We experimentally investigate the first-order polarization-mode dispersion (PMD) tolerance of two polarization-multiplexed (POLMUX) RZ-DQPSK signals at overall 40 Gb/s. The polarization demultiplexing is enabled by an automatic endless polarization stabilizer. Time-interleaving the two orthogonally polarized RZ-DQPSK signals minimizes the crosstalk due to the non-ideal polarization stabilization, while it represents the worst-case for the PMD-induced crosstalk. Bit-error rate measurements are performed both in back-to-back and after 25-km standard single-mode fiber. The PMD tolerance is evaluated as a function of the instantaneous differential group delay, introduced by a first-order PMD emulator. 40-Gb/s POLMUX RZ-DQPSK is more sensitive to PMD than single-polarization 20-Gb/s DQPSK, while it is more PMD-tolerant than 40-Gb/s NRZ-OOK. Besides, its chromatic dispersion robustness is similar to the single-polarization 20-Gb/s DQPSK. The combination of POLMUX and DQPSK is therefore very promising in view of transmission systems at high bit-rate.
©2008 Optical Society of America
The transmission capacity is strongly limited by chromatic dispersion (CD) in optical amplified systems, where the fiber attenuation is overcome by using optical amplifiers. The tolerated amount of CD is inversely proportional to the square of the bit rate. To reduce the CD detrimental effect, dispersion-compensating fibers (DCFs), optical compensators  and electronic compensators  can be used.
A further limiting factor of the transmission capacity is the polarization-mode dispersion (PMD). The tolerated differential-group delay (DGD), expressing the robustness to first-order PMD, scales with the inverse of the bit rate. The PMD compensation is a critical task, since the PMD is a stochastic phenomenon. Several optical and electrical adaptive techniques for PMD mitigation  have been proposed, actually featuring a relevant degree of complexity.
Nowadays, for high-capacity optical transport, the request for advanced modulation formats  more robust to the transmission impairments than the common on-off keying (OOK) format is increasing. This trend is particularly evident in view of upgrading the present 10-Gb/s OOK installed systems to 40 Gb/s, or even to 100 Gb/s for next-generation 100G Ethernet transport . In the case of 1-bit per-symbol formats, such as the standard OOK and the duobinary/phase-shaped binary transmission modulation formats, 40-Gb/s system implementation is challenging owing to factors such as the need of high-speed optoelectronics and the reduction of CD and PMD tolerance. In particular the increased bit rate yields a major complexity and cost of the electronic and optical circuits in CD and PMD compensator implementation. Further penalties can arise in dense wavelength division multiplexed (DWDM) systems in the process of channel multiplexing and demultiplexing.
The highly spectrally-efficient multilevel formats, characterized by a larger number of bits per-symbol at lower symbol rate, show a narrower optical spectrum and a higher robustness towards CD and PMD with respect to binary modulation at the same bit rate. Therefore the use of these multilevel formats appears promising, although it implies higher complexity in the transmitter/receiver side and worse bit error rate performance.
In recent years, differential quadrature phase-shift keying (DQPSK) [6–7], characterized by 2 bits per-symbol, has been proposed for optical communication systems and its transmission performance widely studied. In most cases DQPSK format is exploited with return-to-zero (RZ) pulse shaping, which gives better sensitivity  and fiber nonlinearities tolerance  than non-return-to-zero (NRZ) shaping [9–10].
In order to further double the transmission capacity, DQPSK format can be combined with polarization multiplexing (POLMUX) [11–15]. In POLMUX DQPSK systems 4 bits per-symbol are encoded, so the spectral efficiency is quadrupled with respect to OOK, reducing the bandwidth requirements for modulators/detectors and greatly enhancing the robustness towards CD. As the PMD tolerance is concerned, the situation is more critical in POLMUX systems, since, in spite of the positive effect of reducing the symbol rate, the PMD induces a coherent crosstalk between the demultiplexed channels [16–19].
