5-17
Now that you have learned about portable equipment, let's look at one of the newest areas of
communications. You are going to learn the fundamentals of how a laser works and how it may be used in
the field of communications.
Q9. What are the three main design considerations of portable equipment?
LASERS
The word LASER is an acronym for light amplification by stimulated emission of radiation. The
laser is widely used in industry, and experimental work is being done with it in communications. You will
find a laser is similar to uhf and microwave power sources and could replace either of them in point-to-
point communications.
THEORY OF OPERATION
Lasers take energy at (or near) the visible light spectrum and convert it to a very narrow and intense
beam in the same region. A close relative of the laser is the light emitting diode (LED). The LED takes dc
or low frequency ac power and converts the energy into visible light.
The principle of the laser is somewhat similar to that of a very high-Q cavity resonator. Chapter 1 of
NEETS, Module 11, Microwave Principles, explains cavity resonators. The laser is shock-excited by a
spark transmitter. This transmitter is called a spark transmitter because it uses the discharge of a capacitor
through an inductor and a spark gap as a source of rf. While the input energy of the laser covers a wide
band of frequencies, the output is on one frequency. Energy outputs of the laser are either INCOHERENT
or COHERENT. For example, if you turn on a transmitter with no modulation, you will get coherent
radiation. When you connect a noise source to an antenna, the result is incoherent radiation.
Lasers can be either cw or pulsed. Actually, lasers are little different from conventional oscillators.
However, the way lasers convert energy from one form to another is quite different. In conventional
oscillators, dc power from the collector is converted to rf energy. The frequency is for the most part
independent of the molecular or atomic structure of the generator. This is not true for the laser. Laser
conversion takes place directly within the molecular structure of a crystal or gas. The external circuits
have little effect on actual output frequency. The fact that the light from an LED is always the same color
results from similar conditions. In a laser, incoherent light excites the electrons in the atoms to higher
energy levels than they normally would have. The new energy states are unstable and the electrons drop
down to lower energy levels. Energy is then released in the form of light.
Figure 5-18 shows the ends of the crystal or glass tube laser with light waves reflecting back and
forth between two mirrored surfaces. One mirror is only partially reflective, and light energy is
transmitted through it to form the light beam. You will find that power sources for lasers include flash
tubes or, in the case of diode-type lasers, dc power supplies.
