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Optical regenerative nonreturn-to-zero (NRZ) to return-to-zero (RZ) format conversion using a lithium niobate phase modulator and a lithium niobate intensity modulator is proposed and demonstrated. The key advantage of the proposed format converter is that the converted RZ signal has a very small pulse width, which can be multiplexed to a higher bit rate using optical time division multiplexing technology. The operation can greatly reduce the timing jitter of the degraded NRZ signal due to the regenerative property of the proposed scheme. Besides, the format converter can also support multi-channel operation. An experiment is performed with the feasibility of the scheme demonstrated.
©2010 Optical Society of America
1. Introduction
Digital optical communications primarily employ conventional data modulation format of either non-return-to-zero (NRZ) in a wavelength division multiplexing (WDM) network or return-to-zero (RZ) in an optical time division multiplexing (OTDM) network. Considering the different scale and requirement of the future optical networks, the two modulation formats may be selectively used [1]. In this regard, all-optical NRZ-to-RZ format conversion is of great importance to transparently and seamlessly connect the optical networks operating with different modulation formats. Since the optical NRZ signals introduced to the format converter may be degraded by long-distance fiber transmission, it is desirable that the format converter has the capability to restore the quality of degraded signals. The converted RZ signal from the NRZ-to-RZ format converter is also required to have a small pulse width so that it can be multiplexed to be a higher bit rate signal in an OTDM network. In addition, the format converter should be able to simultaneously convert multi-channel NRZ signals to RZ signals due to the multi-channel nature of the WDM networks. Previously, optical NRZ-to-RZ conversion has been demonstrated by various approaches including the use of high nonlinearity fiber (HNLF) [2], semiconductor optical amplifier (SOA) [2–6], microring resonator [7], optoelectronic oscillators [8,9] and optical modulator [10]. However, the schemes in [2–7] which are based on optical nonlinearity, would introduce serious interchannel crosstalk when applied to a multi-channel system. The methods proposed in [8–10] can support multi-channel operation, but the generated RZ signals have large pulse widths.
In this paper, we propose and demonstrate a regenerative NRZ-to-RZ format converter based on a lithium niobate (LiNbO3) phase modulator (PM) and a LiNbO3 intensity modulator (IM) followed by a section of dispersion compensating fiber (DCF). The structure is similar to the schemes for the generation of ultrashort optical pulses [11–14]. By carefully adjusting the modulation index of the PM and the length of the DCF, a low-timing-jitter RZ signal with a very small pulse width is obtained from a degraded NRZ signal. This RZ signal is further multiplexed to have a much higher data rate. The high quality of the converted RZ signal ensures the excellent transmission performance of the signal in a fiber link. Besides, multi-channel operation can also be achieved based on the proposed format converter.
2. Principles
Figure 1 shows the schematic of the proposed NRZ-to-RZ format converter consisting of a LiNbO3 PM and a LiNbO3 IM followed by a DCF. A local electrical clock generated by a radio frequency (RF) source is split into two signals to drive the two modulators. The relative phase of the two signals is adjusted by an electrical phase shifter. In practice, the local electrical clock can be extracted from the incident NRZ signal using an optoelectronic oscillator, as demonstrated in [8,9]. The principle of the proposed method can be understood by the schematic shown on the right of Fig. 1. First, the phase modulation in the PM introduces a periodic nonlinear positive and negative chirp across the input NRZ signal. Then, the following IM acts as a pulse carver to carve the NRZ signal into a RZ signal with a duty cycle of 50%. At the same time, the timing jitter of the NRZ signal is greatly suppressed by the synchronous modulation in the IM [15]. The phase difference between the driving signals to the PM and the IM is controlled and optimized by the phase shifter so that the negative chirp part is selected by the IM while the positive chirp part is suppressed. After passing through the DCF with positive dispersion, the chirped RZ signal is compressed and the duty cycle is greatly reduced. In our scheme, the dispersion medium is crucial for the generation of RZ signal with a small duty cycle. It should be noted that a PM and a dispersion medium can realize optical Fourier transformation and reduce the timing jitter of an optical signal [16].
Fig. 1 The schematic of the proposed NRZ-to-RZ format converter based on cascaded LiNbO3 modulators. Dotted line: chirp of the signal; solid line: waveform of the signal.
