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We proposed and demonstrate an all-optical demodulation and format conversion scheme for multi-channel (carrier suppressed) return-to-zero differential phase shift keying ((CS)RZ-DPSK) signals. By utilizing a single delay interferometer (DI) with half bit delay, multi-channel (CS)RZ-DPSK signals can be demodulated simultaneously at the destructive port of the DI, with the corresponding converted nonreturn-to-zero differential phase shift keying (NRZ-DPSK) signals obtained at the constructive port. The proposed multi-channel operation has been demonstrated for 6*20 Gb/s RZ-DPSK and 6*40 Gb/s CSRZ-DPSK signals, with ~0.8 and 1.2 dB average power penalties for the format conversions respectively.
©2011 Optical Society of America
1. Introduction
All-optical modulation format conversion is a key function in providing flexible management and interface for wavelength division multiplexed (WDM) and optical time division multiplexed (OTDM) networks [1]. In the past, many researchers had demonstrated various conversion schemes for on-off keying (OOK) signals [2–6]. Compared with the OOK format, differential phase shift keying (DPSK) format has attracted more attention nowadays due to its superior transmission performance and improved receiver sensitivity with balanced detection [7], and thus many papers proposed and demonstrated format conversion between OOK and DPSK [8–10] or between different types of DPSK formats [11,12]. Recently, several papers proposed and demonstrated format conversion and other signal processing for multi-channel OOK and DPSK operation based on a single device [13–17]. These provide much more flexibility for future network interface, enable optical parallel processing, reduce system cost and thus have very promising merits.
In this paper, we propose and demonstrate a parallel multi-channel return-to-zero DPSK (RZ-DPSK) signal processing, using a single fiber based delay interferometer (DI). By utilizing a DI with a half bit delay, simultaneous demodulation and RZ-DPSK to nonreturn-to-zero DPSK (NRZ-DPSK) format conversion can be achieved for multi-channel input. The principle for demodulation is based on multi-channel destructive interference, while the format conversion is using the multi-channel constructive interference. The proposed scheme is applicable to RZ-DPSK signal with different duty-cycles. For demonstration, 6*20 Gb/s RZ-DPSK signals with 33% duty-cycle and 6*40 Gb/s RZ-DPSK signals with 66% duty-cycle (i.e. carrier suppressed DPSK (CSRZ-DPSK)) are demodulated and converted to corresponding NRZ-DPSK signals, with small power penalties respectively. Clear open eye diagrams show a good performance for the proposed multi-channel demodulator and format converter.
2. Operation principle and experimental setup
The experimental setup is shown in Fig. 1 . 6 DFB lasers at different wavelength, which suit the ITU grid and with optimal channel spacing, are combined by an array waveguide grating (AWG) and modulated by two Mach-Zehnder modulators (MZMs) to generate the multi-channel RZ-DPSK signals with different duty-cycles and different bit-rates. The pseudorandom bit sequence (PRBS) length is 231-1. The generated signals are amplified to an optimal power by an erbium-doped fiber amplifier (EDFA) and an attenuator (ATT). Then the multi-channel signals are processed through a DI with half bit delay. Due to the periodical filtering property of the DI, the multi-channel demodulated signals are obtained at the destructive port, while the converted NRZ-DPSK signals can be obtained at the constructive port at the same time. The demodulated and converted outputs are demultiplexed by two AWGs with channel bandwidth of 50GHz, which perform further narrow filtering for the processed signals. The final results can be analyzed by the Optical Spectra Analyzer (OSA) and the Communication Signal Analyzer (CSA) respectively. The bit error ratio (BER) can be analyzed by the Error Analyzer (EA) after a single-end receiver. Although the balance detection will have a 3dB receive sensitivity improvement for DPSK signal, the single-end can also reflect the conversion performance.
In our experiment, the six channels are from a same pattern generator which means they are correlated. However, our proposed format conversions are based on a passive DI, and the conversion principle is based on the interference between adjacent pulses within the single channel. The crosstalk between different channels is very small if the channel spacing is relatively large. Thus, the decorrelation is not necessary for our scheme.
