Optical Amplifiers for Multi Mode / Multi Core Transmission

Optical Amplifiers for
Multi Mode / Multi Core Transmission
Peter M. Krummrich
Technische Universitaet Dortmund, Friedrich-Woehler-Weg 4, 44227 Dortmund, Germany
peter.krummrich@tu-dortmund.de
OFC/NFOEC Technical Digest © 2012 OSA
Abstract: Optical amplifier concepts based on multi mode and multi core active fibers are
compared. From an amplifier perspective, the multi mode fiber type has a better potential for cost
and energy efficient spatial division multiplexing.
OCIS codes: (060.2330) Fiber optics communications, (060.2320) Fiber optics amplifiers and oscillators
1. Introduction
The bandwidth demand in optical data transport networks has grown exponentially with an increase of 40 % to 60 %
per annum for more than two decades and this trend will most likely continue in the coming years [1]. Conventional
technologies to increase fiber capacity such as more bandwidth efficiency by higher order modulation and more
channels by wider wavelengths bands are destined to reach physical, energy efficiency and cost limits [2]. Spatial
division multiplexing (SDM) has been proposed as an alternative approach to increase fiber capacity [3,4], which
may help to overcome limits foreseen for the aforementioned techniques.
However, SDM cannot provide a commercially viable solution, unless concepts are found to reduce the cost and
energy per transmitted bit together with the fiber capacity increase [5]. As in case of the transition from single
channel to wavelength division multiplexed (WDM) multi channel transmission, one key element will be the optical
amplifier which needs to be capable of processing all transmitted channels simultaneously. Promising results
concerning lumped erbium-doped fiber (EDF) based amplification have been published recently for multi core as
well as multi mode active fiber types [6-10]. However, the concept of multi mode amplification is not new – it has
rather found a new application in SDM.
According to the author's knowledge, the first experimental demonstration of optical amplification in multi mode
EDF dates back to the year 1991 [11]. At this time, the motivation for investigating multi mode amplification was
not given by capacity increase, but rather by overcoming loss limitations in applications using large core multi mode
transmission for cost and/or robust optical path reasons, such as local area networks (LAN) or fiber sensors. The
concept of using multi mode cores for signal amplification is also quite common in high power applications [12],
albeit with a different focus. The large mode field area of large core fibers helps to overcome issues with high spatial
power densities, even if only the fundamental mode is used.
Even distributed Raman amplification in few mode fiber has already been demonstrated [13]. With the fiber
tapers presented in [6], a straight forward implementation for the other transmission fiber type, multi core, comes to
mind, where the pump/signal combiners are realized in single mode / single core technology. For backward
pumping, they can be inserted at the output of the transmission fiber span after the individual signals are coupled
from the multi core transmission fiber into single core fibers by the taper.
However, even with these very promising results, more work has to be done because several key topics need
further investigation. As mentioned above, the commercial success of SDM will strongly depend on the capability of
SDM to increase fiber capacity together with a reduction of the cost and energy per transmitted bit. For this purpose,
optical amplifiers have to be developed with the ability to process a large number of SDM and WDM channels
simultaneously. These amplifiers have to cost less and consume less electrical power than a given number of single
mode / single core WDM amplifiers required to process the same number of channels.
This invited paper focuses on concepts for cost reduction and energy efficiency improvement of optical
amplifiers capable of combining SDM and WDM operation as one of the most important issues to be solved. Other
important issues which also have to be studied are equalization of gain and noise figures of the SDM channels for
full and partial channel loading, the impact of spatial hole burning, and channel power transients resulting from the
interaction of channels – just to mention a few examples.
2. Cost and energy efficiency of erbium doped fiber amplifiers
Erbium doped fiber amplifiers (EDFA) provide the most widely deployed solution for lumped signal amplification
in case of single core / single mode WDM transmission. They also possess great potential for extending their
application towards SDM in combination with WDM. For better compatibility and more efficient coupling, the
active fiber type of the amplifier should coincide with the type of the transmission fiber. As it is not decided yet,
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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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OFC/NFOEC Technical Digest © 2012 OSA
a reduction of the cost and energy per bit compared to the case with parallel single core and single mode WDM
amplifiers.
Further improvement of cost and energy efficiency can be achieved by amplifier module design optimization [8].
The cores of multi mode EDF can be pumped using multi mode pump lasers. Direct pumping of the cores of multi
core fibers needs single mode pump sources and cladding pump approaches are rather inefficient in this case. The
potentially better electrical to optical power conversion efficiency of multi mode pumps favors using the multi mode
active fiber type in preamplifier applications.
