INTRODUCTION
The aim of this chapter is to discuss laser applications in optical communication systems and the advantages it will offer over other conventional systems. However, to understand the role of lasers in optical communication, it is necessary to understand the importance of optical (or light wave) communication and also the basics of an optical communication system.
Then only we can get the full picture of the role played by lasers in optical communication systems.
OPTICAL ( OR LIGHTWAVE ) COMMUNICATION
Communication may be broadly defined as the transfer of information from one point to another . From time to time scientists have tried to design and improve communication system by which information can be sent from one place to another. Any communication system consists of three parts viz.
(i) transmitter
(ii) transmission channel (may be either a guided transmission line such as a wire or waveguide) and
(iii) receiver
Basic Communication Using a transmission line, the signal gets progressively attenuated and distorted. So, the improvement in the communication process would mean improving the transmission fidelity and at the same time to improve the data rate of transmission.
Communication Using Lasers
In case of lasers, the optical frequencies are extremely large (~ 10¹5 Hz) in comparision of conventional radio waves (-10 ०6 Hz) and microwave (-10 °6 Hz). Due to this fact, a laser beam acting as a carrier wave is capable of carrying a large number of information as compared to radiowaves and microwaves. The lasers have the following significant advantages:
1. Ability to carry a large number of information
2. Narrow spectral linewidth
3. Very high concentration of optical power
4. Coherence, allowing use of techniques such as frequency modulation and superheterodyne detection. These are very useful in electronic communication.
5. The spatial coherence of laser offers achievement of high antenna gains with very small antenna size.
Optical Communication
1. Information system source or message origin.
The information signal source may be voice, music, video signals, digital data, etc. which is in analog form to be transmitted. This is converted from analog signal to electrical signal.
2. Transmitter The transmitter consists of a drive circuit and a light source. The drive circuit transfers the electric input signals into digital pulses. The light sources convert the digital pulses into optical pulses. The light source usually used LED (light emitting diode) or Laser.
3. Optical fiber The optical fiber is used as a propagation medium. It acts as a wave guide and transmits the optical pulses towards the receiver by the principle of total internal reflection.
4. Receiver It consists of a photo or light detector, an amplifier and a signal restorer. The photo detector is a receiver which receives the optical pulses and converts them into electrical pulses. The amplifier amplifies the signal strength. The amplified electrical signal is converted from digital signal to analog signal with the help of signal restorer. In this way, the original electrical signal is obtained in analog form with same information.
Important Aspects
Each channel in communications needs a bandwidth (range of frequencies around the central transmission frequency). Optical frequencies (in the visible or near-infra-red spectrum region), are very high frequencies (10¹4-10¹5 Hz). The bandwidth of voice communication over phone lines is only about 10 kHz. Thus, the number of phone conversions that can be sent over the optical communications system is in large numbers. Diode lasers can be modulated at speeds of tens of Giga-Hertz (100 Hz). Their light can be transmitted over tens of kilometers without the need for amplification. Thus, optical communications provide the perfect solution for reliable high volume communication.
Another important aspect of optical communication is that light waves cannot travel far in open atmosphere as the energy gets very rapidly dissipated. Hence , some kind of guiding channel is needed for light also . (just like a metal wire for guiding electric current). Optical fiber provides the necessary wave guide for light (optical) frequencies. Since, we use optical fibres, such communication systems are often called optical fiber communication systems.
TYPES OF OPTICAL COMMUNICATION APPLICATIONS
We can divide optical communication applications into following categories:
(1) Communication between two space-crafts: Here the communication channel is free space. The problems associated with such applications are that of pointing and locating the highly directional and thin optical beam by the receiver.
(2) Point-to-point communication: In such communication we use optical waveguides such as optical fiber to provide highly reliable point-to-point communication. The requirements associated with such applications are:
(i) low-loss in guides,
(ii) simple and cheap repeaters and terminals,
(iii) guides with characteristics suitable for low-cost installation.
(3) Communication through atmosphere: In such applications the atmosphere (or the ocean) is a part of the optical propagation path. Due to turbulent nature of atmosphere and presence of rain, fog, etc. , the lightwave after passing through some distance in the atmosphere , suffers a low - frequency noise modulation . In addition, it can even happen that the beam completely misses the receiver. When a space craft enters the atmosphere, there is a tremendous heating of the body of the space craft. Due to this a very high - temperature plasma is formed around the vehicle . It becomes very difficult to communicate with the space craft using radio waves because of the presence of the plasma. The plasma acts as a conductor for the radio waves. As such these waves are not transmitted through the plasma layer. The phenomenon is known as "blackout" during the re-entry of the space craft. However, the plasma is transparent to optical frequencies (-10¹5 Hz). Therefore, we can use laser beam to communicate from the space craft to the ground section during the re-entry period.
ADVANTAGES OF OPTICAL FIBER COMMUNICATION
(i) The optical communication has far more information carrying capacity a compared to other systems.
(ii) Smaller size and weight of the systems make optical communication more suitable in space and aeronautical applications.
(iii) Optical fiber has lower cost of cables per unit length compared to that of metal counter part.
(iv) Optical fibers (glass or plastic) are insulators. No electric current flows through them, either due to transmitted signal or due to external radiation striking the fibre. In addition, the optic wave within the fiber is trapped, so none leaks out during transmission to interfere with signals in other fibers.
(v) Light cannot couple into the fiber from its side. Thus, a fiber is well protected from interference and coupling with other communication channels.
(vi) When high voltage lines are present, a wire communication link could shon circuit the lines by falling across them, causing considerable damage. This problem disappears with fibers.
(vii) As the fibers do not radiate energy within them, it is difficult for an intruder to detect the signal being transmitted. So, they offer high degree of security and privacy.
(viii) They can withstand extreme temperatures before deter rating. Temperature approaching 800°C leave glass fiber unaffected.
(ix) Corrosion due to water or chemicals is less severe for glass than for copper.
(x) There is no need for additional equipment to protect against grounding and voltage problems.
