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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 24 — Nov. 19, 2012
  • pp: 27051–27061
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Modeling and characterization of a few-mode EDFA supporting four mode groups for mode division multiplexing

Guillaume Le Cocq, Laurent Bigot, Antoine Le Rouge, Marianne Bigot-Astruc, Pierre Sillard, Clemens Koebele, Massimilliano Salsi, and Yves Quiquempois  »View Author Affiliations


Optics Express, Vol. 20, Issue 24, pp. 27051-27061 (2012)
http://dx.doi.org/10.1364/OE.20.027051


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Abstract

Numerical and experimental study of a Few-Mode (FM) Erbium Doped Fiber Amplifier (EDFA) suitable for mode division multiplexing (MDM) is reported. Based on numerical simulations, a Few-Mode Erbium Doped Fiber (FM-EDF) has been designed to amplify four mode groups and to equally amplify LP11 and LP21 mode groups with gains greater than 20 dB and with a differential modal gain of less than 1 dB. Experimental results confirmed the simulations with a good concordance. This modal gain equalization is obtained by tailoring the erbium spatial distribution in the fiber core with a ring-shaped profile.

© 2012 OSA

1. Introduction

First and foremost, the algorithm used to design the FM-EDF will be discussed and an erbium doping profile adapted to small DMG between LP11 and LP21 modes will be investigated. Then, the characteristics of the FM-EDFA will be presented and, finally, amplification of LP11 and LP21 modes alone or together will be reported and compared to numerical simulations. Results for LP01 and LP02 will also been presented.

2. Theoretical model and Design

Fig. 1 (a) Schematic of the fiber cross section used to design the FM-EDFA with a ring-shaped erbium doping profile. (b) Differential modal gain between LP11 and LP21 modes. (c) and (d) Gains for LP11 and LP21 modes as a function of external (Rde) and internal (Rdi) radii. The blue ellipses represent areas for which gains are equal. Simulations are performed for a 200 mW pump power at 980 nm equally distributed on each mode. Input signal power is −11.55 dBm per mode at 1550 nm. For each modeling, the length of the FM-EDF is chosen so as to obtain the maximum gain for LP21 mode.

3. Fiber realization

Fig. 2 Refractive index and erbium doping profile of the FM-EDF.

4. Amplifier set-up

To experimentally characterize the gain performances of the FM-EDF, pump (974 nm laser diode from Oclaro) and signal beams (81640A tunable laser source from Agilent) are multiplexed in free space, and then, injected into 6 m of FMF especially designed for weakly-coupled MDM transmissions. This FMF is a step index fiber with core radius equal to 7.5 μm and core/cladding refractive index difference equal to 9.7×10−3. A complete characterization of this fiber is reported in reference [13

13. P. Sillard, M. Astruc, D. Boivin, H. Maerten, and L. Provost, “Few-mode fiber for uncoupled mode-division multiplexing transmissions,” ECOC 2011, paper Tu.5.LeCervin.7 (2011).

]. As shown on Fig. 3, the FMF is spliced to a 3 m-long piece of FM-EDF, which is also spliced to another 6 m-long piece of FMF. The output end was angle-cleaved so as to prevent laser effect and it is connected to the input port of an Optical Spectrum Analyzer (6370 from Yokogawa) or an IR camera (C10633 InGaAs camera from Hamamatsu). Phase plates are used to shape the desired signal field profile: either the LP11 (2-quadrant phase plate) and/or the LP21 modes (4-quadrant phase plate) are then injected in the FMF. LP01 and LP02 modes cannot be tested alone with this set-up (as it is explained later in this paper) and no demultiplexing set-up is used to separate modes at the output. Coupling losses of the pump beam are about 1.1 dB. At signal wavelength, coupling losses depend on which mode is excited: without phase plate and in centered injection conditions, these losses are equal to 2.1 dB. With a 2-quadrant phase plate, LP11 mode is excited with 10 dB coupling losses and with a 4-quadrant phase plate, LP21 is excited with 16 dB coupling losses. These high coupling losses can be explained by the fact that it was not possible to conserve a 4f setup (meaning that the phase plate is not at the focal length of the two lenses, due to the bulky micropositioner and dichroic miror obstruction). Nevertheless, such coupling losses are not a limiting factor in the context of this experiment where the goal is to study the characteristics of the FM-EDFA. Note that potential alternatives exist to overcome the drawbacks of phase plates such as mode-selective couplers [14

