Introduction - Optical Signal Processing by Silicon Photonics - page 4

substrate. Thus integrating both active and passive devices are monolithically
onto a single substrate resulting a multilayered integration then these devices are
normally referred to as IP devices, while when both active and passive elements
fabricated as individual devices are interconnected together they form larger IO
devices or circuits. Thus IP can also be seen as a process for the miniaturization
and integration of optical systems on a single substrate, and therefore it is consid-
ered as a further development of IO technology, not necessarily as an alternative
technology. Both IO and IP allow processing to be performed on optical signal
by requesting to provide an alternative to the conversion of an optical signal back
into the electrical system earlier. Hence thin transparent dielectric layers on planar
substrates which act as optical waveguides are used in IO and IP to produce small-
scale and miniature optical components and circuits.
The developments in IO have crossed the stage where both signal process-
ing and logic functions can be physically imagined. Furthermore, such devices
are considered as the building blocks for future digital optical computers.
Nevertheless, quite a number of these devices can be combined and closely linked
with developments in light wave communication employing optical fibers.
A most important factor in the development of IO is that it is highly incompat-
ible with multimode fiber systems because it is basically based on single-mode
optical fiber. In fact IO did not make a major contribution to early deployed opti-
cal fiber systems. The initiation, however, of single-mode transmission technology
further motivated work in IO to supply devices and circuits for these more devel-
oped third-generation systems. In addition, the continuous expansion of single-
mode optical fiber communications has helped in creating a developed market for
such IO components. More likely the new generations of optical fiber communica-
tion systems employing coherent and possibly soliton transmission will lean heav-
ily on IO and IP techniques for their implementation.
The other major feature provided by optical signals while interacting within a
responsive medium is the capability to make use of light waves of distinct frequen-
cies (or wavelengths) within the same guided wave channel or device. Such fre-
quency division multiplexing makes possible the information transfer capacity far
better than anything offered by electronics. Moreover, in signal processing language
it allows parallel access to information points within an optical system. This option
for prevailing parallel signal processing coupled with ultrahigh speed operation offers
remarkable potential for applications within both communications and computing.
The devices of interest in IO and IP are often the counterparts of microwave
or bulk optical devices. These include junctions and directional couplers, switches
and modulators, filters and wavelength multiplexers, lasers and amplifiers, detec-
tors and bistable elements. It is envisaged that developments in this technology
will provide the basis for the next generation of optical networks. The technol-
ogy associated with the design and fabrication of IP circuits and devices highly
depends upon different factors that mostly result from the characteristics of the
substrate material on which the various devices are to be fabricated. The IP pro-
cess may require serial, parallel or hybrid integration of independent devices.
In serial integration of devices, different elements inside optical chip can be