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We report on the performance of 100Gb/s coherent non return-to-zero (NRZ-) polarization division multiplexed (PDM-) quadrature phase shift keying (QPSK) transmission over 16x100km of standard single mode fibre under constraints of typical transparent terrestrial networks, employing Erbium-Doped Fibre Amplifiers. We first evaluate the impact of cross non linear effects onto the performance of 100Gb/s coherent PDM-QPSK signals and we investigate the impact of shifting one of the polarization multiplexed tributaries by half a symbol duration with respect to the other one. Finally we show that this solution is robust against channel-to-channel cross-talk from transparent nodes and does not suffer from performance degradation stemming from co-propagating 40Gb/s channels.
©2009 Optical Society of America
1. Introduction
In order to cope with the need for capacity, the development of 100Gb/s transponders for the next few years appears to be of interest for deployment plans of many carriers. As the move to 100Gbit/s channel rate in WDM systems is generally motivated by the need for an increase of the total system capacity, a technique fitting with the existing 50GHz-grid infrastructures is desirable. Among the different modulation formats and detection techniques recently proposed to meet the challenge of boosting the total system capacity [1–5], polarization-division multiplexing (PDM-) quadrature phase shift keying (QPSK) associated with coherent detection and digital signal processing (DSP) has attracted much attention. This approach is compatible with spectral efficiencies of at least 2bit/s/Hz, which is naturally advantageous for dense WDM systems, and has an excellent robustness to linear impairments. Indeed, chromatic dispersion (CD) can be fully compensated in the digital domain using dedicated finite impulse response (FIR) filters with appropriate length, and polarization mode dispersion (PMD) amounts greater than 20ps can be mitigated using blind equalization based on adaptive FIR filter [1, 6, 7]. Regarding the tolerance to non linear effects, which determines the maximum reach in long-haul WDM systems, transmission distances up to 2,500km have been reported in pioneering experiments [1, 2] at 100Gbit/s. However, these experiments were realized using Raman amplification in order to improve the optical signal-to-noise ratio (OSNR) at the receiver.
In this paper we focus on the performance of 100Gb/s coherent (NRZ-)PDM-QPSK transmission over 16x100km of standard single mode fibre (SSMF) under the constraints of typical transparent terrestrial networks, employing Erbium-Doped Fibre Amplifiers (EDFAs). After having assessed the impact of non linear effects in WDM systems using coherent detection, we investigate whether shifting one the polarization multiplexed tributaries by half a symbol duration with respect to the other one can be helpful to contain the impairments related to non linear effects. Then we evaluate the impact of channel-to-channel cross-talk from transparent nodes, as well as the impact of co-propagating a 100Gb/s channel together with 40Gb/s neighboring channels.
2. Set-up configuration
As depicted in Fig. 1, our test-bed consists of 79 conventional DFB lasers, spaced by 50GHz and separated into two independently modulated, spectrally interleaved combs, plus one narrow linewidth (~100kHz) tunable external cavity laser (test channel), at 1545.72nm. The light from each set is sent to a distinct QPSK modulator operating at 28Gbaud (or 56Gb/s). The modulators are fed by 215-1-bit-long sequences at 28Gb/s, including forward error correction (FEC) and protocol overhead. Polarization multiplexing is then performed by dividing and recombining the QPSK data into a polarisation beam combiner (PBC) with an approximate 100 symbol delay, yielding PDM-QPSK data at 112Gb/s. Here, by choosing polarization maintaining fibres with appropriate lengths before the PBC, the two orthogonal polarization tributaries can be either temporally aligned or interleaved by half a symbol (~18ps). In the following, aligned-PDM-QSPK refers to signals with polarization tributaries pulse-to-pulse aligned, in contrast with interleaved-PDM-QSPK that refers to signals with polarization tributaries temporally interleaved by half a symbol. The corresponding eye diagrams observed on an Agilent digital communications analyzer are shown in the insets a) and b) of Fig. 1. When the tributaries are temporally aligned, the eye diagram exhibits a high extinction ratio and clearly defines the five transition states between symbols of PDM-QPSK format, as shown in the inset (a) of Fig. 1. On the contrary, when the tributaries are interleaved by half a symbol, a cw-like eye diagram is observed, as shown in the inset (b) of Fig. 1.
