photonic and laser

Although fiber optic communications is just one aspect of the broader topic of photonics,
we will emphasize it in this book since it is a well-established and increasingly important
technology. The beginnings of optical communications can be traced to the inventor of
the telephone, Alexander Graham Bell. In 1880, Bell invented a device he termed the
photophone, which allowed information to be transmitted through air on a beam of modulated
sunlight. The modulated light was detected by the photoacoustic effect, in which a
sound wave is produced inside a closed gas cell when modulated light is absorbed inside
the cell. Although this was a clever device, it was much less practical than the telephone,
and was not developed further.
It was not until the 1960s that optical communications was considered seriously again,
this time motivated by two parallel developments. The laser had been developed at the beginning
of the decade, and this provided a light source that was powerful and highly directional,
both valuable characteristics for sending a light signal over long distances.
Sending a laser beam through the air, however, has obvious limitations as a practical
communications source over long distances. What was needed was a way to guide light
over a controlled path for distances measured in miles rather than feet. It was proposed in
1966 by Kao and Hockham that glass, if sufficiently purified, could form such a light
guide by confining the light to the central region of an optical fiber through the principle
of total internal reflection (TIR). Although this theoretical paper suggested the possibility
of optical fiber communications, the attenuation of light in the glasses available at that
time was too great to make the scheme practical.
To quantify the degree of light attenuation in glass, we digress from the historical development
to define the decibel, or dB, which is commonly used to characterize attenuation.
If light of power Pin is incident on a length of fiber, and light of power Pout exits the
far end of that fiber, then the dB loss is defined as
dB loss = 10 log10
 (1-1)
which can also be written in the form
= 10–(dB loss/10) (1-2)
From this definition, you can see that a factor of 10 drop in power corresponds to a 10
dB loss, a factor of 100 drop corresponds to a 20 dB loss, and so on. In an electrical circuit,
power is proportional to the square of the voltage, so in electrical engineering the dB
loss is often defined in terms of a voltage ratio as
dB loss = 10 log10
= 20 log10
 (1-3)
The utility of the decibel concept becomes apparent when loss elements are cascaded.
Suppose there are two fiber lengths, as shown in Fig. 1-1, with losses of (dB loss)1 and
(dB loss)2 respectively. Light of power P1 enters the first fiber, and light of power P2 exits
this fiber. The light power P2 then enters the second fiber, and exits the second fiber with
power P3. The dB losses for the individual fiber sections are