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We propose an all-optical modulation format conversion scheme from non-return-to-zero on-off-keying (NRZ-OOK) to return-to-zero (RZ) multiple-level phase-shift-keying (PSK) based on nonlinearity in optical fiber. The proposed conversion scheme is numerically investigated and experimentally demonstrated. We experimentally demonstrate error-free operation of NRZ-OOK/RZ- binary PSK conversion at 10.7 Gb/s. The operation of the NRZ-OOK/RZ-quadrature PSK conversion is shown by eye opening after balanced receiving at a symbol rate of 10.7 Gsymbol/s. In addition, we demonstrate the feasibility of the modulation format conversion from NRZ-OOK to RZ-8-levels PSK by numerical simulation.
©2007 Optical Society of America
1. Introduction
Optical communication systems have been primarily adopting on-off-keying (OOK) signals, which convey the information in the intensity, in either non-return-to-zero (NRZ) or return-to-zero (RZ) pulse format. Recently, advanced optical modulation formats have attracted an increased attention [1]–[3]. Some of these formats carry information through OOK, but also modulate the phase without carrying information in order to enhance signal’s robustness to fiber dispersion, nonlinearity, and optical filtering. This group includes duobinary, alternate mark inversion, chirped-RZ, and carrier-suppressed-RZ (CSRZ). In contrast, phase-shift-keying (PSK) signals convey the information in the phase itself. Due to the difficulty of generating absolute phase reference in direct-detection, the phase of the preceding bits is used as a relative phase reference for demodulation. This results in differential-PSK (DPSK), which carries information in the form of phase change between the consecutive bits. DPSK has the advantage of requiring 3 dB received power less than OOK to reach a given bit-error-rate (BER). Moreover, it has robustness to fiber nonlinear effects. Recent studies have revealed that DPSK shows better transmission performance than conventional OOK for the long-haul transmission [4], [5]. More advanced modulation format, differential quadrature PSK (DQPSK), appears to be promising technique in order to exploit the better receiver sensitivity and to secure the compatibility with 50 GHz channel spacing in ultra-dense wavelength division multiplexed transmission [6]. One of the most straightforward advantages of the DQPSK compared to DPSK is the increase of the tolerance to chromatic dispersion and polarization mode dispersion by approximately a factor of 2 [7]. Moreover, differential 8-levels PSK (D8PSK) transmission, which has very high spectral efficiency, has been experimentally demonstrated [8]. These advanced PSK formats will be adopted to long-haul transmission systems depending on the type of network.
In future optical networks, different modulation formats may be selectively used depending on the network size and the bit rate. The importance of all-optical modulation format conversion, transparently connecting different kinds of modulation formats in edge nodes, is growing. So far, various kinds of all-optical format conversions have been investigated, e.g., RZ-OOK to NRZ-OOK using semiconductor optical amplifier (SOA) loop mirror [9] and SOA-Mach-Zehnder interferometric (MZI) wavelength converter [10]; between RZ-OOK and CSRZ-OOK using SOA-loop-mirror [11] and frequency-shift-keying (FSK)-to-PSK [12]. All-optical OOK to binary PSK (BPSK) conversion using SOA-MZI wavelength converter [13] and highly nonlinear fiber (HNLF) [14] have been reported, for its attractive potential to connect cost-effective OOK based metro area networks (MANs) to robust PSK based long-haul backbone networks as shown in Fig. 1. To the author’s knowledge, however, any all-optical modulation format converter from OOK to QPSK and 8PSK has not been reported yet.
In this paper, we investigate a novel all-optical modulation format conversion from NRZ-OOK to RZ-multiple-levels PSK, including RZ-QPSK and RZ-8PSK, based on nonlinearity in optical fiber. This paper is organized as follows: in Section 2, we explain the basic principle of the proposed modulation format conversion. In Section 3, we then demonstrate the feasibility of the NRZ-OOK/RZ-BPSK conversion by numerical simulations and experiments. An error-free operation of the modulation format conversion at 10.7 Gb/s is demonstrated. In Section 4, we then describe the NRZ-OOK/RZ-QPSK conversion in a similar way of the section 3. Eye opening of the converted RZ-QPSK signal after balanced receiving at a symbol rate of 10.7 Gsymbol/s is demonstrated. In Section 5, we show the feasibility of the NRZ-OOK/RZ-8PSK conversion by numerical simulations.
