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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 3 — Jan. 30, 2012
  • pp: 2668–2680
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Mode-division multiplexed transmission with inline few-mode fiber amplifier

Neng Bai, Ezra Ip, Yue-Kai Huang, Eduardo Mateo, Fatih Yaman, Ming-Jun Li, Scott Bickham, Sergey Ten, Jesús Liñares, Carlos Montero, Vicente Moreno, Xesús Prieto, Vincent Tse, Kit Man Chung, Alan Pak Tao Lau, Hwa-Yaw Tam, Chao Lu, Yanhua Luo, Gang-Ding Peng, Guifang Li, and Ting Wang  »View Author Affiliations


Optics Express, Vol. 20, Issue 3, pp. 2668-2680 (2012)
http://dx.doi.org/10.1364/OE.20.002668


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Abstract

We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fiber’s LP01 and two degenerate LP11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

© 2012 OSA

1. Introduction

As the capacity of optical systems based on single mode fiber (SMF) approach the nonlinear Shannon’s limit [1

1. D. Qian, M.-F. Huang, E. Ip, Y.-K. Huang, Y. Shao, J. Hu, and T. Wang, “101-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB5.

], further capacity growth requires new transmission paradigm. Time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) are examples of parallel transmission in time/frequency which have been successfully employed in SMF systems. To further increase the number of parallel channels, the spatial dimensions may be exploited. Indeed, space-division multiplexing (SDM) is gaining prominence as the most promising technology for overcoming the capacity crunch. To date, SDM transmission has been reported for three fiber types: these are weakly coupled multicore fibers (MCF) [2

2. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7×97×172-Gb/s SDM/WDM/PDM) QPSK transmission through 16.8-km heterogeneous multi-core fiber,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB6.

], strongly coupled MCF [3

3. R. Ryf, A. Sierra, R.-J. Essiambre, A. H. Gnauck, S. Randel, M. Esmaeelpour, S. Mumtaz, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, T. Hayashi, T. Taru, and T. Sasaki, “Coherent 1200-km 6×6 MIMO mode-multiplexed transmission over 3-core microstructured fiber,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.1.

], and multimode fibers (MMF) [4

4. S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). [CrossRef] [PubMed]

,5

5. E. Ip, N. Bai, Y. Huang, E. Mateo, F. Yaman, S. Bickham, H. Tam, C. Lu, M. Li, S. Ten, A. P. T. Lau, V. Tse, G. Peng, C. Montero, X. Prieto, and G. Li, “88×3×112-Gb/s WDM Transmission over 50-km of Three-Mode Fiber with Inline Multimode Fiber Amplifier,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.2.

]. Weakly coupled MCFs behave like parallel SMF channels, and have the lowest decoding complexity per bit. However, the achievable information capacity per unit area is low. By contrast, both strongly coupled MCF and MMF require multiple-input multiple-output (MIMO) detection techniques with high decoding complexity, but can achieve higher information capacity per unit area.

2. Few-mode EDFA

A few-mode erbium-doped fiber amplifier (FM-EDFA) was constructed out of 15 meters of few-mode erbium-doped fiber. Originally intended as a step-index profile fiber with a uniformly doped core, the actual refractive index profile measured in the radial direction is shown in Fig. 1(a)
Fig. 1 (a) Refractive index of MM-EDF, (b) Normalized radial intensity profiles of signal and pump modes.
. The FM-EDF supports two mode groups at the signal wavelength (around 1550 nm): LP01(s) and LP11(s), and four mode groups at the pump wavelength (around 980 nm): LP01(p), LP11(p), LP21(p) and LP02(p). Normalized intensity distributions for the signal and pump modes of interest are shown in Fig. 1(b).