The principle for the DI based DPSK demodulation is well-known and it is straightforward to extend this technique to multi-channel operation [18]. For conventional demodulation, the delay of the DI is one bit, and the Alternative Mark Inversion (AMI) signal can be achieved at the destructive port. If the delay time is less than one bit, the AMI signal will be still obtained with amplitude ripples, which can be reduced by the subsequent filtering.
It is also well-known that the constructive interference can be applied to RZ-OOK to NRZ-OOK format conversion, both for single [2] and multi-channel [13] operation. Here, for the first time to our best knowledge, we extend the constructive interference to DPSK signal format conversion. Taking one channel for example, Fig. 2 represents the process of the proposed format conversion. By controlling the working condition of the DI, the original RZ-DPSK signal (Fig. 2(a)) will interfere with the signal after a half bit delay (Fig. 2(b)), and as a result the constructive output is shown as the blue line in Fig. 2(c). The output amplitude will only be determined by the relative phase difference between the original and delayed pulses, while the phase information is preserved as the input RZ-DPSK signal.
For the constant phase part of the original signal, the constructive interference can be easily achieved by controlling the DI, leading to a constant amplitude output with ripples on the top of the NRZ-DPSK. The ripples are due to the duty cycle of the input is not large enough, and can be improved by further narrow filtering [2]. In our experiment, the subsequent filtering can be achieved by the AWG, which is utilized for demultiplexing. On the other hand, for the phase transit part of the original signal (the phase changes between 0 and π), the original and the delayed pulse will experience destructive interference under the constructive condition, which leads amplitude dip in the NRZ-DPSK output. As a result, the converted NRZ-DPSK signal is with the same amplitude property (i.e. the dips between phase transits) as a conventional NRZ-DPSK signal generated by a push-pull MZM.
3. Results and discussions
For demonstration, RZ-DPSK signals with different bit-rates and duty cycles are utilized. For RZ-DPSK signals with 33% duty cycle, 6 channel signals at 20 Gb/s with 200 GHz channel spacing are processed by a DI with 40 GHz free spectra range (FSR). The spectra are presented in Figs. 3(a) to 3(c), showing the original input 6 RZ-DPSK signals, the demodulated signals and the converted multi-channel NRZ-DPSK signals respectively. By demultiplexing with an AWG with 50 GHz bandwidth, the corresponding eye diagrams for two selected channels (Ch. 1 and Ch. 2) are shown in Figs. 3(d) to (i). Figures 3(d) and (g) are the eye diagrams of Ch.1 and Ch.2 input RZ-DPSK signals, Figs. 3(e) and (h) are the corresponding demodulated AMI eye diagrams, and Figs. 3(f) and (i) are the corresponding converted NRZ-DPSK eye diagrams.
Fig. 3 Measured spectra and eye diagrams for 6-channel RZ-DPSK processing at 20 Gb/s. (a)-(c) Spectra of the original input 6 RZ-DPSK signals, the demodulated signals and the converted multi-channel NRZ-DPSK signals; (d) to (f) the eye diagrams of the Ch.1 RZ-DPSK, the demodulated AMI and the converted NRZ-DPSK signals; (g) to (i) the eye diagrams of the Ch.2 RZ-DPSK, the demodulated AMI and the converted NRZ-DPSK signals.
As we analyzed, the constructive interference is insensitive for the signal phase, and thus the proposed multi-channel signal processing is also capable for RZ-DPSK signals with 66% duty cycles (i.e. CSRZ-DPSK). We further use a DI with 80 GHz FSR to simultaneously demodulate and convert 6 CSRZ-DPSK signals at 40 Gb/s (400 GHz channel spacing). The measured spectra and eye diagrams (Ch. 2 and Ch. 5) are shown in Fig. 4 . Figures 4(a) to (c) show the spectra of the original input 6 CSRZ-DPSK signals, the demodulated signals and the converted multi-channel NRZ-DPSK signals respectively. Figures 4(d) and (g) are the eye diagrams of Ch.2 and Ch.5 input RZ-DPSK signals, Figs. 4(e) and (h) are the corresponding demodulated AMI eye diagrams, and Figs. 4(f) and (i) are the corresponding converted NRZ-DPSK eye diagrams. Successful demodulation and format conversion can also be achieved for multi-channel CSRZ-DPSK signals.
