An easy way to understand how fiber optics work is to visualize peering into a very long tube the inside of which is coated with a perfectly mirrored surface. One mile away, at the opposite end, a friend shines a bright flashlight into the tube. Because the tube is internally coated with a perfect mirror, you will see his light perfectly at your end – regardless of how many twists and turns the pipe takes! Now if your friend flashes the light off and on repeatedly (simulating a binary off/on electronic pulse), you’ll see this “digital” light data at your end of this internally mirrored pipe – literally, at the speed of light.
Most optical fibers made for communications contain silica glass that consists of a solid inner core surrounded by a cladding layer of glass, with a lower index of refraction than the core. The boundary between the core and the cladding causes an internal reflection so that light entering the core at one end remains trapped until it emerges at the other end.
Light sent through the fiber is most commonly generated by either an LED or a laser. These specialized optical transmitters “flash” the light to represent digital binary data, either on or off. The modulated light is sent at very fast data transmission rates, typically from 125 Mbs (millions of bits per second) to 10Gbs (billions of bits per second) – and faster. The light “data” passes through the entire length of the fiber and is detected at the other end by an optical receiver that converts the pulsing light back into an electrical signal.
Optical fibers are designed to operate in either “multimode” or “singlemode” applications. Singlemode fibers will only accept light rays that enter parallel to the axis of the fiber’s core. Multimode fibers will accept light rays entering at angles of up to 25 degrees off-axis. By accepting a wider range of angular displacement, the light rays entering at wider angles must travel a longer distance for a given length of fiber. The difference in distance results in a minute variation in arrival time for light rays entering at different angles. Variations in this arrival time become proportionally greater as the length of the fiber and/or the data rate increases. Singlemode fiber overcomes this limitation of multimode fiber and can achieve much longer transmission ranges; however, due to the accuracy necessary to produce light entering parallel to the fiber core’s axis, single mode systems are typically more expensive.
One common concern about using fiber optic cable is its durability. Fiber cable comes with various types of jacketing which can provide equal if not greater durability than its copper equivalents. A single strand of glass fiber is only slightly larger in diameter than a human hair. This fiber “core” is surrounded by layers of protective material. Standard fiber is usually installed with a PVC jacket. “Plenum cable,” a higher-rated grade, comes with a fire-retardant coating (usually Teflon) so that it does not emit toxic gasses and smoke, should it burn. “Tactical fiber,” with the highest grade jacketing, is specifically designed for quick and easy deployment in rugged, harsh environments. It is specifically engineered and manufactured to meet the stringent environmental and mechanical requirements of the United States military. These various grades of jacketing provide increasing levels of durability, but all are as flexible as their equivalent diameter copper cables.
The most common types of fiber optic connectors used are “ST” and “SC” connectors. Both can be field-terminated and are the most useful in permanent fiber installations. TFOCA (Tactical Fiber Optic Cable Assembly) and TFOL (Tactical Fiber Optic Link) connectors provide a higher level of durability. These connectors are designed to be used with tactical fiber in harsh military field applications and have been adapted for use in numerous demanding commercial applications as well.
The Benefits of Using Fiber Optic Cable
There are many benefits to using fiber optic cable instead of copper cable. Some of the most important advantages concern fiber’s inherently superior dielectric properties. Since optical fiber has no metallic components, it is unsurpassed for providing complete electrical isolation as well as noise immunity.
Electrical isolation is most important when it comes to eliminating ground loops. A ground loop is a condition where an unintended connection to the ground is made through an interfering electrical conductor. Generally, a ground loop connection exists when an electrical system is connected in more than one way to an electrical ground. Since there is no electrical conduction through fiber cable, equipment grounded at one end of the connection is completely isolated from the ground at the other end. Ground loops can be an especially irritating source of headaches in even the simplest sound systems. By using optical fiber signal transmission, you can eliminate these major sources of problems – entirely.
Another advantage of optical fiber is its immunity to external noise. Electrical noise, also known as EMI (electromagnetic interference), and RFI (radio frequency interference), are electrical signals that produce undesirable effects and otherwise disrupt audio and data systems. Sources of EMI/RFI include lighting equipment, computers, electric motors, radio and TV broadcasts. Fluorescent lights and power lines are a common source of annoying 60 Hz hum. Lightning can also be a common natural source of audio and data system interference and disruption. The interference from all these sources modifies and interacts with data signals in metal cables, causing data errors and transient unreliability. Even traditional high-quality “balanced” copper cables are susceptible to EMI/RFI and lightning problems. In summary, fiber optic cables are totally immune to any extraneous electrical fields, so they carry only clean signals.
The low signal attenuation performance and superior signal integrity found in fiber optic systems facilitates much longer runs for signal transmission than metal-based systems. While single-line, voice-grade copper systems require in-line signal repeaters for satisfactory performance over long distances, it is common for multimode optical systems to extend to two kilometers (km) – or about 1.25 miles. Singlemode fiber systems to reach up to twenty or more km – about 12.5 miles – with no active or passive processing. Emerging technologies for fiber optics promise even greater distances in the future.
The long, continuous lengths and small diameters of fiber optic cable runs, provide numerous advantages for installers and end-users. Since today’s applications require an ever-increasing amount of bandwidth, it is important to consider space constraints. It is commonplace to install new fiber optic cabling within existing HVAC duct systems. The relatively small diameter and light weight of optical cables make such installations more practical and also saves valuable electrical conduit space.
System designers typically plan optical systems that will meet growth needs for a fifteen- to twenty-year life span. Although sometimes difficult to predict, potential growth can be accommodated by installing spare fiber cables for future requirements. Installation of spare fibers today is more economical than installing additional ones later. In addition, with the use of multiplexing technology, additional channels can be carried over the same fiber cable by simply upgrading the hardware at either end.
Fiber optics is affordable today, as the price of electronics falls and optical cable pricing remains low. In many cases, fiber solutions are actually less costly than copper. As bandwidth demands increase rapidly with technological advances, fiber will continue to play a vital role in the long-term success of more reliable telecommunications.