To characterize the FM-EDFA, we use the experiment setup shown in Fig. 2
Fig. 2 Experiment setup.
. A tunable C-band external cavity laser (ECL) is amplified and passed through a polarization controller (PC), and is then spatially transformed by a mode multiplexer. The mode multiplexer splits the input signal into two paths. The single-mode fiber in each path is terminated by a beam collimator (BC) with focal length of 11 mm. At one of the paths, a phase plate (PP) converts the LP01 Gaussian beam into an LP11 mode. The two beams are passively combined by a passive beam splitter (BS) and launched into two meters of undoped FMF of the same type as the 50-km FMF used in the transmission experiment described in Section 3 which supports the propagation of LP01 and LP11 modes at the signal wavelength. At the FM-EDFA, the FMF is terminated at another 11-mm BC. A dichoric mirror (DM) spatially combines the signal beam with a 980 nm forward pump beam, which is collimated by an 8-mm BC followed by an optional PP. The spatially overlapping signal and pump are launched into the FM-EDF whose ends are spliced with short sections of undoped FMF. At the output of the FM-EDFA, another 980nm pump beam is collimated by an 8-mm BC and spatially transformed by an optional PP. After reflection by a DM, the beam is launched into the output of FM-EDF as a counter propagating pump. We angle-cleave the output facet of the FM-EDF to minimize reflections back into the amplifying medium, and terminated the fiber at an 11-mm BC. Two DMs are used to filter the unused pump, allowing the signal power and beam profile to be measured.

3. Phase plates

4. Transmission experiment

At the receiver, the mode demultiplexer is a mirror image of the transmitter’s mode multiplexer: we terminate the FMF at an 11-mm BC and use BSs to split the signal into three tributaries. In the LP11o and LP11e tributaries, the signal is spatially demodulated using phase plates. All three tributaries are then coupled back into single mode fibers using 11-mm BC. These signals are amplified and then filtered by wavelength selective switches (WSS). The channel of interest is downconverted to electrical baseband by mixing the signals with a common local oscillator (LO) laser using three polarization-and-phase diversity hybrids and twelve photodetectors, recovering the in-phase (I) and quadrature (Q) components of the six spatial-polarization modes. The electrical signals are sampled using three quad-channel sampling oscilloscopes with sampling rates and electrical bandwidths of 40 GSa/s and 16 GHz.

For data recovery, the digital signal processing algorithm shown in Fig. 7
Fig. 7 Digital signal processing architecture.
is used. First, we upsample the signals to M=2 times the baud rate (56 GHz). Optional frequency-domain equalizers (FDE) are used to compensate the average chromatic dispersion at (CD) of the LP01 and LP11 mode groups (see Section 6), followed by a 6×6 multiple-input multiple-output (MIMO) time-domain equalizer (TDE) that demultiplexes the six spatial-polarization modes. The operation of the equalizer can be generalized from [11

11. E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. 25(8), 2033–2043 (2007). [CrossRef]

]. Let yk=[y1,kyNm,k]T be the vector of received samples used to recover symbol k, where Nm=6 is the number of modes, and yi,k=[yi,Mk+Lyi,MkL]T is a vector of N=2L+1 complex-valued samples received at mode i at M times baud rate. The equations for MIMO equalization and equalizer update are given by:

x^k=WTyk,and
(1)
WW+2μyk*εkT.
(2)

In Eq. (1), x^k=[x^1,kx^Nm,k]T is the equalized symbol, and W is the equalizer matrix
W=[W11W1NmWNm1WNmNm]
(3)
comprising Nm × Nm components Wij, each of which is a time-domain column vector of length N – i.e., Wij=[Wij,LWij,L]T. In the least-means square (LMS) update equation shown in Eq. (2), μ is the step size, and εk the error vector obtained for symbol k. In decision-aided (DA) mode, the transmitted symbol xk is known, so εk=xkx^k; in decision-directed (DD) mode, a symbol decision [xk]D is made on the equalized symbol xk, and the error εk=[xk]Dx^k is the difference between the decision and its equalized value. For this experiment, we used a training sequence of 20,000 symbols for initial convergence of the TDE, at which point, the equalizer is switched to decision-directed (DD) adaptation. To ensure non-degeneracy of the outputs, we check the cross-correlation of the six output tributaries. In the experimental results presented in Sections 5 and 6, twenty data sets of 32,768 symbols were captured. For each data set, bit-error rate (BER) and Q-factor were computed from the second-half of the data after convergence of the TDE, (i.e., 655,360 bits per spatial-polarization modes were evaluated).

