Th.2.A.3.pdf
Wavelength-Tunable Burst-Mode Receiver with Correlation-Based Polarization Separation (1)
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J. Gripp , J. E. Simsarian , S. Corteselli , T. Pfau (1) (2)
Alcatel-Lucent, Bell Laboratories, 791 Holmdel Rd., Holmdel, NJ 07733, U.S.A. Alcatel-Lucent, Bell Laboratories, 600-700 Mountain Ave., Murray Hill, NJ 07974, U.S.A. Email:
[email protected]
Abstract We demonstrate a fast wavelength-tunable burst-mode receiver for DP-QPSK optical packets. Training symbols are used for frequency offset recovery, packet alignment, and rapid polarization separation using 64-bit cross correlations. Introduction Metropolitan networks that use electronic packet and circuit switches for aggregation of data into optical wavelengths are inefficient due to the electronic processing of through traffic that is not added or dropped at a node. Optical packet networks enable sub-wavelength optical bypass so that through traffic does not consume network-interface bandwidth and electronicswitch resources. The advent of coherent transmission with polarization division multiplexing has improved the spectral efficiency of wavelength division multiplexed optical 1 transport , and has made possible a network based on optical packet selection with a burstmode coherent receiver using a wavelength2 tunable local oscillator (LO) . Fast-converging digital signal processing must be used at the receiver to rapidly recover the optical packet data. An important step in the packet processing is separating the two signal polarizations since the typical dual-polarization 1 constant modulus algorithm (CMA) can have long convergence times and may even fail to 3 recover both polarizations simultaneously . 4 Recent work by Maher et al. used 25 different initial test polarization rotation angles and the fastest converging one was selected; however, this technique does not find the optimal initial 5 rotation angles. Vacondio et al. implemented a real-time receiver that finds the optimal rotation
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angles for polarization separation, but additional optical carvers are needed at the transmitter to 6 extinguish one of the polarizations. Dischler used synchronization symbols for adaptation of the receiver in optical orthogonal frequency division multiplexed signals without wavelength switching. Here we demonstrate the use of training sequences and cross-correlations for fast recovery of dual polarization quadrature phase shift keyed (DP-QPSK) signals following nanosecond timescale LO wavelength switching. The generation of the training symbols does not require any additional optical hardware and induces minimal optical power variations in the signal. Experiment and Results Figure 1a shows a diagram of the experimental setup. The packet generator uses two fieldprogrammable gate arrays (FPGAs) to generate the optical packets, and has four independent outputs to drive the in-phase (I) and quadrature (Q) components of the x and y polarization I/Q modulators, IX, QX, IY, and QY. While the outputs 7 can generate 10 Gb/s signals , we operate them at 5 Gb/s for compatibility with the sampling rate of the FPGA receiver. The light from the signal laser is split, modulated, and polarization combined to create the 5 Gbaud DP-QPSK signal. The relatively low baud signal and sampling rate put tighter requirements on the LO laser linewidth and wavelength accuracy. An
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Fig. 1 a) Diagram of the experimental setup. b) Received optical packets with the signal laser at O2 with LO laser switching, c) The signal laser is at O1 with LO laser switching, and d) expanded view of the packet headers and switching between the wavelengths.
