Open Access
Add to BibSonomyAbstract
We propose a new automatic optical polarization demultiplexing scheme for polarization division multiplexed (PDM) signals, which uses the radio frequency (RF) power from a low frequency RF power detector as a control signal. This scheme is based on the intrinsic characteristics of PDM signals and does not need to add a special signal at a PDM transmitter. The effectiveness of this demultiplexing method is experimentally demonstrated in a 2×10 Gb/s on-off-keying (OOK) PDM transmission system.
©2009 Optical Society of America
1. Introduction
Polarization division multiplexing (PDM), which simultaneously transmits two channels of an identical wavelength in orthogonal states of polarization (SOPs), can double the spectral efficiency of fiber-optic communication systems, therefore attracts much attention [1–5]. As the SOP of a signal changes randomly with wavelength and time and cannot be maintained in a transmission link, automatic polarization demultiplexing has to be performed at the receiver side to separate the two polarization-distinguished channels, either in the electronic domain for coherent detection or in the optical domain for direct detection. Compared with electronic polarization demultiplexing in coherent detection, optical polarization demultiplexing has its own advantages. For example, it does not need high-speed digital signal processing and is almost independent of bit rates, which could be desired features in some high capacity applications.
Previous techniques for automatic demultiplexing include: (i) imposing radio-frequency (RF) tones at the transmitter side using amplitude modulation, phase modulation or frequency modulation to identify the two polarizations [6–9]; (ii) using different power levels for the two polarizations at transmitter [10–11]; (iii) using RF power over the whole RF signal bandwidth as the feedback signal [12]. Each technique has at least one of these drawbacks: (i) extra penalty is induced for the signal at one of the polarizations before transmission; (ii) the transmitter needs to be delicately designed to impose physical difference between channels; (iii) high-speed electronics are needed to process the demultiplexing control signal.
In this paper, we propose a new automatic polarization demultiplexing scheme for PDM signals in optical domain without any special treatment of the signals at the transmitter side and only low frequency electronics are needed to control the demultiplexing process.
2. Analysis of the theoretical approach
A conceptual diagram of a PDM system is shown in Fig. 1. The PDM signal consists of two orthogonal channels A⃗ and B⃗, which are from the same continuous wave (CW) laser. The 3dB polarization-maintaining (PM) coupler splits the CW carrier with equal power, and then the two parts of the CW carrier are modulated with data A and B, respectively. Before the two channels are multiplexed at the polarization beam combiner (PBC), the differential time delay between the two branches is set to be τ 0. We use non-return-to-zero (NRZ) on-off-keying (OOK) modulation as an example in the analysis. For simplicity, we assume that each channel is linearly polarized and PDL and PMD are neglected. At the receiver side, a polarization controller (PC) and a polarization beam splitter (PBS) are applied to demultiplex the two channels, and then the output electrical field at port A of the PBS can be expressed as:
where MA(t) and MB(t) represent the modulation envelopes of the channels A⃗ and B⃗, respectively, θ is the angle between the SOP of the channel A⃗ and the polarizer at port A of the PBS in the Jones space, E 0 is the amplitude of the optical fields of both channels, ω 0 is the center frequency of the CW carrier, and ϕ(t) is the random phase fluctuation. We assume ideal rectangular pulse shape, which has {MA(t),MB(t)} ∈ {0,1} and (〈∙〉 denotes the average). The photo-detector converts EAout(t) into the photocurrent. By virtue of the Wiener-Khintchine theorem, the power spectrum of the photocurrent is given by the Fourier transform of its autocorrelation function, which can be expressed as [13]
where e is the electron charge, σ is the detector responsivity (Typically 0.5~4A/W at 1550nm), GE (1)(0) and GE (2)(τ) are the first and second order correlation function of the optical field as
By substituting (3) into (2) and then applying the Fourier transform, the spectrum of the photocurrent can be obtained [14]
where Sd(ω) stands for direct intensity detection term, Sbeat(ω)is the optical beating term, Sshot(ω) denotes the shot noise term and it is the Fourier transform of GE (1)(0). Since Sshot(ω) ≪ Sbeat(ω), S(ω) can be expanded as [13–15]
where Δω is the laser linewidth, SM(ω)is the spectrum of the modulation envelope
and Scorr(ω) is the spectrum generated from the beating of the correlated optical carrier of the two channels and it can be expressed as
Omitting the direct current component, the dependence of S(ω) on θ is shown in Fig. 2, in which the phase mismatch of the optical carriers of the two channels is assumed as cos(ω 0 τ 0) = 0.5. For θ = 0° or 90°, the power of the RF spectrum is the minimal, and for θ = 45°, it is the maximal. The inset of Fig. 2 shows both the theoretical and experimental results of the frequency components within 400MHz of the RF spectrum. In other modulation schemes the exact shape of SM(ω) will be changed and it will lead to the change of S(ω), but there always is a power difference of the RF spectrum between different launching angles.