The electric field of the optical signal at the output of the IM can be written as
where Ein(t) is the electric field of the input NRZ signal and ωm is the angular frequency of the electrical clock. α and β are the modulation indices of the PM and IM, respectively, are defined bywhere Vm 1, Vm 2 are the driving voltages and Vπ 1, Vπ 2 are the half-wave voltages of the PM and IM, respectively. The propagation of the optical signal in the DCF can be described by the nonlinear Schrödinger equation:where A is the slowly varying pulse envelop, β 2 is the dispersion parameter, γ is the nonlinear parameter, and αL is the loss of the DCF. Since the DCF used in the scheme is relatively short, the fiber loss and nonlinearity parameter are neglected in our simulation.Since the pulse width of the converted RZ signal is important for further signal multiplexing, the influence of α and the length of the DCF on the pulse width is numerically investigated. Figure 2 shows the minimum pulse width of the converted RZ signal that can be achieved and the corresponding DCF length as a function of α. In the calculation, the data rate of the incident signal is assumed to be 10 Gb/s. As can be seen, both the minimum pulse width and the optimal fiber length decrease with α. When α exceeds 4, the pulse width is as small as 4 ps. However, the pedestal may exist in the converted RZ signal since the nonlinear chirp induced by the PM could not be completely compensated by the DCF even with an optimal length. The insets in Fig. 2 show the eye diagrams of the converted RZ signal when α = 1 and α = 5, respectively. Obvious pedestal can be seen in the RZ signal generated with α = 1 while the RZ signal obtained with α = 5 is almost pedestal-free. To evaluate the impact of α on the quality of the RZ signal, the time-bandwidth product and the pedestal of the converted RZ signal against α is calculated, with the results shown in Fig. 3 . Since the converted signal is Gaussian-like [14] and the Gaussian pulse is considered to be pedestal-free, the pedestal of the converted RZ signal is evaluated by comparing the converted signal with a Gaussian pulse which has the same peak power and pulse width. A p-factor is thus introduced and defined as
where PRZ is the average power of the ‘1’ bit in the converted RZ signal and PG is the average power of an ideal Gaussian pulse which has the same peak power and pulse width as the converted RZ signal. Based on the numerical simulation, it is found that a larger α leads to the generation of higher-quality RZ signals. As can be seen from Fig. 3, for the case of α = 5, the time-bandwidth product of the converted RZ signal is approximately 0.44 which is almost equal to that of an ideal transform-limited Gaussian pulse. At the same time, the pedestal is calculated to be less than 4% which has little negative influence on the signal quality. The results indicate that the quality of the converted RZ signal would not be limited by the proposed scheme if a large α is used. The stability of the system can also be improved by using a large α since the corresponding optimal length of the DCF is greatly reduced.3. Experimental results and discussion
Experiments based on the experimental setup shown in Fig. 1 are performed to demonstrate the proposed method. Two continuous-wave (CW) light at wavelengths of 1543.86 nm and 1549.44 nm, respectively, are generated by two DFB laser diodes and modulated with a 10-Gb/s 231−1 pseudo random bit sequence (PRBS) at a LiNbO3 IM. The generated NRZ signals are transmitted over a dispersion-managed (DM) fiber link of 80 km. The average power of each channel is amplified to 4.5 dBm by an erbium-doped fiber amplifier (EDFA) before injected into the format converter. The dispersion value of the DCF is 100 ps/nm. The half-wave voltages of the PM and IM are 3.5 and 5.5 V, respectively, and the powers of the driving signals are 200 and 60 mW, respectively. The converted RZ signals are amplified by a second EDFA to an average power of 0 dBm, filtered and transmitted over another DM fiber link. The eye diagrams of the optical signals are observed by a sampling oscilloscope (Agilent 86100A) and the bit error rate (BER) curves are measured by a bit-error rate tester (Agilent N4901B) combined with a receiver without an optical preamplifier.
