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Polarization interleaving to reduce inter-channel nonlinear penalties in polarization multiplexed transmission D. van den Borne, S. L. Jansen, G. D. Khoe, H. de Waardt
COBRA institute, Eindhoven University of Technology, The Netherlands email:
[email protected],
[email protected],
[email protected],
[email protected]
S. Calabró, N. E. Hecker-Denschlag
Siemens AG, ICN, Carrier Products, D-81539 Munich, Germany email:
[email protected],
[email protected]
Abstract: We investigate through simulations and experiments inter-channel nonlinear penalties in 2x10Gbit/s NRZ polarization-multiplexed transmission. We show that the inter-channel nonlinear penalties can be partially mitigated by polarization interleaved transmission of the polarization-multiplexed channels. 2004 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation; (060.4370) Nonlinear optics, fibers;
1. Introduction Increasing the spectral efficiency of optical transmission by using alternative modulation formats is a topic which received considerable research interest in recent years. Alternative modulation formats allow for a higher spectral efficiency through a narrower spectrum, e.g. duobinary and filtered CS-RZ, or multilevel transmission such as DQPSK and polarization multiplexing (POLMUX). POLMUX is a well-known technique to increase spectral efficiency and can be used to effectively double fiber capacity. It has been used successfully in both record breaking laboratory experiments [1,2] as well as field trails [3]. However POLMUX has so far not found its way into commercial optical transmission systems, mainly due to its vulnerability to PMD [4] and multi-channel nonlinear transmission penalties [5,6]. In this paper we show that polarization interleaving of the POLMUX channels can reduce the influence of interchannel nonlinear transmission penalties. We show through simulations that transmission tolerances can be increased considerably. Experimental results with different numbers of co-propagating channels and channel spacings confirm the potential of the proposed method. 2.
Interleaved polarization multiplexing
In non-POLMUX transmission phase insensitive detection is used, resulting in a low-pass behavior characterizing the influence of XPM [7]. The low-pass behavior of XPM leads to a penalty which scales strongly with the channel spacing. For POLMUX transmission this is not the case, as the nonlinear phase shift results in a change in the state of polarization (SOP). The nonlinear phase shift depends on the transmitted bit-sequence in the co-propagating channels which leads to a noise-like change of the SOP and hence depolarization. This cross polarization modulation (XPolM) penalty dramatically influences polarization de-multiplexing at the receiver and dominates over the normal XPM penalty in multi-channel POLMUX transmission [5]. Note that XPolM penalties are not POLMUX specific but arise in all modulation formats which use polarization sensitive detection, as for example shown in [8].
Fig. 1. Simulation results showing eye opening penalty (EOP), for transmission over 100km SSMF with different numbers of copropagating channels and a 100GHz channel spacing (a) POLMUX (b) non-POLMUX (c) interleaved POLMUX transmission.
In POLMUX transmission the XPolM penalty depends on the total launch power of all co-propagating channels and is independent of the channel spacing. Fig. 1a shows the influence of XPolM on POLMUX signals after transmission over 100km SSMF with a 100GHz channel spacing. The simulation results include the influence of
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cross-polarization nonlinear interaction but neglect the influence of fiber birefringence. Inter-channel nonlinear transmission penalty increase with the number of co-propagating channel and scales accurately with the total launch power into the fiber. For non-POLMUX transmission on the contrary, as shown in figure 1b, only a modest XPM penalty is evident with an increasing number of co-propagating channels. When comparing single channel and 9 channel WDM transmission, for a 1dB eye opening penalty (EOP), the total inter-channel nonlinear penalty is significantly smaller for non-POLMUX (3dB) versus POLMUX (11.2dB) transmission. The XPolM transmission penalty in POLMUX transmission therefore decreases transmission tolerances to such an extent that the suitability of POLMUX transmission for long-haul transmission is questionable. Interleaved POLMUX transmission deterministically spreads out the SOP of the channels over the Poincaré sphere, by changing the SOP of the even wavelength channels with respect to the uneven channels. Polarization interleaved POLMUX transmission bears a strong similarity to polarization interleaved transmission for nonPOLMUX transmission, where adjacent channels are transmitted in orthogonal SOP’s. This reduces the influence of inter-channel transmission penalties such as XPM and FWM. In polarization interleaved