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We demonstrate an optical Nyquist pulse TDM (Nyquist OTDM) transmission at 160 Gbaud with a substantial increase in the dispersion tolerance compared with a conventional OTDM transmission. Optical Nyquist pulses can be bit-interleaved to ultrahigh symbol rate without suffering from intersymbol interference due to its zero-crossing property at every symbol interval. This allows the signal bandwidth to be greatly narrowed compared to typical pulse waveforms such as Gaussian or sech profile. By virtue of this property, a dispersion tolerance over ± 8 ps/nm was successfully realized in 160 Gbaud, 525 km transmission.
©2012 Optical Society of America
1. Introduction
An increase in spectral efficiency allows both capacity expansion within a finite transmission bandwidth, and an enhanced tolerance to chromatic dispersion (CD) and polarization-mode dispersion (PMD) in an ultrahigh-speed transmission. Recently, a number of electrical CD and PMD compensation techniques have been demonstrated along with advances in digital coherent technologies [1, 2]. Although these techniques can eliminate the need for optical compensation techniques, it is difficult to apply them to ultrahigh-speed systems with a symbol rate beyond the speed and bandwidth limits of electronic devices such as A/D converters and digital signal processors (DSP).
Optical time division multiplexing (OTDM) is a simple and direct approach to achieving an ultrahigh symbol rate in a single wavelength channel, and thus it can overcome these limitations. OTDM is also compatible with coherent transmission, either by using a coherent homodyne receiver in conjunction with a pulsed local oscillator (LO), which also functions as an OTDM demultiplexer [3], or by combining a conventional OTDM demultiplexer, an RZ-CW converter, and a coherent receiver with a CW-LO [4]. However, in both cases, the signal distortion has to remain within the OTDM symbol period, otherwise individual tributaries cannot be demultiplexed accurately. This indicates that it is necessary to adopt CD and PMD compensation optically in front of the coherent receiver. It is therefore desirable to use an optical pulse with large CD and PMD tolerance. However, typical pulse waveforms such as Gaussian or sech profiles have rapidly decreasing tails, which generally occupy a large bandwidth in the frequency domain and thus may not be an appropriate waveform in terms of CD and PMD tolerance.
We recently proposed a new type of optical pulse, which we call an “optical Nyquist pulse”, whose shape is given by the sinc-function-like impulse response of the Nyquist filter [5]. With a conventional digital Nyquist filtering technique [6–9], which is employed for a baseband data sequence in a DSP, the symbol rate is limited to the speed of electronics. By contrast, optical Nyquist pulses can be easily multiplexed to an ultrahigh symbol rate by employing OTDM. The optical Nyquist pulse allows the signal bandwidth to be narrowed without intersymbol interference (ISI), and therefore tolerance to CD and PMD can be greatly improved. Furthermore, by combining this scheme with QAM, it is possible to achieve an ultrahigh bit rate and spectral efficiency simultaneously. In this paper, based on the generation of an optical Nyquist pulse and its OTDM multiplexing and demultiplexing as presented in [5], we realize the 160 Gbaud transmission of Nyquist OTDM signals over 525 km for the first time, and demonstrate a substantial improvement in dispersion tolerance compared with that of a conventional OTDM transmission with Gaussian pulses.
2. Experimental setup for 160 Gbaud OTDM transmission using optical Nyquist pulse train
Figure 1(a) shows our experimental setup for the 160 Gbaud OTDM transmission of optical Nyquist pulses over 525 km. In the transmitter, we first generated an optical Nyquist pulse train at a repetition rate of 40 GHz from a mode-locked fiber laser (MLFL) that emits 40 GHz, 1.8 ps Gaussian pulses at 1545 nm. The Gaussian pulses were transformed into a Nyquist profile by using a spectrum manipulation technique based on the spatial intensity and phase modulation of spectral components with a liquid crystal spatial modulator [10]. Here, the amplitude waveform and its frequency spectrum are defined as follows:
where T is the symbol period, and α (0 ≤ α ≤ 1) is known as a roll-off factor [11]. Here we set T = 6.25 ps for 160 Gbaud OTDM, and α = 0.5. The generated waveform is shown in Fig. 1(b), which accurately fits the profile in Eq. (1). It should be noted that in Eq. (1) the pulse power has an oscillating tail but it becomes zero at t = nT (n: integer), and therefore there is no ISI at every symbol interval after the OTDM despite a strong overlap with neighboring pulses.
