1. Introduction

Nonlinear optical signal processing functionalities, such as wavelength conversion, regeneration, format conversion and so on, may find some applications in future all-optical networks [1–4]. Conversions between various data formats, such as return-to-zero (RZ) and non-return-to-zero (NRZ), on-off-keying (OOK) and phase-shifted-keying (PSK), provide flexibilities for system performance optimization and network reconfigurability [5–8]. On the other hand, wavelength or signal multicasting that is accomplished by simultaneously replicating one data wavelength onto multiple ones, can significantly improve the efficiency and throughput of wavelength-division-multiplexing (WDM) optical networks [9]. Therefore, multi-function signal processing approaches are highly desirable to meet requirements for dynamic networks.

So far various schemes of NRZ-to-RZ format conversion have been demonstrated for either single-channel [5–8] or multiple channels [10–13]. Wavelength multicasting can be achieved through different mechanisms either in fiber or integrated devices (mostly semiconductor optical amplifier, SOA) [14–16]. Sometimes wavelength conversion and multicasting can be simultaneously accomplished through multiple pumps [17–19], but generally limited to several data channels.

In this paper, based on various effects in a highly nonlinear fiber (HNLF), such as four-wave-mixing (FWM), cross-phase-modulation (XPM) and cross-gain-modulation (XGM), we demonstrate an efficient optical signal processing scheme at 10-Gb/s that can simultaneously enable both NRZ-to-RZ format conversion and wavelength multicasting functionalities. Using only one pump channel, we covert the original NRZ data to RZ signal, and more importantly, the converted RZ signal can be multicast onto up to eight channels with six of them error-free (i.e. 10⁻⁹ BER) as we set the wavelength spacing between the pump and the NRZ channel to 0.8 nm. Five out of seven channels can be error-free as the spacing set to 0.4 nm as well.

2. Operation principle

The principle or major mechanisms of proposed scheme is shown in Fig. 1 . In order to achieve NRZ-to-RZ format conversion, we have to use at least one pump channel (λ_P) that is modulated by the synchronized clock signal and the to-be-converted NRZ channel (λ_S). Therefore, under different nonlinear regions (if we control the optical power well), there are three possible nonlinear effects: (i) FWM—most likely will happen between λ_S and λ_P near the zero-dispersion wavelength, and the generated wavelengths of other channels can be expressed as $\frac{1}{{\lambda }_{\text{S}}}+\frac{1}{{\lambda }_P}=\frac{1}{{\lambda }_{Li}}+\frac{1}{{\lambda }_{Ri}}$(Li and Ri correspond to the i-th channel of the short- and long-wavelength side, respectively). (ii) XGM—the pump channel that originally carries clock signal may be converted to the reversed RZ channel due to the XGM effect between the NRZ signal and the clock under high power. (iii) XPM—the NRZ signal may be broadened due to XPM and if a detuned optical filter is used for the broadened spectrum, we can obtain a generated RZ signal as well. All these effects are observed and demonstrated in our approach.

 

Fig. 1 Principle of the proposed scheme for NRZ-to-RZ format conversion and one-to-many multicasting based on various effects in highly nonlinear fiber. FWM: four-wave-mixing; XGM: cross-gain-modulation; XPM: cross-phase-modulation

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3. Experimental setup and results

The experimental setup is shown in Fig. 2 . An external cavity tunable laser (ECL) is modulated at 10-Gb/s by a Mach-Zehnder modulator (MZM1) with 2³¹-1 pseudorandom bit sequences (PRBS) to generate NRZ-OOK signal. The pump channel from a DFB laser is modulated by MZM2 with the 10-GHz clock signal. A variable optical delay line (VDL) is used to synchronize both channels for the format conversion. Through polarization controllers (PC1-PC4) to optimize the polarization states of the pump and NRZ channels, we combine both channels into a high power EDFA (output power can be 2W) followed by 1-km highly nonlinear optical fiber (HNLF) [20]. The zero dispersion wavelength, dispersion slope, and nonlinear coefficient of the highly nonlinear fiber are 1556 nm, 0.02 ps/nm²/km, and 30 (W·km)⁻¹, respectively. Other modules or equipments shown in the figure are used to facilitate different measurements, including a MS9710C optical spectrum analyzer (OSA), an optical tunable filter, a variable optical attenuator (VOA), the bit-error-rate tester (BERT) and a high-speed oscilloscope (86100C with 65-GHz optical bandwidth). The wavelength measurement may not be that accurate due to the resolution of our OSA (0.05-nm). An optical filter with tunability in both wavelength and bandwidth is used before the receiver to optimize the performance, especially for the converted channel based on the XPM effect.