POLMUX transmission a signal is transmitted in both orthogonal SOP’s of each co-propagating wavelength channel. The decrease in transmission penalties is therefore based on the observation that the sum of the Stokes vectors of two co-propagating wavelength channels is constant during transmission, which cancels the influence of XPolM. Using the Manakov equation [9], one can show that the nonlinear polarization shift induced on a channel A through a second copropagating channel B is equal to ∂SA/∂z = 8/9γP0(SA x SB), where Si denotes the Stokes vector of channel i, γ the fiber nonlinearity, P0 the channel power and z is the transmission distance. Assuming a sum vector S0 with S0 = ½(SA+ SB) this can be shown to be equal to ∂SA/∂z = 16/9γP0(SA x S0), which only depends on SA and the sum vector S0. When channels A and B now have an orthogonal SOP, the sum vector S0 equals zero and the nonlinear polarization shift only depends on channel A itself. Using a similar argument it can be shown that also for multiple co-propagating channels polarization interleaving minimizes the variance in nonlinear polarization shift. The results depicted in Fig. 1c illustrate the influence of polarization interleaving on POLMUX transmission. The linear polarization components of the even channels are 90° rotated with respect to the uneven channels, where the circular polarization component is equal for all channels. The channel launch power resulting in a 1dB EOP for 9 co-propagating channels is increased from 2.4dB to 10.2dB when comparing non-interleaved and interleaved POLMUX transmission. Note than for polarization interleaved POLMUX transmission the nonlinear penalty does not longer scale with the total launch power but with the channel power, similar to non-POLMUX transmission. 3.
Experimental setup
We experimentally verified the principle of interleaved POLMUX transmission with the setup depicted in Fig. 2. A Mach-Zehnder interferometer NRZ modulates between 3 and 9 CW channels, generated by a distributed feedback laser and spaced on a 50GHz grid, at a 9.95328Gbit/s bit rate (PRBS 231-1). The interchannel nonlinear transmission penalties are measured for the center channel at 1551.6nm. Using the POLMUX modulator, discussed in more detail in [3], the 10Gbit/s NRZ modulated signals are subsequently polarization-multiplexed to 2x10Gbit/s.
Fig. 2. Experimental setup, PM phase modulator, PC polarization controller, PBS polarization beam splitter.
The Mach-Zehnder interferometer is polarization sensitive; hence after modulation all channels have an identical linear polarization component. The circular polarization component is not controlled due to the unavailability of circular polarizer. Consequently, the channels are launched with a random circular polarization state. The POLMUX channels are then polarization interleaved using a 50GHz interleaver by splitting the channels into two subsets and afterwards combining all channels using a second 50GHz interleaver. During the measurement the SOP of one of the subsets is adjusted such that the measured bit-error-rate (BER) is minimized. The signals are transmitted over 100km SSMF and matching DCF. To produce sufficient nonlinear interactions without relying on long-haul transmission an equal launch power is used for both the SSMF and DCF fiber. After transmission the center channel is filtered out using a narrowband 0.2nm optical filter, subsequently manually polarization de-multiplexed to 10Gbit/s and detected using a standard 10Gbit/s receiver and BER tester. 4.
Measurement results
Fig. 3a shows the influence of polarization interleaved transmission, measured for 3, 5 and 9 co-propagating POLMUX channels and a 50GHz channel spacing. In contrast to the simulation results the measured BER scales
JWA41 with the number of co-propagating channels both with and without polarization interleaved transmission. For a 10-6 BER, a power gain of approximately 1dB is measured. But with an increasing number of co-propagating channels the beneficial influence of polarization interleaved transmission decreases. The influence of the channel spacing is evident from the measurement results depicted in Fig. 3b, where we compare a 50GHz with a 300GHz channel spacing for 3 co-propagating channels. In non-interleaved transmission the XPolM penalty is not dependent on the channel spacing and for both a 50GHz and 300GHz channel spacing a similar nonlinear tolerance is measured. However with interleaved transmission the power gain for a 10-6 BER decreases from 1.2dB to 0.2dB with an increase in channel spacing from 50GHz to 300GHz. Comparing figures 3a and 3c shows the difference between interleaved POLMUX and non-POLMUX transmission. Clearly, the number of co-propagating channels influences the transmission penalty much less, due to the absence of an XPolM penalty. The power gain due to polarization interleaved transmission is 0.9, 0.6 and 0.5dB for respectively 3, 5 and 9 channels, smaller than the results obtained for POLMUX transmission.