For both cases of aligned- and interleaved-PDM-QPSK, the two combs are combined with a 50GHz interleaver. The resulting multiplex is boosted through a dual-stage EDFA incorporating -400ps/nm dispersion compensating fibre (DCF), passed into a low-speed (<10Hz) polarisation scrambler, and sent into a re-circulating loop. The re-circulating loop incorporates four 100km-long spans of SSMF, separated by home-made dual-stage EDFAs with 15dBm output power. Each EDFA includes a spool of DCF for partial dispersion compensation yielding 220ps/nm residual dispersion per round trip. A 50GHz spacing wavelength selective switch (WSS) is also inserted in the loop and is used to perform channel power equalization. Here, all the channels are passed through the same output port of the WSS. In all experiments, we measure the performance after four loop round-trips, i.e. at a transmission distance of 1600km. We vary the power per channel at each fibre input by reducing the number of channels from 80 to 20. The test channel power is set at the same level as all the channels.
At the receiver side, the channel under study is selected by a 0.4nm bandwidth filter and sent into the coherent receiver. Such a receiver consists of a polarization beam splitter (PBS) followed by two coherent mixers based on free-space optics, one for each received polarization state. Coherent mixers then combine the signal with a narrow linewidth (~100kHz) tuneable continuous wave laser achieving a phase offset of 90° between each of their four outputs, so as to supply the in-phase and quadrature components. These components are detected by four conventional 40GHz-bandwidth balanced photodiodes, digitized by four analog-to-digital converters (ADC) of a Tektronix DPO 72004 oscilloscope, operating at 50Gsamples/s with a 20GHz electrical bandwidth, and stored by sets of 2,000,000 of samples (which correspond to a time slot of 40μs). Due to polarization scrambling, each recording corresponds to an arbitrary received state of polarization. For each bit-error rate (BER) measurement, five sets of 2,000,000 of samples are stored and processed off-line on a computer (see [1, 2] for more details) for re-sampling at twice the symbol rate (56Gsample/s), digital CD mitigation, polarization demultiplexing and equalization based on Constant Modulus Algorithm [8]. Then carrier phase estimation (CPE) is performed using the Viterbi and Viterbi algorithm [9], and symbols are finally identified for BER measurement. The computed BER are averaged over the five measured sets and subsequently converted into Q2-factors.
3. Transmission experiment
3.1 Tolerance to non linear impairments
Recently, the temporal interleaving of polarization tributaries of PDM-QPSK signals with Return-to-Zero pulse carving and differential detection (RZ-PDM-DQPSK) has been demonstrated [10, 11] to improve the tolerance to non linear impairments. According to this, we investigate the benefit of interleaving the multiplexed polarization tributaries by half a symbol in the context of coherent PDM-QSPK signals but without RZ pulse carving. We restrict here the investigation to (NRZ-)PDM-QPSK format, which is more convenient for the short-term development of 100Gb/s coherent PDM-QPSK transponders since the use of a RZ pulse-carver introduces additional issues for the transponder realization such as the increase of losses, consumption as well as cost. Nevertheless, the potential of interleaving the multiplexed polarization tributaries by half a symbol with coherent detection and RZ-PDM-QPSK format has been addressed elsewhere [12].