14. J. D. Love and N. Riesen, “Mode-selective couplers for few-mode optical fiber networks,” Opt. Lett. 37, 3990 (2012). [CrossRef] [PubMed]

] or asymetric Y-junction [15

15. N. Riesen, J. D. Love, and J. W. Arkwright, “Few-mode elliptical-core fiber data transmission,” IEEE Photon Tech. Lett. 24, 344–346 (2012). [CrossRef]

].

Fig. 3 Experimental set-up used to test the FM-EDF. Pump at 974 nm and signal at 1550 nm are multiplexed into a 3 m-long piece of FM-EDF spliced to FMF at the input and the output. Output is imaged on an IR camera or connected to an optical spectrum analyzer. The signal mode profile can be selected by using phase plate.

The FMF has a step index profile, whereas FM-EDF has not (Fig. 2). So as to be sure that the signal modes are well injected in the FMF, and also conserved when they experience a splice between FMF and FM-EDF, they have been individually imaged on the camera, successively before the splice (FMF) and after the splice (FM-EDF). On Fig. 4, it can be seen that LP01, LP11 and LP21 are well injected in the FMF, and that the LP11 and LP21 modes are conserved when they undergo a splice between two different fibers. This can be explained by overlap integrals: the LP11 and LP21 modes can only couple with themselves if the splice is perfectly centered. We report the coupling efficiency between FMF and FM-EDF modes in Tab. 1 and Tab. 2. Mode coupling efficiency factors (Γij)2 were calculated as the square of the overlap integrals Γij between FM-EDF and FMF transverse mode field profiles (Eq. (1)), for pump and signal wavelengths. Then, these factors were reported (in percent) in Tab. 1 and Tab. 2, for both wavelengths respectively.
(Γij)2=(EiΨjdS)2Ei2dSΨj2dS
(1)

Fig. 4 Mode image capture recorded at 1550 nm, with an infrared camera: before splice (output of the FMF) and after splice (output of the FM-EDF).

Table 1. Theoretical coupling efficiency between FM-EDF modes and FMF modes at signal wavelength, when these two fibers are butt-coupled. Values are calculated using Eq. (1) and mode field profiles of the fibers are calculated by FEM.

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Table 2. Theoretical coupling efficiency between FM-EDF modes and FMF modes at pump wavelength, when these two fibers are butt-coupled. Values are calculated using Eq. (1) and mode field profiles of the fibers calculated by FEM.

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Fig. 5 Experimental and simulated pump intensity profile before splice (output of FMF) and after splice (output of FM-EDF), at 974 nm.

5. Experimental characterization

The FM-EDF has been characterized by measuring the gain of each signal mode (LP11, LP21 but also LP01 and LP02), individually injected at 1550 nm, as a function of the total pump power coupled in the FM-EDF (Fig. 6). Gain values were obtained by comparing output (i.e. full length) and input (after a few centimeters) spectra in the EDF. Injection and splice were checked at the beginning and at the end of each experiment in order to validate the results. Then, these results have been compared to simulations performed using the experimental parameters: experimental refractive index/erbium profiles of the FM-EDF, signal/pump mode profiles obtained by FEM, modal composition of the pump beam deduced from Tab. 2. Pump and signal powers were measured after a few centimeters of FM-EDF with a powermeter, at the end of each experiment. The simulations reported in the following of this article were performed using these measured pump/signal powers. Note that no degree of freedom was left in order to fit the experimental data.

Fig. 6 Gain evolution at the output of a 3 m-long FM-EDFA as a function of pump power. Signal power was about −10 dBm. Signal mode is LP21 mode (a), LP11 mode (b) or LP01 & LP02 modes (c).

