Page 217 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 211

HE 11 mode (see Fig. 9.9) has two propagation vectors that are ab-
solutely identical for a symmetric fiber. If the fiber loses its circular
symmetry, these two modes separate, which means that they will
have different characteristic group velocities. A light pulse coupled
into a fiber will split its power between these two modes. The effect of
the difference in group velocities is that the part of the light pulse in
one polarization mode will travel faster than the remainder of the
light pulse in the other mode. At the end of the fiber, the pulse will ap-
pear smeared out in time. Just like chromatic dispersion, this effect
becomes more important as the modulation frequency increases.
The circular symmetry of an optical fiber can be changed by many
things. To be sure, there are imperfections in manufacture. However,
strains induced by cabling the fiber, a truck passing over a buried ca-
ble, local heating during the day, in fact, almost any kind of perturba-
tion, will distinguish the two modes, and thus also change the orienta-
tion of their principal axes in the fiber. Polarization-mode dispersion
is not static but rather unpredictable, and, in fact, is quite insidious.
To be able to send signals at bit rates above 20 GHz, polarization-
mode dispersion must be compensated for. This means that you have
to monitor the channel performance continuously and compensate for
the measured pulse broadening by inducing a polarization mode dis-
persion of the opposite sign. Achieving this compensation in a com-
pact and efficient way poses a significant challenge to today’s optical
fiber engineers.
In summary, the demand for more capacity in the optical fiber
telecommunications system can be answered in two ways: sending the
information at higher and higher bit rates or sending multiple wave-
lengths over the same fiber. When the bit rate is increased, dispersion
effects, in which the pulse width broadens as it propagates down the
fiber, also become more important. At 20 GHz and above, it is disper-
sion and not loss that will determine the maximum transmission dis-
tance before the signal needs to be reconditioned. One solution to the
dispersion problem is to send more information using multiple wave-
lengths of light for each channel rather than raising the bit rate. A
number of problems associated with this approach appear: the need
for separate receivers to detect and to recondition each signal chan-
nel, the need to replace each such repeater unit every time the bit rate
is changed, and intractably large and complicated switching and sig-
nal processing circuits. In 1986, an idea that was 25 years old was re-
discovered: the all-optical amplifier. In an instant, all of these prob-
lems vanished as it was demonstrated that the laser (remember that
laser stands for Light Amplification by Stimulated Emission of Radia-
tion) was capable of amplifying simultaneously a signal consisting of
many wavelengths without having to do any detection or demodula-

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