Similar arguments apply to the optimization of booster stages, where most attention has to be paid to the pump to
signal power conversion efficiency due to the high total signal output powers. Also in this application, the multi
mode active fiber type provides potential energy efficiency advantages due to its better compatibility with multi
mode pump sources.
3. Distributed Raman amplification
A similar comparison of the pump power efficiency of multi mode and multi core active fiber types based on the
required pump spatial power density has also been carried for distributed Raman amplification in the transmission
fiber [14]. Cladding pump approaches were found to result in excessively high total pump power requirements of
several ten Watts. For direct pumping of the cores, still several Watts of total pump power have to be expected.
Total pump powers predicted by the spatial power density based estimation agree well with the experimental results
reported in [13].
Total pump powers of several Watts are not desirable but can be realized with currently available pump
technology. Third order Raman pumps are already using such total power levels. With respect to the comparison of
multi mode and multi core active fiber types, a better pump power efficiency was found for the multi mode type as
in case of the erbium doped fiber. Again the higher packing density of modes in the multi mode fiber leads to
reduced total pump powers.
3. Summary and conclusions
The realization of cost and energy efficient optical amplifiers will play a crucial role for the commercial success of
deploying spatial division multiplexing for increasing fiber capacity. Results from concept studies comparing
amplifier designs based on multi mode and multi core active fiber types reveal a better potential of the multi mode
fiber type for lumped erbium-doped fiber based amplification as well as distributed Raman amplification in the
transmission fiber. From an amplifier perspective, the multi mode fiber type has to be considered as the better
candidate for cost and energy efficient fiber capacity increases based on spatial division multiplexing.
4. References
[1] A. Chraplyvy: "The Coming Capacity Crunch" 35th European Conference on Optical Communication (ECOC 2009), paper 1.0.2
[2] P. J. Winzer: "Challenges and evolution of optical transport networks", 36th European Conference on Optical Communication (ECOC
2010), Torino, Italy, Sept. 19 – 23, We.8.D.1
[3] S. Berdaguè, P. Facq: "Mode Division Multiplexing In Optical Fibers", Applied Optics (1982), Vol. 21, No. 11, pp. 1950 – 1955
[4] M. Koshiba, K. Saitoh, Y. Kokubun: "Heterogeneous multi-core fibers: proposal and design principle", IEICE Electronics Express (2009),
Vol. 6, No. 2, pp. 98 – 103
[5] P. M. Krummrich: "Spatial multiplexing for high capacity transport", Optical Fiber Technology (2011), in print
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and noise properties of an erbium-doped multicore fiber amplifier", Optics Express (2011), Vol. 19, Iss. 17, pp. 16715–16721
[7] N. Bai, E. Ip, T. Wang, G. Li: "Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump", Optics Express
(2011), Vol. 19, Iss. 17, pp. 16601–16611
[8] P. M. Krummrich: "Optical amplification and optical filter based signal processing for cost and energy efficient spatial multiplexing",
Optics Express (2011), Vol. 19, Iss. 17, pp. 16636–16652
[9] Y. Yung, S.-u. Alam, Z. Li, A. Dhar, D. Giles, I. Giles, J. Sahu, L. Grüner-Nielsen, F. Poletti., D. Richardson: "First demonstration of
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Montero, X. Prieto, G. Li: "88 x 3 x 112-Gb/s WDM transmission over 50-km of three-mode fiber with inline multimode fiber amplifier",
37th European Conference on Optical Communication (ECOC 2011), Geneva, Switzerland, Sept. 18 – 22, Th.13.C.2
[11] G. Nykolak, S. A. Kramer, J. R. Simpson, D. J. DiGiovanni, C. R. Giles, H. M. Presby: "An Erbium-Doped Multimode Fiber Amplifier",
IEEE Trans. Phot. Technol. Lett. (1991), Vol. 3, No. 12, Dec., pp. 1079 – 1081
[12] C. D. Stacey, J. M. Jenkins: "Demonstration of fundamental mode propagation in highly multimode fibre for high power EDFAs",
Conference on Lasers and Electro-Optics Europe (CLEO 2005), Munich, Germany, June 17, p. 558
[13] R. Ryf, A. Sierra, R.-J. Essiambre, S. Randel, A. Gnauck, C. A. Bolle, M. Esmaeelpour, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka,
D. Peckham, A. McCurdy, R. Lingle: "Mode-equalized distributed Raman amplification in 137-km few-mode fiber", 37th European
Conference on Optical Communication (ECOC 2011), Geneva, Switzerland, Sept. 18 – 22, Th.13.K.5
[14] P. M. Krummrich, K. Petermann: "Evaluation of Potential Optical Amplifier Concepts for Coherent Mode Multiplexing", Conference on
Optical Fiber Communication (OFC 2011), March 6-10, Los Angeles, CA, USA, OMH5
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