It is seen on Fig. 6(a) and 6(b) that the gains are nearly equal for LP11 and LP21 modes, with a maximum DMG of 1 dB for all gain values greater than 10 dB, and a maximum gain close to 20 dB. The threshold of the amplifier is about 40 mW. A good accordance between experiments and simulations has to be noticed, with a maximum difference between experimental and theoretical gains of 1.5 dB for all gain values greater than 0 dB. Situation is slightly different for LP01 mode because, experimentally, both LP01 and LP02 modes were excited in the FM-EDF. It can be seen on Fig. 6 (c) that an average gain of 15 dB is experimentally observed. Simulations for both LP01 and LP02 modes are reported in Fig. 6 (c) and show that these two modes can be amplified with gains larger than 10 dB.

Fig. 7 Evolution of the SNR deterioration for a 3 m-long FM-EDFA as a function of pump power. Signal power was about −10 dBm. SNR deterioration is reported for LP21 mode (a) and LP11 mode (b).

Finally, simultaneous amplification of LP11 and LP21 modes has been tested by adding a second signal channel on set-up of Fig. 2. So as to facilitate the measurement, the two modes have been used at two different wavelengths, namely 1554 nm for LP11 and 1550 nm for LP21. The results are presented on Fig. 8.

Fig. 8 (a) Gain as a function of pump power for simultaneous amplification of LP21 and LP11 modes. The two modes are tested at different wavelengths, namely 1554 nm for LP11 and 1550 nm for LP21. FM-EDF was 3.4 m-long and signal power was −17 dBm per mode (b) Input and output spectra for about 100 mW pump power, both for experiments and simulations.

Even if this primary design of FM-EDFA was to obtain similar gains for the LP11 and LP21 modes, it can be used for MDM transmissions using LP21, LP11, LP01 and LP02 modes, since all these modes are amplified. LP01 and LP02 gains were not measured in our set-up due to mode profile mismatching. However, note that this FM-EDF has been employed in a 6 modes amplifier set-up where the ring-doped FM-EDF was concatenated with an other FM-EDF (which has a flat erbium doping profile), so that the first part of the amplifier amplifies LP11 and LP21 modes and the second part re-amplifies LP01 and LP02 modes that suffer from lower gains [18

18. M. Salsi, R. Ryf, G. Le Cocq, L. Bigot, D. Peyrot, G. Charlet, S. Bigo, N. K. Fontaine, M. A. Mestre, S. Randel, X. Palou, C. Bolle, B. Guan, and Y. Quiquempois, “A six-mode erbium-doped fiber amplifier,” ECOC 2012, Post-Deadline paper, Th.3.A.6 (2012).

].

6. Conclusion

Acknowledgment

This work has been supported by the French government, in the frame of STRADE research project (ANR-09-VERS-010). We also acknowledge financial support from the Ministry of Higher Education and Research, the Nord-Pas de Calais Regional Council and the FEDER through the “Contrat de Projets Etat Region (CPER) 2007-2013”.

References and links

1.

A. Chralyvy, “The coming capacity crunch,” ECOC 2009, p.1 (2009).

2.

P. Sillard, “New fibers for ultra-high capacity transport,” Opt. Fiber Technol. 17, 495–502 (2011). [CrossRef]

3.

B. Zhu, T. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112 Tb/s space-division multiplexed DWDM transmission with 14 b/s/Hz aggregate spectral efficiency over a 76.8 km seven-core fiber,” Opt. Express 19, 16665–16671 (2011). [CrossRef] [PubMed]

4.

C. Koebele, M. Salsi, D. Sperti, P. Tran, P. Brindel, H. Mardoyan, S. Bigo, A. Boutin, F. Verluise, P. Sillard, M. Astruc, L. Provost, F. Cerou, and G. Charlet, “Two mode transmission at 2x100 Gb/s, over 40 km-long prototype few-mode fiber, using LCOS-based programmable mode multiplexer and demultiplexer,” Opt. Express 19, 16593–16600 (2011). [CrossRef] [PubMed]

5.

C. Koebele, M. Salsi, L. Milord, R. Ryf, C. A. Bolle, P. Sillard, S. Bigo, and G. Charlet, “40 km transmission of five mode division multiplexed data streams at 100 Gb/s with low MIMO-DSP complexity,” ECOC 2011, paper Th.13.C.3 (2011).