Fig. 1 Experimental setup for 160 Gbaud Nyquist pulse transmission over 525 km (a), and the waveforms of 40 GHz and 160 Gbaud OTDM Nyquist pulses (b).
The 40 GHz optical Nyquist pulses were then DPSK modulated at 40 Gbit/s, 215–1 PRBS, and multiplexed to 160 Gbaud using a delay-line bit interleaver. An eye diagram of the obtained Nyquist OTDM signal is shown in Fig. 1(c). The OTDM signal becomes an analog-like continuous data stream, and the eye diagram appears greatly distorted due to interference. However, as indicated by the blue dots, no ISI occurs and a constant level is maintained at every symbol interval. The 160 Gbaud Nyquist pulses were transmitted over a 525 km dispersion-managed transmission link, where each 75 km span comprises a 50 km single-mode fiber (SMF) and a 25 km inverse dispersion fiber (IDF) that compensates for the dispersion and dispersion slope of the SMF simultaneously. The launched power into each span was + 6 dBm, and the span loss was compensated for with EDFAs.
In the receiver, the transmitted Nyquist pulse was first demultiplexed from 160 to 40 Gbaud. Here, unlike conventional OTDM demultiplexing, we adopted an optical sampling technique so that only data at the ISI-free point could be extracted from the continuous data stream. As an ultrashort optical sampler, we employed a nonlinear optical loop mirror (NOLM) switch. The NOLM was composed of a 100 m highly nonlinear fiber (HNLF) with γ = 17 W−1km−1, a dispersion slope of 0.03 ps/nm2/km, and a zero-dispersion wavelength of 1554 nm. The sampling pulse source was a 40 GHz MLFL emitting a 720 fs pulse at 1563 nm, which was PLL operated with a 40 GHz clock extracted from the transmitted data. This enabled a switching gate width of 1.0 ps. At the NOLM output, the sampled signal was separated from the sampling pulse with a 15 nm optical filter, followed by preamplification, demodulation with a one-bit delay interferometer (DI), and balanced detection.
3. Experimental results
We manually added residual dispersion to the transmission link and evaluated the dispersion tolerance of the 160 Gbaud signal in a 525 km transmission. Figure 2 (a-1) and (b-1) show the Nyquist OTDM signal waveforms after a 525 km transmission with residual dispersions of 0 and + 3 ps/nm, respectively. The corresponding demultiplexed waveforms are shown in Fig. 2(a-2) and (b-2), respectively. For comparison, we carried out the same experiment with a 1.8 ps Gaussian pulse, where we adopted an electro-absorption modulator as a conventional OTDM demultiplexer with a gate width of 4.8 ps. The results are shown in Fig. 3 . From these figures, it can be seen that Gaussian pulses suffer from strong ISI due to a dispersion-induced pulse overlap even with such a small residual dispersion as shown in Fig. 3(b-1) and (b-2), while the dispersion does not lead to significant distortions with Nyquist pulses as shown in Fig. 2(b-1) and (b-2), resulting in greater resilience to residual dispersion. Figure 4(a) and (b) show the bit-error rate (BER) characteristics corresponding to these transmissions. The additional power penalty associated with the residual dispersion is only ~1 dB with Nyquist pulses as shown in Fig. 4(a). On the other hand, the dispersion leads to a large power penalty and an error floor as shown by the red curve in Fig. 4(b) in spite of the use of an EAM as a demultiplexer instead of a NOLM, which should offer better BER performance because there is less OSNR degradation during demultiplexing.