At the first step, single-channel operation is investigated, i.e., only the channel at 1543.86 nm is switched on. Figure 4(a) and 4(b) show the eye diagrams of the input NRZ signal and the converted RZ signal, respectively. The timing jitter of the NRZ signal and the converted RZ signal is measured to be 3.4 and 1.4 ps, respectively, using the sampling oscilloscope. The Q2-factors are 25.3 dB for both the NRZ and RZ signals, with the Q-factors measured by the optical sampling oscilloscope. To examine the multiplexing performance, the RZ signal is polarization controlled and passively multiplexed to be a 40-Gb/s signal using a fiber-based OTDM multiplexer, with the eye diagram shown in Fig. 4(c). The multiplexed signal is observed to have uniform amplitude and a high extinction ratio. Figure 4(d) shows the eye diagram of the converted RZ signal after transmission over 80-km DM fiber link. The waveform of the transmitted signal does not greatly distort except the increment of the timing jitter. To study the tolerance of the format converter to the degradation of the input signal, the NRZ signal is transmitted through an 80-km DM fiber link. Figure 4(e) and 4(f) show the eye diagrams of the input degraded NRZ signal and the converted RZ signal, respectively. The timing jitter of the degraded NRZ signal is reduced from 6.2 to 1.5 ps, and the Q2-factors is improved from 20.2 to 22.8 dB after the format conversion. The converted RZ signal is also multiplexed to be a 40-Gb/s signal, with the eye diagram shown in Fig. 4(g). The eye diagram of the RZ signal transmitted over another 40-km DM fiber link is shown in Fig. 4(h), which shows no degradation of the signal quality compared to the signal shown in Fig. 4(d). The study confirms the high performance of the format converter. The BER performance of the single-channel format conversion is shown in Fig. 5 . The receiver sensitivity of the original NRZ signal and the converted RZ signal is −14.3 and −20.5 dBm, respectively, corresponding to a negative power penalty of −6.2 dB. The difference of the receiver sensitivity between the NRZ and RZ signal may relate to the low duty cycle of the converted RZ signal. If the receiver is optimized and an optical filter with an optimized bandwidth for the NRZ signal is used, the difference of the receiver sensitivity can be reduced. The BER curve of the converted RZ signal transmitted over 80-km fiber link is also plotted. The power penalty for the 80-km transmission is only 0.5 dB. For the format conversion of the degraded NRZ signal, the receiver sensitivity of the NRZ signal after 80-km transmission is decreased to −12.9 dBm. After the format conversion, the receiver sensitivity is dramatically improved to −19.5 dBm, corresponding to a negative power penalty of −6.6 dB, which is better than the back-to-back case. The measurement indicates the regenerative property of the format converter and its high tolerance to the degradation of the input signal. Furthermore, the converted RZ signal is successfully transmitted through a 40-km DM fiber link with a power penalty of only 0.4 dB, which confirms the high quality of the signal after format conversion.
Fig. 4 Eye diagrams of (a) the input NRZ signal, (b) the converted RZ signal, (c) the multiplexed 40-Gb/s signal and (d) the RZ signal after 80-km transmission. Eye diagrams of (e) the input NRZ signal after 80-km transmission, (f) the converted RZ signal, (g) the multiplexed 40-Gb/s signal and (h) the RZ signal after 40-km transmission.
To demonstrate the feasibility of multi-channel operation, simultaneous dual-channel format conversion is performed. The minimum channel spacing of the input NRZ signals required by the format converter is determined by the spectral width of the converted RZ signal. The eye diagrams of the dual-channel input signals are shown in Fig. 6(a) and 6(d), respectively. Double-trace fall edges can be seen in the eye diagrams of the NRZ signals with a timing jitter of 3.5 and 3.4 ps, respectively. After the format conversion, the timing jitter is measured to be 1.4 ps for both signals in the two channels, as shown in Fig. 6(b) and 6(e). The eye diagrams of the multiplexed 40-Gb/s RZ signals are shown in Fig. 6(c) and 6(f). Figure 7 shows the spectra of the dual-channel converted RZ signals. It can be seen that the two channels have almost the same spectral profile and power. It should be noted that the flatness of the dispersion profile of the DCF is crucial for the multi-channel format conversion with uniform performance. The BER performance of the dual-channel format conversion is shown in Fig. 8 . The back-to-back power penalty is −6 and −6.4 dB, respectively, and the difference of receiver sensitivity between the two channels is reduced from 0.6 dB for the NRZ signals to 0.2 dB for the converted RZ signals. The converted RZ signals are also transmitted over an 80-km DM fiber link. Error-free transmission of the two channels is achieved with power penalties of only 0.2 and 0.5 dB, respectively.
Fig. 6 (a)(b) input NRZ signals (c)(d) converted RZ signals and (e)(f) OTDM signals of Ch.1 and Ch.2 respectively.
4. Conclusions
We have proposed and demonstrated optical regenerative NRZ-to-RZ format conversion based on cascaded PM and IM. RZ signal with low timing jitter and small duty cycle was obtained using the proposed scheme. The format conversion of the NRZ signals before and after 80-km fiber transmission had negative power penalties. The subsequent transmission of the corresponding converted RZ signals in 80-km and 40-km DM fiber link had power penalties of 0.5 and 0.4 dB, which demonstrated the excellent performance of the proposed format converter. In addition, simultaneous dual-channel NRZ to RZ format conversion was performed to demonstrate the feasibility of multi-channel operation based on the proposed scheme. The performance of the signal in each channel was almost the same as that of the single-channel operation.
Acknowledgments
This work was supported by the National Science Foundation of China (NSFC) projects 60736036 and 61077055, 863 project 2009AA01Z256, Beijing project YB20091000301, and the “973” Major State Basic Research Development Program of China project 2011CB301700.
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