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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 4 — Feb. 13, 2012
  • pp: 3541–3549
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First-order polarization mode dispersion compensator using two independent feedback signals enabling separation of principal states of polarization and differential group delay controls

Ki Ho Han and Wang Joo Lee  »View Author Affiliations


Optics Express, Vol. 20, Issue 4, pp. 3541-3549 (2012)
http://dx.doi.org/10.1364/OE.20.003541


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Abstract

We propose and demonstrate a novel first-order polarization mode dispersion (PMD) compensator separating principal-states-of-polarization (PSP) control from differential-group-delay (DGD) control by using two independent feedback monitoring signals. To verify the proposed operating principle, we fabricated an automatically adaptive module-type 40-Gb/s PMD compensator on printed circuit boards. The results show that unlike previous typical compensation method of controlling alternately PSP and DGD, the proposed scheme provides independent continuous PSP tracking regardless of DGD control by delay line under rapidly varying PSP condition, thus resulting in stable compensated signal with reduction in compensation time.

© 2012 OSA

1. Introduction

With growing demand for larger capacity in backbone fiber-optic networks, bit rate per a wavelength channel has recently been evolving from 10 Gb/s to 40 Gb/s or higher [1

1. M. Chacinski, U. Westergren, L. Thylen, B. Stoltz, J. Rosenzweig, R. Driad, R. E. Makon, J. Li, and A. Steffan, “ETDM transmitter module for 100-Gb/s ethernet,” IEEE Photon. Technol. Lett. 22(2), 70–72 (2010). [CrossRef]

,2

2. G. Raybon, P. J. Winzer, A. A. Adamiecki, A. H. Gnauck, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, C. R. Doerr, R. Delbue, and P. J. Pupalaikis, “All-ETDM 80-Gbaud (160-Gb/s) QPSK generation and coherent detection,” IEEE Photon. Technol. Lett. 23(22), 1667–1669 (2011). [CrossRef]

]. In such high-speed optical transmission system, polarization mode dispersion (PMD) is becoming a main factor limiting its performance [3

3. C. D. Poole and R. E. Wagner, “Phenomenological approach to polarisation dispersion in long single-mode fibres,” Electron. Lett. 22(19), 1029–1030 (1986). [CrossRef]

12

12. B. W. Hakki, “Polarization mode dispersion compensation by phase diversity detection,” IEEE Photon. Technol. Lett. 9(1), 121–123 (1997). [CrossRef]

]. PMD means pulse distortion due to differential group delay (DGD) between two mutually orthogonal principal states of polarization (PSP) occurring in a birefringent fiber-optic link, which is induced mainly by environment change, birefringent optical components, and asymmetric fiber core. Under a same DGD, higher bit-rate system suffers more impairment due to smaller signal pulsewidth. Therefore, PMD compensation is essential for transmission systems at 40 Gb/s or beyond.

To solve this problem, we propose a novel compensator using two independent feedback signals. One is used only for controlling a PC to align PSP to PBS, and the other for adjusting a delay line to offset DGD. Then, the two PSP and DGD controls can be separated and operated independently of each other due to use of the two separate feedback signals, thus preventing PSP tracking loss regardless of a slow delay line even though PSP changes rapidly and considerably during the delay line motion. To experimentally demonstrate the proposed scheme, we fabricated a first-order 40-Gb/s PMD compensator in module type on printed circuit boards. The compensator showed a very fast response time of less than 2 μs for PSP tracking and, regardless of relatively slow delay line control, maintained stable compensated output signal under randomly fast varying PSP condition.