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complex expected y-polarization sequence. Before performing the cross correlations, the offset frequency between signal and LO must first be removed. The blue trace in Fig. 2a shows the recovered frequency offset, 'f, during the header for switching from O to O. The frequency offset is measured using 'f 'I/(2S*T), where 'I is the phase change in radians between consecutive sampling points. 'f is averaged over 256 samples. Because there is no data modulation, the fourth power typically used in QPSK frequency recovery is not needed, thereby increasing the frequency capture range to ±1/(2T). It is evident in Fig. 2a that 'f is drifting due to thermal transients in the LO laser following the wavelength switching. The correlation occurs in section Sc of Fig. 2a, so we predict 'fc in that region using the equation, 'fc 'f2 – ('f1 – 'f2)*'tc2/'t21, where 'f1 and 'f2 are the frequency offsets in the regions S1 and S2, respectively, and 'tc2 and 't21 are the time differences between regions Sc and S2, and S2 and S1, respectively. The red trace in Fig. 2a shows 'fc. The left column of Fig. 2b shows examples of the cross correlations when the optical signal to noise ratio (OSNR), referenced to 0.1 nm, is 8.0 dB. The presence of peaks in the cross terms Cyx and Cxy indicates mixing of x and y polarization in the transmission link. We detect the correlation peak by monitoring the sum of the squared magnitudes of all four correlations. Then the polarization angles Tx = arctan(Cyx/Cxx), Ty = arctan(Cxy/Cyy), Ix = arg(Cxx/Cyx)/2, and Iy = –arg(Cyy/Cxy)/2 are obtained from the correlation values at the peak position. The polarization of the received data is rotated back to its original state by applying the inverse Jones matrix -1 –jϕ jϕ –jϕ jϕ J = [[e cosT, e sinT], [–e sinT, e cosT]], where T = (Tx + Ty)/2 and I= (Ix + Iy)/2. The right column of Fig. 2b shows correlations after 2 the back rotation. The cross terms |Cxy| and 2 2 2 |Cyx| are reduced and |Cxx| and |Cyy| are increased, indicating that the polarization axes are aligned. Fig. 2c shows the accuracy of the c)
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amplified spontaneous emission (ASE) noise source adds noise to the signal before it is received by a polarization-diversity 90-degree optical hybrid together with a fast-tunable LO, followed by four balanced detectors. The LO is a 16-channel digitally tunable external cavity laser with narrow linewidth and wavelength 8, 9 switching time < 10 ns . The received signals are converted by four 8-bit analog-to-digital converters (ADCs), operated at sampling rate, 1/T = 10 GSamples/sec that are connected to a 10 circuit board with four FPGAs . The received data is extracted from one of the FPGAs and processed offline on a computer. An example of the recovered optical packets is shown in Fig. 1b-d. The LO laser repeatedly switches wavelength between O1 at 1554.13 nm and O2 at 1555.74 nm with a packet length of 2.56 Ps. For Fig. 1b, the signal laser is set to O2, and for Fig. 1c, the signal laser is set to O1. The times when there is no signal in Fig. 1b-c are caused by the signal-LO interference having a larger frequency than the receiver bandwidth. Fig. 1d shows an expanded region of the combination of traces in Fig. 1b-c. During the header between optical packets, all of the modulator components are held on one of the QPSK symbols for 70 ns. Within 6 ns of the end of the packet data, the wavelength of the LO begins switching, and the remainder of the header time is used to recover and track the offset frequency between the signal and LO lasers. 15 The packet consists of segments of four 2 -1 pseudo-random bit sequences (PRBS). The segments in QX, IY, and QY are shifted relative to IX by 24576, 4096, and 20480 bits, respectively. The first 64 symbols of this payload form two distinct complex correlation patterns, one for each polarization. We perform four complex cross-correlations Cxx, Cxy, Cyy, and Cyx between the received data and two pre-calculated correlation patterns that correspond to a system with no polarization rotation. For example, Cxy is the cross correlation of the complex received x signal from the digital receiver with the 64-bit
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Fig. 2: a) 'f versus time during the packet header, regions S1 and S2, and during the correlation pattern, region Sc. b) Left column: magnitudes of the cross-correlations squared before back rotation. Right column: magnitudes of the cross correlations squared after polarization back rotation. c) Calculated errors in the recovered I and T versus incoming T for different lengths of the correlation pattern, CL.
