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We demonstrate simultaneous NRZ-to-RZ conversion for 16 DWDM channels, using a single SOA and a subsequent delay interferometer (DI) acting as a comb-like filter to control the obtained pulse-width for all of the channels. The SOA is operated in deep saturation, resulting in weak cross gain modulation and cross phase modulation induced crosstalk between different NRZ channels. By adjusting the detuning between the peaks in the DI spectrum and each corresponding carrier, good quality RZ signals with different duty cycles can be achieved. Bit-error-rate measurements show negative power penalties for the obtained RZ signals with different duty cycles. Significant timing jitter reductions for all channels show the good regenerative performance of the proposed converter.
©2008 Optical Society of America
1. Introduction
All-optical networks are likely to be a hybrid of wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) networks, adopting the advantages of both technologies. The return-to-zero (RZ) format is widely used in OTDM networks due to its tolerance to fiber nonlinearities in spite of dispersion-induced effects, while the non-return-to-zero (NRZ) is preferred in WDM networks for its ease of implementation, relatively high spectral efficiency and timing-jitter tolerance. Therefore, format conversion between NRZ and RZ is a desirable function at the nodes of OTDM and WDM networks. Several papers have demonstrated all-optical format conversions from NRZ-to-RZ [1–5]. These approaches, though achieving the format conversions based on various techniques, remain inherently single-channel operation and thus cannot offer the advantage of parallel optical processing. On the other hand, only a few papers have demonstrated multi-channel NRZ-to-RZ conversions on WDM grid although most of them are with limited channels and large wavelength spacing [6, 7]. Multi-wavelength all-optical conversions between NRZ and RZ modulation formats with tunable duty cycle are highly desirable for WDM network and all-optical parallel signal processing.
In this paper, for the first time to the best of our knowledge, we demonstrate simultaneously 16*10Gb/s (with wavelength spacing of 0.8nm on ITU DWDM grid) NRZ to RZ format conversions, based on a single semiconductor optical amplifier (SOA) and a subsequent delay interferometer (DI) to control the duty cycle of the resulting 16 channels RZ signals. By saturating the SOA with a high power optical sine-wave clock signal, the 16 NRZ channels only experience cross phase modulation (XPM) induced by the clock signal, with very tiny cross gain modulation (XGM) and XPM effect between different channels. Therefore the 16 channels can carry different data. Different output duty cycles can be achieved by adjusting the detuning between the peaks in the DI spectrum and each corresponding carrier. Experimental results agree well with simulations at 10 Gb/s. Bit-error-rate (BER) results show that the conversions can be achieved with negative power penalty, for output RZ signals with different duty cycles. The root-mean-square (RMS) timing jitter for all the channels is also significantly reduced by the proposed format converter. The XPM-based multi-channel scheme is robust in terms of having a rather simple structure, being easy to integrate, can be operated over a broad wavelength band, and has potential to operate at 40Gb/s and beyond [4].
2. Operation principle and simulated results
As shown in Fig. 1, the principle is similar to the non-inverted wavelength conversion based on a SOA and a filter [8]. The SOA, acting as a nonlinear element, causes the spectrum of the input NRZ signal to be broadened due to the XPM effect. The DI filter, which has a comb-like spectral response, is used to extract the specific desired spectra from the broadened spectra, for each of the 16 channels at the same time. An additional tunable filter with 0.3nm bandwidth is used to filter out one of the converted channels for evaluation. Simply speaking, the NRZ signals will generate transient frequency shifts within the control signal duration and the filter, with proper detuning, will only transmit frequency shifted components caused by the XPM. For multi-channel operation, in order to minimize the XGM induced inter-channel crosstalk, the SOA should be deeply saturated by the control signal (clock signal).
The pulse-width of the converted RZ signal is proportional to the pulse width of the clock signal, and a short clock pulse of several picoseconds is generally necessary to achieve enough XPM effect and hence achieve RZ signal with small duty cycle [4, 6]. However, in this case, an additional short pulse source is needed which will increase the system cost. Alternatively, a detuned filter after the SOA can be used to overcome the reduced XPM induced by the sine wave clock signal. Besides avoiding the expensive short pulse source, this method offers another important advantage: the pulse width of the final RZ signal can be finely controlled by choosing different filter detuning, which gives the conversion flexibility.
Table 1. Parameter values used in simulation
Using the parameters given in Table 1, Fig. 2 shows the simulated eye diagrams for one of the 16 channels. With a given filter detuning (0.2nm), Figs. 2(a), (b) and (c) are the converted RZ signals from one of the 16 channels, for clock input power of 5, 9 and 12dBm respectively. We can see that, with increasing clock power, the XGM induced crosstalk reduces gradually, resulting in the reduction of the output RZ amplitude jitter. Please note that the bit patterns of the 16 channels are different and the 16 channel NRZ signals can be converted to RZ signals individually, when the SOA is deeply saturated.
