40Gbit/s all-optical format conversion from the carrier-suppressed return-to-zero (CS-RZ) to the non-return-to-zero (NRZ) is proposed and demonstrated with a temperature-controlled all-fiber delay interferometer (DI) and narrow-band filters. The NRZ signals can be achieved at two different wavelengths simultaneously from the original CS-RZ, with polarization and input power independence. The operation principle is theoretical analyzed with the help of numerical simulation and spectra analysis. Simulated results are well coincidence with experimental results. The format conversion can be achieved with power penalty of 1.6dB
© 2007 Optical Society of America
To release the potential of optical transmission systems and achieve higher transmission capacity, a lot of research on novel modulation formats other than traditional non return-to- zero (NRZ) and return-to-zero (RZ) has been done in recent years. Among these new formats, carrier-suppressed return-to-zero (CS-RZ) shows good tolerance to some linear and nonlinear impairments and is widely used in newly reported 40-Gb/s experiment systems . On the other hand, the transmission systems employing different modulation formats require different optimal settings, such as different dispersion and nonlinearity management. The RZ/CS-RZ format is widely employed in Optical Time Division Multiplexing (OTDM) networks as it has large tolerance to polarization mode dispersion (PMD), and NRZ format is spectrally efficient and thus is better suited for Dense Wavelength Division Multiplexing (DWDM) system. Thus, modulation format conversion will become an important interface technology for future optical networks that will include different formats.
Previous reports have been realized all-optical format conversions between RZ and NRZ using different techniques [2–9]. Recently, the format conversion from RZ to CS-RZ has been demonstrated for the first time using the CS-RZ clock , and Li et al. has demonstrated the NRZ to CS-RZ conversion and the conversions between the RZ and CS-RZ based on semiconductor optical amplifier loop mirror . These converted signals from these methods would be generated at one wavelength. Furthermore, to our best knowledge, there is no report on all-optical format conversion from the CS-RZ to the NRZ.
The all fiber delay interferometer (DI) has been used to perform the format conversions from the NRZ to RZ  and the RZ to NRZ . In this paper, we report an all-optical CS-RZ to NRZ format conversion at 40Gbit/s with DI by cascading narrow-band filters. The passive converter, without exploiting any active device, has extremely simple configuration and no additional noise; it is transparent to bit-rates and input powers, and can be operated with polarization independent. Furthermore, by controlling the DI and the filters properly, the NRZ signals can be achieved with small power penalty at two different carrier wavelengths. We analyze this process with the help of spectra evolution analysis (i.e. the spectra before and after conversion), which can illustrate the conversion principle simply.
2. Experimental setup and operation principle
The experimental setup is shown in Fig. 1, which includes a fiber DI and two tunable narrowband filters. The SHF optical communication system is used to generate input CS-RZ signal at the center wavelength of 1559.64nm, and an average power of 0dBm. Its bit rate is fixed at 40Gbit/s in our experiment. The signal power can be controlled by followed erbium doped fiber amplifier (EDFA) and the attenuator (ATT). The signal is preprocessed by the DI, which is a comb filter from the point of view of the spectrum domain. Two followed 0.3nm narrowband filters, with center wavelength of 1559.48nm and 1559.8nm respectively, are used to perform the complete conversions. Our proposed DI has a time delay Δt about 12.5ps in one arm which is corresponding to about 2.6mm fiber length difference, and the wavelength spacing Δλ is approximate 0.64nm. The other arm is temperature-controlled to change the refractive index, thus the phase difference (Δψ) between the two arms will change, resulting in the comb-like transmission spectrum would be shifted without distortion, as well as the center wavelengths of the filters can be adjusted to satisfy the input signals. So our converter can be operated with wavelength transparency. Furthermore, due to the all-passive devices, the converter is polarization and power insensitive. When Δφ and Δt are appropriate, the CS-RZ to NRZ conversions can be achieved at two output ports of the DI corresponding to two different wavelengths. The results can be analyzed by the Optical Spectrum Analyzer (OSA) and Communication Signal Analyzer (CSA) respectively. The bit error ratio (BER) can be measured by the Error Analyzer (EA).
The operation principle can be analyzed in frequency domain. Take one output port of the DI for example, Fig. 2 shows the simulated results for the eye diagrams and the spectra of the format conversion, it can help us to get a comprehensive understanding of the conversion.
As we know, the carrier of the CS-RZ signal is suppressed while the interval of two sidebands is equal to the modulation bitrate, it is 40GHz in our experiment.
By controlling the operation temperature of the comb-filter (DI), one of the notch wavelengths aims at one sideband of the spectrum of the CS-RZ, while one of the peak wavelengths aims at the other sideband, which will be the carrier wavelength of the converted NRZ. The wavelength spacing between the adjacent peak (or notch) in the transmission spectrum of the DI is governed by the following equation:
where c is the velocity of light in vacuum, λ is the carrier wavelength of CS-RZ signal. According to Eq. (1), Δλ is determined by Δt, which is fixed to TB/2 (TB is the bit period) in our experiment to ensure the spacing between the adjacent peak and notch is 0.32nm [i.e. Δλ/2, corresponding to 40GHz frequency spacing, as shown in Fig. 2(a)]. Thus, one spike of the CS-RZ will be suppressed, while the other can be enhanced.
