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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 12 — Jun. 11, 2007
  • pp: 7407–7414
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All-optic scheme for automatic polarization division demultiplexing

X. Steve Yao, L.-S. Yan, B. Zhang, A. E. Willner, and Junfeng Jiang  »View Author Affiliations


Optics Express, Vol. 15, Issue 12, pp. 7407-7414 (2007)
http://dx.doi.org/10.1364/OE.15.007407


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Abstract

We describe a cost effective scheme to automatically separate two polarization channels in a polarization division multiplexing (PDM) system, without having to modify the existing transmitter or receiver electronics or software. We experimentally validate the concept by achieving an extinction ratio of more than 28-dB between two demultiplexed channels. Finally, we successfully demonstrate the PDM scheme in a 1.12-Tb/s (14×2×40-Gb/s) system over 62-km of transmission fiber.

© 2007 Optical Society of America

1. Introduction

Increasing the transmission capacity or spectral efficiency of an existing fiber system without having to change any part of transmission hardware or software is an attractive proposition for carriers or system operators, because it can significantly reduce the system “down” time and minimize the equipment and installation cost for system upgrade. One method for doubling the system capacity or spectral efficiency is polarization-division-multiplexing (PDM), in which two independently modulated data channels with the same wavelength, but orthogonal polarization states are simultaneously transmitted in a single fiber [1–12

1. S. Bhandare, D. Sandel, B. Milivojevic, A. Hidayat, A. A. Fauzi, H. Zhang, S. K. Ibrahim, F. Wust, and R. Noe, “5.94-Tb/s 1.49-b/s/Hz (40×2×2×40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km,” IEEE Photon. Technol. Lett. 17, 914–916 (2005). [CrossRef]

]. At the receiver end, the two polarization channels are separated and detected independently. Ideally, the operator only needs to add a transceiver (identical to the existing ones in the system) and an associated polarization multiplexer/demultiplexer at each end of the fiber link, while leaving the rest of the system, including fibers, amplifiers, repeaters, wavelength MUX/DMUX, optical add/drop multiplexers (OADM), switching optics, and even the network management software, unchanged, as shown in Fig. 1, or with minimal modification. Other methods of increasing system spectral efficiency, such as reducing the channel wavelength spacing or increasing transceiver bit rate, require significant system re-design, and are therefore not suitable for the upgrade of existing systems, although they may be feasible for the implementation of new systems.

Fig. 1. Illustration of a polarization division multiplexing (PDM) system

Significant challenges remain for the practical deployment of PDM systems, including finding an effective polarization demultiplexing solution and overcoming polarization cross-talk between the two polarization channels induced by polarization-mode-dispersion (PMD) and polarization-dependent-loss (PDL). In this paper, we will concentrate on a cost-effective polarization demultiplexing solution, assuming that the PMD and PDL of the system is sufficiently low for polarization multiplexed transmission.

Polarization multiplexing is straightforward, requiring only a polarization beam combiner (PBC) to combine two channels with orthogonal polarizations, as shown in Fig. 1. However, separating the two channels with acceptable cross-talk at the receiving end is not trivial, because the polarization states of the two channels are no longer linear, and change rapidly with time. It is possible to monitor the cross-talk of the two channels in real time and then use the monitored information to dynamically control the states of polarization (SOP) of the two polarization channels in order to separate them with a polarization beam splitter. So far, no good method has been found to monitor the cross-talk optically; therefore, one must rely on the detected electronic signal in the receiver to monitor cross-talk. Previously proposed schemes include (i) monitoring of clock tone or pilot tones [5–8

5. A. R. Chraplyvy, A. H. Gnauck, R. W. Tkach, J. L. Zyskind, J. W. Sulhoff, A. J. Lucero, Y. Sun, R. M. Jopson, F. Forghieri, R. M. Derosier, C. Wolf, and A. R. McCormick, “1-Tb/s transmission experiment,” IEEE Photon. Technol. Lett. 8, 1264–1266 (1996). [CrossRef]

] (ii) multi-level electronic detection [9–10

9. M. I. Hayee, M. C. Cardakli, A. B. Sahin, and A. E. Willner, “Doubling of bandwidth utilization using two orthogonal polarizations and power unbalancing in a polarization-division-multiplexing scheme,” IEEE Photon. Technol. Lett. 13, 881–883 (2001). [CrossRef]

], and (iii) cross-correlation detection of the two demultiplexed channels [11

11. R. Noe, S. Hinz, D. Sandel, and F. Wust, “Crosstalk detection schemes for polarization division multiplex transmission,” J. Lightwave Technol. 19, 1469–1475 (2001). [CrossRef]

]. Each of these schemes has one or more of the following drawbacks: (i) it requires high-speed electronics, thereby making it bit-rate dependent; and more importantly (ii) it requires modification or even significant re-design of existing transceivers, making it more difficult to upgrade existing systems.