On the other hand, with a fixed optimal input clock power (12dBm), Figs. 2(d), (e) and (f) show the converted RZ signals with different duty cycles, for filter detuning of 0, 18 and 35GHz (0, 0.15 and 0.28nm), respectively. Although the pulse width is suppressed as the filter detuning increases, the detuning cannot be too large or else there will be crosstalk from the adjacent channel.
3. Experimental setup and Results
The experimental setup is shown in Fig. 3. 16 WDM channels (wavelength from 1547.79 to 1559.79nm with spacing of 0.8nm) are coupled into two MZMs with two AWGs. The odd channels are driven by the 10Gb/s data (PRBS 231-1) from an Anritsu 10G pattern generator; while the even channels are driven by data. An optical delay line is added to the odd channels to ensure decorrelation from the even. Another CW beam (1546.99 nm) is fed into a third MZM, which is driven by the 10GHz sinusoidal clock signal, to obtain an optical clock signal as the control input to the SOA. The delay of the RF clock can be adjusted to synchronize the NRZ signals. Then the signals are launched into SOA via a WDM coupler. The average powers of the NRZ signals and the clock signal are approximately 3 and 13 dBm respectively before the SOA. The SOA is a Kamelian device biased at 160mA. A fiber based DI with free spectral range (FSR) of 0.8nm is used to extract part of each of the broadened DWDM channel spectra. By controlling the operating temperature, its transmission peaks are adjusted to be offset from each carrier wavelength with optimal detuning. Thus, the chirp induced on each channel is transmitted, while the original spectral components are suppressed to some extent and hence format conversions from NRZ to RZ can be achieved. A subsequent tunable filter with a 0.3nm 3dB bandwidth is used to filter out one of the converted channels for evaluation.
For practical all-optical format conversion, the clock can be recovered from one of the NRZ signals, as we have demonstrated in [9]. Since clock extraction is not the purpose of this experiment, for simplicity we obtained the optical clock by modulating a MZM with the RF clock signal directly from the pattern generator.
Since the SOA is deeply saturated by the clock signal, input NRZ signals are only modulated by the XPM, almost without amplitude modulation. Thus, the 16 channels can carry different patterns, which have been confirmed by simulation. However, for experimental convenience, only two patterns are used here.
The insets in Fig. 3 show the measured bit stream, for both odd and even channels, with optimal filter detuning. Results show that the proposed converter works well for different bit patterns simultaneously. The measured spectra before the SOA, after the SOA, after the DI and one of the converted spectra (channel 4) are shown in Fig. 4. It is obvious that all the NRZ spectra are broadened due to the modulation of the clock signal after the SOA, regardless of the spacing between the clock signal and each NRZ signal.
Fig. 4. Spectra of 16 DWDM channels (a) before SOA (b) after SOA (c) after DI and (d) one of the converted channels (channel 4).
Taking one of the 16 channels (channel 4) as a representative example, Fig. 5 shows the evolution of the obtained duty cycle with increasing DI and the following filter detuning. It can be seen clearly that the pulse width of the converted RZ without detuning is 80 ps, which is wider than the pulse width of the pump clock signal. In other words, the modulation induced by the clock signal is very slight. The pulse width can be significantly compressed to 47 ps with a filter detuning of 0.25nm. Although larger detuning can further compress the pulse width, the cross talk from adjacent channel becomes significant.
Figure 6 plots the BER measurements for the format conversions, which are taken for channels 3 (odd channel) and 16 (even channel) for each of the three different duty cycles. We can see that negative power penalty can be achieved, for the RZ signals with duty cycles of 47, 58 and 70%, respectively. The back to back eye diagram of NRZ signal from one of the 16 channels is also shown in Fig. 6.
Since external optical clock signal is used, our proposed converter has regenerative properties and suppresses timing jitter. To further investigate the retiming performance, the input 16 channel NRZ signals are distorted by attenuating their power and passing them through an EDFA to add ASE. Figure 7 presents the RMS timing jitter measurements, a reduction of timing jitter, from approximately 7 to 2ps, can be observed for all channels.
4. Conclusion
In conclusion, we have demonstrated simultaneous NRZ to RZ conversions of 16 DWDM channels, using a single SOA and a detuned DI filter. The multi-channel conversions are based on XPM in the deeply saturated SOA. By adjusting the detuning of the carrier and subsequent optical DI filter, RZ signals can be obtained with different duty cycles. Negative power penalties are observed, for achieved RZ signals with different duty cycles. Greater than 3 times timing jitter reduction can be achieved for all 16 channels after the conversion.
Acknowledgement
This work was supported by National High Technology Developing Program of China (Grant No 2006AA03Z0414), the Engineering and Physical Sciences Research Council (EPRSC) of UK and the China Scholarship Council.
References and links
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