Electro-Optics - Applied Optics Fundamentals and Device Applications - page 53

2.7 MultiplexingandDemultiplexing:Information
DistributionTechniques:WDMSchemes
There are many advantages to wavelength division multiplexing (WDM) sys-
tems where lightwaves with different wavelengths, each carrying different
information, are transmitted simultaneously through a single fiber. WDM
data communication systems rely on the use of optical MUXs and DEMUXs
designed to assure low insertion loss per wavelength channel, high isolation
between channels, small size, and high reliability.
Several fundamental elements that have been considered for WDM appli-
cations are prisms, gratings, and interference filters, although other novel
approaches exist. These components are the passive optical components that
can perform the efficient multiplexing and demultiplexing of light sources
into and out of trunk interconnections. In a data interconnect system, WDM
is most advantageously utilized with optical fibers as opposed to other inter-
connection schemes.
2.7.1 Prisms
These MUX/DEMUX devices make use of the birefringement properties
of various transparent materials. In general, this uses the principle of the
dependence of optical speed in the material on wavelength. Two compa-
nies released representative product information on such devices [81]. GTE
designed a two-channel system that can utilize source wavelengths that are
13-23 nm apart and have spectral widths of up to 6.5 nm. This is accomplished
by using adjustable length polarizing prisms. The prism length is adjusted
such that the two wavelengths emerge perpendicular to one another. ADC
fiber optics offers both passive and active MUX/DEMUX devices. The pas-
sive device utilizes a biconical taper technique in which the coupling length
of the device is wavelength dependent. Each wavelength, therefore, will exit
at a different port of the device. The active device utilizes a dichroic filter
to separate the different wavelengths. Although this type of filter is gener-
ally bulky, the ADC device is produced in a high-precision injection-molded
plastic package that is small enough to be PC board compatible.
2.7.2 Gratings
In grating MUXs and DEMUXs, the mixture of wavelengths arriving in the
transmission fiber or from the laser is separated and passed on to the receiver
fibers. The light emerging from the transmission fiber or laser can be colli-
mated by lenses before striking the grating. The angle at which the light is
diffracted by the grating in relation to the optical axis depends on the wave-
length. Several different gratings are currently available that have different