a) Definition
Optical fiber communications systems (OFCS) consisting of diode lasers emitting infrared light signals at wavelengths chosen for transmission with minimal loss in high purity glass fibers.
The high bandwidth and low noise of optical systems led to their rapid adoption as well as landline communications networks.
Optical fiber is a technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated light waves.
b) Advantages of Fiber Optic Systems
Fiber optic transmission systems – a fiber optic transmitter and receiver, connected by fiber optic cable – offer a wide range of benefits not offered by over traditional metal communications lines (traditional copper wire or coaxial cable) These include:
- Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data and deliver it with greater fidelity.
- Fiber optic cables are less susceptible than metal cables to interference. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind.
- Fiber optic cables are much thinner and lighter than metal wires. Data can be transmitted digitally (the natural form for computer data) rather than analogically. Can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data.
- As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil or exposed to most corrosive atmospheres in chemical plants without significant concern.
- Since the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.
- Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.
- Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.
c) Disadvantage
The main disadvantage of fiber optics is that the cables are expensive to install. In addition, they are more fragile than wire and are difficult to split.
Fiber optics is a particularly popular technology for local-area networks. In addition, telephone companies are steadily replacing traditional telephone lines with fiber optic cables. In the future, almost all communications will employ fiber optics.
d) Structure
The basic point-to-point fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver.
Figure 1. Point-to-point fiber optic transmission system
The Optical Transmitter: The transmitter converts an electrical analogue or digital input signal into a corresponding optical signal (modulated light). The source of the optical signal can be either a light emitting diode, or a solid-state laser diode. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers.
Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 2 shows a graphic representation of these two basic schemes.
Figure 2 Basic Optical Modulation Methods
The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD).
LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input and stable in terms of light output versus ambient operating temperature.
LDs have very small light emitting surfaces and can couple many times more power to the fiber than LEDs. LDs are also linear in terms of light output versus electrical current input, but unlike LEDs, they are not stable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability. LEDs and LDs are modulated in one of two ways; on add off, or linearly. Figure 4.25 shows simplified circuitry to achieve either method with an LED or LD:
Figure 3 Methods of Modulating LEDs or Laser Diodes
Figure 3, a transistor is used to switch the LED or LD on and off in step with an input digital signal. This signal can be converted from almost any digital format by the appropriate circuitry, into the correct base drive for the transistor.
The operational amplifier circuit of figure 3 accomplishes linear modulation of an LED or LD. The inverting input is used to supply the modulating drive to the LED or LD while the non-inverting input supplies a DC bias reference.
Digital on/off modulation of an LED or LD can take a number of forms. The simplest, as we have already seen, is light-on for a logic "1", and light-off for a logic "0". Two other common forms are pulse width modulation and pulse rate modulation.
In the former, a constant stream of pulses is produced with one width signifying a logic "1" and another width, a logic "0". In the latter, the pulses are all of the same width but the pulse rate changes to differentiate between logic "1" and logic "0". Figure 4 shows all of the above modulation methods as a function of light output.
Figure 4 Various Methods to Optically Transmit Analog Information
The Fiber Optic Cable: The cable consists of one or more glass fibers, which act as waveguides for the optical signal.
Fiber optic cable is similar to electrical cable in its construction, but provides special protection for the optical fiber within. For systems requiring transmission over distances of many kilometers, or where two or more fiber optic cables must be joined together, an optical splice is commonly used.
The Optical Receiver: The receiver converts the optical signal back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.
Typical bandwidths for common fibers range from a few MHz per km for very large core fibers, to hundreds of MHz per km for standard multimode fiber, to thousands of MHz per km for single-mode fibers.
And as the length of fiber increases, its bandwidth will decrease proportionally. For example, a fiber cable that can support 500 MHz bandwidth at a distance of one kilometer will only be able to support 250 MHz at 2 kilometers and 100 MHz at 5 kilometers.