1. Introduction

Future all-optical networks are quite likely to combine elements of wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM), adopting the advantages of both technologies. The return-to-zero (RZ) format is widely used in OTDM networks due to its tolerance to fibre 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 may become an important interface technology between OTDM and WDM networks. For several years, researchers have demonstrated all-optical format conversion between NRZ and RZ signals [1-4]. On the other hand, the development of commercial electronic techniques has shown advantages over optoelectronic signal processing at high speed [5]. As a result, hybrid format conversions also received much interest [6, 7]. These approaches, however, to date have been for single-channel operation and thus cannot offer the advantage of parallel optical processing. Noting the growing demands of dense WDM (DWDM) networks, several papers recently have proposed and demonstrated multi-channel format conversions on the ITU grid [8-10]. Some of these further demonstrated the conversions with signal regeneration [9, 10]. However, besides the limited numbers of channels and large wavelength spacing operation, these approaches suffer from some drawbacks, such as TOAD architectures with large footprints, high cost and complexity and so on.

Recently, we demonstrated semiconductor optical amplifier (SOA) based all-optical format conversion for 16 DWDM channels each at 10Gb/s modulation rate [11]. In that paper, however, only preliminary results without a transmission performance test were achieved, and large optical clock power was necessary to mitigate the crosstalk induced by cross gain modulation in the SOA operation. In this paper therefore, we propose and demonstrate a simple optoelectronic NRZ-to-RZ regenerative conversion for multi-channel DWDM signals at both 10 and 20Gb/s, using a single phase modulator (PM) and a subsequent fibre delay-interferometer (DI) acting as a detuned filter for all the channels at the same time. By driving the PM with a local RF clock signal and optimizing the operation point of the DI, simultaneous NRZ to RZ format conversion can be achieved with some regenerative properties for all input channels. Format conversion for 16 channels at 10Gb/s and 8 channels at 20Gb/s (both 100GHz channel spacing) successfully show good performance and applicability of the technique for multi-channel operation at different bit-rates. Partial regeneration is demonstrated for the 16 channels at 10Gb/s, for 50 and 75km single mode fibre (SMF) transmission without dispersion compensation. Here BER results show 3.5 and 4.2 dB power penalty improvement, respectively. Compared with previous schemes, the proposed regenerative format converter benefits from a simple configuration for multichannel operation at different bit-rates, low power consumption and has a good potential for integration.

2. Experimental setup and operation principle

The experimental setup is shown in Fig. 1. 16 continuous wave (CW) channels from a DWDM laser array, at wavelengths from 1547.79 to 1559.79 nm with 100GHz spacing, are coupled via an array waveguide grating (AWG) into a Mach-Zender modulator (MZM). The MZM is driven by a 10 Gb/s 2³¹-1 PRBS pattern from an MP1763C Anritsu pattern generator (For 20Gb/s operation, only 8 channels are used for proof of concept, and the wavelengths are from 1550.19 to 1555.79nm with 100GHz spacing, and the MZM is driven by 2³¹-1 PRBS pattern from an Agilent pattern generator). The polarisation of the signals input to the MZM is optimised with a polarisation controller (PC). A span of standard single mode fiber (SMF) with 50 or 75 km length is used to degrade the signals. A simple multi-channel format converter consisting of an EDFA, a PC, a PM driven by a local 10/20GHz RF clock signal and a DI is used to perform the 3R regeneration for all the channels. The NRZ signals are temporally aligned and the delay of the RF clock signal is changed to achieve synchronization of the incoming signals. Due to the induced phase modulation in the clock driven PM, the mark levels (“1”s) of the input NRZ signals experience frequency chirp and power reallocation, hence resulting in a spectral broadening. The subsequent DI, with comb spacing of 100GHz, is used to perform detuned filtering for each of the 16 channels. By controlling the operating temperature, its transmission peaks are adjusted to be offset from each carrier wavelength with optimal detuning. The chirp induced on each channel is passed, while the original degraded components are suppressed to some extent. Thus, format conversions for all the channels at the same time can be achieved. The NRZ to RZ conversion process also results in partial regeneration, such as extinction ratio (ER) improvement and timing jitter reduction, since the original distortion parts are suppressed to some extent due to the detuned filtering. The detuning value is proportional to the chirp. A Gaussian filter with 0.3nm 3dB bandwidth is used to filter out one of the regenerated channels for evaluation. The insets in Fig. 1 indicate the bit pattern before and after the regenerative format conversion.

 

Fig. 1. Experimental setup

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Please note that all the channels can carry different data patterns, which has been confirmed by simulation. However, for experimental convenience, only one pattern is used here due to limits on available equipment.