2. Operating principle

Figure 2(a)
Fig. 2 Simulated results. (a) Filtered RF power spectra versus angle between PSP and PBS axes for given 2-GHz filter bandwidth, 20-ps DGD, and 30-degree launch angle for 40-Gb/s NRZ signal. (b) RF power filtered at 40 GHz versus angle between PSP and PBS axes for various DGD values. (c) DOP of DDL output signal versus DGD for various launch angles while keeping PSP parallel to PBS axes.
shows simulated RF power from one PBS axis, bandpass-filtered at various frequencies, as a function of angle θ between output PSP and PBS when given 2-GHz BPF bandwidth, 20-ps DGD and 30-degree launch angle to input PSP for 40-Gb/s non-return-to-zero (NRZ) modulation format. The simulation was carried out using the model described in [22

22. K. H. Han and W. J. Lee, “Tracking and separation of time-varying principal states of polarization in optical fiber link for first-order PMD compensation and its filter-dependent performance,” Opt. Fiber Technol. 14(4), 268–274 (2008). [CrossRef]

]. At θ=nπ/2 (n: integer), two orthogonal PSPs are completely aligned to PBS axes and separated into two fast and slow PSPs via PBS. As shown in Fig. 2(a), only the RF signal filtered at 40 GHz shows convergence to minimum at θ=nπ/2, which can be reached by adjusting the PC in a way to minimize the RF feedback signal by dithering process. At other frequencies, no minimum-converging points can be found at θ=nπ/2. Therefore, RF signal filtered at clock frequency can be used as a feedback signal for separation of two fast and slow PSPs via PBS. Figure 2(b) shows RF power filtered at 40 GHz with 2-GHz bandwidth as a function of the angle θ for various DGDs. Regardless of DGD value, the RF power filtered at clock frequency shows minimum when PSP are aligned with PBS. The largest contrast or dynamic range in RF feedback power is obtained at a 12.5-ps DGD from which almost symmetric contrast with DGD is observed as shown in Fig. 2(b). Figure 2(c) shows simulated DOP at the DDL output in regard to DGD for various launch angles to input PSP after completion of PSP and PBS alignment. This DGD means residual DGD during compensation, which is sum of fiber DGD and differential delay time, and can be reduced to zero by adjusting DDL in a direction against birefringence of fiber link. The graphs show that DOP decreases with increasing DGD and the steepest inclination is obtained for a 45-degree launch angle. Convergence to DOP maximum means complete DGD canceling, which can be attained by adjusting the delay line in a way to maximize the DOP by dithering algorithm. If a return-to-zero (RZ) signal is used, the center of BPF for feedback signal should be twice the clock frequency due to a half times smaller pulsewidth than NRZ signal but the control algorithm is the same as the case of NRZ signal. We assume fully compensated chromatic dispersion (CD). If residual CD is large, the RF feedback signal may be affected by RF components induced by CD.

3. Experiment and results

Figure 4(a)
Fig. 4 (a) RF feedback signal measured timewise by digital oscilloscope during scan and tracking processes when a filter of 2-GHz bandwidth and 40-GHz center frequency is used. (b) Time variation of RF feedback signal when applying a control voltage to LN PC to find a response time of PSP control.
shows oscilloscope view of temporal variation of RF feedback signal measured by RF detector during scan and real-time tracking for a 25-ps DGD. The scan time was measured to be 50 ms, which can be varied and become larger with smaller step angle and larger average number. Following scan, the real-time tracking process was shown to begin where minimum feedback RF power was obtained. Figure 4(b) shows a measured response time of PSP control which is defined to be time taken from point of applying control voltage to LN PC until when RF detector detects the filtered feedback signal. The result showed a very fast response time of less than 2 μs, which is expected to allow tracking of rapid PSP variation up to submillisecond in fiber link.

Figures 5(a)
Fig. 5 3-dim and 2-dim view of measured RF feedback signal as a function of angles of half-wave and quarter-wave plates for given (a) 10-ps and (b) 20-ps DGDs, respectively.
and 5(b) show RF feedback signal detected while scanning two wave plates up to 180 degrees for 10-ps and 20-ps DGDs, respectively. The bottom in the graphs shows the contour of the RF power. The results show that regardless of DGD values, the convergent minimum point occurs two times and four times during 180-degree rotation of QWP and HWP, respectively. The two PSPs were confirmed to be aligned to PBS at those minimums and real-time tracking starts at one of them.