In the following sections, we first describe theoretically the new polarization demultiplexing scheme based on channel power imbalance. We then show our experimental validation of the scheme by separating two polarization multiplexed channels with a power imbalance of just above 0.5 dB. Finally, we show the results of a successful system demonstration of the scheme by doubling the transmission capacity of a DWDM system containing 14 WDM channels at 40-Gb/s per channel over 62-km LS (LEAF-Submarine by Corning Inc.) fiber transmission.

2. Description of the Concept

Mx=12[1100110000000000]
(1)
My=12[1100110000000000]
(2)

Optical data stream i with an arbitrary SOP Si can be expressed in Stokes space as:

Si=[Si0Si1Si2Si3]=[PiPicos2εicos2θiPicos2εisin2θiPisin2εi]
(3)

where Pi is the optical power, εi is the ellipticity angle, and θi is the orientation angle of the ith optical data stream. After passing through the PBS, the Stokes vectors along the x and y directions are MxSi and MxSi, respectively, where the first row of each Stokes vector represents the optical power along the corresponding direction. In particular,

Pix=12Pi(1+cos2εicos2θi)
(4)
Piy=12Pi(1cos2εicos2θi)
(5)
Fig. 2. Conceptual diagram of proposed Polarization DEMUX using automated feedback control (solid-line: optical path; dotted-line: electronic control). PBC: polarization combiner, PBS: polarization splitter, BS: beamsplitter, DPC: dynamic polarization controller, G1 & G2: electrical amplifiers, PD1 & PD2: photodetectors.

Since the two data streams are incoherent with each other (they are either generated using two independent laser sources or are rendered incoherent by other means), the optical powers emerging along the x and y axes of the PBS are

Px=P1x+P2x=12P1(1+cos2ε1cos2θ1)+12P2(1+cos2ε2cos2θ2)
(6)
Py=P1y+P2y=12P1(1+cos2ε1cos2θ1)+12P2(1+cos2ε2cos2θ2)
(7)

For the two optical data streams with orthogonal SOPs S⃑1 and S⃑2, the following relationships hold:

ε2=ε1
(8)
{θ2=θ1+12π,0θ112πθ2=θ112π,12π<θ1π
(9)

Substituting Eq.(4) and Eq.(5) into Eq.(6) and (7) yields

Px=12(P1+P2)+12(P1P2)cos2ε1cos2θ1)
(10)
Py=12(P1+P2)12(P1P2)cos2ε1cos2θ1)
(11)

To automatically and effectively separate the two orthogonal data channels, we monitor the relative optical power levels of the two channels by coupling a small amount of the signal power into two low-speed photodetectors (PD1 and PD2). Through low-noise electronic circuits (G1 and G2), the power difference between the two polarization states (Px - Py) is translated into the voltage difference (V 1-V 2), expressed as:

V1V2=α1Pxα2P2=12(α1α2)(P1+P2)+12(α1+α2)(P1P2)cos2ε1cos2θ1
(12)

where α1 and α2 are the response coefficients of the two photodetectors and their corresponding amplification circuits. Through electronic gain balancing or software calibration, they can be adjusted to be equal (α1 = α2 = α) and therefore the voltage difference of Eq. (12)becomes

ΔV=V1V2=α(P1P2)cos2ε1cos2θ1
(13)

As long as there is a power difference between the two channels (P 1 - P 2 ≠ 0), the voltage difference between the two power monitors depends on the orientation angle θ and ellipticity angle ε, which can be changed by the dynamic polarization controller. Therefore, by maximizing the calibrated voltage difference ΔV, we can effectively minimize the crosstalk and readily separate the two orthogonal channels by forcing either a positive maximum [(θ = 0°, ε = 0°) or (θ = ±90°, ε = ±90°)] or a negative maximum [(θ = ±90°, ε = 0°) or (θ = 0°, ε = ±90°)]. The positive maximum corresponds to a Stokes vector of (1,0,0) while the negative maximum corresponds to a Stokes vector of (-1,0,0).

