Applications of optical phase conjugation in robust optical transmission systems S.L. Jansen1, D. van den Borne2, P.M. Krummrich3*, S. Spälter3, H. Suche4, W. Sohler4, G.D. Khoe2, H. de Waardt2, I. Morita1 and H. Tanaka1 1: KDDI R&D Laboratories, Saitama, Japan, email:
[email protected] 2: Eindhoven, University of Technology, the Netherlands 3: Nokia Siemens Networks, Munich, Germany 4: Applied Physics, University of Paderborn, Germany * At the time of this work, the author is now with the University of Paderborn
ABSTRACT In this paper, WDM transmission experiments are discussed showing simultaneous compensation of nonlinear effects and chromatic dispersion through optical phase conjugation (OPC). The performance of OPC and DCF for chromatic dispersion compensation are compared in a wavelength division multiplexed (WDM) transmission link with 50-GHz spaced 42.8-Gb/s RZ-DQPSK modulated channels. The feasible transmission distance for a Q-factor ~10 dB is limited to approximately 5,000 km and 3,000 km for the OPC and the DCF based configuration, respectively. When the Q-factor as a function of the transmission distance is observed, at shorter distances, the Q-factor of the OPC based configuration is about 1.5 dB higher than that of the DCF based transmission system. Up to 2,500-km transmission a linear decrease in Q is observed for both configurations. After 2,500-km transmission, the Q-factor of the DCF based configuration deviates from the linear decrease whereas the OPC based performance is virtually unaffected. Keywords: Fiber optics communications; phase conjugation; spectral inversion; nonlinear transmission; dispersion compensation. 1. INTRODUCTION Optical phase conjugation (OPC) is a well know technique to compensate for deterministic impairments in fiber-optic transmission systems. OPC takes the complex conjugate from the phase response of the signal without changing the amplitude response. Several techniques have been proposed to phase conjugate an optical signal. Most of the realized OPC units are based on one of the following processes: four-wave mixing (FWM) in highly nonlinear fibers (HNLF) [1], FWM in a semiconductor optical amplifier (SOA) [2], FWM in silicon waveguides [3], parametric difference frequency generation (DFG) in periodicallypoled aluminum gallium arsenide (AlGaAs) [4] or parametric DFG in a periodically-poled lithium-niobate (PPLN) waveguide [5]-[7]. Advantages of a PPLN waveguide for OPC are that negligible noise is added to the phase conjugated signal and that it allows for high conversion efficiencies [5]. As well, since the PPLN waveguide is instantaneous and phase sensitive in its response, the OPC through a PPLN waveguide is transparent to data rate and modulation format [7] and no third order nonlinear impairments such as SPM and cross-phase modulation (XPM) occur in the phase conjugation process. By placing a phase conjugator in the middle of a transmission link, OPC regenerates the signal indirectly: deterministic phase impairments that occurred in the first part of the transmission link (before conjugation) are cancelled by impairments that occur in the second part of the link (after conjugation). Initially, OPC was
proposed to compensate for chromatic dispersion [8]. In this configuration the inline dispersion compensating fibers (DCFs) are omitted, which saves the extra loss through DCFs potentially resulting in a higher optical signal-to-noise ratio (OSNR) after transmission. As well, as can be seen in Fig. 1, a simplified design of the inline dispersion map and inline amplifiers is realized through mid-link OPC. Since the dispersion accumulates linearly along the transmission line, the phase conjugator needs to be placed in the middle of the link to obtain full dispersion compensation. Therefore OPC for chromatic dispersion compensation is typically called “mid-link OPC”. It has been shown however that when the residual dispersion is compensated for, deviations from the middle of the transmission line give negligible penalty [9]. Apart from chromatic dispersion compensation, OPC can be employed to compensate for deterministic nonlinear impairments resulting from the Kerr effect. The compensation of self phase modulation (SPM) induced impairments has extensively been studied [10, 11]. At high data-rates, for example 40 Gbit/s and more, transmission is mostly in the pseudo-linear regime, where intra-channel nonlinear impairments dominate the transmission penalty for state-of-the-art systems [12]. Several experiments have been reported showing that intra-channel nonlinear impairments can be compensated for by using OPC [13-15], significantly extending transmission reach. Recently, it has been shown that OPC can compensate as well for impairments resulting from nonlinear phase noise [16, 17]. Nonlinear phase noise can severely impair the performance of phase shift keyed transmission systems [18]. Hence, the application of OPC can relax link design. Simultaneous compensation of nonlinear effects and chromatic dispersion can be realized with a single OPC unit and has been shown in [13, 19]. Next generation transmission systems will likely employ advanced modulation formats. A modulation format that recently received a lot of interest is