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Gain optimisation of an erbium doped fibre for remote pumping system
Electronics Letters
Abstract
The authors investigate the optimum length of erbium doped fibre
(EDF) for a remote pumping system. The optimum EDF gain yielding the
minimum total noise figure (NF) is calculated and experimental data
support the theoretical analysis
In this paper, a new technique of designing repeaterless transmission links using a remotely-pumped erbium-doped fibre amplifier (R-EDFA) is discussed. The transmission losses before and after the R-EDFA are varied using variable optical attenuators and the bit error rate test is executed to monitor the performance of the repeaterless transmission systems. The optimised location of R-EDFA for the longest transmission distance can be experimentally determined by determining the threshold of bit error rate. The proposed technique is also verified in practical repeaterless transmission systems using real transmission fibres.
Noise figure characteristics of erbium-doped fibre amplifiers (EDFAs) with different optical feedback directions, namely counter- and co-feedback, and without feedback are presented. It was found that the noise figure exhibited the pump power and the feedback direction dependence although the population has been clamped after the onset of laser oscillation. Departure of the noise figure for two different feedback directions was found to be increasing with the pump power. The dependency of the depth of dip for the noise figure on the feedback direction at the saturated input signal power was also observed.
Numerical solution of a general rate-equation model of an erbium-doped fiber amplifier highlights several important features of the device. A difference in strong-signal gain between copropagating and counterpropagating signal pump schemes in the presence of pump excited-state absorption is predicted, and this is confirmed by experiment. The detriment in the noise figure previously associated with counterpropagating schemes for only small-signal conditions is shown to be similar for strong signals, corresponding to a power-amplifier operating regime. In addition, a correction to the noise figure is given for the limit of a low-gain amplifier.
A rate equation model is used to calculate the excess noise figure
for erbium-doped fibre amplifiers. It is shown that erbium amplifiers
can operate with high gain and can have noise figures only 0.2 dB above
the quantum limit
Applications of semiconductor laser amplifiers in intensity modulated digital optical transmission systems were studied theoretically. An optical linear amplifier repeater between electronically regenerating terminal repeaters and an optical linear preamplifier in front of a photodetector in an electronically regenerating repeater are discussed. Both traveling-wave type and Fabry-Perot cavity type laser amplifiers are considered. The noise and error rate performance in these systems are evaluated using formulations for semiconductor laser amplifiers. The mean and variance in the optical amplifier output photons calculated by the photon master equation [1] is used to obtain the worst case variance in the equalized output voltage [2] for these systems. The required receiving power reduction from direct detection scheme by a preamplifier system and the repeater spacing expansion between two electronically regenerating terminals by an optical linear amplifier repeater system are delineated.