost assistance when performing fiber work.