Figure 7
Fig. 7 Measured DOP versus DGD with or without optical filter. The sideband filtering provides the largest dynamic range and the highest resolution to DGD tracking.
shows DOP measured before the compensator with DGD varied up to 25 ps by using a polarization analyzer with or without a FBG optical bandpass filter (OBPF) before the compensator. The reason why, to obtain Fig. 7, the OBPF in Fig. 3(a) was placed before the compensator and then DOP was measured before the compensator is that in configuration as Fig. 3(a), the total DGD after the compensator is sum of DGD by the emulator and differential delay time by the delay line of the compensator and thus each DOP after the compensator for each fixed total DGD value cannot be obtained since the delay line moves automatically and adaptively. The measurement was done to find an optimum filter (OBPF) condition to enhance DOP dynamic range. The three DOP curves showed decrease with DGD increase regardless of the filter use. This is due to the fact that different wavelength within an optical signal band produces, via interaction with DGD, different polarization state at output of birefringent fiber link, represented as polarization diffusion on Poincare sphere. The extent of the polarization diffusion intensifies as DGD increases, thus worsening DOP. In the case of using the sideband filter whose center is 0.2 nm off that of the optical signal, the DOP showed larger dynamic range than that without filter because the center components in the signal band become weakened and comparable to its side components through the filtering due to the filter shape similar to Gaussian type. The larger DOP dynamic range provides higher resolution and accuracy in one step move in DGD tracking. On the other hand, the filter whose center wavelength is identical to that of the optical signal showed rather smaller DOP dynamic range than the filter-less case because via the filtering the side components in the signal band become more weakened than the center components due to the filter shape so that extent of depolarization is mitigated and hence DOP is increased more than the case without filter under the same DGD.

Figure 8(a)
Fig. 8 (a) Temporal variation of PSP and DOP measured before and after PMD compensator. (b) Measured BER before and after PMD compensation.
shows time variation of PSP (s1, normalized Stokes parameter) and DOP before compensation and DOP after compensation under 25-ps DGD condition. To simulate fast PSP change in fiber link, the motorized PC2 (HP, scan rate 8) behind the PMD emulator was used to provide its fastest speed of random polarization fluctuation. We used the polarization analyzer (HP) to measure states of polarization (SOP) and DOP with its highest 2-ms sampling rate. The normalized Stokes parameter s1 in Fig. 8 shows how fast PSP changes and its maximum speed was measured to be about 0.1/ms. The DOP before compensation was measured to be an average of 0.66 for 25-ps DGD with slight fluctuation. Regardless of the fast PSP variation, the measured DOP after compensation in Fig. 8(a) was maintained within a range of 0.980 to 0.997 with compensated open-eye signal, showing that the speed of the compensator with less than 2 μs response time in PSP control is fast enough to track and keep the PSP fluctuating rapidly at 0.1/ms aligned constantly to PBS axes in real time. Figure 8(b) shows measured bit error rate (BER) before and after PMD compensation with the input power to the preamplifier (PA) varied by an attenuator under various DGD values. The results show that for 12.5-ps DGD, the PMD compensation yielded a 7.2 dB power gain at a 10−11 BER and a 3.4 dB Q-value gain at an input power of −24.5 dBm where optical signal-to-noise ratio (OSNR) after the PA was measured to be 22.6 dB in 0.2-nm resolution.

4. Conclusion

References and links

1.

M. Chacinski, U. Westergren, L. Thylen, B. Stoltz, J. Rosenzweig, R. Driad, R. E. Makon, J. Li, and A. Steffan, “ETDM transmitter module for 100-Gb/s ethernet,” IEEE Photon. Technol. Lett. 22(2), 70–72 (2010). [CrossRef]

2.