6.

N. Bai, E. Ip, T. Wang, and G. Li, “Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump,” Opt. Express 19, 16601–16611 (2011). [CrossRef] [PubMed]

7.

Y. Jung, S. Alam, Z. Li, A. Dhar, D. Giles, I. P. Giles, J. K. Sahu, F. Poletti, L. Gruner-Nielsen, and D. J. Richardson, “First demonstration and detailed characterization of a multimode amplifier for space division multiplexed transmission systems,” Opt. Express 19, B952–B957 (2011). [CrossRef]

8.

Z. Jiang and J.R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large-mode-area Yb-doped fibers,” J. Opt. Soc. Am. B 25, 247–254 (2008). [CrossRef]

9.

C. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers,” J. Lightwave Technol. 9, 271–283 (1991). [CrossRef]

10.

M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15, 3236–3246 (2007). [CrossRef] [PubMed]

11.

Q. Kang, E.L. Lim, Y. Jung, J.K. Sahu, F. Poletti, C. Baskiotis, S.U Alam, and D.J. Richardson, “Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP01 pump configuration,” Opt. Express 20, 20835–20843 (2012). [CrossRef] [PubMed]

12.

M. Salsi, J. Vuong, C. Koebele, P. Genevaux, H. Mardoyan, P. Tran, S. Bigo, G. Le Cocq, L. Bigot, Y. Quiquempois, A. Le Rouge, P. Sillard, M. Bigot-Astruc, and G. Charlet, “In-line few-mode optical amplifier with erbium profile tuned to support LP01, LP11 and LP21 mode groups,” ECOC 2012, paper Tu.3.F.1 (2012).

13.

P. Sillard, M. Astruc, D. Boivin, H. Maerten, and L. Provost, “Few-mode fiber for uncoupled mode-division multiplexing transmissions,” ECOC 2011, paper Tu.5.LeCervin.7 (2011).

14.

J. D. Love and N. Riesen, “Mode-selective couplers for few-mode optical fiber networks,” Opt. Lett. 37, 3990 (2012). [CrossRef] [PubMed]

15.

N. Riesen, J. D. Love, and J. W. Arkwright, “Few-mode elliptical-core fiber data transmission,” IEEE Photon Tech. Lett. 24, 344–346 (2012). [CrossRef]

16.

N. W. Spellmeyer, “Communications performance of a multimode EDFA,” IEEE Photon Tech. Lett. 12, 1337–1339 (2000). [CrossRef]

17.

G. Nykolak, S. A. Kramer, J. R. Simpson, D. J. DiGiovanni, C. R. Giles, and H. M. Presby, “An erbium doped multimode optical fiber amplifier,” IEEE Photon Tech. Lett. 3, 1079–1081(1991). [CrossRef]

18.

M. Salsi, R. Ryf, G. Le Cocq, L. Bigot, D. Peyrot, G. Charlet, S. Bigo, N. K. Fontaine, M. A. Mestre, S. Randel, X. Palou, C. Bolle, B. Guan, and Y. Quiquempois, “A six-mode erbium-doped fiber amplifier,” ECOC 2012, Post-Deadline paper, Th.3.A.6 (2012).

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 25, 2012
Revised Manuscript: October 23, 2012
Manuscript Accepted: November 4, 2012
Published: November 16, 2012

Citation
Guillaume Le Cocq, Laurent Bigot, Antoine Le Rouge, Marianne Bigot-Astruc, Pierre Sillard, Clemens Koebele, Massimilliano Salsi, and Yves Quiquempois, "Modeling and characterization of a few-mode EDFA supporting four mode groups for mode division multiplexing," Opt. Express 20, 27051-27061 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-24-27051


