1. Introduction

Unquestionably, wavelength-division multiplexing (WDM) and optical time-division multiplexing (OTDM) are the two most-established technologies for expanding the capacity in optical fiber communications. The interconnections in future photonic systems and networks is probably the signal interchanges of WDM and OTDM systems, which require not only the wavelength conversions but also the waveform conversions between WDM and OTDM formats. In addition, due to their cost efficiency and simple implementation, nonreturn-to-zero (NRZ) and return-to-zero (RZ) are the popular modulation formats for data communications in present WDM and OTDM systems. This makes the all-optical wavelength and waveform conversions between NRZ and RZ signals at high bit-rate desirable to improve the flexibility and scalability of future all-optical networks. Furthermore, the transmission performance of a communication link is strongly dependent on the pulsewidth of the RZ signals at the transmitter due to the influences of dispersion, nonlinearities, and characteristic of optical receivers [1–5]. Therefore, the achievement of wavelength and waveform conversions with pulsewidth tunability at the same time are particularly desirable for the data interchanges of different modulation formats between network nodes as well as the enhancement of transmission performance through pulsewidth management.

All-optical wavelength and NRZ-to-RZ data format conversion has been realized by Mach-Zehnder interferometer [6], delayed-interferometric-signal wavelength converter (DISC) [7], or nonlinear optical loop mirror (NOLM) [8, 9]. These converters have difficulties in obtaining pulsewidth tunability of the converted RZ signal or give narrow tuning range of less than 50 % duty cycle [7]. Recently, a passive signal converter [10] and a configuration using a phase modulator followed by an interleaver [11] have been proposed to perform tunable pulsewidth NRZ-to-RZ format conversion. However, while the former converter requires a differential encoding of the input NRZ signals, the latter one only obtains the converted signal with the duty cycle larger than 30 % and does not offer a linear adjustment of the pulsewidth because of the filtering characteristic of the interleaver. These devices [10, 11] also lack the function of wavelength conversion due to the nature of the configurations. On top of these, none of the above research has investigated on the transmission characteristics of the format-converted signals, which are crucial for the applications of the devices into practical systems and networks.

In this paper, an all-optical signal processing scheme enabling wavelength conversion and pulsewidth tunable NRZ-to-RZ data format conversion by using nonlinearities in semiconductor optical amplifier (SOA) and highly nonlinear fiber (HNLF) is experimentally demonstrated. Transmission performance of the converted signals is also investigated. Wide pulsewidth tuning range of from 20 % up to 80 % duty cycle is obtained for RZ outputs at 1546 nm with negative power penalties compared to back-to-back NRZ at 1558 nm. The proposed system takes advantages of the fast switching characteristic of HNLF and the wide pulsewidth tunability in SOA-based switch. By avoiding the direct input of data signal into the SOA for preventing the patterning effect [12], which is observed in our first demonstration [13], the scheme is potential for operation of higher speeds (40 Gb/s). For the investigation on transmission characteristics, the receiver sensitivities of the converted RZ signals with each different pulsewidth are measured at every 5 km long standard single-mode fiber (SSMF) transmission up to 65 km distance, corresponding to cumulative dispersion of 1105 ps/nm. The transmission behaviors of the different pulsewidth signals indicate the existence of the optimal pulsewidth in each link with specific cumulative dispersion. The improvement of transmission performance through the tunable pulsewidth management of the converted RZ from the NRZ data format signal makes our proposed scheme distinctive from previous research in Ref. [1–5], in which the performance optimization is applicable not only at pulse source of the transmitter, but also at photonic node of the network.

2. Principle and experimental setup

The proposed scheme is shown in Fig. 1. The SOA-based switch, which employs a delayed Sagnac interferometer [14] using cross-phase modulation (XPM) in SOA, acts as a pulsewidth tunable clock generator. The fiber-based switch using four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) performs an AND logic function [1] between input NRZ and the converted RZ clock from the former switch.

As described in Ref. [14], the SOA-based switch consists of a short fiber loop with a polarization beam splitter (PBS), an SOA placed at an arbitrary position in the loop, and a variable delay line (DL). The continuous-wave (CW) probe through a circulator and PBS is divided into two orthogonally polarized waves propagating in opposite directions, clockwise (CLW) and counterclockwise (CCW) in the loop. These two waves experience XPM in the SOA which is nonlinearly excited by the input RZ clock traveling CLW in the loop. A polarization controller (PC) and a polarizer are used to convert the phase difference between the CLWand CCW waves to optical intensity. Since the switching principle is based on the optically induced differential phase modulation (DPM), adjusting the time delay of the delay line (DL) changes the time interval of the DPM and thus allows wide pulsewidth tunability of the RZ clock train.