In that purpose, we first assess the impact of non linear effects onto aligned-PDM-QPSK. Fig. 2 shows the performance of 100Gb/s coherent aligned-PDM-QPSK signals versus launched power for the test channel surrounded by neighbors of the same format, or by cw neighbors. This latter case corresponds to emulating the propagation of a single channel. It can be seen that the presence of modulated neighbors only brings 1dB Q2-factor penalty onto the optimum performance of 100Gb/s coherent aligned-PDM-QPSK, which is in line with numerical analysis performed in [13]. This small impact of Cross Phase Modulation (XPM-) related impairments can be attributed to the relatively high walk-off between the WDM channels for a 28Gb/s symbol rate. Then, we measured the performance of 100Gb/s coherent interleaved-PDM-QPSK signals versus launched power. This result is depicted in Fig. 2 and we can observe that at higher launched powers, coherent interleaved-PDM-QPSK appears less impacted by cross non linear effects than aligned-PDM-QPSK. However, regarding the optimum performance, both modulation formats show the same optimum Q2-factor at 9.6dB. Therefore, unlike the case of RZ-PDM-QPSK associated with differential detection, the temporal interleaving of polarization multiplexed tributaries does not bring significant benefits for optical transmission of 100Gb/s PDM-QPSK with coherent detection. We focus next on 100Gb/s coherent PDM-QPSK with aligned polarization tributaries only.
Fig. 2. Measured performance of 100Gb/s coherent PDM-QSPK in single channel transmission and in WDM transmission with aligned or interleaved polarization tributaries.
3.2 Impact of in-band cross-stalk and co-propagating 40Gb/s channels
We now investigate the robustness of 100Gb/s coherent (aligned-) PDM-QPSK against the constraints of terrestrial networks. In a first step we measure the performance of 100Gb/s coherent PDM-QPSK across transparent nodes, affected by in-band cross-talk. Cross-talk is emulated by the help of the WSS. In addition to channel power equalization and optical filtering, the WSS can also emulate cross-talk stemming from transparent nodes when odd and even channels are passed through distinct output ports before being uncorrelated and recombined through a 3dB coupler, as described in Fig. 3. In a second step, the test channel at 100Gb/s is surrounded by neighbors at 40Gb/s, as expected during smooth network upgrades. For that purpose, the 79 DFB lasers are modulated with Differential Phase Shift Keying (DPSK) at 43Gb/s as described in Fig. 3. In contrast with the case when surrounding channels are modulated with polarization multiplexing, a low speed polarization scrambler is inserted on the odd channels path in order to randomly vary the polarization state between odd and even channels. The DPSK spectra are truncated by the interleaver and when passed into the 50GHz-spaced wavelength slots of the WSS.
Fig. 3. Experimental setup to assess the impact of in-band cross-talk and co-propagating 40Gb/s DPSK channels onto the performance of 100Gb/s coherent PDM-QSPK.
Figure 4 represents the performance for the two successive experiments versus launched power. The solid curve with circles describes the performance in homogeneous WDM transmission of 100G coherent PDM-QPSK when traveling across 4 nodes. This curve is very similar to the curve of Fig. 2, which underlines the resistance of 100Gb/s coherent PDM-QPSK systems against in-band cross-talk. The dashed curve with triangles describes the measured performance of 100Gb/s coherent PDM-QPSK when surrounded by 40Gb/s DPSK channels. This curve shows a very similar trend as the one obtained with 100Gb/s neighboring channels, with a nearly identical optimum performance of 9.5dB. The slight horizontal shift observed between the two curves is attributed to measurement uncertainties. This result demonstrates the full compliancy of the 100Gb/s coherent PDM-QPSK solution with 40Gb/s DPSK channels, which is of importance for future capacity upgrade in long-haul DWDM optical networks.
Fig. 4. Measured performance of 100Gb/s coherent PDM-QSPK with in-band cross-talk when surrounded either by other 100Gb/s coherent PDM-QSPK or by 40Gb/s DPSK channels.
4. Conclusion
We have shown that interleaving polarization tributaries of 100Gb/s coherent PDM-QPSK signals without RZ carving does not bring sufficient benefits to improve the transmission reach obtained when polarization tributaries are pulse-to-pulse aligned. Besides, we have demonstrated that 100Gb/s coherent PDM-QPSK solution is very robust to in-band cross-talk and is fully compliant with co-propagating 40Gb/s DPSK channels, revealing its potential for future upgrade of high spectral efficiency systems with the constraints of terrestrial networks.
Acknowledgment
This work has been supported by the French government, in the frame of the COHDEQ 40 and TCHATER projects.
References and links
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