Obviously, POLMUX requires polarization demultiplexing to receive the two orthogonally polarized channels at the same wavelength. Actually optical signal propagation in the fiber is susceptible of random changes of the state of polarization (SOP) due to external disturbances such as temperature variation and mechanical perturbation. Therefore automatic endless SOP tracking and stabilization are needed at the end of the fiber link, before polarization demultiplexing. Previous works on POLMUX DQPSK mainly make use of manually adjustable fiber polarization controllers and just a few [11–13] exploit an automatic polarization controller, which is unavoidable for in-field application. POLMUX system performance is thus influenced by the non-ideal behavior of the SOP stabilizer; a relevant issue is the interaction of the non-ideal polarization stabilization with PMD, causing additional coherent crosstalk between the polarization channels after demultiplexing.
In the present work we experimentally investigate the first-order PMD tolerance of a 40-Gb/s transmission system obtained by combining POLMUX with the RZ-DQPSK modulation format, at a symbol rate of 10 Gbaud. An automatic endless polarization stabilizer, based on the control method described in , is used at the receiver before polarization demultiplexing. For minimizing the contribution to the coherent crosstalk caused by the non-ideal polarization stabilization, the two POLMUX RZ-DQPSK channels are timeinterleaved , so that a half-symbol delay separates neighboring orthogonally-polarized RZ pulses. Time-interleaving is a very particular condition: it shows the maximum PMD-induced eye-opening penalty (EOP) as a function of the interpulse delay as reported in , while, on the contrary, it minimizes the EOP due to fiber nonlinearities, thanks to the nearly constant intensity. Therefore, the aim of our work is to measure, for the first time to the best of our knowledge, the PMD-induced penalties in a 40-Gb/s POLMUX RZ-DQPSK system in presence of an automatic endless polarization stabilizer and in case of time-interleaving. The bit-error rate (BER) versus optical signal-to-noise ratio (OSNR) at the receiver is measured for different values of DGD, both in back-to-back condition and after 25 km of uncompensated standard single-mode fiber (SSMF), corresponding to about 400 ps/nm of CD. For comparison purposes we also measure the first-order PMD tolerance of NRZ and RZ single-polarization DQPSK at 20 Gb/s, and of NRZ-OOK at 10 Gb/s.
2. Experimental system set up exploiting an endless polarization stabilizer
Figure 1 shows the employed experimental set up. The 1-MHz DFB laser radiation at 1548.51 nm is RZ shaped by means of a RZ carver, consisting in a lithium niobate Mach-Zehnder modulator (MZM) driven by a 10-GHz clock signal, achieving a duty cycle of 50% and an extinction ratio of 13 dB. DQPSK modulation is obtained by the cascade of a MZM, giving π-phase modulation, and a phase modulator (PM) performing π/2-phase modulation. These two latter modulators are driven by two identical 10-Gb/s 27-1 pseudo-random bit sequences (PRBS), mutually delayed of 10 bits and with bit transitions synchronized by an RF phase shifter.
The 20-Gb/s RZ-DQPSK signal is split by a coupler in two replicas uncorrelated by means of a fiber spoil, with a delay of about 15 µs. The two replicas, equalized in power, are orthogonally polarized by manually adjustable fiber polarization controllers (PCs) and recombined by a pig-tailed micro-optic polarization beam combiner (PBC). A fine adjustable delay line is employed to time-interleave the symbols of the two orthogonally polarized RZ-DQPSK channels. A 1-MHz pilot tone, with a peak-to-peak intensity equal to 10% of the signal mean value, is superimposed to one of the two channels by an electro-absorption modulator (EAM), in order to label the channel whose SOP is to be monitored and stabilized. Both POLMUX channels, with an overall capacity of 40 Gb/s, are then optically boosted by an erbium-doped fiber amplifier (EDFA) and transmitted over an uncompensated SSMF link. Nonlinear effects are negligible due to low launch power. During fiber propagation the SOPs of the two channels evolve randomly, but they almost maintain their mutual orthogonality.
