The reasons for fiber optic cable as a transmission medium providing a high degree of immunization to noise (EMI) as opposed to other transmission media all stem from the use of light to convey the information (signals) and the construction of the medium (the fiber optic cable).
Fiber Optic Lines
A fiber optic cable is comprised of varying numbers of bundled fiber optic lines (see Figure 1). Each fiber optic line has a core made of an incredibly thin long strand of optically pure glass or plastic capable of carrying digital information over very long distances through the propagation of light carrying signals. The key components of a fiber optic line include:
- Inner Core - The inner core is usually made of very long strands of optically pure glass or plastic. This is where the light travels
- Outer Insulating Jacket - Usually made of Teflon or PVC and helps to protect the other layers from mechanical damage and moisture
- Kevlar Fibers - Helps to strengthen the cable and prevent breakage
- Plastic Coating - Cushions the fiber core against shock damage as well as adding waterproofing functionality
- Optical Cladding Layer - Surrounds each individual glass or plastic strand. Without this cladding the propagation of the light signals down the length of the fiber cannot occur. This is the solid blue/gray layer in Figure 1 and that to which the arrow labeled “Cladding” is pointing at in Figure 2.
Fiber Optic Cables
Hundreds and sometimes even thousands of these individual fiber optic lines are bundled together to form a fiber optic cable. A protective jacket then encases (an additional outer covering) the bundles of fiber optic lines to form what we know as a fiber optic cable. The jacket also serves to keep all of the constituent fiber optic lines together in a neater more easily managed package.
Types of Optic Fibers
Optic fibers commonly come in one of two forms:
- Single-Mode Fibers - These fibers have small cores that are about 9 microns in diameter and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers). Communications links greater than 200m are one of the major uses of single-mode optic fiber cables.
- Multi-Mode Fibers - Multi-Mode fibers have larger cores than single-mode fibers that are about 62.5 microns in diameter and transmit infrared light (wavelength = 850 to 1,300 nm) from Light-Emitting Diodes (LED). They also support multiple transmission paths hence their name multi-mode fibers. Their main applications are for communications over short distances or for applications requiring high power transmissions.
Fiber Optic Cable Connectors
While there are a number (20+) of different types of fiber optic connectors on the market, the majority of connections have predominantly used either one of two types.
Note also that many of the various connectors are technology or proprietary specific and are incompatible with other systems. For example, Fiber Distributed Data Interface (FDDI) has its own specific connector, as does Toshiba's fiber optic digital audio (TOSLINK connector) which is shown in Figure 3.
The primary roles played by these fiber optic connectors are to terminate the ends of an optical fiber and mechanically couple (join) and align the cores of the fibers in order to facilitate the passage of light (the signal). They also provide a connectivity system that is far easier and quicker to implement than splicing allows.
The main differences between the various types of optic fiber cable connectors are their physical dimensions and methods of coupling. Because of this, most organizations will elect to use one type of connector on an organization-wide basis for all of their fiber optic connectivity requirements.
Whenever it is not possible to do so then it makes sense to select a standard alternative connector for specific connections and the primary connector for the rest. A common scenario in which the need to use different connector types based on the type of cable in question arises when using both single-mode and multi-mode fiber optic cables within the same production environment implementation.
The two predominant fiber optic cable connectors are:
- ST Connector - Traditionally, the barrel shaped (similar to a BNC connector) ST connector (Fig.4) has been the most commonly used type of optic fiber cable connector
- SC Connector - The newer square faced SC connector (Fig.5) is rapidly becoming the optic fiber cable connector of choice, as it is considerably easier to install (particularly in confined spaces)
Fiber Optic Cable - Signal Propagation
The optical cladding used in fiber optic cable construction is an overlooked (not glamorous) yet key factor in this technology. To understand how it works we need to explore a little bit into the world of radiation physics.
In order to ease the pain somewhat I will keep it simple and tell this tale in pictures but first we need to clarify precisely what it is that we are referring to when we are using the terms “critical angel” and “total internal reflection”. Yes, this is crucial if we are to understanding the propagation of signal down a fiber optic cable.