In this experiment, the coupling loss between single-mode fiber and the LP01 mode of the FMF is 1.8 dB, while the coupling loss between single-mode fiber and the LP11 of the FMF (the loss due to the phase plate is negligible) is 3.5 dB. The losses of the beam splitter are 2.8 dB (through) and 4.0 dB (reflected). The attenuation of the FMF are ~0.22 dB/km for the LP01 mode and ~0.25 dB/km for the LP11 mode. Summing these losses, the LP11o and LP11e modes have the highest total end-to-end loss of ~29.1 dB. The gains of the transmitter’s single-mode fiber amplifiers were set so that all three spatial modes have the same power at the output of the FMF (i.e., before the FM-EDFA).

5. Experimental results

Figure 8
Fig. 8 BER vs. OSNR.
shows BER vs. OSNR curves for: (i) single-mode fiber back-to-back (BTB), (ii) MDM transmission BTB (via 1 meter of FMF), and (iii) after transmission for channel 38 at 1550.12 nm. It is observed that at a target BER of 10−3, the OSNR penalty for cases (i) and (ii) with respect to the theoretical additive white Gaussian noise-limited BER vs. OSNR curve for QPSK are 1.2 dB and 2.8 dB, respectively. The similar penalties experienced by all three spatial modes in case (ii) indicate the channel matrix is approximately unitary. For case (iii), we used a TDE of 301 taps per tributary to compensate MGD and CD (i.e., no FDE was used). The length of the TDE was sufficient to overcome the MGD of the channel (see Section 6). The larger OSNR penalties for the higher-order LP11o and LP11e modes are caused by a combination of mode coupling at the MM-EDFA and mode-dependent gain in the amplifying medium causing the channel matrix to be non-unitary.

Figure 9
Fig. 9 Time-domain equalizer taps after convergence.
shows an example of the TDE obtained after convergence. The subplot at the intersection of the i-th row and j-th column is the vector Wij=[Wij,LWij,L]T as outlined in Section 4. In this experiment, we used M=2 times oversampling and N=301 taps for the TDE, which corresponds to a time span of 151 symbols sufficient to overcome the channel’s MGD. The vertical axis in each subplot is the value (real and imaginary components shown) of the filter coefficient, and the horizontal axis is the time index LlL. We aligned the training symbols with the received signal in such manner that the coefficient at l=0 is about halfway between the arrival times of the LP01 and LP11 modes. The coefficients l>0 are the causal coefficients, and the coefficients l<0 are anti-causal coefficients.

Figure 11
Fig. 11 Q vs. Launch Power after transmission.
shows Q vs. launch power after transmission. It is observed that the Q-factor is still increasing at the highest power, indicating that the system remains in the linear regime even when the transmitter’s EDFAs are set to their maximum output powers. System performance is therefore limited by the combined amplified spontaneous emission (ASE) of the FM-EDFA and single-mode EDFAs. In Fig. 12
Fig. 12 Measured BER for all WDM channels after transmission. Insets: Constellation diagrams of best and worst modes.
, the BERs after transmission is shown for all the channels at the optimal launch power of −0.5 dBm/λ. It is observed that the BER of all modes of all the WDM channels are below the threshold of 3.8×10−3 for 7% hard-decision forward-error correction (HD-FEC) code. The BERs at the short wavelengths (right-hand side) are slightly higher due to the gain vs. wavelength characteristic of the MM-EDFA. The constellations of the best (Ch. 48) and worst (Ch. 88) channels are shown in the insets.

From the TDE taps observed in Fig. 9, it is possible to achieve significant reduction in algorithmic complexity by keeping only those equalizer taps with significant energy around each peak. Additionally, the number of significant taps at each peak can be reduced by compensating known CD using the FDE shown in Fig. 7. Figure 13
Fig. 13 Q penalty vs. No. of taps per tributary of 6 × 6 equalizer.
shows Q penalty vs. number of taps (Ntaps) per TDE tributary, where half of the taps are allocated around each peak centered at the LP01 and LP11 arrival times. It is observed that to achieve Q penalty less than 1 dB, around 80 taps per tributary is required.

