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Optical Data Transmission with Fluctuating Frequency Offset and Strong Phase Noise James Mountjoy1 , Anthony J. Walsh2 , Barry Cardiff3 , John A. O’Dowd4 , L. P. Barry2 , Andrew Ellis25 and Anthony Fagan1 1 University College Dublin, Dublin, Ireland. 2 Dublin City University, Dublin, Ireland. 3 Now with S3 Group, Sandyford, Dublin, Ireland. 4 Now with FAZTech Research Limited, 9B Beckett Way, Park West, Dublin 12, Ireland. 5 Now with Aston University, Birmingham B4 7ET, England.
[email protected]
Abstract: We examine data transmission during the interval immediately after wavelength switching of a tunable laser and, through simulation, we demonstrate how choice of modulation format can improve the efficacy of an optical burst/packet switched network. © 2013 Optical Society of America OCIS codes: (060.5060) Phase Modulation; (140.3600) Lasers, tunable; (060.4265) Networks; wavelength routing.
1.
Introduction
Fast switching semiconductor wavelength tunable lasers, e.g. Sampled Grating Distributed Bragg Reflector (SG-DBR), have been the subject of research for the last few decades. A key application area for such lasers is high-speed optical burst switched networks and optical packet switched networks [1]. Typically, in such networks, data transmission is suspended for hundreds of nanoseconds during switching periods (until the laser has been locked to the destination wavelength) impacting the overall throughput of the network. In this paper, we evaluate the temporally-resolved error rate performance of different modulation schemes during these switching transient intervals. Our results show that Double Differential Phase Shift Keying (DDQPSK) can deliver error-free performance within 2 ns of the commencement of a laser switching event. For the purpose of this investigation SG-DBR lasers have been used, and the modulation schemes considered are Quadrature Phase Shift Keying (QPSK), Differential QPSK (DQPSK), and DDQPSK. Whilst it is known that the additive white Gaussian noise (AWGN) performance of the differential schemes is inferior to QPSK [2, 3], we examine the performance of these schemes under conditions that would typically be endured in the moments after a wavelength switch, such as a large dynamic frequency offset (FO) with an associated high level of stochastic phase noise. The comparison is done through simulation based on an experimental characterisation of a SG-DBR laser which has just switched wavelength. 2.
Signal Characteristics
The carrier signal immediately after a switching event of the tunable laser shows an initial offset to the desired frequency which the laser is switching to. It can be seen in Fig. 1 that the instantaneous frequency is liable to change suddenly and severely, then gradually settles to the desired frequency. This presents the problem that there is a signal on which data is to be modulated which has the following undesirable characteristics: it appears at an unknown FO but
(a)
(b)
Fig. 1: Ensemble average (of 80 bursts) of the frequency deviation transient after a wavelength switch from (a) 1548nm to 1561nm and from (b) 1561nm to 1548nm
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within an expected range; it is liable to change frequency rapidly and it has a severe level of phase jitter. The challenge addressed in this paper is to investigate appropriate modulation schemes for this type of signal. 3. 3.1.
Modulation Schemes Quadrature Phase Shift Keying with Second Order Digital Phase-Locked Loop
Due to the coherent nature of QPSK, it is necessary to implement a second order digital phase-locked loop (PLL) in the receiver to track both phase and frequency errors. The PLL was tuned to tolerate the random phase jitter of the tunable laser while tracking the FO [4]. 3.2.
Differential Quadrature Phase Shift Keying with First Order Digital Phase-Locked Loop
Differential encoding is well known to be effective when there is a static phase offset without the need of a PLL [2]. However, in this case, where there is a FO, a first order PLL is necessary to correct for a phase offset. One of the advantages of a differential scheme is that there is no error propagation, which is useful for a signal that is liable to change suddenly. Note that a first order PLL may not perform well when there is a large shift in instantaneous frequency. One of the disadvantages of DQPSK is that when AWGN is the only impairment there is a ∼ 3dB penalty compared to the QPSK case [2]. 3.3.
Double-Differential Quadrature Phase Shift Keying
Originally designed to deal with Doppler shift in satellite communications [5], DDQPSK is rarely considered in the modern communications community since it performs significantly worse than QPSK and DQPSK when AWGN is the only impairment [3]. The structure of a DDQPSK encoder is simply a concatenation of two DQPSK encoders. DDQPSK is particularly useful because for a static FO it provides perfect correction and DDQPSK will correct for a linearly changing FO with a remaining phase offset. It does not suffer from error propagation, though it does suffer from error multiplication due to a symbol’s reliance on its previous two symbols which contributes to the poorer performance in AWGN [3]. More recently, work has been done to improve robustness of DDQPSK to AWGN for optical communications with a static FO [6]. 4. 4.1.