In order to optically demultiplex the two orthogonally polarized channels at the same wavelength, we employ a novel control scheme able to achieve endless polarization stabilization , based on the cascade of two similar stages. Each stage reduces by one the number of degrees of freedom of the SOP motion and contains a pair of birefringent elements with fixed eigenstates and controllable phase retardation, in order to perform the SOP transformation. The birefringent elements are realized as bulk magneto-optic polarization rotators (i.e. birefringent elements with circular eigenstates) by exploiting the Faraday effect in bismuth substituted rare-earth iron garnet, which demonstrates high magneto-optic effect and low insertion loss. Each stage comprises also a quarter wave plate suitably oriented and inserted between the two rotators . The polarization rotation of each variable Faraday rotator is controlled by the electrical current in the coils surrounding the garnets The role of the first stage is to map the evolutions of the input SOP onto the fixed meridian of the Poincaré sphere, said Γ in Fig. 2, representing the elliptical SOPs with diagonal axes. Then the second stage stabilizes the output SOP in the vertical state (point V in Fig. 2). The maximum current intensity required by the variable Faraday rotators is lower than 30 mA. Polarization demultiplexing is performed by a polarizing beam splitter (PBS) with a polarization extinction ratio (PER) higher than 25 dB. In response to an endlessly varying input SOP at a speed of 30 rad/s, under locking conditions, a PER higher than 14 dB is obtained due to the non ideal behavior of the stabilizer optical components, such as the hysteresis of magneto-optic polarization rotators, the non-uniform magnetic field in the garnets and the critical alignment of the quarter wave plates.
The polarization stabilizer is introduced at the end of the SSMF link, to compensate for random polarization fluctuations. The SOP of the channel identified by the pilot tone is stabilized into the vertical state, and consequently the SOP of the other channel is stabilized into the horizontal state. A PBS then demultiplexes the two POLMUX channels at the same wavelength. The time-interleaving of the two orthogonally polarized RZ-DQPSK channels reduces the inter-channel crosstalk caused by the finite polarization extinction ratio of the employed stabilizer.
A variable amount of amplified spontaneous emission (ASE) noise is added by a first variable optical attenuator (VOA) to the single demultiplexed DQPSK channel at 20 Gb/s, in order to modify the optical signal-to-noise ratio (OSNR), measured by an optical spectrum analyzer (OSA) with 0.5-nm resolution. Before optical filtering (1-nm bandwidth), a second VOA allows to fix the power at the receiver. The in-phase and quadrature components of the single demultiplexed RZ-DQPSK signal can be detected by a pair of Mach-Zehnder delay interferometers (MZDIs) with 1-symbol delay and biased respectively at a phase difference of ±π/4. In our experiments we have considered a single MZDI realized in SiON integrated-optic technology with 100-ps delay and thermally biased. The two output ports of the MZDI are connected to a balanced differential detector with 10-GHz bandwidth. The BER measurements are performed at different OSNR levels, while the average optical power at each MZDI output port is kept equal to -13 dBm. The errors are counted by entering the error detector the calculated bit sequence to be received.
In order to check the tolerance of the transmission system towards PMD, a first-order PMD emulator is inserted after the PBC. This emulator consists in a variable delay line between two orthogonal linear SOPs. Thus a controlled amount of DGD between two polarization axes, i.e. the two orthogonal eigenSOPs of the emulator, is introduced. Then the system performance is evaluated for different values of DGD in the worst case of SOP orientation of the POLMUX channels with respect to the polarization axes of the emulator. This worst condition corresponds to POLMUX channels with elliptical SOPs characterized by axes oriented at ±45° with respect to the PMD emulator axes, and is obtained by means of a manually adjustable fiber PC placed before the emulator.
3. System performance measurements
At first, we have measured the required OSNR to achieve the reference 10-6 BER as a function of the instantaneous DGD both in back-to-back condition and after uncompensated 25 km of SSMF for 20-Gb/s DQPSK signal, both in case of RZ and NRZ shaping. The experimental set up is the same shown in Fig. 1, provided that the 20-Gb/s RZ-DQPSK transmitter is connected directly to the PMD emulator, without performing polarization multiplexing. Besides, the SOP stabilizer is not present at the receiver. The 10-Gb/s NRZ-OOK performance has been also measured in back-to-back and after 25 km. The experimental performance (Fig. 3) shows that 20-Gb/s DQPSK (both NRZ and RZ) have a higher PMD tolerance than 10-Gb/s NRZ-OOK; in fact a 30-ps DGD causes for 10-Gb/s NRZ-OOK a penalty almost equal to 3 dB, while for 20-Gb/s DQPSK the penalty reduces to less than 2 dB. Yet, as expected, RZ-DQPSK is more affected by CD impairments.