 

Fig. 1. Scheme of the wavelength and pulsewidth tunable NRZ-to-RZ conversion by combination of SOA- and fiber-based switches. PC: Polarization Controller, PBS: Polarization Beam Splitter, BPF: Band Pass Filter, DL: Delay Line.

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In the fiber-based switch, the input NRZ signal is overlapped in time with the converted RZ clock from the SOA-based switch. These signals at different wavelengths are combined by a coupler and then amplified by an erbium-doped fiber amplifier (EDFA) before injection into HNLF to generate a sufficient FWM output. PCs are used to maximize the interaction between the two inputs. An optical band pass filter (BPF) is used to select the FWM-generated RZ signal. Since the output signal has almost the same waveform as the converted RZ clock, tuning the delay line in the SOA-based switch enables us to widely adjust the pulsewidth of the output RZ signal with the same data of the input NRZ signal.

Table 1. Characteristics of highly nonlinear fiber.

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In the experiment, 10 Gb/s NRZ data and RZ clock signals are generated at 1558 nm and 1542 nm, respectively. For the timing regeneration was not considered in this research, the clock recovery scheme is not employed in the demonstrated setup. Instead of this, a direct synchronization is made between the incoming NRZ data and the RZ clock. An NRZ-to-RZ converter based on terahertz optical asymmetric demultiplexer (TOAD) with clock all-optically recovered from the NRZ signal has been demonstrated in Ref. [9]. Pulsewidth of the optical clock is 20 ps. The CW probe wavelength is set at 1552 nm. The NRZ data is modulated by pseudorandom bit sequence (PRBS) with a pattern length of 2³¹-1. In the SOA-based switch, while the injected CW probe power is 0 dBm, the input RZ clock power is optimized at 2 dBm to obtain the maximum extinction ratio of the converted signal. The recovery time and injection current of the SOA are 100 ps and 200 mA, respectively. An EDFA along with a BPF amplifies the converted RZ clock to the same power level as the input NRZ at 0 dBm. In the fiber-based switch, a 500 m long HNLF with characteristics shown in Table 1 is used for FWM process. The EDFA is set to produce a total injected power of 20 dBm for the combined signals. After the fiber-based switch, a FWM signal is generated at 1546 nm. A 30 GHz-bandwidth sampling oscilloscope is used to monitor the eye patterns of the output signals. The transmission performance of the converted signals is investigated over standard single-mode fiber (SSMF) with the dispersion of 17 ps/nm/km at 1550 nm with distance up to 65 km without dispersion compensation. The launched power into the transmission link is set at 2 dBm and a non-preamplifier 10 Gb/s receiving scheme is employed to detect the output signals.

3. Performance of the wavelength and pulsewidth tunable NRZ-to-RZ converter

 

Fig. 2. Eye diagrams of the converted RZ signals for various time delay settings of (a) 20 ps, (b) 40 ps, (c) 60 ps, and (d) 80 ps (50 ps/div).

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Figure 2 shows eye diagrams of the NRZ-to-RZ conversion at 10 Gb/s for various time delay settings. The converted RZ signals with different pulsewidths corresponding to the delay settings of 20 ps, 40 ps, and 60 ps are observed. For the case of 80 ps time delay setting, the pulsewidth of output signal is slightly narrower than the expected value of 80 ps. That is because in such a wide delay setting the RZ clock from SOA-based switch is not fully overlapped by the mark level of the input NRZ signal, which has slow leading and trailing edges. However, this obstacle can be simply overcome by using NRZ signal with steeper leading and trailing edges. The clear eye openings without patterning effect in Fig. 2, unlike in the previous proposal [13], show that the current system is capable of performing a widely and linearly width-tunable conversion with high quality. The improvement of eye patterns in comparison with results in Ref. [13] is due to the rearrangement of the switches, in which the SOA-based switch with function of pulsewidth tunability is placed before the fiber-based one performing the NRZ-to-RZ format conversion [12]. The replacement of data signal by the clock train for activation of the cross-phase modulation in SOA is the key to prevent the patterning effect on the converted signals. This principle makes the present system relatively independent on the data bit-rate. It is therefore easy for the proposed scheme to be upgraded to higher speed because of the possibility of the SOA-based fiber loop mirror in tunable pulsewidth clock generation at speed of more than 10 Gb/s, and the fast switching characteristic based on four-wave mixing in fiber. Since the rising and falling edges of the converted RZ signal are critically dependent on the width of the input clock signal, the present system is also potential for even wider pulsewidth tuning range, which is smaller than 20% and over 80% duty cycles, if a narrower-width input RZ clock is utilized instead of the one of 20 ps-width in this experiment.

