1. Introduction

All-optical modulation format conversion is a key function in providing flexible management and interface for the wavelength division multiplexing (WDM) and the optical time division multiplexing (OTDM) networks. In the past, many researchers had demonstrated various conversion schemes for the on-off keying (OOK) signals [1–4]. Compared with the OOK format, the differential phase shift keying (DPSK) format has attracted more attention nowadays due to its superior transmission performance and improved receiver sensitivity with balanced detection [5], and thus many papers proposed and demonstrated format conversion between different types of DPSK formats [6–8]. Recently, several papers proposed and demonstrated format conversion for multi-channel OOK signals based on a single device [9–11]. These provide much more flexibility for future network interface, enable optical parallel processing, reduce system cost and thus have very promising merits.

Usually, the nonlinear effect preserving the phase information (for instance four-wave-mixing) in a nonlinear device is utilized to perform the format conversion for phase modulated signals. In this paper, for the first time to our best knowledge, we propose and demonstrate an all-optical parallel format conversion for multi-channel non return-to-zero DPSK (NRZ-DPSK) to return-to-zero DPSK (RZ-DPSK) format conversion, using single device. The conversions are based on the cross phase modulation (XPM) in a single semiconductor optical amplifier (SOA), which is optimized to work at deep saturation to mitigate the undesired crosstalk induced by the cross gain modulation (XGM). The multi-channel input signals are phase modulated by a synchronous optical signal with large power, and a subsequent offset filter is used to extract the chirped part to perform the pulse carving function. Generally, the XPM process will introduce a phase shift to the signal which suffers the phase modulation. This is undesirable for the phase modulated signal. However, an identical phase shift will be added to both “0” and “pi” of the original DPSK signal, resulting in the phase difference and thus the modulation information being preserved after the XPM. 6-channel NRZ-DPSK signals at 40 Gb/s can be converted to the corresponding RZ-DPSK signals using a synchronous optical clock signal. The bit error ratio (BER) measurements show a good conversion performance of the proposed format conversion scheme.

2. Experimental setup and operation principle

The experimental setup is shown in Fig. 1 . Six WDM channels (wavelength from 1540 to 1559 nm with spacing of 3.2nm, 1550nm exclusive) are coupled into a push-pull Mach-Zender modulator (MZM) using an arrayed waveguide grating (AWG). The 6 CW lights are driven by the 40 Gb/s data (PRBS 2³¹-1) from a same pattern generator, and thus the decorrelation is necessary to emulate the real application where the bit streams of each channel are different. A span of 1km single mode fiber (SMF-28) is utilized to achieve the decorrelation for the multi-channel input signals. The eye diagrams of the NRZ-DPSK signals after the decorrelation are shown as the inset. Another CW beam (1550 nm) is fed into a phase modulator (PM), which is driven by the synchronous 20 GHz sinusoidal clock signal (the highest clock signal available from the pattern generator), to obtain a CW signal with sinusoidal phase modulation. A subsequent DI with 40 GHz free spectral range (FSR) is used to obtain a 40 GHz optical clock signal using the destructive interference. The delay of the RF clock can be adjusted to synchronize with the NRZ-DPSK signals. A WDM synchronization scheme consisting of two AWGs and six tunable optical delay lines are used to compensate the differences of the wavelength dependent group velocity through the transmission link, by performing the demultiplexing, the delay and re-multiplexing respectively. The six input signals are firstly demultiplexed by an AWG, and then delayed separately using six optical delay lines. Considering the relative delay induced by the SMF, the delay values of the delay lines for each channel are chosen to ensure that all the six channels are decorrelated and aligned in time domain when they reach the SOA. Finally, the delayed signals are multiplexed by an AWG again. Then the signals are launched into the SOA via a WDM coupler. The SOA is a CIP nonlinear device (CIP SOA-NL-1550) biased at 210mA. The average powers of the six the NRZ-DPSK signals and the clock signal are approximately 3 and 13 dBm respectively. A subsequent filter with 1nm bandwidth is used to extract the broadened part and achieve the pulse carving. Thus, the chirp induced on each channel is transmitted, while the original spectral components are suppressed to some extent, and hence the format conversions from NRZ-DPSK to RZ-DPSK can be achieved from the point of view of amplitude. Meanwhile, the relative phase difference (data information) has been preserved, due to the same phase shift for all the bits, as the schematic constellations in Fig. 1 shown.

 

Fig. 1 the experimental setup and operation principle

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Since our proposed format conversion is based on an offset filtering after the XPM, the channel spacing is a tradeoff of the crosstalk and the spectral efficiency. If the channel spacing is too small, there will be crosstalk from the adjacent channel while the detuned filtering is performed. On the other hand, the channel bit rate is 40 Gb/s and the modulation is performed after the multiplexing at the AWG, and thus the spectra width for each channel is not narrow. Based on the two reasons above, we choose 3.2 nm channel spacing to ensure a good conversion with less crosstalk. Although the power of the clock signal is as large as 13 dBm, the four wave mixing (FWM) effect is actually not too serious, since the power of each NRZ-DPSK signals is relatively small and the channel spacing is large.

