Optical-Cable Construction

he core is the highly refractive central region of an optical fiber through which light is transmitted. The standard telecommunications core diameter in use with SMF is between 8 m and 10 m, whereas the standard core diameter in use with MMF is between 50 m and 62.5 m. Figure 3-4 shows the core diameter for SMF and MMF cable. The diameter of the cladding surrounding each of these cores is 125 m. Core sizes of 85 m and 100 m were used in early applications, but are not typically used today. The core and cladding are manufactured together as a single solid component of glass with slightly different compositions and refractive indices. The third section of an optical fiber is the outer protective coating known as the coating. The coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber. The buffer coating could also be constructed out of one or more layers of polymer, nonporous hard elastomers or high-performance PVC materials. The coating does not have any optical properties that might affect the propagation of light within the fiber-optic cable. During the installation process, this coating is stripped away from the cladding to allow proper termination to an optical transmission system. The coating size can vary, but the standard sizes are 250 m and 900 m. The 250-m coating takes less space in larger outdoor cables. The 900-m coating is larger and more suitable for smaller indoor cables.

The Physics Behind Fiber Optics

A fiber-optic cable is composed of two concentric layers, called the core and the cladding, as illustrated in Figure 3-1. The core and cladding have different refractive indices, with the core having a refractive index of n1, and the cladding having a refractive index of n2. The index of refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum. The actual speed of light in a vacuum is 300,000 kilometers per second, or 186,000 miles per second. Read More

Fiber-Optic Applications

Telecommunication applications are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs.

Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service between central office switches at local levels, and sometimes as far as the neighborhood or individual home (fiber to the home [FTTH]).

Optical fiber is also used extensively for transmission of data. Multinational firms need secure, reliable systems to transfer data and financial information between buildings to the desktop terminals or computers and to transfer data around the world. Cable television companies also use fiber for delivery of digital video and data services. The high bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as high-definition television (HDTV) telecasts.

Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated tollbooths, and changeable message signs, also use fiber-optic-based telemetry systems.

Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used in most modern telemedicine devices for transmission of digital diagnostic images. Other applications for optical fiber include space, military, automotive, and the industrial sector.

Disadvantages of optical fibers compared to wires

* higher cost
* need for more expensive optical transmitters and receivers
* more difficult and expensive to splice than wires
* at higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an imperfection can destroy several metres per second "fiber fuse" detection circuitry at the transmitter can break the circuit and halt the failure to minimize damage.
* cannot carry electrical power to operate terminal devices (Note: current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; increased use of fiber-based intermediate systems)

Advantages of optical fibers over wires

• low loss, so repeater-less transmission over long distances is possible
• large data-carrying capacity (thousands of times greater)
• immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation)
• high electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials
• light weight
• signals contain very little power

Why would I use an APC connector?

APC, or angled polish connectors, minimize back reflection, ensuring the light does not reflect back into the transmitter or cable once it reaches the receive end. APC Connectors are typically used for application specific single mode projects.

What does fast axis and slow axis mean for PM?

Fast and slow refers to the alignment or propagation direction (plane) of the light. Fast axis aligns the internal stress rods in a horizontal fashion, while slow axis aligns stress rods in a vertical fashion.