 

Fig. 3. Measured BERs of the converted RZ signals with different time delay settings. The inset shows BER comparisons of the 20 ps converted RZ signal with the original NRZ, and the conventional 20 ps RZ data signal generated from EAM.

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To evaluate the performance of the proposed conversion in detail, the bit-error-rate (BER) characteristics of the converted RZ signals with different pulsewidths are measured in comparison with the back-to-back NRZ data signal and the conventional 10 Gb/s RZ data signal with results in Fig. 3. The conventional RZ data signal with the pulsewidth of 20 ps is generated at 1552 nm by electroabsorption modulator (EAM) and LiNbO ₃ modulator (LNM). As shown in Fig. 3, the converted RZ signals at the BER of 10^-9 have power penalties of -1.7 dB, -1.6 dB, -1.4 dB, and -1.2 dB compared with the original NRZ signal, for the time delay settings of 20 ps, 40 ps, 60 ps, and 80 ps, respectively. The negative power penalties obtained from the proposed scheme are similar to the results in previous numerical analysis [5] and experimental research [10, 11], in which there is an improvement in receiver sensitivity of the pulsewidth-tuned RZ signal compared to NRZ. The typical reason of the improvement is that at the same received average power, the peak-to-peak power of the RZ signal is larger than that of the NRZ since the RZ owns the shorter pulsewidth. Further reason is the bandwidth limitation of the electrical components in the present receiver causing the rise in intersymbol interference (ISI) and the drop in receiver sensitivity when detecting the signal with wider pulsewidth. The narrower pulsewidth signal thus owns the better sensitivity.

It is also seen in the inset of Fig. 3 that while the receiver sensitivity of the conventional RZ at the BER of 10^-9 is -22.3 dBm, the 20 ps converted one offers an error-free operation with received power at -22.7 dBm. The better BER performance of the 20 ps converted RZ than the conventional RZ data signal with the same pulsewidth results from two reasons. First, it is well known that the wavelength conversion based on XPM in SOA additionally yields signal regeneration [15–17]. Therefore, after the SOA-based switch, the 20 ps width-tuned clock has a higher extinction ratio than the input 20 ps clock train, which is also used for the generation of the conventional 20 ps RZ data signal. Second, because the power of the output FWM product is dependent to the square of the input NRZ power which is the pump wave in the fiber-based switch, there is also a distinction ratio improvement over the FWM process [18–20]. The coupling of the signal regenerative features from both SOA-based and fiber-based switches is believed to be the cause for the better performance of the converted signal from our proposed scheme than the RZ signal with the same pulsewidth generated by the conventional way. The results on the BER characteristics of the converted signals again indicate the good performance of the present system in offering at the same time the wavelength conversion, the NRZ-to-RZ data format conversion, and the pulsewidth tunability functions.

Although not demonstrated in the present work, the tunable-wavelength conversion is also possible with proposed scheme. Adjusting the wavelength of the converted RZ clock through changing the CW wavelength from the SOA-based switch makes it possible to tune the wavelength of the converted RZ data signal. The wavelength tunability range is restricted only by the FWM-based switch, not by the SOA-based one since the wavelength conversion using XPM in SOA gives larger wavelength hopping than usingFWM in HNLF [17, 21]. According to our numerical results analyzing the FWM efficiency of the employed HNLF, using the same approach as in Ref. [22], an efficiency of approximately 15% is obtained with a converted wavelength range of 17 nm, corresponding to frequency shift of 2100 GHz. It should be noted that the converted wavelength range in the current experimental setup is double the wavelength spacing of input signals which are the NRZ signal and the pulsewidth-tuned RZ clock. As a result, the expected wavelength tunability range of the proposed wavelength and format converter is around 17 nm, certainly with sufficient input power for the FWM process.

4. Transmission performance and optimal pulsewidth

 

Fig. 4. Receiver sensitivity at BER of 10^-9 of the original NRZ and the converted RZ signals with different time delay settings Δt for various SSMF transmission lengths.