G. Raybon, P. J. Winzer, A. A. Adamiecki, A. H. Gnauck, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, C. R. Doerr, R. Delbue, and P. J. Pupalaikis, “All-ETDM 80-Gbaud (160-Gb/s) QPSK generation and coherent detection,” IEEE Photon. Technol. Lett. 23(22), 1667–1669 (2011). [CrossRef]

3.

C. D. Poole and R. E. Wagner, “Phenomenological approach to polarisation dispersion in long single-mode fibres,” Electron. Lett. 22(19), 1029–1030 (1986). [CrossRef]

4.

C. D. Poole, R. W. Tkach, A. R. Chraplyvy, and D. A. Fishman, “Fading in lightwave systems due to polarization-mode dispersion,” IEEE Photon. Technol. Lett. 3(1), 68–70 (1991). [CrossRef]

5.

H. Bülow, “System outage probability due to first- and second-order PMD,” IEEE Photon. Technol. Lett. 10(5), 696–698 (1998). [CrossRef]

6.

H. Kogelnik, P. J. Winzer, L. E. Nelson, R. M. Jopson, M. Boroditsky, and M. Brodsky, “First-order PMD outage for the hinge model,” IEEE Photon. Technol. Lett. 17(6), 1208–1210 (2005). [CrossRef]

7.

P. Boffi, M. Ferrario, L. Marazzi, P. Martelli, P. Parolari, A. Righetti, R. Siano, and M. Martinelli, “Measurement of PMD tolerance in 40-Gb/s polarization-multiplexed RZ-DQPSK,” Opt. Express 16(17), 13398–13404 (2008). [CrossRef] [PubMed]

8.

N. Gisin, R. Passy, J. C. Bishoff, and B. Perny, “Experimental investigations of the statistical properties of polarization mode dispersion in single mode fibers,” IEEE Photon. Technol. Lett. 5(7), 819–821 (1993). [CrossRef]

9.

L. E. Nelson, R. M. Jopson, H. Kogelnik, and G. J. Foschini, “Measurement of depolarization and scaling associated with second-order polarization mode dispersion in optical fibers,” IEEE Photon. Technol. Lett. 11(12), 1614–1616 (1999). [CrossRef]

10.

J. M. Gené and P. J. Winzer, “First-order PMD outage prediction based on outage maps,” J. Lightwave Technol. 28(13), 1873–1881 (2010). [CrossRef]

11.

M. Karlsson, “Polarization mode dispersion induced pulse broadening in optical fibers,” Opt. Lett. 23(9), 688–690 (1998). [CrossRef] [PubMed]

12.

B. W. Hakki, “Polarization mode dispersion compensation by phase diversity detection,” IEEE Photon. Technol. Lett. 9(1), 121–123 (1997). [CrossRef]

13.

F. Heismann, D. A. Fishman, and D. L. Wilson, “Automatic compensation of first order polarization mode dispersion,” in Proc. European Conference on Optical Communication, ECOC98, 529–530 (1998).

14.

R. Noe, D. Sandel, M. Yoshida-Dierolf, S. Hinz, V. Mirvoda, A. Schopflin, C. Gungener, E. Gottwald, C. Scheerer, G. Fischer, T. Weyrauch, and W. Haase, “Polarization mode dispersion compensation at 10, 20, and 40 Gb/s with various optical equalizers,” J. Lightwave Technol. 17(9), 1602–1616 (1999). [CrossRef]

15.

H. Sunnerud, M. Chongjin Xie, M. Karlsson, R. Samuelsson, and P. A. Andrekson, “A comparison between different PMD compensation techniques,” J. Lightwave Technol. 20(3), 368–378 (2002). [CrossRef]

16.

N. Y. Kim, D. Lee, J. Park, and N. Park, “Comparisons on PMD-compensation feedback methods for bandwidth-rich transmission formats,” IEEE Photon. Technol. Lett. 16(6), 1597–1599 (2004). [CrossRef]

17.