3. Results and discussion

First, we perform multi-channel format conversion for the back to back condition. Figure 2(a) to (c) shows the back to back spectra of the 16 channel 10Gb/s NRZ signals, the corresponding signals after the PM and the converted RZ signals after the DI. It is shown that the spectrum of each channel is broadened due to the phase modulation after the PM, and the specific parts (red shifted part of each channel) of the broadened spectra are extracted by the DI. Figure 2(d) shows one of the 16 converted spectra after the filter. For 8 channels at 20Gb/s operation, Fig. 3(a) to (d) show the similar spectra evolutions, indicating the conversions are achieved successfully at higher bit-rate.

A 0.3nm optical filter is then used to select one of the multiple channels. Figure 4(a) to (d) show the eye diagram of original NRZ signal at 10Gb/s, the converted RZ signal at 10Gb/s, the original NRZ signal at 20Gb/s and the converted RZ signal at 20Gb/s, respectively.

 

Fig. 2. Spectra of the 16*10Gb/s signals (a) before the regenerator (b) after the PM (c) after the DI (d) one of the regenerated signals

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Fig. 3. Spectra of the 8*20Gb/s signals (a) before the regenerator (b) after the PM (c) after the DI (d) one of the regenerated signals

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Fig. 4. Eye diagram of (a) original NRZ signal at 10Gb/s, (b) the converted RZ signal at 10Gb/s, (c) the original NRZ signal at 20Gb/s and (d) the converted RZ signal at 20Gb/s.

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In order to test the regeneration performance of the proposed multi-channel approach, we transmit the 16 channels NRZ signals at 10Gb/s over different lengths (50/75km) of SMF without dispersion compensation to test the degradation tolerance.

 

Fig. 5. Eye diagrams for one channel at (a) 50km transmission (b) 75km transmission (c) regenerated signal after 50km (d) regenerated signal after 75km Time scale: 50ps/div

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For clarity, we measure one of the 16 10Gb/s channels (Channel 5). Figure 5(a) and (b) show the eye diagrams of the NRZ signals after 50km SMF and after 75km SMF, respectively. The corresponding regenerated signals after 50 and 75km transmission are shown in Fig. 5(c) and (d); significant improvements are obtained with timing jitter reduction and ER enhancement. Figure 6 shows BER curves for channel 5 back-to-back, after the two degrading stages and after regeneration, showing 3.5 and 4.3dB improvements in sensitivity for 50 and 75km transmission without dispersion compensation respectively. Even considering the expected receiving sensitivity difference between NRZ and RZ formats, the partial regeneration performance is still good. Figure 7 shows the rms timing jitter reduction and ER improvement for all channels, with 50 and 75km transmission respectively. About 2 and 3dB ER improvement and 5 and 6ps jitter reductions can be observed after the regenerative conversions, for the two transmission cases respectively.

 

Fig. 6. BER measurements for channel 5

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Fig. 7. Measured timing jitter and ER improvement

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In the experiment, the delay of the RF clock should be adjusted to achieve synchronization of the incoming signals. The best output quality will be achieved when every clock pulse exactly aims at the center of each NRZ pulse. If misalignments occur, the quality of the output signal will degrade. We further investigate the sensitivity to bit misalignments by simulating the output Q factor with different clock delay. The simulation is performed via VPI transmission Maker 7.5^TM software and the simulation parameters are the same as the experimental ones. For one channel of 16*10Gb/s NRZ signals after 50km SMF transmission, the mismatch between the RF clock signal and the NRZ signal is adjusted gradually. The degradation of output Q factor is 4.7dB when the mismatch is adjusted from 0 (the clock pulse exactly aims at the center of NRZ pulse) to 33% (the offset between clock pulse and NRZ pulse center is 33ps at 10Gb/s).

4. Conclusions

In conclusion, we propose and demonstrate a simple and novel optoelectronic NRZ-to-RZ multi-channel conversion scheme at different bit-rates, with partially regenerative properties. By properly adjusting the detuning between the DI filter and corresponding NRZ carriers, format conversions can be achieve successfully by inputting 16*10Gb/s and 8*20Gb/s NRZ signals to a RF clock driven phase modulator and a subsequent DI with FSR equals to the input channel spacing. The regeneration performance of the multi-channel format converter is confirmed by transmitting 16 DWDM channels NRZ signals at 10Gb/s with 100GHz spacing, where BER results show 3.5 and 4.2 dB negative power penalties, for 50 and 75km SMF transmission, respectively. Also, 2 and 3dB ER improvements and 5 and 6ps timing jitter reduction can be found for all the input channels, after the two transmission cases respectively.