3. Experimental verification

We implemented this simple yet novel demultiplexing scheme with a digital signal processor (DSP) based circuit, a fiber squeezer polarization controller, a polarization beamsplitter, and two monitoring photodetectors, as shown in Fig. 2. The two monitoring photodetectors obtain the voltage difference defined in Eq. (13) and feed it back to the DSP circuit. Our DSP firmware then automatically instructs the polarization controller to control the state of polarization to maximize the voltage difference (either a positive maximum or a negative maximum). The two polarization channels are considered to be separated when the maximum voltage difference is reached and maintained.

First we evaluate the concept using two static wavelength channels (~ 1-nm separation) launched at two orthogonal polarization states with different power levels. The reason for the use of different wavelengths is to distinguish the two polarization channels with an optical spectrum analyzer (OSA). This does not affect the result because the scheme is wavelength independent. After the polarization demultiplexer, we measure the extinction ratio (ER) using an optical spectrum analyzer, as shown in Fig. 3. A stable ER of >28-dB can be achieved when the power difference between the two channels is higher than 0.5 dB. For higher power differences (e.g. >1-dB), a stable ER >35 dB is possible. The remaining crosstalk is mainly due to electronic noise and the limits of the feedback accuracy, with the upper limit determined by the ER of the PBS inside the polarization demultiplexer (>40-dB). Because the scheme is wavelength independent, the experiment indicates that two polarization channels of the same wavelengths can also be effectively separated.

Fig. 3. Concept proof using two static wavelength channels with different power levels: here the power difference is ~0.5-dB (before Pol. DEMUX), and an ER of ~ 28 dB is achieved with the proposed polarization demultiplexing scheme (after Pol. DEMUX). >35-dB ER is possible as the power difference increases.

Fig. 4. Evaluation of proposed polarization demultiplexing scheme in a single-channel 10-Gb/s RZ back-to-back transmission setup. (a) Power penalties of both polarization channels (PDM_H and PDM_V) as a function of power difference between them. (b) Bit-error-rate (BER) curves as the power difference between two orthogonal channels is set to 0.5 dB (i.e. the power of PDM_V is 0.5-dB higher than that of PDM_H); the corresponding power penalties for the two orthogonal channels compared to the case without PDM are ~ 0.25 dB and 0.75 dB, respectively. Square: back-to-back without PDM. Circle: PDM-V. Triangle: PDM-H.

4. 1.12-Tb/s PDM transmission

Fig. 5. Experimental demonstration of 14-channel 1.12-Tb/s WDM-PDM transmission: (a) experimental setup; (b) optical spectrum of all 14 channels.

Fig. 6. Transmission results (a) Power penalties of both polarization channels (PDM_V and PDM_H) for all 14 wavelength channels compared to the back-to-back PDM system sensitivity measured at 10-9 BER. (b) Typical BER curves of one wavelength channel with eye diagrams inserted: bk_bk (back-to-back case without PDM transmitter); PDM bk_bk (back-to-back case with PDM transmitter).

5. Discussion

6. Summary

We have described a new polarization demultiplexing scheme for separating two polarization multiplexed channels based on channel power imbalance. We validated the scheme with a power imbalance of just above 0.5 dB in a system containing a single wavelength channel. Finally, we successfully demonstrated the scheme in a DWDM system containing 14 WDM channels of 40-Gb/s per channel over 62-km LS fiber transmission.

Acknowledgments

We thank Zhangfeng Wang, A. Belisle, Lynn Lin of General Photonics Corporation for helping on the electronic and mechanical design, and Teichen Wang and Shoufeng Lan of Tianjin University for some experimental efforts.

References and Links

1.

S. Bhandare, D. Sandel, B. Milivojevic, A. Hidayat, A. A. Fauzi, H. Zhang, S. K. Ibrahim, F. Wust, and R. Noe, “5.94-Tb/s 1.49-b/s/Hz (40×2×2×40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km,” IEEE Photon. Technol. Lett. 17, 914–916 (2005). [CrossRef]

2.

L. Leng, S. Stulz, B. Zhu, L.E. Nelson, B. Edvold, L. Gruner-Nielsen, S. Radic, J. Centanni, and A. Gnauck, “1.6-Tb/s (160×10.7 Gb/s) transmission over 4000 km of nonzero dispersion fiber at 25-GHz channel spacing,” IEEE Photon. Technol. Lett. 15, 1153–1155 (2003). [CrossRef]

3.