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The transmission performance is investigated by transmitting the converted RZ signals with 20 ps, 40 ps, 60 ps, and 80 ps time delay settings Δt through SSMF up to 65 km long without dispersion compensation. The error-free sensitivities of the received signals are measured at every 5 km propagation. As shown in Fig. 4, while propagating over SSMF links, the signals of different pulsewidths give different behaviors of the receiver sensitivity in comparison with the NRZ signal. Although the 20 ps RZ signal shows the highest sensitivity for transmission less than 5 km, it is rapidly degraded for longer distances due to fiber dispersion. On the other hand, the converted signals of 40 ps, 60 ps, and 80 ps pulsewidths retain better performance than the NRZ signal for longer distance transmission. While the best behavior for fiber lengths less than 30 km can be seen with the 40 ps signal, the highest receiver sensitivity for links between 35 km and 50 km is achieved with the signal of 60 ps pulsewidth. In the case of converted signal at 80 ps delay setting, the negative-power penalty against the NRZ signal is only achieved for links less than 40 km. Over 50 km transmission the NRZ signal with its better dispersion tolerance shows a better sensitivity than the compared convertedRZ signals. The obtained results indicate that for each transmission distance with particular cumulative dispersion there is existence of an optimal pulsewidth for the converted RZ signal to obtain highest receiver sensitivity. The optimal pulsewidth is shifted to the longer time delay setting when the cumulative dispersion becomes larger [3]. Consequently, the improvement of the transmission performance could be completed by the optimization of RZ data format using tunable pulsewidth management by our proposed converter.

 

Fig. 5. The optimal pulsewidths of the converted RZ signal and its corresponding power penalties at BER of 10^-9 compared with the conventional NRZ signal in various SSMF transmission lengths.

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Figure 5 shows the optimal pulsewidths for the converted RZ signal to obtain the best performance in various SSMF links and its power penalties at BER of 10^-9 against the NRZ transmission. As the transmission distance increases along with the accumulation of fiber dispersion, the optimal pulsewidth linearly varies from 20 ps to 70 ps for links to 50 km long. Over 50 km distance, the best performance is found at RZ signals within 70 ps and 75 ps width. In addition, by the tunable pulsewidth management, the system clearly shows the improved performance in comparison with the NRZ transmission, in which the power penalty remains negative on links until 60 km long with the cumulative dispersion up to 1020 ps/nm. This can not be seen in Fig. 4. For instance, at 50 km transmission with dispersion of 850 ps/nm, while the NRZ transmission performance starts to surpass the 60 ps width-tuned signal as seen in Fig. 4, the power penalty in Fig. 5 is gained by 0.5 dB as the pulsewidth is optimized at 70 ps. The results again present the good transmission performance of the converted signals from the proposed wavelength and pulsewidth tunable NRZ-to-RZ conversion even under the circumstances of largely cumulative dispersion.

The decisive factor to the performance improvement and the various transmission characteristics of the converted RZ signals is believed to be the combination of pulsewidth, cumulative dispersion, and frequency chirp induced by the converter. While the RZ signal with shorter pulsewidth gives better sensitivity than with longer pulsewidths, the influence of dispersion on the shorter pulsewidth signal is stronger. That makes the optimal pulsewidth for the highest sensitivity shifted to the longer pulsewidths when the cumulative dispersion becomes larger as seen in Fig. 4 and 5. Besides, the consequences of red-chirp, induced by SOA in the converter, at the leading and trailing edges of the converted RZ partly changes the waveform of the signal when interacting with the anomalous dispersion of the fiber [23]. The slower propagations of the red-chirped edges relative to chirp-free center of the pulses cause a steepening at the leading edge and a broadening of the trailing edge. The chirp also leads to the pulse peaking effect and then additionally improves the receiver sensitivity thank to the pulsewidth narrowing. The effects make the BERs of the optimized-pulsewidth signals significantly improved on SSMF links shorter than 30 km as shown in Fig. 5. The magnitude of negative power penalties observed in these cases is larger than 1.5 dB. However, there is a case that the interaction of chirp with fiber dispersion can not improve the transmission performance as shown in Fig. 4 for the case of 80 ps delay setting. The chirp content of the converted signal causes a degradation of the performance for links over 40 km and thus reduces the dispersion tolerance of the converted signal. As a result, the improvement of transmission performance critically depends not only on the interaction of the pulsewidth and the cumulative dispersion, but also on chirp characteristic in the converted signal.

5. Conclusion

We have demonstrated a wavelength and NRZ-to-RZ data format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches, and investigated on its transmission performance on various SSMF links without dispersion compensation. The negative power penalties of the converted signals compared with the back-to-back NRZ signal proved the high quality operation of the proposed converter in a wide tuning range of pulsewidth. It is also possible for the device to operate at higher bit-rate because of the capability of the SOA-based in generation of clock pulse over 10 Gb/s, and the fast response of the fiber-based switch. The obtained results indicated that the improvement of transmission performance can be done by the optimization of RZ data format using tunable pulsewidth management by our proposed converter. The decisive factor to the performance improvement is the coupling of pulsewidth, cumulative dispersion, and chirp in the converted signal.