Abstract:
Wavelength-division multiplexing (WDM) technology effectively utilizes the enormous
bandwidth of an optical fiber by multiplexing numerous channels at different
wavelengths. However, a high power WDM system suffers performance degradation due
to several nonlinear interactions between the information bearing lightwaves and the
transmission medium. These optical nonlinearities can lead to interference, distortion and
excess attenuation of optical signals. Among the nonlinear effects, stimulated Raman
scattering (SRS) is found to be the ultimate power-limiting phenomenon for a wide band
WDM system. The SRS effect causes frequency conversion of light and results in excess ""\
attenuation of short-wavelength channels in wavelength-multiplexed systems, and thus
induces interchannel modulation between each WDM channel. The analysis shows that
the product of total power and total bandwidth must be smaller than 500 GHz-W to
reduce the degradation due to SRS to an acceptable level.
In all the analyses of the Raman crosstalk reported so far, the experimentally found
Raman gain profile of silica is approximated by a triangular function, where the peak
Raman gain occurs at 15 THz spacing between channels and no Raman gain is assumed at
larger spacing. However, for a wide-band WDM system, this approximate model of the
Raman gain becomes insufficient to depict the system limitations. Therefore, an improved
model is proposed for the Raman gain profile of silica, which includes, in addition to the
linearly increasing function assumed in previous works, a linearly decreasing function and
an exponentially decreasing function to approximate the tail of the Raman gain profile.
The model is used to calculate the power depletion of the shortest-wavelength channel due to the SRS effects from all other channels in a wide-band WDM system. The
channels are considered to be equally spaced. Expression for power depletion and the
power penalty suffered by the system due to SRS crosstalk is evaluated. Based on this
improved model, the performance analysis of a WDM system is modified and
generalized.
The probability density function (PDF) of the Raman crosstalk is approximated by a
Gaussian distribution. Thus the probability of error due to SRS crosstalk is evaluated. A
statistical analysis of the WDM system performance is developed to estimate the system
bit error rate, which in turn is used to estimate the power penalty suffered by the system.
Finally, the system limitations for a given maximum penalty are quantified.