Fig. 4 Measured spectra and eye diagrams for 6-channel CSRZ-DPSK processing at 40Gb/s. (a)–(c) The spectra of the original input 6 CSRZ-DPSK signals, the demodulated signals and the converted multi-channel NRZ-DPSK signals; (d) to (f) the eye diagrams of the Ch.2 CSRZ-DPSK, the demodulated AMI and the converted NRZ-DPSK signals; (g) to (i) the eye diagrams of the Ch.5 CSRZ-DPSK, the demodulated AMI and the converted NRZ-DPSK signals.
In the experiment, the channel spacing for the input signals is determined by the channel bit rate and the DI FSR. Generally, the channel spacing should be the minimal common multiple of the two parameters. Thus, the channel spacing should be 200 and 400 GHz for 6*20 Gb/s and 6*40 Gb/s respectively. The limited spectral efficiency will limit the transmission capacity in real WDM application. One possible solution might be dividing the whole channels into odd channels and even channels. Each series of channels are processed separately and combined afterwards.
It should be noted that the proposed multi-channel processing has good tolerance to wavelength mismatch when filtering by the DI, meaning that the scheme will still work well while the laser wavelength drifts within certain range. This can be confirmed by the results of Ch. 5 in Fig. 4(b). From the spectrum, we can see that the although the carrier of Ch. 5 is offset to the ITU grid, resulting in a wavelength mismatch while processing by the DI, the final demodulation and the conversion after the AWG is still good, since the narrow filtering by the AWG can mitigate the asymmetric of the output spectrum within certain range.
To further investigate the proposed parallel signal processing, we measure the power penalty of each channel and plot an average BER for the 6*20 Gb/s and 6*40 Gb/s processing, as shown in Figs. 5(a) , (b), (c) and (d). For each case, the measurements are performed for the input (CS)RZ-DPSK signals, the demodulated signals from the 0.5 bit DI destructive port and the converted NRZ-DPSK signals respectively. In Figs. 5(a) and (b), the round and rectangle marks represent the power penalties for the (CS)RZ-DPSK-to-AMI and the (CS)RZ-DPSK-to-NRZ-DPSK respectively. It is clear that the power penalty of each channel is almost the same in 20 Gbit/s processing (shown in Fig. 5(a)), while that of the Ch. 5 is slightly large in 40 Gbit/s processing (shown in Fig. 5(b)). The relatively large value for Ch. 5 is due to the mismatch of the laser wavelength and DI transmission file. In Figs. 5(c) and (d), the average BER curves are plotted for the two cases respectively. For 6*20 Gb/s operation (Fig. 5(c)), results show the average power penalties for the 0.5 bit DI demodulation and conventional 1 bit DI demodulation is ~0.5 dB, while that of the format conversion are ~0.8 dB. For 6*40 Gb/s operation (Fig. 5(d)), the results are ~0.6 and ~1.2 dB for the demodulation and format conversion. One of the demodulated eye diagrams for the converted NRZ-DPSK signals are also shown in the figures as insets.
Fig. 5 Measured power penalty in each channel for (a) multi-channel RZ-DPSK processing at 20 Gb/s and (b) multi-channel CSRZ-DPSK processing at 40 Gb/s. Measured average BER results for the (c) multi-channel RZ-DPSK processing at 20 Gb/s and (d) multi-channel CSRZ-DPSK processing at 40 Gb/s.
4. Conclusion
A single DI based multi-channel DPSK signals processing has been proposed and demonstrated. Thanks to the periodic filtering property of the DI, the demodulations are based on destructive interference for the multi-channel signals, while the format conversions are based the constructive interference. Simultaneous demodulations and format conversions can be achieved for 6*20 Gb/s RZ-DPSK signals and 6*40 Gb/s CSRZ-DPSK signals by utilizing suitable DIs with half bit delay. BER measurements show small and reasonable power penalties for the proposed multi-channel processing.
Acknowledgments
This work was supported by National Basic Research Program of China (Grant No. 2011CB301704), National Natural Science Foundation of China (NNSFC) (Grant No. 61007042), the State Key Laboratory of Advanced Optical Communication Systems and Networks (Grant No. 2008SH10), the Fundamental Research Funds for the Central Universities (Grant No. HUST 2010QN041), and the Doctoral Program Foundation of Institutions of Higher Education of China (Grant No. 20090142110052).
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