Why Not Use Raman Amplifier to Extend the CWDM Network Reach?

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In comparison with the long-haul DWDM
network that uses the thermo-electric coolers to stabilize the laser emissions
essential, the CWDM network is a more economical solution that features wider
wavelength spacing, allowing the wavelength fluctuation of uncooled directly
modulated laser diodes (DMLs). But on the other hand, the CWDM network exists
the limitation for the uncooled DMLs’ output power and the additional loss of CWDM
Mux Demux and optical add/drop modules. These make the CWDM loss budget
limited to < 30 dB and the CWDM reach within 80 km. Moreover, when the
insertion loss of the dark fiber is higher than our expectation, a decreasing
transmission distance may occur. Hence, here offers the Raman amplifier (see
the following figure) to extend the CWDM network reach, as an ideal solution.

 

What’s Raman Amplifier?

 

Raman amplifier, also referred to as
RA, is a kind of optical fiber amplifier based on Raman gain, which is used for
boosting optical signals and finally achieving a longer transmission distance.
Different from the erbium-doped fiber amplifier (EDFA) and semiconductor
optical amplifier (SOA), the RA intensifies the signals through the nonlinear
interaction between the signal and a pump laser within an optical fiber, as
shown in the figure below.

 

At present, two kinds of Raman
amplifiers are available on the market, the distributed and lumped Raman
amplifiers. As for the distributed Raman amplifier (DRA), it uses the optical
fiber as the gain medium to multiplex the pump wavelength with signal
wavelength, so that the optical signals can be boosted. With regard to the
lumped one (LRA), it requires a shorter length of optical fiber for the signal
amplification. Both of these two Raman amplifiers are suitable for amplifying
CWDM signals and extending the CWDM network reach.

 

Why Raman Amplifier Is Used for
Amplifying CWDM signals?

 

As we know, the EDFA and SOA are able
to strengthen the CWDM signals. But why it is not recommendable for the CWDM
network? In fact, they can not perform as well as the RA in the CWDM network
for some limitations, which can be learned from the following figure.

 

The figures above shows various gain
bandwidths of these three optical fiber amplifiers for CWDM network, but only
the gain bandwidth the RA offers meet the CWDM network demands. To fully serve
the CWDM network, the RA usually optimizes the pumping lightwave spectrum to
extend the usable optical bandwidth. As for the EDFA, its gain bandwidth can
not match well with the channel spacing of the CWDM network requirements. And
for the SOA, although it offers the gain bandwidth fit enough for the CWDM
network, it is still not suggested for the inherent technical limitations. In
details, the SOA has a relatively low saturation power but a high noise figure
and polarization sensitivity, compared to other two amplifiers. Hence, the RA
is undoubtedly the best choice to strengthen the CWDM signals and lengthen the
CWDM network reach.

 

How Does Raman Amplifier Benefit CWDM
Network?

 

In order to study the benefit of RA for
the CWDM network, here offers two sets of research data about the receiver
sensitivity, for a bit-error rate (BER) of 10-9 using a pseudo-random bit
sequence (PRBS) with a 231-1 word length.

 

From the figure above, we can learn
that the first set of data is resulted from the four channel CWDM network
without use of the RA, while the second utilizes the RA. In order to check
whether the Raman amplifier benefits the CWDM network, we can take the data of
100km CWDM transmission through singlemode fiber (SMF) as an example. The power
penalty of the transmission with a RA are separately -34.4 dBm, -34.2 dBm,
-33.2 dBm and -32.3 dBm. It is 0.3 dBm better than the power penalty of the
transmission without a RA, at least. Except that, we can also learn that the
CWDM network with a RA can transmit the signals through the SMF at lengths up
to 150m without any repeater stations, while the network without the RA cannot.

 

Conclusion

 

The Raman amplifier is an ideal
alternative to the repeater in CWDM network, for intensifying the CWDM signals
and extending the CWDM network reach. By using the Raman amplifier, the loss
budget of the CWDM network can be increased, which finally achieves a longer
transmission. Meanwhile, from the view of cost, the RA and the repeater are
almost the same, but the repeater stations should cost much more for
constructing and maintaining. Moreover, using the RA in the CWDM network can
also gain the loss compensation of OADM. Then, why not use Raman amplifier to
extend your CWDM network reach?