Thus, by using the band-pass filter (indicated as ‘Extraction Window’ in Fig. 2) to select the particular range of the RF spectrum and applying electrical amplification, the power level of the newly generated RF signal can be treated as a direct indication of the misalignment between the PDM signals and the PBS. The automatic demultiplexing is achieved by continually adjusting the polarization controller before the PBS to minimize the RF signal.
3. Experiment results
The experiment setup for demonstration of the demultiplexing scheme is shown in Fig. 3. A CW light is modulated with 231-1 pseudo-random binary sequence (PRBS) 10-Gb/s data to generate a 10-Gb/s NRZ-OOK signal. The signal is split by a 3dB coupler into two branches. A delay line (DL) measured as 26ns is inserted in one branch to make the bit sequences of the two channels synchronized (the relative time delay τ 0 = nTb, where Tb is the bit period and n is an integer) in the time domain to maximize the interferometer crosstalk, and a variable optical attenuator (VOA) is applied in the other branch to equalize the power of the two channels. In each branch the signal is adjusted with a PC to get the proper polarization, and then is combined with a PBC.
Transmission is performed in a four span all-Raman amplified straight line system. Each span consists of a 100-km non-zero dispersion shifted fiber (NZDSF) and a dispersion compensation fiber (DCF) with residual dispersion per span of 30ps/nm. Neither pre-compensation nor post-compensation is used. Both the NZDSF and DCF are pumped at transparency. At each span, the signal power launched into the NZDSF is set to be -5dBm, and the output of the NZDSF is set to be -7dBm. After transmission, the signal is loaded with amplified spontaneous emission (ASE) noise to get a certain optical signal-to-noise ratio (OSNR) and selected with a 5-nm ASE filter and a 0.2-nm tunable grating filter (JDSU TB9). The selected signal is amplified to about 5dBm and then sent to the demultipelxer.
As a part of the demultiplexer, a commercially available PC, Adaptif A3000 (also called polarization stabilizer), with an analog control input port is used. The PC is placed in front of the PBS for stabilizing the SOPs of the PDM signals. The output optical signal of one PBS port is firstly converted to electrical signal by a 15GHz photo-detector, and then the output RF signal is filtered within 10kHz to 1MHz and amplified, and detected with a low frequency RF power detector to generate the control signal, as shown in Fig. 4. The control signal is sent to the analog input port to control the PC. The SOP stabilizing takes less than 5s with the current PC and the control program.
The RF spectrum of the photocurrent is shown in Fig. 5, in the cases of two critical launching angles between the PDM signals and the PBS. Within the certain bandwidth, the RF power level is maximal when θ = 45° or minimal when θ = 0°/90°, and the power ratio for this two cases is larger than 10dB. The upper-right insets of Fig. 5 show the eye diagrams of the output optical signal at one PBS port.
The bit-error rate (BER) curves versus OSNR with 0.1nm bandwidth resolution are shown in Fig. 6. For comparison, we also give the results of using the RF power from a wideband RF detector for feedback control [12]. In the case of using wideband RF power detector, we insert a RF detector with a bandwidth up to 8000MHz between the photo-detector and the band-pass filter in Fig. 4. The results measured with only manual PC adjustment are marked as ‘Manual’. It shows that in the back-to-back condition the penalty is zero at 10-3 BER and ~1dB at 10-9 BER, and in the 400km transmission condition the penalty is ~0.2dB at 10-3 BER and just slightly above 1dB at 10-9 BER. Using the wideband RF power or low frequency RF power as feedback control does not have significant differences in performance. This is because most of the interferometric beating effect is concentrated at low frequencies. Note that the exact structure the RF spectrum is affected by modulation format and the time delay between the two polarizations of a PDM signal. Therefore, for different transmitters the best spectrum extraction window varies. The detailed analysis of transmitter dependence of the interfering term will be investigated in our future work.
Note that additional information is needed in a real system to identify each polarization. For example, one can add additional low-speed phase or amplitude modulation on one polarization in the optical domain, or use a special coding in one polarization and do the channel identification in the electrical domain.
