Electro-Optics - Applied Optics Fundamentals and Device Applications - page 54

diffraction efficiencies. One such MUX/DEMUX uses a chirped grating. This
is an effective means of demultiplexing several channels, although gratings
are somewhat bulky in comparison to integrated structures.
2.7.3 Bandpass Filters
In practice, bandpass filters, although functional in optical multiplexing, are
a bulky WDM format. Several combinations of filters must be used depend-
ing on the number of channels multiplexed. Filter architectures could not be
used for LSI board applications; however, some combination of WDM using
a combination of two time division multiplexed (TDM) channels, where two
channels are on different wavelengths, may be possible. It is not possible
to wavelength multiplex a great number of channels using bandpass filters
due to signal degradation encountered when passing through several filters.
This is caused by the limited transmission ratio achievable with an interfer-
ence filter. This ratio is 85% at best, although the maximum rejection of other
wavelengths, determined by the reflection ratio, can be 100%.
2.7.4 TDM Schemes
TDM has important applications for LSI interconnects. When multiplexing
a large number of signal channels, it is necessary to TDM to reduce inter-
connect complexity. Typical requirements are such that as many as 64 chan-
nels, each operating at 25 MHz, need to be transmitted. One possibility is to
TDM the 16 bit address, data, and control words into four channels and then
those four channels into one channel. This output channel would then be
modulated at 1.6 GHz, which is entirely within the modulation rates attain-
able with laser diodes. Several commercially available GaAs universal shift
regulators would meet the needs of this type of TDM.
2.8 Electro-OpticandAcousto-OpticModulators
Electro-optic and acousto-optic modulators are the most highly developed
types of modulators commercially available for digital communications. The
drawback for these devices is that they operate in general on a power to
bandwidth ratio of approximately 1 mW/MHz. The bandwidth of acousto-
optic modulators is usually a small fraction of the acoustic frequency. This
is highly prohibitive for data communications in the gigahertz band. The
power requirements of these devices can also be prohibitive [82]. They are
additionally quite bulky, although developments by some researchers to a
rate above 1 Gb/s with order of magnitudes less drive power are available.