Fig. 2 160 Gbaud Nyquist pulses after 525 km transmission with residual dispersion of 0 (a-1) and 3.0 ps/nm (b-1). (a-2) and (b-2) are the corresponding demultiplexed waveforms.
Fig. 3 160 Gbaud Gaussian pulses after 525 km transmission with residual dispersion of 0 (a-1) and 3.0 ps/nm (b-1). (a-2) and (b-2) are the corresponding demultiplexed waveforms.
Fig. 4 BER characteristics for 160 Gbaud-525 km transmission with Nyquist (a) and Gaussian (b) pulses.
We further examined the dispersion tolerance by measuring the receiver sensitivity for the two pulses at various residual dispersion values. Here we defined the receiver sensitivity as the received optical power needed to achieve a BER of 10−9. Figure 5 shows a comparison of the receiver sensitivities. It can be seen that a Gaussian pulse leads more rapidly to performance degradation, and error-free performance was not achieved for a residual dispersion of ± 4 ps/nm and beyond. On the other hand, a Nyquist pulse is still tolerable even for a dispersion of ± 8 ps/nm, and the degradation is much slower against the residual dispersion. It should be noted that simply reducing the signal bandwidth by employing broader Gaussian pulses does not contribute to performance improvement, but rather leads to greater impairments due to the larger ISI associated with a strong pulse overlap regardless of the dispersion.
Fig. 5 Comparison of the receiver sensitivity between Nyquist and Gaussian pulses for various residual dispersion values.
A numerical analysis of the dispersion dependence of the Nyquist pulse train was already reported in Ref [5], where we showed that a Nyquist pulse has better ISI characteristics than Gaussian pulses. However, to take advantage of the zero-ISI property of optical Nyquist pulses in a long-haul transmission, it is important to determine the degree to which the periodic zero crossing in the pulse tail is maintained during propagation. We carried out a numerical analysis of the propagation of a single optical Nyquist pulse to clarify the influence of dispersion on the zero-crossing property and its periodicity. We found that the increase in the dispersion preserves the periodicity in the tail oscillation, while it causes a gradual increase in the zero level.
Figure 6 shows the ratio of the Nyquist pulse intensity at the adjacent symbol point t = T with respect to the peak intensity, η = |u(T)|2/|u(0)|2, for various dispersion values. The red curves show η for optical Nyquist pulses with α = 0, 0.5, and 1, and the blue curves are the results for Gaussian pulses with FWHMs of 2.5, 3.5, and 4.5 ps. In general, the Nyquist pulses can maintain a lower η than the Gaussian pulses when the dispersion is increased. This numerical analysis supports the present experimental demonstrations as regards the increased tolerance to dispersion with the optical Nyquist pulse.
Fig. 6 Growth of the zero level at the adjacent symbol point t = T against dispersion. The red and blue curves are the results for optical Nyquist and Gaussian pulses, respectively.
We have also analyzed the interplay between dispersion and nonlinearity, and found that the zero-crossing point starts to deviate but its magnitude and direction depend on the sign of the dispersion. Detailed results will be presented elsewhere.
4. Conclusion
We realized a 160 Gbaud transmission of optical Nyquist pulses, and demonstrated their dispersion tolerance in a 525 km transmission. Optical Nyquist pulses were found to be tolerant to a residual dispersion of over ± 8 ps/nm by virtue of their narrow bandwidth and without sacrificing ISI. This is more than twice the tolerance that can be achieved with Gaussian pulses. This scheme is potentially scalable to a higher symbol rate per channel such as 1 Tbaud, and is also expected to offer a large tolerance to higher-order dispersion and PMD. Moreover, it offers the possibility of achieving a spectral efficiency approaching the Shannon limit by employing coherent QAM.
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