L. Xu, H. Miao, and A. M. Weiner, “All-order polarization-mode-dispersion (PMD) compensation at 40 Gb/s via hyperfine resolution optical pulse shaping,” IEEE Photon. Technol. Lett. 22(15), 1078–1080 (2010). [CrossRef]

18.

X. S. Yao, X. Chen, T. J. Xia, G. Wellbrock, D. Chen, D. Peterson, P. Zhang, A. Belisle, L. Dong, and T. Yu, “In-service light path PMD (polarization mode dispersion) monitoring by PMD compensation,” Opt. Express 18(26), 27306–27318 (2010). [CrossRef] [PubMed]

19.

H. Bülow, W. Baumert, H. Schmuck, F. Mohr, T. Schulz, F. Kuppers, and W. Weiershausen, “Measurement of the maximum speed of PMD fluctuation in installed field fiber,” in Proc. Optical Fiber Communication Conference, OFC 99, 83–85 (1999).

20.

D. S. Waddy, L. Chen, and X. Bao, “Polarization effects in aerial fibers,” Opt. Fiber Technol. 11(1), 1–19 (2005). [CrossRef]

21.

K. Ogaki, M. Nakada, Y. Nagao, and K. Nishijima, “Fluctuation differences in the principal states of polarization in aerial and buried cables,” in Proc. Optical Fiber Communications Conference, OFC 2003, 14–15 (2003).

22.

K. H. Han and W. J. Lee, “Tracking and separation of time-varying principal states of polarization in optical fiber link for first-order PMD compensation and its filter-dependent performance,” Opt. Fiber Technol. 14(4), 268–274 (2008). [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
(260.2030) Physical optics : Dispersion

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 2, 2011
Revised Manuscript: January 21, 2012
Manuscript Accepted: January 23, 2012
Published: January 30, 2012

Citation
Ki Ho Han and Wang Joo Lee, "First-order polarization mode dispersion compensator using two independent feedback signals enabling separation of principal states of polarization and differential group delay controls," Opt. Express 20, 3541-3549 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-4-3541