return-to-zero differential quadrature phase-shift-keying (RZ-DQPSK). RZ-DQPSK has a favorable spectral width making it robust against narrow band filtering. Furthermore it has a higher polarization mode dispersion (PMD) and chromatic dispersion tolerance at the same effective data rate as binary modulation, possibly easing deployment of 40 Gbit/s transmission over legacy fiber [20, 21]. Therefore we believe that especially for 40 Gbit/s transmission, the RZ-DQPSK modulation format offers significant advantages in providing a robust solution for high-capacity long-haul transport. A concern however with RZ-DQPSK is possible impairments by SPM induced nonlinear impairments, such as nonlinear phase noise. Since OPC can compensate for SPM induced nonlinear impairments, we investigated the combination of OPC with RZ-DQPSK. In this paper, we compare the performance of an OPC based system with the performance of a ‘conventional’ DCF based transmission line for 42.8 Gbit/s RZ-DQPSK. 2. EXPERIMENTAL SETUP The experimental setups of the DCF and the OPC based configurations are depicted in Fig. 1. At the transmitter, 52 wavelengths on a 50-GHz ITU grid are modulated using two parallel modulator chains for separate modulation of the even and odd channels. The RZ-DQPSK modulator chain starts with a MachZehnder modulator (MZM) carving a pulse with a 50% duty cycle. The second modulator is an integrated DQPSK modulator with two parallel MZMs within a super Mach-Zehnder structure. The relative phase shift between the two parallel modulators is π/2. A 21.4-Gbit/s data streams is created by electrically multiplexing two 10.7-Gbit/s PRBS signals with a length of 215-1 and a relative delay of 16 bits. This 21.4-Gbit/s data stream is split and fed to both inputs of the 42.8-Gbit/s DQPSK modulator with a relative delay of 10 bits of the bit sequences for de-correlation. After modulation, the even and odd channels are combined with a 50-GHz interleaver. A polarization beam splitter (PBS) ensures that all channels are copolarized for worst-case interaction.
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Fig. 1: Experimental setup The re-entrant re-circulating loop consists of three 94.5-km spans of SSMF with a chromatic dispersion of ~16 ps/nm/km and an average span loss of 21.5 dB (including 1.5 dB loss of couplers required for Raman amplification). The loss of the SSMF spans is compensated for by using a hybrid Raman/EDFA structure for signal amplification. The average on/off Raman gain of the backward pumped Raman pumps is ~12 dB. A loop-synchronous polarization scrambler (LSPS) is used to reduce the statistical correlation of loop-induced polarization effects. Power equalization of the WDM channels is provided by a channel based DGE with an effective bandwidth of 42 GHz, hence spectral filtering of the signals occurs with every re-circulation. In the OPC based configuration, no DCF is used after each span. Mid-link, after 8 circulations, the signals are fed through the re-entrant branch for OPC. First, the 26 channels used to balance the signal in the amplifiers (from 1530.8nm to 1540.6nm) are removed using a band selection filter (BSF). The 26 remaining channels (from 1546.1 nm to 1556.1 nm) are phase conjugated. OPC is realized by second harmonic generation and DFG in a periodically-poled lithium-niobate (PPLN) waveguide. A polarization-diversity PPLN subsystem is used to overcome the polarization dependence of the PPLN waveguide, with a polarization dependent loss of less than 0.4 dB [19]. The output from an ECL laser at 1543.4 nm, amplified to 500 mW, is used as pump signal and the input power per channel is approximately 10 mW before the polarization beam splitter. The optical spectrum after conjugation is depicted in Fig. 2a, showing a conversion efficiency of -7.2 dB. After the PPLN subsystem, the pump is suppressed through a pump block filter (PBF) and the original channels are removed using another BSF. Finally, the phase conjugated channels (ranging from 1530.8 nm to 1540.6 nm) of the PPLN subsystem are recombined with the original channels (ranging from 1546.1 nm to 1556.1 nm) to balance the signal propagating through another 8 circulations in the re-circulating loop. The optimum input power for the OPC configuration was determined to be -3 dBm/channel [19]. In the DCF based configuration, the PPLN for chromatic dispersion compensation is removed and DCF modules are inserted after each span, hence creating a periodic dispersion map. The pre-compensation and inline undercompensation is 1020 ps/nm and ~33 ps/nm/span, respectively. 20% of the DCF is placed between the Raman pump and the first stage of the inline amplifier to balance the DCF insertion loss. In order to optimize the performance of the DCF based transmission system, the optical input power in the SSMF, the inline dispersion map and the pre-compensation is optimized at 2,260-km transmission distance. The optimal input power is determined to be -3.5 dBm/channel. This optimal input power is 0.6 dB lower than the input power used in the OPC based transmission experiment (-3 dBm). The optimal inline
dispersion map is found to be an under-compensation of around 33 ps/nm/span. The pre-compensation is found optimal at -1020 ps/nm [22].