 

Fig. 2 Experimental setup. ECL: external cavity (tunable) laser; DFB: distributed-feedback laser; HNLF: highly nonlinear fiber; MZM: Mach-Zehnder modulator; VOA: variable optical attenuator; BERT: bit-error-rate tester; OSA: optical spectrum analyzer; VDL: variable (optical) delay line

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First we set the wavelength spacing between the pump channel and the NRZ channel to 0.4-nm near the zero-dispersion wavelength of the HNLF (i.e. 1555.96nm and 1556.36nm for the pump and the NRZ channel, respectively). As we vary the input power into the HNLF, the generated FWM spectrum, as well as the performance of converted channels, changes as shown in Fig. 3(a) . For the power of 20-dBm, there are seven converted RZ channels, with five of them can be error-free (Fig. 3b). The BER of other two channels can reach 10⁻⁶ (error-free is achievable if using FEC technology). As the power increases (i.e. to 23 dBm), the FWM spectrum will be distorted due to increased channel broadening effects (first-left and first-right channels in Fig. 3a). However, it should be noted that under a certain pump power (e.g. 20-dB), the original clock channel can be converted to RZ channel as well (Fig. 4 . a1 to Fig. 4. b1) due to the XGM effect (but cannot reach error-free for this case), the data pattern is reversed for the converted channel though (i.e. “1”s become “0”s and “0”s become “1”s) and should be processed as a special case for the multicasting function (i.e. reversed logics at the receiver side). In addition, the original NRZ channel can be converted to RZ data if we use detuned filtering (1556.45nm) based on the XPM effect (Fig. 4a2 to Fig. 4b2).

 

Fig. 3 Results for the wavelength spacing between the pump and signal channels as 0.4-nm (a) spectra under different pump power; (b) measured BER performance for all the channels as the pump power of 20 dBm (among seven converted RZ channels, five can be error-free).

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Fig. 4 Typical eye diagrams for different channels under 20-dBm pump power as the wavelength spacing of 0.4-nm. (a1) original pump signal (clock); (a2) original data signal (NRZ); (b1) converted RZ signal from the clock through XGM; (b2) converted RZ signal from the original NRZ through XPM (detuned optical filtering is required)

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Next we change the wavelength spacing to 0.8-nm (i.e. 1556.00nm and 1556.80nm for the pump and the NRZ channel, respectively). The generated spectra and BER performance of all-channels are shown in Fig. 5 . Out of eight converted channels, six can be error-free with the other two reach 10⁻⁷. Note that the converted RZ channel (1556.00-nm) from the original clock channel can be error-free for this case while with ~3-dB power penalty difference compared to other converted error-free channels. The converted RZ channel from the original NRZ channel is also error-free with slightly detuned filtering (1556.86-nm). The eye diagrams of all eight converted channels are shown in Fig. 6 . If we increase the pump power, the performance of some channels may be better (e.g. the converted RZ channel from the clock one), while some may get degraded due to increased XPM or SPM effect (referred to Fig. 5a).

 

Fig. 5 Results for the wavelength spacing between the pump and signal channels as 0.8-nm (a) spectra under different pump power; (b) measured BER performance for all the channels as the pump power of 20 dBm (among eight converted RZ channels, six can be error-free).

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Fig. 6 Typical eye diagrams for different channels under 20-dBm pump power as the wavelength spacing of 0.8-nm (1556.00-nm corresponds to the original clock or pump channel; 1556.86-nm corresponds to the converted RZ channel from the original NRZ channel based on XPM effect and detuned filtering).

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Furthermore, we evaluate the conversion (or FWM) efficiency crossing the gain spectrum of the high power EDFA. Figure 7 illustrates the generated multicasting ability of the proposed approach as we shift the center wavelengths of the clock and NRZ channels. The wavelength spacing is fixed to 0.8-nm and the pump power is set to 20-dBm for all measurements. In general, we can always obtain the format conversion and multicasting functionalities, but there do exist some regions with high efficiency, i.e. from 1535 nm to 1555 nm. Near the zero-dispersion wavelength region with very small dispersion slope, the deviation of the efficiency may come from the characteristics of the high power EDFA itself. Therefore, better results (better converted BER, more multicast channels) may be possible though optimizing the EDFA noise control.

 

Fig. 7 Generated multicasting spectra as the wavelengths of the pump (clock) and the NRZ data shift crossing the EDFA gain spectrum (dark line). The wavelength spacing between the clock and NRZ channels is fixed to 0.8-nm, while the input power into the HNLF is set to 20-dBm.

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4. Conclusion

We demonstrated a scheme that can achieve NRZ-to-RZ format conversion and multicasting from one to six data channels using a HNLF and one single pump. The approach incorporates several nonlinear effects including FWM, XGM and XPM to perform desired optical signal processing functionalities.

For the wavelength spacing between the clock (pump) channel and the NRZ channel as 0.8-nm, we obtained eight converted RZ channels with six of them error-free at 10-Gb/s. Such scheme may be a good candidate for future all-optical networks in terms of function integration.