Y. Frignac, G. Charlet, W. Idler, R. Dischler, P. Tran, S. Lanne, S. Borne, C. Martinelli, G. Velth, A. Jourdan, J. Hamaide, and S. Bigo,, “Transmission of 256 wavelength-division and polarization-division-multiplexed channels at 42.7Gb/s (10.2 Tb/s capacity) over 3×100km of TeraLight fiber,” in Proc. OFC’02 Postdeadline, Anaheim, CA, paper FC5 (2002).

4.

D. Borne, S. L. Jansen, E. Gottwald, P. M. Krummrich, G. -D Khoe, and H. Waardt, “1.6-b/s/Hz spectrally efficient 40 × 85.6-Gb/s transmission over 1,700 km of SSMF using POLMUX-RZ-DQPSK,” in Proc. OFC’06 Postdeadline, Anaheim, CA, paper PDP34 (2006).

5.

A. R. Chraplyvy, A. H. Gnauck, R. W. Tkach, J. L. Zyskind, J. W. Sulhoff, A. J. Lucero, Y. Sun, R. M. Jopson, F. Forghieri, R. M. Derosier, C. Wolf, and A. R. McCormick, “1-Tb/s transmission experiment,” IEEE Photon. Technol. Lett. 8, 1264–1266 (1996). [CrossRef]

6.

P. M. Hill, R. Olshansky, and W. K. Burns, “Optical polarization division multiplexing at 4 Gb/s,” IEEE Photon. Technol. Lett. 4, 500–502(1992). [CrossRef]

7.

S. G. Evangelides, L. F. Mollenauer, J. P. Gordon, and N. S. Bergano, “Polarization multiplexing with solitons,” J. Lightwave Technol. 10, 28–35 (1992). [CrossRef]

8.

K. Iwatsuki, K. Suzuki, S. Nishi, and M. Saruwatari, “80 Gb/s Optical soliton transmission over 80 km with time/polarization division multiplexing,” IEEE Photon. Technol. Lett. 5, 245–247(1993). [CrossRef]

9.

M. I. Hayee, M. C. Cardakli, A. B. Sahin, and A. E. Willner, “Doubling of bandwidth utilization using two orthogonal polarizations and power unbalancing in a polarization-division-multiplexing scheme,” IEEE Photon. Technol. Lett. 13, 881–883 (2001). [CrossRef]

10.

Y. Han and G. Li, “Experimental demonstration of direct-detection quaternary differential polarization-phase-shift keying with electrical multilevel decision,” Electron. Lett. 42, 109–111 (2006). [CrossRef]

11.

R. Noe, S. Hinz, D. Sandel, and F. Wust, “Crosstalk detection schemes for polarization division multiplex transmission,” J. Lightwave Technol. 19, 1469–1475 (2001). [CrossRef]

12.

G. Charlet, H. Mardoyan, P. Tran, A. Klekamp, M. Astruc, M. Lefranc¸ois, and S. Bigo, “Upgrade of 10 Gbit/s ultra-long-haul system to 40 Gbit/s with APol RZ-DPSK modulation format,” Electron. Lett. 41, 1240–1241 (2005). [CrossRef]

13.

X. Steve Yao, US patent application #60/575,127 (2005).

14.

L.E. Nelson and H. Kogelnik, “Coherent crosstalk impairments in polarization multiplexed transmission due to polarization mode dispersion,” Opt. Express 7, 350–361 (2000). [CrossRef] [PubMed]

15.

L.E. Nelson, T. N. Nielsen, and H. Kogelnik, “Observation of PMD-Induced coherent crosstalk in polarization-multiplexed transmission,” IEEE Photon. Technol. Lett. 13, 738–740 (2001). [CrossRef]

16.

D. Borne, N. E. Hecker-Denschlag, G. D. Khoe, and H. Waardt, “PMD-induced transmission penalties in polarization-multiplexed transmission,” J. Lightwave Technol. 23, 4004–4015 (2005). [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: February 20, 2007
Revised Manuscript: April 20, 2007
Manuscript Accepted: May 25, 2007
Published: June 1, 2007

Citation
X. Steve Yao, L.-S. Yan, B. Zhang, A. E. Willner, and Junfeng Jiang, "All-optic scheme for automatic polarization division demultiplexing," Opt. Express 15, 7407-7414 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-12-7407


