Introduction - Optical Signal Processing by Silicon Photonics - page 3

Fig. 1.2 Basic diagram of digital communication
analog optical fiber communication links are generally limited to shorter distances
and lower bandwidth operation than digital links (Fig. 1.2 ).
Communication while using an optical carrier wave guided along a glass fiber has
a number of awfully striking features, several of which were apparent when the tech-
nique was originally born. Furthermore, the advancements in the technology have
surpassed even the most positive predictions; it has created many additional advan-
tages. Hence it is functional to consider the qualities and special features offered by
optical fiber communications over more usual mediums for electrical communica-
tions. The optical carrier frequency in the range 176 to 375 THz (generally in the
infrared region) yields much greater probable transmission bandwidth than conven-
tional metallic cable systems, or even millimeter wave radio systems. Indeed, today
the typical bandwidth multiplied by length product for an optical fiber link incorpo-
rating fiber amplifiers was 5,000 GHz km in contrast with the typical bandwidth-
length product for coaxial cable of around 100 MHz km. Hence at this time optical
fiber is already signifying more than a factor of 50,000 bandwidth improvement over
coaxial cable while also providing this superior information-carrying capacity over
much longer transmission distances.
Integrated technology for optical devices has developed within optical fiber
communication to make fabrication of a complete signal possible on a single chip.
Integration for such devices has become a confluence of several optical or pho-
tonic disciplines. Both Integrated optics (IO) and Integrated Photonics (IP) tech-
nologies can be made distinguishable from one another by mean of control of the
optical devices. The IO terminology is determined by electronic control of the
optical devices and operation of IP devices is controlled by photons. The IP does
not consider any optoelectronic conversion hence this technology is also termed as
'all-optical' too. Both IO and IP use planar waveguide technology in order to pro-
vide the interconnections between optical components that include the basic com-
ponents for guiding and control of optical signals. IP technology, however, makes
the fabrication of subsystems possible and systems can be considered as a single