Fig. 2: Optical spectra; (a) after the PPLN subsystem (b) at the receiver after transmission over 4,500 km (0.01nm resolution bandwidth) At the receiver, the chromatic dispersion is optimized per channel using a variable dispersion compensator and the channel selected with a 0.2-nm channel-selection filter (CSF) to select the desired channel. The signal is split and one part is used for clock recovery. The other part is fed to a two-bit (94 ps) MachZehnder delay interferometer (MZDI) to convert the signal from phase to intensity modulation followed by a balanced detector. The signal is de-multiplexed to 10.7 Gbit/s and evaluated using a BER-tester. The performance of the two tributaries is averaged through loop operation; hence only one 10.7-Gbit/s tributary of the 21.4-Gbit/s signal is measured. 3. EXPERIMENTAL RESULTS The performance of 42.8 Gbit/s RZ-DQPSK transmission is evaluated after 4,500 km transmission for the 26 phase conjugated channels. In Fig. 3, the Q-factors of both the in-phase and quadrature channels are depicted. Both in-phase and quadrature components show similar performance, with a slightly better performance for the in-phase tributary due to modulator imperfections. The worst Q-factor measured is 9.6 dB; hence all measured channels are well above the FEC limit of a concatenated forward error correction (FEC) code with 7% redundancy [19]. The received OSNR of all channels after 4,500 km transmission is approximately 16 dB on average, which corresponds to a 2-dB OSNR penalty compared with the back-toback performance. This indicates only minor influence of nonlinear phase noise distortions. The performance as a function of transmission distance is depicted in Fig. 4 for the OPC and the DCF based transmission system. The number of channels is reduced to 18 in this experiment, 9 channels that are phase conjugated (from 1549.32 nm to 1552.52 nm), and an additional 9 channels (from 1534.25 nm to 1537.40 nm) to balance the amplifiers. By adjusting the number of loop re-circulations, the transmission performance of the center channel (1535.8 nm) is measured from 1,700 km up to 5,700 km. Fig. 3b further depicts the performance of 21.4 Gbit/s RZ-DQPSK, as reported in [23]. Comparing the Q-factor decrease of 42.8 Gbit/s and 21.4 Gbit/s with transmission distance clearly illustrates that doubling the data rate and spectral efficiency results in a decrease in feasible transmission distance by about a factor of two.
Fig. 3: (a) In-phase and quadrature Q-factors for all channels of the configuration with OPC after 4,500-km transmission and (b) Q-factor as a function of transmission distance.
Comparing the performance of the OPC and the DCF-based configuration at 42.8 Gb/s, it can be seen that at low transmission distances, the performance difference is about 1 dB in Q-factor penalty. After 2,500-km transmission, the Q-factor of the DCF based configuration deviates from the linear decrease whereas the OPC based performance is virtually unaffected. As a result, the feasible transmission distance of the DCF and the OPC-based transmission systems is about 3,000 km and 5,500 km, respectively. We conjecture that the performance degradation of the DCF aided transmission results from SPM induced nonlinear impairments, including nonlinear phase noise. In the OPC aided transmission experiment the SPM induced nonlinear impairments resulting from both modulator imperfections and transmission line are reduced through mid-link OPC [16], [24], resulting in an increased transmission reach. 4. OUTLOOK With the experiments discussed in this paper, we show that OPC is an effective technique to compensate for nonlinear impairments in fiber-optic transmission systems. However, OPC compensates for deterministic impairments only. The most significant impairment that fundamentally can not be compensated for is polarization-mode dispersion (PMD). Recently many different methods have been proposed to compensate for linear impairments (PMD and chromatic dispersion) in the electrical domain, such as for instance coherent detection in combination with FIR filters [25] and maximum-likelihood sequence estimation (MLSE) [26]. The dispersion of the complete transmission line can be compensated for with these techniques, but this would require powerful electrical dispersion compensators (EDCs) that are expensive and high power consuming [27]. By placing one OPC in the middle of such a transmission link, the gross dispersion is compensated for and the nonlinear tolerance can be increased. A “weak” EDC can subsequently be used to compensate for residual chromatic dispersion and PMD [27]. Another method to effectively compensate for PMD and chromatic dispersion is by using an advanced modulation format such as orthogonal frequency division multiplexing (OFDM) [28]. However, low input powers are required to minimize nonlinear impairments. We conjecture that by adding OPC to such a transmission link the nonlinear tolerance could be significantly enhanced. 5. CONCLUSION Transmission of 50-GHz spaced, 42.8-Gb/s DQPSK is presented. In this experiment OPC is used to compensate for chromatic dispersion as well as nonlinear impairments and successful transmission of 26 WDM channels over 4,500-km is demonstrated. For OPC, one polarization independent PPLN subsystem is used to compensate for the gross chromatic dispersion of the transmission line (more than 70,000ps/nm). When the performance of OPC aided transmission is compared to ‘conventional’, optimized DCF aided
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