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Raman fiber amplifiers (RFAs) have become increasingly important in optical communication systems and optical
networks to compensate for the fiber loss and/or splitting loss. Comparing to the conventional rare-earth doped fiber
amplifiers, RFAs have flexible signal gain band and low noise figure (NF) level [1].
Recently, we reported an RFA with signal/pump double-pass the gain medium scheme by utilizing an optical circulator
(OC) as a signal/pump reflector [7]. The experiments have successfully confirmed that the pumping efficiency
improvement for this kind of RFA is more efficiency than other types of RFAs. What's more, the pumping efficiency
improvement and the NF suppression can be realized simultaneously. Although it is crucial to numerically predict the
characteristics of RFA such as signal power and noise figure (NF) versus pump wavelength, pump power, gain medium
characteristic and so on, the optimum design of RFA parameters has not yet been addressed.
In this paper, we preliminary describe the numerical simulation method based on a set of coupled steady-state equations
to estimate the characteristics of signal/pump double-pass RFA. First we assume that the ASE level combines other
noises is 30 dB lower than that of the input signal [8], so only the forward and backward signal/pump are considered.
This is a typical boundary condition problem and Newton method is used. As we get the distribution of pump power and
signal power along the DCF, the noise item at different frequency can be found by the relaxation method. After several
iterations, all WDM channel signals and noise are convergent as predict. In this paper, in order to optimize the length of
the DCF to get the best performance, we suggest a proper definition of NF called effective NF. During the simulation,
the length of SMF is kept constant, so DCF length becomes the only dominant variable parameter to affect the gain and
NF of the RFA. To verify the algorithm above, the parameters are set equal to those in Ref. [7], and the simulation
results are in very good agreement with those of experimental data. To show the optimum DCF length under a fixed
pump power and pump wavelength, we calculate results of both the signal output power and NF versus DCF length at
three signal wavelengths. We find that the largest signal output power may appear at the length of DCF is 3.6 km, and
the lowest NF could be found as the DCF length is 4 km. In general, DCF has a larger gain coefficient than that of the
SMF, but the attenuation coefficient of the former is also higher than that of SMF. So, there is a trade-off when selecting
the length of DCF. From the simulation results, we conclude that the optimum length of DCF is 3.8 ± 0.2 km. DCF
length of around 3.8 km is an ideal value to realize the best characteristics for the RFA.
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