Optical Amplifiers for Multi Mode / Multi Core Transmission

whether the multi core or the multi mode fiber type will feature better transmission performance, both types should
also be considered for optical amplification. Fig. 1 shows a schematic sketch of the two fiber types – a fiber with
multiple cores, each guiding only a single mode (MSC) and a fiber with a single core, which guides multiple modes
(SMC).
Multi single mode core fiber (MSC)
Single multi mode core fiber (SMC)
Fig. 1: Schematic sketch of multi core and multi mode fiber types
The optimization of cost and energy efficiency should consider the purpose of the amplifier stage – whether the
main function consists of low noise amplification of low power signals occurring at the output of a transmission
span as in case of a preamplifier stage or boosting the channel powers to the relatively high levels required at the
input of a transmission fiber span as in case of a booster stage.
For the preamplifier stage, the main design target is given by realizing a high level of inversion of the lower and
upper laser level erbium ion population densities in a cost and energy efficient way. The required spatial pump
power density for this purpose can be estimated from a simplified rate equation model [14]. A comparison of the
total pump powers required to achieve a level of inversion resulting in low noise amplification with different active
fiber types and designs has revealed that the multi mode fiber type possesses a better potential for pump power
efficiency than the multi core fiber type.
Fig. 2 shows a plot of the required pump powers for the case of direct pumping of the cores in multi core and
multi mode active fibers. For the multi core fiber, the required pump power increases proportionally to the number
of cores. If the modes of the multi mode fiber are counted with two orthogonal polarizations each to achieve the
same capacity as in case of the multi core fiber, the required pump power also increases approx. proportional to the
number of modes, but with a reduced slope. The lower total pump power required for the multi mode fiber type
compared to a multi core fiber with the same capacity results from the higher packing density of modes. A lower
fraction of the pump radiation is guided outside the core area, where it does not contribute to the inversion of erbium
ions.
Power in mW
1400
1200
1000
800
600
400
200
0
0 10 20 30 40
No. of modes
Multi core fiber
Multi mode fiber
Fig. 2: Total pump power required for low noise amplification in a preamplifier stage
as a function of the number of modes in multi core and multi mode fiber types
OFC/NFOEC Technical Digest © 2012 OSA
According to the comparison, preamplifier stages with the multi mode fiber type need approx. 30 % less total pump
power than stages processing the same number of SDM and WDM channels with a multi core active fiber. The
actual power ratio depends on the design of the two active fiber types. The resulting reduction in required electrical
power supply is by far smaller than the one achievable by replacing single channel optoelectronic regenerators with
multi channel optical WDM amplifiers. Nevertheless, it can provide a helpful contribution towards the reduction of
cost and energy per bit. Multi core amplifiers with a given number of cores basically need the same total pump
power as individual single core WDM amplifiers for the same number of channels. The reduction in pump power
achievable by processing all channels in a single multi mode core, which can be considered as integration, results in
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