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References

  1. S. Bhandare, D. Sandel, B. Milivojevic, A. Hidayat, A. A. Fauzi, H. Zhang, S. K. Ibrahim, F. Wust, and R. Noe, "5.94-Tb/s 1.49-b/s/Hz (40x2x2x40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km," IEEE Photon. Technol. Lett. 17, 914-916 (2005). [CrossRef]
  2. L. Leng, S. Stulz, B. Zhu, L.E. Nelson, B. Edvold, L. Gruner-Nielsen, S. Radic, J. Centanni, and A. Gnauck, "1.6-Tb/s (160x10.7 Gb/s) transmission over 4000 km of nonzero dispersion fiber at 25-GHz channel spacing," IEEE Photon. Technol. Lett. 15, 1153-1155 (2003). [CrossRef]
  3. Y. Frignac, G. Charlet, W. Idler, R. Dischler, P. Tran, S. Lanne, S. Borne, C. Martinelli, G. Velth, A. Jourdan, J. Hamaide, and S. Bigo, "Transmission of 256 wavelength-division and polarization-division-multiplexed channels at 42.7Gb/s (10.2 Tb/s capacity) over 3x100km of TeraLight fiber," in Proc. OFC’02 Postdeadline, Anaheim, CA, paper FC5 (2002).
  4. D. Borne, S. L. Jansen, E. Gottwald, P. M. Krummrich, G.-D, Khoe, and H. Waardt, "1.6-b/s/Hz spectrally efficient 40 x 85.6-Gb/s transmission over 1,700 km of SSMF using POLMUX-RZ-DQPSK," in Proc. OFC’06 Postdeadline, Anaheim, CA, paper PDP34 (2006).
  5. A. R. Chraplyvy, A. H. Gnauck, R. W. Tkach, J. L. Zyskind, J. W. Sulhoff, A. J. Lucero, Y. Sun, R. M. Jopson, F. Forghieri, R. M. Derosier, C. Wolf, and A. R. McCormick, "1-Tb/s transmission experiment," IEEE Photon. Technol. Lett. 8, 1264-1266 (1996). [CrossRef]
  6. P. M. Hill, R. Olshansky, and W. K. Burns, "Optical polarization division multiplexing at 4 Gb/s," IEEE Photon. Technol. Lett. 4, 500-502 (1992). [CrossRef]
  7. S. G. Evangelides, L. F. Mollenauer, J. P. Gordon, and N. S. Bergano, "Polarization multiplexing with solitons," J. Lightwave Technol. 10, 28-35 (1992). [CrossRef]
  8. K. Iwatsuki, K. Suzuki, S. Nishi, and M. Saruwatari, "80 Gb/s Optical soliton transmission over 80 km with time/polarization division multiplexing," IEEE Photon. Technol. Lett. 5, 245-247 (1993). [CrossRef]
  9. M. I. Hayee, M. C. Cardakli, A. B. Sahin, and A. E. Willner, "Doubling of bandwidth utilization using two orthogonal polarizations and power unbalancing in a polarization-division-multiplexing scheme,"IEEE Photon. Technol. Lett. 13, 881-883 (2001). [CrossRef]
  10. Y. Han and G. Li, "Experimental demonstration of direct-detection quaternary differential polarization-phase-shift keying with electrical multilevel decision," Electron. Lett. 42, 109-111 (2006). [CrossRef]
  11. R. Noe, S. Hinz, D. Sandel, and F. Wust, "Crosstalk detection schemes for polarization division multiplex transmission," J. Lightwave Technol. 19, 1469-1475 (2001). [CrossRef]
  12. G. Charlet, H. Mardoyan, P. Tran, A. Klekamp, M. Astruc, M. Lefranc¸ois and S. Bigo, "Upgrade of 10 Gbit/s ultra-long-haul system to 40 Gbit/s with APol RZ-DPSK modulation format," Electron. Lett. 41, 1240-1241 (2005). [CrossRef]
  13. X. Steve Yao, US patent application #60/575,127 (2005).
  14. L.E. Nelson, and H. Kogelnik, "Coherent crosstalk impairments in polarization multiplexed transmission due to polarization mode dispersion," Opt. Express 7, 350-361 (2000). [CrossRef] [PubMed]
  15. L.E. Nelson, T. N. Nielsen, and H. Kogelnik, "Observation of PMD-Induced coherent crosstalk in polarization-multiplexed transmission," IEEE Photon. Technol. Lett. 13, 738-740 (2001). [CrossRef]
  16. D. Borne, N. E. Hecker-Denschlag, G. D. Khoe, and H. Waardt, "PMD-induced transmission penalties in polarization-multiplexed transmission," J. Lightwave Technol. 23, 4004-4015 (2005). [CrossRef]

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