HAMS (in my experience) have the reputation of being curious about things technical. Here is an article about fiber optics. I wrote this piece with a view to those amongst us that "do" radios for a living. Part 2 follows shortly.

Many radio providers are faced with increasingly congested sites, higher levels of RFI/EMI and co-channel interference. If this weren't bad enough, many large building owners are looking for 'seamless' internal radio communications for services such as SMR, PCS and cellular. Bringing the outside antenna and inside service user together requires a wide range of technologies, with fiber optic based packages finding increasing favor. This article, part 1 of 2 parts, will examine fiber optic basics - including how fiber 'works'; common types of fibers, connectors and transmitter/receivers; safety and testing considerations; ending with some URLs pointing to Internet sites for further study. Part 2 will focus on some specific applications and equipment.

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? Allow me to pose this question - have you ever been outside on a sunny day and looked into a pool of water? 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 the density of the media in which the light is 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. As you move around the pool the amount of floor visible to you changes as the angle of the light changes relative to your position. What in the world does this have to do with fiber optic cable you ask? Everything. Fiber optic cable works on the principal of reflections of light caused by changes in media density.

Commercial, long haul fiber optic cable is made of ultra pure glass. During manufacture, this glass is pulled into a hair-thin core strand, then clad with an additional layer of glass - the outside layer or cladding having a different density or refractive index. See fig 1, the inner core of the glass fiber cable carries the photonic energy, the cladding provides the difference in density allowing 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 has been engineered into the cable, cable connector, transmitter or receiving device used. See the sidebar for the math used to calculate the Numerical Aperture and illustrations.

Fiber optic cable comes in several 'sizes' and types. The most efficient cable is "single mode" fiber. The core of single mode fiber is approximately 5 to 8 m in diameter and the cladding is 125 m in diameter. This yields excellent characteristics: a small NA, wide bandwidth and low attenuation. Fed by a LASER device, this cable typically operates at a wavelength of 1300 or 1550 nanometers (nm). The cable covering is usually yellow in color, a holdover from when the Bell system was the primary user of this type of cable.

Multimode fiber has a core diameter of 50 to 62.5 m and a cladding diameter of 125 m. This yields a larger NA, narrower bandwidth and higher losses within the cable. Fed by LED devices, this cable typically operates at a wavelength of 850 or 1300 nm. It is usually orange in color. Finally, the lowest grade of fiber has a core diameter of 62.5 to 100 m and a cladding diameter of 125 m. It is usually gray in color. This final type of cable is suitable only for jumper cables or very short runs as it has high losses and narrow bandwidth. The fiber core and cladding are engineered to the application and can be glass over glass, glass covered by plastic cladding and 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 pull member of metal wire or a strand Kevlar is usually added and an outside PVC coating, colored for identification, completes the cable. These cables have at least 2 fibers or can have as many as 256 fibers, the larger cables found in CATV or data service applications. Fiber optic cable can have many different 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 those based on a "ferrule". 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, it has a precision hole through the center. Type designations can be two digit alpha, such as ST, FC/PC but can also be longer as in 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, assure quality and increase production, many companies purchase finished connectors that have a 6 to 12 inch 'pigtail'. This connector 'pigtail' 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 from 'bad' connectors.

Now that the cable is completed, what devices are attached. Transmitters are either Light Emitting Diodes 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, use little drive power . LED transmitters offer no hazard to the technician or user. I know of no LED based amplifier systems.

LASER diode based systems, modulated by several different schemes, offer excellent (very low) spectral dispersion, and 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 plus side here is extremely long distance propagation of signal, on the order of 80 to 100 Km before amplification is required. There are several LASER pumped erbium-doped amplifiers on the market, while not something the average LMR/SMR technician will use, they are exciting as a demonstration of advanced physics in use.

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" -- the LASER energy is contained within the fiber. The equipment should have 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 - as a reminder here are some sample rules:-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.-technicians shall not look into fiber cables, equipment apertures or connectors unless system/cable is KNOWN to be de-energized.-fiber optic connectors and cable ends must be properly cleaned before attachment to system equipment.-anti-static protective wrist straps must be worn when handling circuit packs or boards.Certain other Government and Industry standards exist for worker protection. The sidebar #2 references are included 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.

Testing in a fiber optic systems is pretty much limited to continuity checks and loss measurements. These tests are performed by using an Optical Time Domain Reflectometer (OTDR) or a calibrated source and 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 $20K or more, 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 utilize fiber optic cable in their system. If so, they may be a source of low-cost assistance when performing fiber work.

SIDEBAR 1 URLs for further readinghttp://www.fotec.com/index.htmloffers a "Lennie Lightwave" testing primer and an extensive line of fiber optic test equipment. While biased toward FOTEC equipment, does offer good primer material.http://corning.com/prod_svcs/index.htmlselect the Fiber 101 to learn about the history of optic cable and how Corning glass is used to make the cable.http://www.metrotek.com/test.htmhere is one source of low cost fiber optic test equipment.Further work with a search engine using "keyword search" for fiber optic, optic cable, fiber optic test equipment or glass fiber cable will give many more sites to visit - enjoy.

SIDEBAR 2 LASER safety regulationsU.S. Department of Health and Human Services, Title 21 CFR, Subchapter J. Part 1040.10, Performance Standard for Laser Products, and Part 1040.11, Special Purpose Laser Products. Note: Most communication equipment falls under these rules.

OSHA regulationsU.S. Department of Labor: Guidelines for Laser Safety and Hazard Assessment, OSHA Instructional PUB 8-1.7, Directorate of Technical Publications, August 19,1991

OSHA Instruction CPL 2-2.20B CH-2; Chapter 17: Laser Hazards, April 19, 1993, Directorate of Technical Support

U.S. Department of Labor, Bureau of Labor Standards, Safety and Health Regulations for Construction, Section 518.54 Non-Ionizing Radiation, Code of Federal Regulations (CFR), 36 (75): pp.7348-7349, Saturday, April 17, 1971

U.S. Department of Labor: Laser Construction Standard (non-ionizing radiation), Occupational Safety & Health Administration (DOL/OSHA): 29 CFR 1926.54

U.S. Department of Labor: Laser Eyewear Standard, Occupational Safety & Health Administration. Note: These rules are for high power, exposed LASER systems.

SIDEBAR 3 mathBandwidth of the fiber cable is a function of the spectral width (SW) of the source, dispersion (DISP) inherent in the cable itself and length (L) of the cable run.

BW= 0.187/(Disp)(SW)(L)

Numeric Aperture is a unitless measure. It is found as a function of the refractive indices of the cable in question.Find the square root of the sum [n12 - n22] where "n" is the refractive index.. Then, find the cone of acceptance with q = arcsin (NA) and thus NA = sin q - the answer is in degrees.

His articles appear in a variety of publications from 73-Ham Radio Fun to Disaster Recovery Journal and he is a long time contributing editor for Mobile Radio Technology. He teaches part time with the University of Alaska system and lectures at other post-secondary institutions.