6. Fiber characterization

The FMF has a graded index core that was optimized to simultaneously achieve large effective area, low MGD and low coupling between the LP01 and LP11 modes. The calculated effective areas of the LP01 and LP11 modes are 137 and 183 μm2, respectively, and the calculated MGD and chromatic dispersion are plotted in Fig. 14
Fig. 14 Predicted (a) differential mode group delay and (b) chromatic dispersion.
. The MGD of all the channels can be estimated experimentally from the equalizer coefficients after convergence. Figure 15(a)
Fig. 15 Measured (a) differential MGD and (b) dispersion characteristic for experimental FMF.
shows the results, where it is observed that MGD varies from 50 to 80 ps/km across the C-band. In addition, the use of training symbols enables measurement of the change in group delay with channel wavelength, and hence the CD of each mode of the FMF. The results are shown in Fig. 15(b). It is observed that the CD of the LP01 and LP11 modes at 1550 nm are around 20.5 and 19.8 ps/nm/km, respectively. The fluctuations of the data points from the regressive lines is due to measurement “noise” which arises from temperature drift as the channels are swept − i.e., small changes in either refractive index and fiber length will cause a change in the arrival time of the signal (group delay) independent of chromatic dispersion. To reduce measurement noise, we captured the channel sweep in ascending order of channel number as rapidly as possible to reduce thermal fluctuations. The measured differential MGD and chromatic dispersion agree well with the calculated values.

To characterize the fiber attenuation, we took OTDR traces with different offset launch positions with a standard single mode fiber. When there is no offset, it is expected that most light is launched into the LP01 mode. When the offset increases, more and more light will be launched into the LP11 mode. Figure 16
Fig. 16 Measured OTDR traces at different offset launch positions.
shows OTDR traces for seven different launch positions. The attenuation increases slightly from 0.243 dB/km to 0.252 dB/km from center to 12 μm offset, indication very small attenuation difference between the LP01 and LP11 mode.

7. Conclusion

Our experiment was ultimately limited by ASE noise of the receiver’s single-mode EDFAs. We attribute this to the high loss of the receiver’s spatial demultiplexer. To enable longer transmission distance, it is possible to (i) use a spatial hologram to demultiplex the MDM signal without the high losses incurred by the current spatial demultiplexer, and (ii) use a higher-gain FM-EDFA to boost the MDM signal power above sensitivity. In particular, it is necessary for the gain of the FM-EDFA to be at least equal to the span loss in both mode groups of the FMF in order to enable multi-span transmission using a recirculating loop.

References and links

1.

D. Qian, M.-F. Huang, E. Ip, Y.-K. Huang, Y. Shao, J. Hu, and T. Wang, “101-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB5.

2.

J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7×97×172-Gb/s SDM/WDM/PDM) QPSK transmission through 16.8-km heterogeneous multi-core fiber,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB6.

3.

R. Ryf, A. Sierra, R.-J. Essiambre, A. H. Gnauck, S. Randel, M. Esmaeelpour, S. Mumtaz, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, T. Hayashi, T. Taru, and T. Sasaki, “Coherent 1200-km 6×6 MIMO mode-multiplexed transmission over 3-core microstructured fiber,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.1.

4.

S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). [CrossRef] [PubMed]

5.

E. Ip, N. Bai, Y. Huang, E. Mateo, F. Yaman, S. Bickham, H. Tam, C. Lu, M. Li, S. Ten, A. P. T. Lau, V. Tse, G. Peng, C. Montero, X. Prieto, and G. Li, “88×3×112-Gb/s WDM Transmission over 50-km of Three-Mode Fiber with Inline Multimode Fiber Amplifier,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.2.

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(17), 16601–16611 (2011). [CrossRef] [PubMed]

7.

Y. Yung, S. Alam, Z. Li, A. Dhar, D. Giles, I. Giles, J. Sahu, L. Grüner-Nielsen, F. Poletti, and D. Richardson, “First demonstration of multimode amplifier for spatial division multiplexed transmission systems,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.K.4.

8.

R. Ryf, A. Sierra, R. Essiambre, S. Randel, A. Gnauck, C. A. Bolle, M. Esmaeelpour, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, D. Peckham, A. McCurdy, and R. Lingle, “Mode-Equalized Distributed Raman Amplification in 137-km Few-Mode Fiber,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.K.5.

