Page 46 - Optical Communications Essentials

The Behavior of Light

36 Chapter Three

As noted in Chap. 1, all types of waves including light waves can interfere
with one another. Thus if two light waves line up with each other (or are in
phase), they produce a bright spot. However, when two light waves are 180° out
of phase, then the peaks of one wave are aligned with the troughs of the other
wave. In this case the two waves will interfere destructively, thereby canceling
each other out. To explain effects such as these, we need to turn to the electro-
magnetic wave theory or physical optics viewpoint of light. The concepts
involved here are important when we examine the behavior of devices such as
wavelength-sensitive optical couplers.
Whereas the geometric optics approach deals with light rays, the physical
optics viewpoint uses the concept of electromagnetic field distributions called
modes. We will examine the concept of modes in greater detail in Chap. 4 when
discussing optical fibers. Basically the discussion in Chap. 4 shows that modes
are certain allowable distributions of light power in an optical fiber. Later chap-
ters describe other specific physical aspects of the wave theory as they relate to
optical components.

3.2. The Speed of Light
One of the earliest recorded discussions of the speed of light is that by Aristotle
(384 to 347 B.C.), when he quoted Empedocles of Acragas (495 to 435 B.C.) as say-
ing the light from the sun must take some time to reach the earth. However,
Aristotle himself disagreed with the concept that light has a finite speed and
thought that it traveled instantaneously. Galileo disagreed with Aristotle and tried
to measure the speed of light with a shuttered lantern experiment, but was unsuc-
cessful. Finally, about 600 years later in the 1670s, the Danish astronomer Ole
Roemer measured the speed of light while making detailed observations of the
movements of Jupiter’s moon Io.
8
In free space a light wave travels at a speed c 3 10 m/s (300,000,000m/s),
which is known as the speed of light. Actually this is a convenient and fairly
accurate estimate. To be exact, c 299,792,458m/s in a vacuum, which is
equivalent to 186,287.490mi/s, if you prefer those units. The speed of light is
related to the wavelength λ (Greek lambda) and the wave frequency ν (Greek
nu) through the equation c λν.

3.3. Measuring Properties of Light
The physical property of the radiation in different parts of the spectrum can be
measured in several interrelated ways (see the “Measurements in the EM
Spectrum” discussion below). These are the length of one period of the wave,
the energy of a photon, or the oscillating frequency of the wave. Whereas elec-
tric signal transmission tends to use frequency to designate the signal operat-
ing bands, optical communications generally uses wavelength to designate the
spectral operating region and photon energy or optical power when discussing
topics such as signal strength or electrooptical component performance.

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