Optical time division
multiplexing-OTDM
Since
the speed at which electrical signal can modulate optical carriers
is limited, to fully exploit the optical fiber bandwidth, another
optical multiplexing technique is OTDM.
Optical TDM
attempts to copy conventional TDM ideas and realize them optically,
thereby achieving much higher speeds (Fig. 8).
To
overcome the bit-rate limitation imposed by electronics within a
single wavelength bandwidth, OTDM techniques using narrow optical
pulse laser sources have been developed. Since the approach uses
extremely narrow optical timeslots with correspondingly large
bandwidths, a natural increase in the data rates (40 Gb/s–100 Gb/s
or higher) at a single wavelength bandwidth, is obtained by
multiplexing low bit rate tributary data streams directly in the
optical domain. To further increase the capacity, a combination of
OTDM with WDM have been developed as follows:
-
·
7 wavelengths x200 Gb/s (1.4 Tb/s) transmitted through 50 km of dispersion
shifted fiber;
-
·
25 wavelengths x40 Gb/s (1 Tb/s) transmitted over 342 km;
-
·
19 wavelengths x100 Gb/s (3 Tb/s) transmitted over 40 km of dispersion
shifted fiber using supercontinuum WDM sources.
OTDM has features
that make it very attractive as a future network technology:
-
·
Simple access to high
aggregate line rates (100 Gb/s and higher)
-
·
Tributary data rates
at any desired degree of granularity and compatible with existing
technologies such as SDH
-
·
Greatly simplified
amplifier and dispersion management (due to single wavelength
transmission)
-
·
Network nodes that
require electronics operating at the local data rate only
Physical
topologies that can be considered in OTDM are point-to-point, star,
and ring.
The main elements
in OTDM (sub) system are:
-
1. optical pulse source
-
2. optical multiplexer
-
3. optical demultiplexer.
Realization of
OTDM ring networks requires OTDM add-drop multiplexers at each
station on the ring.
Figure 8 -
Point-to-Point transmission system using OTDM. MSSI (mid-span
spectral inversion)
The
technology involved in the implementation of OTDM is very different
from that of WDM. Most of the devices required to implement OTDMA
are still confined to the laboratory. In addition, OTDMA has still a
number of difficult hurdles to solve such as synchronization and the
immature technology of selecting one channel out of multiplexed
picosecond optical pulse data streams. Hence, scope for evolving new
innovative methods exists in order to determine the best trade-off
regarding capacity, performance, flexibility and complexity of a
variety of approaches to network provision
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