9.

J. Liñares, C. Montero, V. Moreno, M. C. Nistal, X. Prieto, J. R. Salgueiro, and D. Sotelo, “Glass processing by ion exchange to fabricate integrated optical planar components: applications,” Proc. SPIE 3936, 227–238 (2000). [CrossRef]

10.

J. R. Salgueiro, V. Moreno, and J. Liñares, “Model of linewidth for laser writing on a photoresist,” Appl. Opt. 41(5), 895–901 (2002). [CrossRef] [PubMed]

11.

E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. 25(8), 2033–2043 (2007). [CrossRef]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

ToC Category:
Transmission Systems and Network Elements

History
Original Manuscript: November 7, 2011
Revised Manuscript: December 12, 2011
Manuscript Accepted: December 13, 2011
Published: January 20, 2012

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Neng Bai, Ezra Ip, Yue-Kai Huang, Eduardo Mateo, Fatih Yaman, Ming-Jun Li, Scott Bickham, Sergey Ten, Jesús Liñares, Carlos Montero, Vicente Moreno, Xesús Prieto, Vincent Tse, Kit Man Chung, Alan Pak Tao Lau, Hwa-Yaw Tam, Chao Lu, Yanhua Luo, Gang-Ding Peng, Guifang Li, and Ting Wang, "Mode-division multiplexed transmission with inline few-mode fiber amplifier," Opt. Express 20, 2668-2680 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-3-2668


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References

  1. D. Qian, M.-F. Huang, E. Ip, Y.-K. Huang, Y. Shao, J. Hu, and T. Wang, “101-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB5.
  2. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7×97×172-Gb/s SDM/WDM/PDM) QPSK transmission through 16.8-km heterogeneous multi-core fiber,” in Proc. OFC (Los Angeles, CA, USA 2011). Paper PDPB6.
  3. R. Ryf, A. Sierra, R.-J. Essiambre, A. H. Gnauck, S. Randel, M. Esmaeelpour, S. Mumtaz, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, T. Hayashi, T. Taru, and T. Sasaki, “Coherent 1200-km 6×6 MIMO mode-multiplexed transmission over 3-core microstructured fiber,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.1.
  4. S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express19(17), 16697–16707 (2011). [CrossRef] [PubMed]
  5. E. Ip, N. Bai, Y. Huang, E. Mateo, F. Yaman, S. Bickham, H. Tam, C. Lu, M. Li, S. Ten, A. P. T. Lau, V. Tse, G. Peng, C. Montero, X. Prieto, and G. Li, “88×3×112-Gb/s WDM Transmission over 50-km of Three-Mode Fiber with Inline Multimode Fiber Amplifier,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.C.2.
  6. N. Bai, E. Ip, T. Wang, and G. Li, “Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump,” Opt. Express19(17), 16601–16611 (2011). [CrossRef] [PubMed]
  7. Y. Yung, S. Alam, Z. Li, A. Dhar, D. Giles, I. Giles, J. Sahu, L. Grüner-Nielsen, F. Poletti, and D. Richardson, “First demonstration of multimode amplifier for spatial division multiplexed transmission systems,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.K.4.
  8. R. Ryf, A. Sierra, R. Essiambre, S. Randel, A. Gnauck, C. A. Bolle, M. Esmaeelpour, P. J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, D. Peckham, A. McCurdy, and R. Lingle, “Mode-Equalized Distributed Raman Amplification in 137-km Few-Mode Fiber,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.K.5.
  9. J. Liñares, C. Montero, V. Moreno, M. C. Nistal, X. Prieto, J. R. Salgueiro, and D. Sotelo, “Glass processing by ion exchange to fabricate integrated optical planar components: applications,” Proc. SPIE3936, 227–238 (2000). [CrossRef]
  10. J. R. Salgueiro, V. Moreno, and J. Liñares, “Model of linewidth for laser writing on a photoresist,” Appl. Opt.41(5), 895–901 (2002). [CrossRef] [PubMed]
  11. E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol.25(8), 2033–2043 (2007). [CrossRef]

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