For a real application, the synchronous optical signal should be extracted using an all-optical clock recovery (CR) scheme. Here, we use a PM and a DI to achieve the 40 GHz sinusoidal pulse train. Although the obtained clock-like signal (as the inset shown in Fig. 1) is with phase transition due to the destructive interference, the conversions will not be affected since the XPM process is insensitive to the phase and only determined by the amplitude. Generally, an all-optical CR scheme extracts the optical clock signal at the same wavelength of the input signal, resulting in the wavelength of the clock signal and one of the input signals overlapped. However, some other CR units can obtain optical signal at a different wavelength (for instance, all-optical CR scheme based on the injection mode-locking SOA fiber ring laser). Another possible solution is first extracting an optical signal at the wavelength of input signal, and then performing an all-optical wavelength conversion for the obtained clock. Theoretically, the phase shift induced by the XPM is determined by the combination of the signal power and the clock power [12]. In our case, the clock power is much higher than the signal power, and thus the phase shift is mainly determined by the clock, meaning that the power of each input signal is not necessary to keep a constant level.

Please note that an optical clock signal with small intensity noise is very important to achieve a good conversion performance, as the clock intensity noise will transfer to the phase noise, which will further transfer to the amplitude noise of the demodulated signal, during the XPM processing. In our experiment, the clock is with good quality (as the inset in Fig. 1 shown), and thus the conversion performance is good. If the input clock signal is with large intensity noise, the converted RZ-DPSK signal after demodulation will also with large intensity noise.

As for the offset filtering, we use a tunable band-pass optical filter to perform the filtering for each channel separately. If a specific comb-like filter can be utilized as [13], the offset filtering can be achieved for all the channels. In this case, the proposed format conversion scheme is potential for integration.

3. Results and discussions

Results show that the proposed conversion scheme works well for all the channels simultaneously. The measured spectra before and after the SOA are shown in Fig. 2(a) and (b) . It is obvious that all the NRZ-DPSK spectra are broadened due to the modulation of the clock signal after the SOA. Generally, a clock signal with very small pulse width is preferred to achieve a stronger XPM effect, resulting in the RZ-DPSK signal with smaller duty cycle. In our experiment, the clock signal is sinusoidal pulse and thus the obtained RZ-DPSK signals are with about 50% duty cycle. On the other hand, the XPM effect is determined by the change of the differential refractive index of the SOA (in other words the change of the SOA carrier density). Accurately, the differential refractive index is a function of the input signal wavelength, and the optimal location of the clock should be at the peak of the differential refractive index change to ensure a strong XPM effect. Because of the limitation of the gain bandwidth of the SOA, we choose the clock signal to locate in the middle of the WDM comb. If we can get an optimal SOA with the peak of the differential refractive index locating at the one side of the WDM signal, we can achieve similar results. For the current SOA, the performance will degrade if the clock at one side of the WDM signals. The specific chirp parts of each channel are then extracted by a subsequent offset filter, the detailed spectrum for one of the converted RZ-DPSK signals is shown in Fig. 2(c), and the filter file is also shown as the red dash line. The corresponding demodulated signal from another one bit delay interferometer is shown in Fig. 2(d), both the alternative mark inversion (AMI) signal from the destructive port and the duobinary (DB) signal from the constructive port are presented.

 

Fig. 2 measured spectra (a) before the SOA (b) after the SOA (c) one of the converted RZ-DPSK (d) the demodulated RZ-DPSK (Res: 0.05nm)

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All the measured eye diagrams are presented in Fig. 3(a) to (f) . From the top down, the six lines of Fig. 3 show the eye diagrams of the input NRZ-DPSK signals, the converted RZ-DPSK signals, the demodulated NRZ-DPSK signals and the demodulated RZ-DPSK signals at the AMI and DB ports, respectively. The full width at half maximal (FWHM) of the pulses after the 40 GHZ FSR DI is about 13~14 ps (measured by the oscilloscope with a bandwidth of 39 GHz).

 

Fig. 3 Measured eye diagrams of all the channels (a) the input NRZ-DPSK signals (b) the converted RZ-DPSK signals (c) the demodulated NRZ-DPSK signals at AMI port (d) the demodulated NRZ-DPSK signals at DB port (e) the demodulated RZ-DPSK signals at AMI port (f) the demodulated RZ-DPSK signals at DB port

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Being processed by a one bit delay interferometer (DI), the converted RZ-DPSK signals can be demodulated for evaluation. The BER measurements are performed for all the channels. Since the balanced detection is not available, the BER curves are plotted using the demodulated signals from one of the DI outputs. For a fair comparison, the AMI ports are utilized since the receiver sensitivities are different for formats with different duty cycle. Results are presented in Fig. 4 , indicating ~-0.8 to −1 dB power penalties for all the 6 channels.

 

Fig. 4 BER measurements for all the channels

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The eye diagram and the BER measurement show that the format conversion can be achieved with negative power penalties. More specifically, the timing jitter of the demodulated signals can be reduced significantly. In our opinion, this is due to the XPM and subsequent offset filtering induced retiming function, and the high quality clock signal is also a main reason for the performance improvement. This is similar to the NRZ-OOK to RZ-OOK conversion with timing jitter improvement, the details can be found in [10].

4. Conclusion

In conclusion, we have proposed and demonstrated a multi-channel all-optical format conversion for phase modulated signals. The parallel conversions are based on the XPM effect in a deep saturated SOA. An identical phase shift will be added to both “0” and “pi” of the original DPSK signal due to the XPM, resulting in the phase difference and thus the modulation information being preserved. 6-channel NRZ-DPSK signals at 40 Gb/s can be converted to the corresponding RZ-DPSK signals simultaneously. The BER measurements show a ~-0.8 to −1 dB power penalty for the proposed conversions.