Th.2.A.3.pdf method in simulation. 'T and 'I denote the maximum deviations of the recovered angles from the actual values as a function of T The maximum deviations were obtained after calculating 360 different values of I at each T. As the length of the correlation pattern, CL, increases, the accuracy of the recovered angles improves. Note that the accuracy is best around T S, as this is the region where the polarizations are the most strongly mixed and all four correlations show distinct peaks. As T approaches or S, two of the four correlations become too small to be accurately measured in the presence of the correlation noise, but since there is less polarization mixing in these regions, the accuracy is not as important. After the polarization back rotation, a twopolarization constant-modulus algorithm (CMA) is used to set the coefficients of a butterfly filter with four 9-tap T/2 finite impulse response (FIR) filters. For the remainder of the packet, the offset frequency and phase is tracked using the Viterbi-Viterbi algorithm and bit errors are 15 counted on the 2 -1 PRBS payload. To test the behavior of the receiver for all polarization angles, we digitally rotate the signal by all T and I to cover the Poincare sphere. Fig. 3a shows the number of symbols after the beginning of the CMA for the bit error ratio (BER) to fall below -3 10 for different T and I when the OSNR is 9 dB. For the top graph of Fig. 3a, the polarization back rotation from the correlations is not applied, so that the CMA does the polarization separation. The bottom graph of Fig. 3a has the correlation back rotation implemented so that the polarizations are separated before the CMA starts. The top graph of Fig. 3a has points with slow CMA settling, and the evolution of the average BER for one of those points is shown in the top graph of Fig. 3b. The bottom graph of Fig. 3b shows the BER settling with the correlation back rotation implemented before the CMA starts. The maximum number of symbols -3 for the BER to be below 10 is reduced from 3840 symbols without the correlation back a)
rotation to 1440 symbols with the correlation back rotation. Furthermore, the correlation back rotation guarantees that the same bit sequence will not be recovered on both x and y outputs of the digital receiver. Fig. 3c shows the BER versus OSNR for the LO laser at a continuous wavelength and switching, with the signal laser continuously at O1 and O2. The BER measurements are done on the packet payloads 1440 symbols after the correlation pattern until the end of the packets. OSNR penalties for LO switching versus continuous wavelength operation were less than 0.5 dB for both wavelengths. Conclusions We demonstrated a fast wavelength-tunable burst-mode receiver for DP-QPSK optical packets. By using a constant header followed by a 64-bit correlation pattern, we recovered the frequency offset, the packet start time and the polarization state. The polarizations were separated using the correlations, which reduced the data recovery times by a factor of 2.7. References [1] P. J. Winzer et al., IEEE J. Lightwave Technol., vol. 28, no. 4, pp. 547-556, 2010. [2] J. E. Simsarian et al., Proc. OFC’10, postdeadline paper PDPB5. [3] C. Xie and S. Chandrasekhar, Proc. OFC’10, paper OMK3. [4] R. Maher et al., IEEE J. Lightwave Technol, vol. 30, no. 24, pp. 3924-3930, 2012. [5] F. Vacondio, et al., Proc. OFC’12, paper OM3H.6. [6] R. Dischler, Proc. ECOC’11, paper Mo.2.A.6. [7] J. E. Simsarian et al., IEEE Photon. Technol. Lett., vol. 23 , no. 16, pp. 1124-1126, 2011. [8] M. Kauer et al., Proc. ECOC’02, vol. 2, paper 3.3.3. [9] G. Souhaité et al., Proc. ECOC’01, paper Tu.F.3.2, pp. 196-197. [10] A. Leven et al., IEEE J. Selected Topics in Quantum Electronics, vol. 16, no. 5, pp. 1227-1234, 2010.
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Fig. 3: a) The number of symbols for the BER to fall below 10 for different T and I without correlation back rotation (top) and with correlation back rotation (bottom). b) BER versus number of symbols after the correlation pattern without correlation back rotation (top) and with correlation back rotation (bottom). c) Packet data BER versus OSNR for the LO continuous (CW) and switching (SW) at O1 and O2.