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References

  1. M. Chacinski, U. Westergren, L. Thylen, B. Stoltz, J. Rosenzweig, R. Driad, R. E. Makon, J. Li, and A. Steffan, “ETDM transmitter module for 100-Gb/s ethernet,” IEEE Photon. Technol. Lett.22(2), 70–72 (2010). [CrossRef]
  2. G. Raybon, P. J. Winzer, A. A. Adamiecki, A. H. Gnauck, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, C. R. Doerr, R. Delbue, and P. J. Pupalaikis, “All-ETDM 80-Gbaud (160-Gb/s) QPSK generation and coherent detection,” IEEE Photon. Technol. Lett.23(22), 1667–1669 (2011). [CrossRef]
  3. C. D. Poole and R. E. Wagner, “Phenomenological approach to polarisation dispersion in long single-mode fibres,” Electron. Lett.22(19), 1029–1030 (1986). [CrossRef]
  4. C. D. Poole, R. W. Tkach, A. R. Chraplyvy, and D. A. Fishman, “Fading in lightwave systems due to polarization-mode dispersion,” IEEE Photon. Technol. Lett.3(1), 68–70 (1991). [CrossRef]
  5. H. Bülow, “System outage probability due to first- and second-order PMD,” IEEE Photon. Technol. Lett.10(5), 696–698 (1998). [CrossRef]
  6. H. Kogelnik, P. J. Winzer, L. E. Nelson, R. M. Jopson, M. Boroditsky, and M. Brodsky, “First-order PMD outage for the hinge model,” IEEE Photon. Technol. Lett.17(6), 1208–1210 (2005). [CrossRef]
  7. P. Boffi, M. Ferrario, L. Marazzi, P. Martelli, P. Parolari, A. Righetti, R. Siano, and M. Martinelli, “Measurement of PMD tolerance in 40-Gb/s polarization-multiplexed RZ-DQPSK,” Opt. Express16(17), 13398–13404 (2008). [CrossRef] [PubMed]
  8. N. Gisin, R. Passy, J. C. Bishoff, and B. Perny, “Experimental investigations of the statistical properties of polarization mode dispersion in single mode fibers,” IEEE Photon. Technol. Lett.5(7), 819–821 (1993). [CrossRef]
  9. L. E. Nelson, R. M. Jopson, H. Kogelnik, and G. J. Foschini, “Measurement of depolarization and scaling associated with second-order polarization mode dispersion in optical fibers,” IEEE Photon. Technol. Lett.11(12), 1614–1616 (1999). [CrossRef]
  10. J. M. Gené and P. J. Winzer, “First-order PMD outage prediction based on outage maps,” J. Lightwave Technol.28(13), 1873–1881 (2010). [CrossRef]
  11. M. Karlsson, “Polarization mode dispersion induced pulse broadening in optical fibers,” Opt. Lett.23(9), 688–690 (1998). [CrossRef] [PubMed]
  12. B. W. Hakki, “Polarization mode dispersion compensation by phase diversity detection,” IEEE Photon. Technol. Lett.9(1), 121–123 (1997). [CrossRef]
  13. F. Heismann, D. A. Fishman, and D. L. Wilson, “Automatic compensation of first order polarization mode dispersion,” in Proc. European Conference on Optical Communication, ECOC98, 529–530 (1998).
  14. R. Noe, D. Sandel, M. Yoshida-Dierolf, S. Hinz, V. Mirvoda, A. Schopflin, C. Gungener, E. Gottwald, C. Scheerer, G. Fischer, T. Weyrauch, and W. Haase, “Polarization mode dispersion compensation at 10, 20, and 40 Gb/s with various optical equalizers,” J. Lightwave Technol.17(9), 1602–1616 (1999). [CrossRef]
  15. H. Sunnerud, M. Chongjin Xie, M. Karlsson, R. Samuelsson, and P. A. Andrekson, “A comparison between different PMD compensation techniques,” J. Lightwave Technol.20(3), 368–378 (2002). [CrossRef]
  16. N. Y. Kim, D. Lee, J. Park, and N. Park, “Comparisons on PMD-compensation feedback methods for bandwidth-rich transmission formats,” IEEE Photon. Technol. Lett.16(6), 1597–1599 (2004). [CrossRef]
  17. L. Xu, H. Miao, and A. M. Weiner, “All-order polarization-mode-dispersion (PMD) compensation at 40 Gb/s via hyperfine resolution optical pulse shaping,” IEEE Photon. Technol. Lett.22(15), 1078–1080 (2010). [CrossRef]
  18. X. S. Yao, X. Chen, T. J. Xia, G. Wellbrock, D. Chen, D. Peterson, P. Zhang, A. Belisle, L. Dong, and T. Yu, “In-service light path PMD (polarization mode dispersion) monitoring by PMD compensation,” Opt. Express18(26), 27306–27318 (2010). [CrossRef] [PubMed]
  19. H. Bülow, W. Baumert, H. Schmuck, F. Mohr, T. Schulz, F. Kuppers, and W. Weiershausen, “Measurement of the maximum speed of PMD fluctuation in installed field fiber,” in Proc. Optical Fiber Communication Conference, OFC 99, 83–85 (1999).
  20. D. S. Waddy, L. Chen, and X. Bao, “Polarization effects in aerial fibers,” Opt. Fiber Technol.11(1), 1–19 (2005). [CrossRef]
  21. K. Ogaki, M. Nakada, Y. Nagao, and K. Nishijima, “Fluctuation differences in the principal states of polarization in aerial and buried cables,” in Proc. Optical Fiber Communications Conference, OFC 2003, 14–15 (2003).
  22. K. H. Han and W. J. Lee, “Tracking and separation of time-varying principal states of polarization in optical fiber link for first-order PMD compensation and its filter-dependent performance,” Opt. Fiber Technol.14(4), 268–274 (2008). [CrossRef]

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