Modern Optical Networks - Optical Signal Processing by Silicon Photonics - page 19

One of the first commercial steps toward an all-optical network was taken
in 1988 when the first optical fiber was installed between Europe and America.
Since then the telecommunication industry has been through a major economic
boom and a crash and is now on the road to recovery. Since the first installation
of optical fibers between the continents several companies and network provid-
ers have installed more fibers leading to a massive increase in capacity. With the
increased internet traffic through the 1990s the demand for more capacity kept
rising and network providers kept delivering. The bandwidth was supplied with
the aid of the newest advances in research such as wavelength division multiplex-
ing (WDM).
With the increase of transmission rates to 10 Gbps and higher, the fiber medium
impairments like dispersion and nonlinearities [ 2 ] start to emerge. On the other
hand, the electronic processing of every packet at a node becomes more challeng-
ing in terms of requirements for electronics, buffering and power consumption,
and processing power. Therefore, the second generation optical networks have
been created to incorporate some of the switching and routing functions in the
optical layer of the network [ 3 ].
The main approach in realizing the second generation optical networks is
frequency division multiplexing in the optical domain. This concept is utilized
in electrical communications to increase the communication channel capac-
ity. Optical frequency division multiplexing involves combining data trans-
mitted on different optical wavelengths within the same fiber, thus it is most
commonly referred to as WDM. It offers a potential for very effective use of
fiber bandwidth directly in the time domain [ 4 ] while allowing for using the
wavelength to perform functions like routing and switching. WDM networks
have been deployed in local metropolitan area networks (MAN) and local area
networks (LAN) over the past few years. Currently, WDM networks are using
hundreds of wavelengths operating at a bit rate of 10 Gbps each. All the rout-
ing and switching is still done electronically and the cost of converting sig-
nals from the optical to the electrical domain and back again is expected to
increase as the bit rate per channel increases. Physical size increase, thermal
dissipation and power consumption increase with increasing bandwidth are
other major limiting factors of electronic switching and routing [ 5 ]. In addi-
tion, it is understood that as transmission rates continue to grow, the speed of
the electronic circuitry will not be able to compete with the speed of the opti-
cal transmission.
In the next generation optical networks [ 6 ], it is expected that all-optical
switches will be used to perform switching and routing functions in the optical
domain. The major advantage to this approach is the tremendous bandwidth poten-
tial. The all-optical approach may lead to potential lowering of cost per band-
width, but it will most certainly help with reduction of the footprint of the gear and
its power consumption and dissipation issues. This is contingent on development
of photonic integrated circuits, which are small in size, consume low energy and
capable of switching large bandwidths of data very fast.