2. Principle of operation
Figure 2 shows the schematic diagram of the proposed modulation format conversion. An RZ pulse sequence and K-channels NRZ-OOK signals are synchronously launched into a HNLF as a probe pulse and control pulses, respectively. The probe pulse is modulated in its phase due to cross phase modulation (XPM) induced by the control pulses. The phase change of the probe pulse due to XPM (Δϕ pro) is described by
where γ is the nonlinear parameter of HNLF, L eff is the effective interaction length of the HNLF, Pk is the peak power of the control pulse in the k-th channel, and Δϕk is the phase change of the probe pulse induced by the control pulse in the k-th channel. As shown in Eq. (1), the phase change of the probe pulse is proportional to the summation of the peak powers of the control pulses. The peak power of the control pulse in the k-th channel, Pk is adjusted so that the phase change of the probe pulse induced by the control pulse in the k-th channel, Δϕk becomes π/2k-1. After passing through the HNLF, the probe pulse has 2K different phases depending on the combination of the control pulses. For example, if P 1 = 28.3[dBm] induced π-phase shift, P 2 = 25.3[dBm] induces π/2 shift and P 1 + P 2 induces 3π/2 shift. Therefore, NRZ-OOK signals can be converted to RZ-2K-levels PSK signal. When K is 1, 2 and 3, the converted signals corresponds to RZ-BPSK, RZ-QPSK and RZ-8PSK signals, respectively. Note here that the plateau shape of NRZ pulse guarantees to generate chirp-free phase change at the center portion of the converted PSK pulse. However, parameters of the HNLF limit the possible number of levels of phase modulation in the converted signal because four wave mixing (FWM) and walk-off between probe pulse and control pulses induce power differences and unstable phase modulation on the converted signal depending on the bit pattern. The converted RZ-multiple-levels PSK signal can be recovered by either delay interferometers with encoding or decoding, or coherent receivers. For example, to recover the original OOK signal from the BPSK converted signal, a toggle flip flop is necessary after the balanced receiver. To recover the original OOK signals from the QPSK or 8PSK signals, more complicated logical operation is required in decoder.
3. NRZ-OOK/RZ-BPSK conversion
3.1. Numerical Simulation
We firstly consider that an NRZ-OOK control signal and an RZ probe pulse sequence are launched into an HNLF. The behavior of the optical pulses propagating in the HNLF is described by the nonlinear Schrödinger equation (NLSE) [15],
We here summarize the units of the quantities appeared in Eq. (2).
| z[m] : | propagation distance, |
| t[s] : | time moving with the group velocity, |
| E(z,t)(|E|2[W]) : | complex envelope of electric field, |
| : | group velocity dispersion, |
| : | nonlinearity, |
| : | fiber loss. |
We calculate the waveform and phase observed at the output of HNLF in the case of NRZ-OOK/RZ-BPSK conversion (K = 1) at the bit rate of 40 Gb/s. The wavelength of the control pulse and the probe pulse are 1548.2 nm and 1555.0 nm, respectively. The peak power and pulse width of the probe pulse are 0.0 dBm and 5.0 ps, respectively. The peak power of the control pulse is set to 28.3 dBm so that the phase shift of the probe pulse induced by XPM is π. The input pulse pattern of the control pulse is fixed as “01100101”. The parameters of the HNLF, D, s, α and length L, at the wavelength 1555 nm are -4.0 ps/nm/km, 24.2 /W/km, 0.24 dB/km and 0.1 km, respectively.
Figure 3 shows the waveform (a) and the temporal phase change (b) of the probe pulse after passing through the HNLF. We observe a stable operation of format conversion from NRZ-OOK to RZ-BPSK. We can observe a periodic clear pulse train in the waveform. The observed small fluctuation of the pulse peaks is caused by the FWM and walk-off between probe pulse and control pulse. At the peak of the probe pulse, the output phase after format conversion is nearly 0 or π depending on the corresponding control pulse of “0” or “1”. These results guarantee the scheme to be a suitable candidate to perform the modulation format conversion from NRZ-OOK to RZ-BPSK for 40Gb/s input signal. In this scheme, we do not need to care about the pattern dependent pulse distortion due to cross gain modulation in SOA devices [13].
Table 1. Parameters of the HNLF @1550 nm.