Successively, 40-Gb/s POLMUX RZ-DQPSK BER curves versus OSNR for 0 km and 25 km of uncompensated SSMF have been measured when the SOP stabilizer described in Section II was active. The performance of the POLMUX RZ-DQPSK channel has been compared, after demultiplexing, with the single RZ-DQPSK channel. POLMUX results refer to the channel labeled by the pilot tone, whose performance is slightly worse than the orthogonally polarized channel (less than 0.5 dB). With 0-ps DGD, Fig. 4 shows an OSNR penalty at 10-6 BER of 1 dB and 2 dB respectively in back-to-back and after 25-km SSMF with respect to the single RZ-DQPSK channel. This penalty is due to the presence of the pilot tone together with the inter-channel crosstalk induced by the finite polarization extinction ratio of the employed SOP stabilizer. Time-interleaving solution reduces the mentioned coherent crosstalk; the achieved performance demonstrates the effectiveness of the proposed endless polarization stabilizer in order to demultiplex POLMUX channels.
Regarding PMD, to better understand how POLMUX technique affects the DGD tolerance, we have experimentally checked the required OSNR to achieve 10-6 BER as a function of the DGD for 0-km and for 25-km propagation (Fig. 4). With respect to single polarization propagation, POLMUX RZ-DQPSK results less tolerant to DGD because the accumulated PMD induces an interchannel crosstalk between the demultiplexed channels. Furthermore time-interleaving condition exploited in our experimentation represents the worst case for this PMD-induced crosstalk. Time-overlapping (i.e., the orthogonally polarized symbols are temporally overlapped) operation was prevented by the limited PER of our stabilizer. In back to back the measured BER shows a floor at 10-8 and at 10-5 respectively with 0-ps DGD and 10-ps DGD.
A CD penalty of about 2 dB with respect to back-to-back is found at 10-6 BER for POLMUX RZ-DQPSK at 40 Gb/s, which is almost the same penalty measured for the single RZ-DQPSK channel at 20 Gb/s.
4. Discussion and conclusion
We have experimentally investigated the first-order PMD tolerance of POLMUX RZ-DQPSK at 10 Gbaud, hence at an overall bit rate of 40 Gb/s. An automatic endless polarization stabilizer has been employed at the receiver to perform polarization demultiplexing. 40-Gb/s POLMUX RZ-DQPSK experimentally demonstrates 1-dB penalty in OSNR at about 10-ps DGD for back-to-back condition, as shown in Fig. 4. POLMUX RZ-DQPSK results more impaired with respect to single polarization RZ-DQPSK at 20 Gb/s owing to the PMD-induced inter-channel crosstalk inherent to POLMUX systems. Furthermore, we have experimentally checked the RZ time-interleaving condition, which is the worst case for PMD-induced crosstalk.
Considering that the DGD tolerance halves when the bit rate doubles, as well known, we infer from the experimental results of Fig. 3 the DGD value corresponding to 1-dB penalty for NRZ-OOK and for single polarization RZ-DQPSK at 40 Gb/s. We find, in back-to-back condition, for 40-Gb/s NRZ-OOK a DGD of about 5 ps and for 40-Gb/s single polarization RZ-DQPSK a DGD of about 13 ps. Hence, at the same bit rate POLMUX RZ-DQPSK is more robust towards PMD with respect to NRZ-OOK, while it is slightly less tolerant with respect to single polarization RZ-DQPSK (in the worst case given by RZ time-interleaving). On the other hand, POLMUX RZ-DQPSK presents the advantage of a very high CD tolerance, as shown in Fig. 4 where POLMUX RZ-DQPSK shows at 40 Gb/s just 2-dB penalty after 25-km SSMF propagation (corresponding to 400-ps/nm CD), while roughly the same penalty is found by single polarization RZ-DQPSK at 20 Gb/s.
In conclusion, combined POLMUX and DQPSK propagation at 4-bit per-symbol appears a very interesting approach to exploit transmission at 40 Gb/s and beyond with good performance in terms of CD and PMD tolerance.
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