Image Processing Reference
In-Depth Information
(Non linear compensation stage)
WDM
coupler
HNLF
1
Bandpass
filter
pump 1
WDM
coupler
y.pol
HNLF
2
dat
a
out
Bandpass
filter
Dispersion compensating
fiber (DCF)
3dB
coupler
x.pol
pump 2
Fig. 8. Block diagram of optical backward propagation module (OBP) (Kumar et al., 2011).
dispersion compensation fibers (DCFs) and non-linear compensation by using HNLFs, as
shown in Fig. 8. In this article the technique is evaluated for 32QAM modulation transmission
with 25G-symbols/s over 800km fiber. The transmission reach without OBP (but with the
DCF) is limited to 240km at the forward error correction limit of 2.1x10
−
3
.Thisisbecause
the multilevel QAM signals are highly sensitive to fiber non-linear effects. The maximum
reach can be increased to 640km and 1040km using two-span OBP (multi-span backward
propagation) and one-span OBP (per-span backward propagation), respectively.
This technique is still in the early stages of development. As DCF in the OBP module can add
additional losses and limit the performance of backward propagation algorithm, as a matter
of fact we have to keep launch power to the DCF low so that the non-linear effects in the DCF
can be ignored.
5. Analysis of step-size selection in 16-QAM transmission
In this section we numerically review the system performances of different step-size selection
methods to implement DBP. We apply a logarithmic distribution of step sizes and numerically
investigate the influence of varying step size on DBP performance. This algorithm is applied
in a single-channel 16-QAM system with bit rate of 112Gbit/s over a 20x80km link of standard
single mode fiber without in-line dispersion compensation. The results of calculating the
non-linearity at different positions, including symmetric, asymmetric, and the modified (
?
)
schemes, are compared. We also demonstrate the performance of using both logarithmic step
sizes and constant step sizes, revealing that use of logarithmic step sizes performs better than
constant step sizes in case of applying the same number of steps, especially at smaller numbers
of steps. Therefore the logarithmic step-size method is still a potential option in terms of
improving DBP performance although more calculation efforts are needed compared with
the existing multi-span DBP techniques such as (Ip et al., 2010; Li et al., 2011). Similar to the
constant step-size method, the logarithmic step-size methods is also applicable to any kind of
modulation formats.
5.1 DBP algorithms and numerical model
Fig. 9, illustrates the different SSFM algorithms used in this study for a span compensated
by 4 DBP-steps. The backward propagation direction is assumed from the left to the right,
as the dashed arrows show. For the constant step-size scheme, step size remains the same
for all steps, while for the logarithmic step-size scheme, step size increases with decreasing
power. The basic principle is well known from the implementation of SSFM to calculate signal
propagation in optical fibers, where adaptive step size methods are widely used. As signal