Fig. 5. Eye diagram and spectrum of converted signal before 1-bit delay interferometer: (a) Eye diagram; (b) Spectrum.
3.2. Experimental Demonstration
Due to the lack of experimental instruments for 40Gb/s system, we demonstrate the proposed converter with 10Gb/s based system. Figure 4 shows the experimental setup for the NRZ-OOK/RZ-BPSK conversion. The NRZ-OOK data signal (control pulse) was generated by modulating a CW light at 1555.3 nm in a lithium niobate intensity modulator (LN-IM) 1 with 10.7 Gb/s pseudo random bit sequence (PRBS) of length 231-1. An RZ probe pulse was generated by modulating a continuous wave (CW) light at 1557.7 nm in an LN-IM 2 by using a 10.7 GHz clock from a pulse pattern generator (PPG). We used the common clock from the PPG only for the experimental convenience to generate synchronized control and probe pulses. However, the clock for the probe pulse can be replaced by an extracted clock from the control pulse using one of reported all-optical clock recovery techniques [16],[17]. Each input polarization was optimized by a polarization controller (PC). The average power of the control and probe pulses launched into the HNLF were 16.1 dBm and 0.8 dBm, respectively. The parameters of the HNLF are summarized in Table 1. The HNLF is a commercially available Sumitomo Electric’s HNL-DSF. The converted signal was detected by a balanced receiver after passing through a 1-bit delay interferometer.
Figures 5 (a) and (b) show the eye diagram and spectrum for 10.7 Gb/s converted signal before the 1-bit delay interferometer, respectively. The periodic RZ pulse train and the carrier suppressed spectrum, which imply the generation of PSK signal, were observed. Since the pulse phases are randomly modulated, the carrier component is suppressed in averaged sense. Figures 6 show the eye diagrams of the converted signal after 1-bit delay interferometer. Eye diagrams of both constructive (a) and destructive (b) output are open clearly. This indicates that the phase of the converted signal is properly modulated as BPSK signal by XPM in HNLF. The walk-off induced a slight fluctuation at the 0 level of the constructive output and the 1 level of the destructive output, which represents the interference between successive symbols of “01” or “10” in the original NRZ-OOK signal. Figure 7 shows a clear eye opening after the balanced receiver. Figure 8 shows the obtained BER of the converted signal. We can see that an error-free operation was achieved. The above mentioned results reveal that the NRZ-OOK signal can be converted into the RZ-BPSK signal using XPM in optical fiber. Of course, we have confirmed that the experimental observations and results of the numerical model are in substantial agreement.
Fig. 6. Eye diagrams of converted signal after 1-bit delay interferometer: (a) Constructive output; (b) Destructive output.
4. NRZ-OOK/RZ-QPSK conversion
4.1. Numerical Simulation
We next consider that 2-channels NRZ-OOK control signals and an RZ probe pulse sequence are launched into a HNLF. We calculate the waveform and phase observed at the output of HNLF in the case of NRZ-OOK/RZ-QPSK conversion (K = 2) at the symbol rate of 40 Gsymbol/s by numerically solving the NLSE (2). Another NRZ-OOK control signal is added to the simulation model of OOK/BPSK conversion described in Section 3.1. The wavelength of added control pulse is set to 1561.7 nm in order to mitigate effects of FWM and walk-off. The peak power of the added control pulses is set to 25.3 dBm so that the phase shift of the probe pulse induced by XPM is π/2. The input pattern of added control pulse is fixed as “11100010”. Other parameters of the input pulses and the HNLF are the same as used in Section 3.1.
Figures 9 show the waveform (a) and the temporal phase change (b) of the probe pulse after passing through the HNLF. The waveform appears as a periodic pulse train since all pulses have almost the same peak power without significant effect of FWM. The observed small fluctuation of the pulse peaks is caused by the FWM and walk-off between probe pulse At the peak of the probe pulse, the output probe pulse after modulation format conversion has the phase of nearly 0, π/2, π or 3π/2 depending on the combination of 2 control pulses. Therefore, we can show the feasibility of the modulation format conversion from NRZ-OOK to RZ-QPSK using the proposed scheme for 40Gsymbol/s. and control pulses.
