Digital Signal Processing Reference
In-Depth Information
attribute of this scheme is that the adverse effects of GVD of HNL glasses can
be changed into an advantage of wavelength division demultiplexing. Resultantly,
it would yield increased maximum transmittable bit-rate in optical communica-
tion by simultaneously demultiplexing optical time division multiplexed signals
and wavelength division—multiplexed signals with an optical Kerr effect-based
demultiplexer.
In order to evaluate different fibers which are constructed from high nonlinear
glasses, we must meticulously understand the problems that need to addressed
and rectified on priority. As mentioned previously, these problems are related to
device length, optical power, the nonlinear-index coefficient, effective mode area
and nonlinear phase shift, the pulse walk-off caused by group velocity mismatch,
the pulse broadening caused by GVD, and the polarization fluctuation caused by
environmental disturbances.
5.4.1 Pulse Walk-Off
As discussed in the previous section, the important feature of GVD is that pulses
at different wavelengths propagate at different speed due to the group velocity
differences. It causes pulse walk-off that plays a central role in nonlinear fiber
switching devices involving two or more closely spaced pulses. In nonlinear fiber
switching devices, they generally use different wavelengths for control and signal
pulses to separate them easily after switching. Therefore, the effect of pulse walk-
off between control and signal pulses seriously affects the switching performance.
The interaction between the two pulses terminates when the fast-moving pulse
completely walks through the slower moving pulse. More specifically, when walk-
off exceeds one time slot of optical time division multiplexing (OTDM) signal
pulses, the control pulse interacts with many other different signal pulses, making
it hard to select only one channel of the signals. Finally, it limits the bit-rate of the
signal and switching bandwidth. This feature is governed by the walk-off param-
eter
d
w
given by:
1
1
v
g
(λ
2
)
d
w
=
β
1
(λ
1
) − β
1
(λ
2
) =
v
g
(λ
1
)
−
The walk-off length for the initial pulse width of
τ
can be defined as
L
d
w
L
W
=
As the walk-off parameter increases and the initial pulse width decreases, the
walk-off length decreases, thus making it hard to achieve high bit-rate and broad
switching bandwidth.
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