4. Conclusion
We have presented an all optical scheme for automatic demultiplexing PDM signals, based on the processing of the inter-channel correlated fields. This scheme does not need any modification of an ordinary PDM transmitter and only requires some low-frequency electrical components at the receiver side. We demonstrated the scheme with a 2×10Gb/s NRZ-OOK PDM system, and the experiment results showed that after a 400km transmission, the penalty induced by the automatic polarization demultiplexing scheme was only ~0.2dB at BER of 10-3 and ~1dB at BER of 10-9.
Acknowledgments
This work was done at Crawford Hill Laboratory, Alcatel-Lucent, USA. The authors are grateful to acknowledge valuable discussions with R. W. Tkach, A. H. Gnauck, L. Möller, and I. Kang. Zinan Wang acknowledges the support from China Scholarship Council, National High-tech Research and Development Program of China (2007AA03Z447) and National Basic Research Program of China (2003CB314900).
References and links
1. A. H. Gnauck, G. Charlet, P. Tran, P. J. Winzer, C. R. Doerr, J. C. Centanni, E. C. Burrows, T. Kawanishi, T. Sakamoto, and K. Higuma, “25.6-Tb/s WDM transmission of polarization-multiplexed RZ-DQPSK signals,” J. Lightwave Technol. 26, 79–84 (2008). [CrossRef]
2. C. Xie, S. Chandrasekhar, and X. Liu, “Impact of Inter-Channel Nonlinearities on 10-Gbaud NRZ-DQPSK WDM Transmission over Raman Amplified NZDSF Spans,” in Proc. ECOC’07, Berlin, Germany, paper Th. 10.4.4 (2007).
3. G. Charlet, J. Renaudier, M. Salsi, H. Mardoyan, P. Tran, and S. Bigo “Efficient mitigation of fiber impairments in an ultra-long haul transmission of 40 Gbit/s polarization-multiplexed data by digital processing in a coherent receiver,” in Proc. OFC’2007, Anaheim, CA, USA, 2007, paper PDP17.
4. 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]
5. Z. Wang and C. Xie, “PMD and PDL Tolerance of Polarization Division Multiplexed Signals with Direct Detection,” in Proc. ECOC’08, Brussels, Belguim, paper We.3.E.2 (2008).
6. P. Martelli, P Boffi, M. Ferrario, L. Marazzi, P. Parolari, S. M. Pietralunga, A. Righetti, R. Siano, M. Torregiani, and M. Martinelli, “Endless Polarization Stabilizer for High Bit-rate Polarization-Division Multiplexed Optical Systems,” in Proc. OFC’2008, San Diego, CA, USA, 2008, paper JWA66.
7. P. S. Cho, G. Harston, C. J. Kerr, A. S. Greenblatt, A. Kaplan, Y. Achiam, G. Levy-Yurista, M. Margalit, Y. Gross, and J. B. Khurgin, “Investigation of 2-b/s/Hz 40-Gb/s DWDM transmission over 4×100 km SMF-28 fiber using RZ-DQPSK and polarization multiplexing,” IEEE Photon. Technol. Lett. 16, 656–658 (2004). [CrossRef]
8. 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]
9. N. E. Hecker, E. Gottwald, K. Kotten, C.-J. Weiske, A. Schopflin, P. M. Krummrich, and C. Glingener, “Automated polarization control demonstated in a 1.28Tbit/s (16×2×40 Gbit/s) polarization multiplexd DWDM field trial,” in Proc. ECOC’01, Amsterdam, Netherlands, paper Mo.L.3.1 (2001).
10. 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]
11. X. S. Yao, L. -S. Yan, B. Zhang, A. E. Willner, and J. Jiang, “All-optic scheme for automatic polarization division demultiplexing,” Opt. Express 15, 7407–7414 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-12-7407. [CrossRef] [PubMed]
12. B. Milivojevic, A. F. Abas, A. Hidayat, S. Bhandare, D. Sandel, R. Noé, M. Guy, and M. Lapointe, “1.6-b/s/Hz 160-Gb/s 230-km RZ-DQPSK polarization multiplex transmission with tunable dispersion compensation,” IEEE Photon. Technol. Lett. 17, 495–497 (2005). [CrossRef]
13. H. Z. Cummins and H. L. Swinney, “Light beating spectroscopy,” Prog. Opt. 8, 133–200 (1970). [CrossRef]
14. P. B. Gallion and G. Debarge, “Quantum phase noise and field correlation in single frequency semiconductor laser systems,” IEEE J. Quantum Electron. 20, 343–349 (1984). [CrossRef]
15. M. Nazarathy, W. V. Sorin, D. M. Baney, and S. A. Newton, “Spectral analysis of optical mixing measurements,” J. Lightwave Technol. 7, 1083–1096 (1989). [CrossRef]