4.2. Experimental Demonstration
Due to the lack of experimental instruments for 40Gb/s system, we demonstrate the proposed converter with 10Gb/s based system. Figure 10 shows the experimental setup for the NRZ-OOK/RZ-QPSK conversion. NRZ-OOK data signal (control pulse) 1 and 2 were generated by modulating coupled CW lights at 1545.3 nm and 1551.9 nm in an LN-IM 1 with 10.7 Gb/s PRBS of length 231-1. These signal 1 and 2 were separated by 3 dB coupler and band-pass filters, and their average powers launched into the HNLF were adjusted to 16.7 dBm and 13.5 dBm, respectively. An RZ probe pulse was generated by modulating a CW light at 1547.8 nm in an LN-IM 2 by using a 10.7 GHz clock from a PPG. The average power of probe pulses launched into the HNLF was -3.0 dBm. We used the same HNLF as in the case of OOK/BPSK conversion described in Section 3.2. The converted signal was detected by a balanced receiver after passing through a 1-bit delay interferometer, in which the phase adjuster has a phase shift of “Δϕ = π/4”.
Fig. 11. Eye diagram and spectrum of converted signal before 1-bit delay interferometer: (a) Eye diagram; (b) Spectrum.
Fig. 12. Eye diagrams of converted signal after 1-bit delay interferometer: (a) Constructive output; (b) Destructive output; (c) Received signal with balanced receiver.
Figures 11 (a) and (b) respectively show the eye diagram and spectrum for 10.7 Gsymbol/s converted signal before the 1-bit delay interferometer. The periodic clear pulse train and the carrier suppressed spectrum were observed. Figures 12 (a) and (b) show similar eye openings after the 1-bit delay interferometer in both constructive and destructive outputs. Figure 12 (c) shows a clear eye opening after the balanced receiver. These results indicate that 2-channel NRZ-OOK signals can be converted into the RZ-QPSK signal with using the proposed scheme.
5. NRZ-OOK/RZ-8PSK conversion
We finally consider that 3-channels NRZ-OOK control signals and an RZ probe pulse sequence are launched into a HNLF. We calculate the waveform and phase observed at the output of the HNLF in the case of NRZ-OOK/RZ-8PSK conversion (K = 3) at the symbol rate of 40 Gsymbol/s by numerically solving the NLSE (2). The third NRZ-OOK control signal is added to the simulation model of OOK/QPSK conversion described in Section 4.1. The wavelength of the third control pulse is 1537.7 nm. The peak power of the third control pulses is set to 22.3 dBm so that the phase shift of the probe pulse induced by XPM is π/4. The input pattern of the third control pulse is fixed as “11001100”. Other parameters of the input pulses and the HNLF are the same as described in Section 4.1.
Figures 13 show the waveform (a) and the temporal phase change (b) of the probe pulse after passing through the HNLF. The waveform appears as a periodic pulse train with a similar shape to the Figs. 3 and 9. The output probe pulse after modulation format conversion has 8 different phases depending on the combination of 3 control pulses. These results show the feasibility of the proposed modulation format conversion from NRZ-OOK to RZ-8PSK and the potential to realize all-optical NRZ-OOK/RZ-multiple-level PSK conversion. Although the experimental demonstration converting 3-channels NRZ-OOK signals to RZ-8PSK signal seems not to be so difficult, the differential detection may require so complicated configuration [8].
6. Conclusion
In this paper, we have proposed, numerically modeled, and experimentally demonstrated all-optical NRZ-OOK to RZ-multiple-levels PSK modulation format conversion using XPM in an optical fiber. Based on numerical simulation, we have shown the feasibility of the modulation format conversion from NRZ-OOK to RZ-BPSK, RZ-QPSK and RZ-8PSK by showing the waveforms and the temporal phase changes of the converted signals in 40Gsymbol/s systems.
In experiments, we have observed clear eye opening and proper optical spectrum of the converted BPSK signal, showing the error-free operation at 10.7 Gb/s. Moreover, clear eye opening and proper optical spectrum of the converted QPSK signal at 10.7 Gsymbol/s have been observed. The proposed modulation format converter has important potential for higher bit rate of 40 Gb/s system or beyond, because phase modulation formats have greater advantages at such a high bit rate. In addition, since the response time of XPM in optical fiber is femto seconds, the converter is not affected by the pattern dependent distortion even in high speed operation. It is an advantage compared with the SOA based modulation format converter [13]. The proposed modulation format conversion scheme will become a key technique at the gateway node interfacing MAN and WAN in the future all-optical networks.
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