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References

  1. A. Chralyvy, “The coming capacity crunch,” ECOC 2009, p.1 (2009).
  2. P. Sillard, “New fibers for ultra-high capacity transport,” Opt. Fiber Technol.17, 495–502 (2011). [CrossRef]
  3. B. Zhu, T. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112 Tb/s space-division multiplexed DWDM transmission with 14 b/s/Hz aggregate spectral efficiency over a 76.8 km seven-core fiber,” Opt. Express19, 16665–16671 (2011). [CrossRef] [PubMed]
  4. C. Koebele, M. Salsi, D. Sperti, P. Tran, P. Brindel, H. Mardoyan, S. Bigo, A. Boutin, F. Verluise, P. Sillard, M. Astruc, L. Provost, F. Cerou, and G. Charlet, “Two mode transmission at 2x100 Gb/s, over 40 km-long prototype few-mode fiber, using LCOS-based programmable mode multiplexer and demultiplexer,” Opt. Express19, 16593–16600 (2011). [CrossRef] [PubMed]
  5. C. Koebele, M. Salsi, L. Milord, R. Ryf, C. A. Bolle, P. Sillard, S. Bigo, and G. Charlet, “40 km transmission of five mode division multiplexed data streams at 100 Gb/s with low MIMO-DSP complexity,” ECOC 2011, paper Th.13.C.3 (2011).
  6. N. Bai, E. Ip, T. Wang, and G. Li, “Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump,” Opt. Express19, 16601–16611 (2011). [CrossRef] [PubMed]
  7. Y. Jung, S. Alam, Z. Li, A. Dhar, D. Giles, I. P. Giles, J. K. Sahu, F. Poletti, L. Gruner-Nielsen, and D. J. Richardson, “First demonstration and detailed characterization of a multimode amplifier for space division multiplexed transmission systems,” Opt. Express19, B952–B957 (2011). [CrossRef]
  8. Z. Jiang and J.R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large-mode-area Yb-doped fibers,” J. Opt. Soc. Am. B25, 247–254 (2008). [CrossRef]
  9. C. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers,” J. Lightwave Technol.9, 271–283 (1991). [CrossRef]
  10. M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express15, 3236–3246 (2007). [CrossRef] [PubMed]
  11. Q. Kang, E.L. Lim, Y. Jung, J.K. Sahu, F. Poletti, C. Baskiotis, S.U Alam, and D.J. Richardson, “Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP01 pump configuration,” Opt. Express20, 20835–20843 (2012). [CrossRef] [PubMed]
  12. M. Salsi, J. Vuong, C. Koebele, P. Genevaux, H. Mardoyan, P. Tran, S. Bigo, G. Le Cocq, L. Bigot, Y. Quiquempois, A. Le Rouge, P. Sillard, M. Bigot-Astruc, and G. Charlet, “In-line few-mode optical amplifier with erbium profile tuned to support LP01, LP11 and LP21 mode groups,” ECOC 2012, paper Tu.3.F.1 (2012).
  13. P. Sillard, M. Astruc, D. Boivin, H. Maerten, and L. Provost, “Few-mode fiber for uncoupled mode-division multiplexing transmissions,” ECOC 2011, paper Tu.5.LeCervin.7 (2011).
  14. J. D. Love and N. Riesen, “Mode-selective couplers for few-mode optical fiber networks,” Opt. Lett.37, 3990 (2012). [CrossRef] [PubMed]
  15. N. Riesen, J. D. Love, and J. W. Arkwright, “Few-mode elliptical-core fiber data transmission,” IEEE Photon Tech. Lett.24, 344–346 (2012). [CrossRef]
  16. N. W. Spellmeyer, “Communications performance of a multimode EDFA,” IEEE Photon Tech. Lett.12, 1337–1339 (2000). [CrossRef]
  17. G. Nykolak, S. A. Kramer, J. R. Simpson, D. J. DiGiovanni, C. R. Giles, and H. M. Presby, “An erbium doped multimode optical fiber amplifier,” IEEE Photon Tech. Lett.3, 1079–1081(1991). [CrossRef]
  18. M. Salsi, R. Ryf, G. Le Cocq, L. Bigot, D. Peyrot, G. Charlet, S. Bigo, N. K. Fontaine, M. A. Mestre, S. Randel, X. Palou, C. Bolle, B. Guan, and Y. Quiquempois, “A six-mode erbium-doped fiber amplifier,” ECOC 2012, Post-Deadline paper, Th.3.A.6 (2012).

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