As shown in Fig. 2(a), the solid line and the dot line represent the simulated spectra for the 40Gbit/s CS-RZ and the proposed DI at optimal condition, respectively. Being filtered by the comb filter, there will be one main spike remains in the spectrum of the signal, which is one of the two sidebands of the input CS-RZ; as the solid line shown in Fig. 2(b). But the higher order spikes in the spectrum can not be suppressed absolutely; this will result in ripples on the waveform in the time domain. The residual spikes will be filtered by the followed narrowband filter [the dot line in Fig. 2(b)], so a better NRZ signal can be achieved, as shown in Fig. 2(c). Figures 2(d), 2(e), and 2(f) show the corresponding eye diagrams. Due to the symmetry of the spectrum of the CS-RZ, for the other output port of the DI at the same time, one notches of the comb filter will aim at another spike in the spectrum of the CS-RZ and the center wavelength of the filter is tuned correspondingly, thus the NRZ can be achieved at another wavelength.
In the time domain, the format conversion process can be analyzed as follows. The CS-RZ pulse entering the DI will be split into two different parts; pulses in one arm will be delayed by half a bit period. At the meanwhile, pulses in the other arm will be Δφ phase shift. For input signal at the center wavelength of 1550nm, when the phase difference Δφ is controlled to be π/2 or 3π/2, the output NRZ waveform represented by Fig. 2(e) will be achieved from one output port of the DI. In the other output port, the NRZ signal can be also achieved, but its output wavelength is different. Although the CS-RZ format has alternating phase of π, through filtering with the DI and the followed narrow band pass filter, the output NRZ pulses will be achieved without obvious phase alternation between the adjacent bits. Thus, the output can be regarded as conventional NRZ which can be demonstrated from its output spectrum. The converted NRZ signals from the two output ports of the DI are based on the same principle and have the same quality.
3. Results and discussion
Figure 3 shows the experimental results for the conversion of the bit stream “1100101011101000” and the eye diagrams. The upper is the input CS-RZ, the middle is the preprocessed signal after the DI, and the lower is the converted NRZ. We can see that, being preprocessed by the DI, the ripples on the top of the waveform are obvious. Then the incomplete converted signal is directly filtered by a followed filter with 3dB bandwidth of 0.3nm, which is approximately 40GHz frequency interval. Thus, a better NRZ signal can be observed. The converted NRZ signals show clear and open eyes, with little ripple on the top. No additional noise and pattern effect can be found. The output extinction ratio (ER) is over 15dB and the Q factor is 9.5.
The spectra of the input CS-RZ signal, the converted NRZ signal together with the optimal transmission spectrum of the DI and the filter are shown and analyzed in Fig. 4. Figures 4(a)–4(c) are the spectra of the conversion at one wavelength, while Figs. 4(d)-4(f) are the conversion spectra at the other wavelength. The spacing of the two wavelengths is 0.32nm. These two conversions are with same principle and output results, excepting the carrier wavelength of converted NRZ signal. From these spectra, we can see that one of the peak wavelengths of DI aim one of the spikes of the CS-RZ signal, while one notch aims at the other, whose spacing is just 0.32nm. One spike can pass through the comb filter, but the other is suppressed due to the transmission characteristic of the DI. So a NRZ-like spectrum can be achieved. Then we adjust the center wavelength of the filter to satisfy the reserved spike, and to eliminate the higher orders, simultaneity. Thus, the NRZ spectrum can be observed. Both conversions show good and open eyes with good ER and Q factors. Figure 5 shows the BER measurements for one of the two conversions. Result shows that the power penalty induced by the pattern converter is about 1.6dB. Considering the sensitivity difference between RZ (including CS-RZ) and NRZ signals of BER test system, the power penalty is very small.
As already discussed, the operation temperature of the DI can be tuned continuously thus its transmission spectrum would be shifted without distortion, as well as the center wavelength of the narrow-band filter can be adjusted to satisfy the input signals. So our format converter can be operated at other wavelengths. Figure 6 shows the measured output Q factor and the ER as a function of the input carrier wavelength (for one output port of the DI). Results show that the output signals can be achieved at different wavelengths with ER of about 15dB and Q factor of about 9.5.
The tunable DI, which is used to preprocess the input CS-RZ signal, is obligatory in our experiment to obtain a good conversion. If we used a single narrow-band filter to perform the conversion, the band-width should be in an exact range to ensure the suppression of the spikes and the reservation of the data modulation information (i.e. the DC component in the spectrum), simultaneously. Figure 7 (a) shows the experimental results for the CS-RZ to NRZ conversion only by using our narrow-band filter. It is clear that the single filter is too wide to suppress the spikes enough, thus the converted signal is not good enough, as shown in the inset of Fig. 7. If a narrower filter is used, although the spikes can be eliminated, the DC components around the carrier will be lost at the same time. This will result in the lost of the data modulation information and the bit stream will be changed. Due to the limitation of our experiment, we present the simulated results instead of the experimental ones in this situation. Figure 7(b) shows the simulated results for the conversion using a filter with 3dB bandwidth of 0.2nm. The results show that it seems impossible to ensure the suppression of the spikes and the reservation of the data modulation information by using a single filter, for the difficulty on the accurately control of the bandwidth of the filter.
Note that, the best candidate for the filter used in the second stage is Gaussian-shaped filter with first order, for the fitness of the transmission spectrum of the filter and the spectrum of the NRZ signal.
In conclusion, we have proposed and demonstrated all-optical format conversion from the CS-RZ to NRZ by using an all fiber delay interferometer and two narrow-band filters. The tunable DI is used to preprocess the CS-RZ, and the followed filter is to perform a complete conversion. Simulation results accord with experimental results very well at 40Gbit/s. NRZ signals at two carrier wavelengths can be achieved simultaneously from the original CS-RZ signal with power penalty of 1.6dB and Q factor of 9.5. The proposed format converter is polarization and wavelength independent.
This work was supported by the National High Technology Developing Program of China (Grant No. 2006AA03Z0414) and the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-04-0715).
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