Fig 3. Experimental results depicting channel power versus BER for both interleaved (⊥) and non-interleaved (||) transmission over 100km of SSMF (a) POLMUX transmission with a 50GHz channel spacing (b) POLMUX transmission with 3 co-propagating channels (c) non-POLMUX transmission with a 50GHz channel spacing.
The experimentally measured power gain due to polarization interleaved transmission is smaller than predicted by the simulation results. Partly this is the result of fiber birefringence, which changes the SOP as a function of wavelength. This randomizes the SOP of co-propagating channels and decreases the influence of polarization interleaving. In interleaved POLMUX transmission not only the nonlinear penalty caused by the adjacent channels but the penalty resulting from all co-propagating channels is decreased. Hence a larger channel spacing or wider transmitted spectrum decreases its influence in the presence of fiber birefringence. 5. Conclusions We have shown through simulations and measurements the influence of inter-channel nonlinear XPolM transmission penalties on POLMUX NRZ transmission. For the first time we showed that XPolM penalties can be reduced in POLMUX transmission through polarization interleaved transmission and we measured about a 1dB power gain. Controlling the circular polarization state could further improve the obtained results and increase POLMUX transmission tolerances. Fiber birefringence reduces the beneficial influence of interleaved POLMUX transmission through a wavelength dependent change of the SOP. However experimental results clearly show that proper control of the launched SOP can extend nonlinear transmission tolerances in POLMUX NRZ transmission. 6. References
[1] A. Chraplyvy, A. Gnauck, R. Tkach, et al., “1 Tb/s transmission experiment ”, IEEE Photon. Technol. Lett., vol. 8, pp. 1264-1266 (1996). [2] S. Bigo, Y. Frignac, G. Charlet, et al., “10.2 Tbit/s (256x42.7 Gbit/s PDM/WDM transmission over 100 km Terraligth fiber with 1.28 bit/s/Hz spectral efficiency ”, in Proc. OFC 2001, PD25, pp.1-3. [3] N. E. Hecker, E. Gottwald, K. Kotten, et al., ”Automated polarization control demonstrated in a 1.28 Tbit/s (16x2x40Gbit/s) polarization multiplexed DWDM field trail ”, in Proc. ECOC 2001, vol. 1, pp. 86-87. [4] L.E. Nelson and H. Kogelnik, “Coherent crosstalk impairments in polarization multiplexed transmission due to polarization mode dispersion”, Optics Express, vol. 7, pp. 350-361 (2000). [5] D. van den Borne, N. E. Hecker-Denschlag, G.-D. Khoe and H. de Waardt, “Cross phase modulation induced depolarization penalties in 2x10Gbit/s polarization-multiplexed transmission”, in Proc. ECOC 2004, Mo4.5.5, pp. 1-3. [6] L. Mollenauer, J. Gordon and F. Heismann, “Polarization scattering by soliton-soliton collisions”, Opt. Lett., vol.20, pp.2060-2062 (1995). [7] C. Furst, J.-P. Elbers, C. Sheerer and C. Glingener, “Limitations of dispersion-managed dwdm systems due to cross-phase modulation”, in proc. LEOS 2000, vol.1, pp. 23-24. [8] B. C. Collings, and L. Boivin, “Nonlinear polarization evolution induced by cross-phase modulation and its impact on transmission systems”, IEEE Photon. Technol. Lett, vol.12, pp. 1582-1584 (2000). [9] D. Wang and C. R. Menyuk, “Reduced model of the evolution of the polarization states in wavelength-division-multiplexed channels”, Opt. Lett., vol. 23, pp. 1677-1679 (1998).