Methods Experimental Data
An experiment was performed using a coherent receiver, and a low linewidth laser as a local oscillator, which captured the In-phase and Quadrature components of the SG-DBR laser in the moments immediately following a wavelength switching event. The data thus obtained are the basis for the simulation work presented in this paper. In the experiment the SG-DBR laser was switched from an initial wavelength λ1 to a wavelength λ2 . The signal was mixed with another laser (with a low linewidth) having a wavelength λLO , and the output, now at an intermediate frequency, was captured as a set of complex samples for post-processing. Naturally, the exact frequency domain trajectory observed depends on the source and destination wavelengths. In this summary paper we concentrate on one specific scenario (80 instances of an SG-DBR switching from 1548nm to 1561nm as in Fig. 1a), comparable results were also obtained for other switching scenarios.
(a) QPSK with 2nd Order PLL
(b) DQPSK with First Order PLL
(c) DDQPSK
Fig. 2: Constellation Graphs for 10 Consecutive Bursts After Switching Events.
SP4D.5.pdf
(a) QPSK with Second Order PLL
(b) DQPSK with First Order PLL
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(c) DDQPSK
Fig. 3: Ensemble average of when errors occurred and the EVM with respect to time (for 80 bursts).
4.2.
Simulation Setup
Taking the data captured above as our transmit carrier signal, a simulation was constructed where said carrier was modulated with each of the three modulation schemes under consideration here. This was done for all data sets and averaged SER results were obtained as a function of time from the switching event. Since it is known how robust each scheme is to AWGN [2, 3], this paper focuses on the effect of the noisy carrier signal without adding the dispersive effects of the channel nor AWGN. 5.
Results
The results in Fig. 2 show constellation diagrams for the three modulation schemes over 10 bursts of the switching signal. Fig. 2a shows a received QPSK constellation for which the symbol error rate (SER) is 0.0137 and the error vector magnitude (EVM) is 3.31%. Fig. 2b shows a DQPSK constellation with a SER of 0.06335 and an EVM of 17.24%. Fig. 2c shows a DDQPSK constellation with a SER of 0.0049 and an EVM of 5.18%. The DDQPSK shows the best overall performance, usually having between 0 and 2 errors per burst. Fig. 3 highlights the points at which the errors occurred (as a percentage of all the errors) during the switching signal and how the EVM changed with time. For DDQPSK (in Fig. 3c) 100% of the errors occurred in the first few nanoseconds of the signal and there were no further errors despite the consistently high EVM. Fig. 3b shows a lot of errors occurring for DQPSK during sudden shifts in frequency. QPSK incurred some errors while the PLL was converging at the start of the signal but performed well once the PLL had locked, with an EVM of less than 10% for the majority of the duration of the burst. 6.
Conclusion
We have studied three modulation formats immediately after a switching event of a wavelength tunable laser to determine the optimum format to employ after switching in an optical burst/packet switched network. We have shown that DDQPSK is a viable scheme which produces good performance (when AWGN is low) on a recently switched laser. A feasible system might include moving through modulation formats using an appropriate protocol. Transmitting data as soon as possible after the switching event will greatly enhance network throughput and bandwidth utilisation, especially as we move towards packet switched systems. References 1. M. Duser and P. Bayvel, “Analysis of a dynamically wavelength-routed optical burst switched network architecture,” Lightwave Technology, Journal of 20, 574–585 (2002). 2. J. Proakis, Digital Communications (McGraw-Hill, 2000), 4th ed. 3. D. van Alphen and W. Lindsey, “Higher-order differential PSK modulation,” Communs., IEEE Trans. (1994). 4. R. Best, Phase-Locked Loops (McGraw-Hill, 1999). 5. M. Pent, “Double differential PSK scheme in the presence of doppler shift,” in “AGARD,” (1979). 6. M. Nazarathy, A. Gorshtein, and D. Sadot, “Doubly-differential coherent 100g transmission: Multi-symbol decision-directed carrier phase estimation with intradyne frequency offset cancellation,” in “Signal Processing in Photonic Communications,” (Optical Society of America, 2010), p. SPWB4.
