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We propose and demonstrate all-optical amplitude regeneration for the wavelength division multiplexing and polarization division multiplexing (WDM-PDM) return-to-zero phase shift keying (RZ-PSK) signals using a single semiconductor optical amplifier (SOA) and subsequent filtering. The regeneration is based on the cross phase modulation (XPM) effect in the saturated SOA and the subsequent narrow filtering. The spectrum of the distorted signal can be broadened due to the phase modulation induced by the synchronous optical clock signal. A narrow band pass filter is utilized to extract part of the broadened spectrum and remove the amplitude noise, while preserving the phase information. The working principle for multi-channel and polarization orthogonality preserving is analyzed. 4-channel dual polarization signals can be simultaneously amplitude regenerated without introducing wavelength and polarization demultiplexing. An average power penalty improvement of 1.75dB can be achieved for the WDM-PDM signals.
©2013 Optical Society of America
1. Introduction
The desire to meet the high-capacity and cost-effective demands of various communication applications has led to the fiber-optic networks with high spectral efficiency (SE) [1]. Besides the wavelength-division multiplexing (WDM) and the optical time division multiplexing (OTDM) techniques, the polarization-division multiplexing (PDM) technology has also attracted much attention as one of the effective multiplexing methods to double the SE by transmitting two different data series using the two orthogonal polarization states with a same wavelength [2, 3]. As a result, the multi-dimensional multiplexing is becoming an effective and successful approach to achieve the record transmission capacity [4]. On the other hand, the all-optical regeneration is a key technique in order to extend the transmission distance and increase the transparency in current phase modulation format based optical networks [5], where the nonlinear phase noise (NPN) is one of the most severe impairments. The accumulation of the NPN can be reduced by limiting the intensity fluctuations of the signal [6], which is a cost-effective and promising technique for the long distance transmission, and a number of techniques addressing this issue have been investigated [7–11]. However, most of the regeneration schemes were only capable for one wavelength or polarization state. Only a few papers had demonstrated regeneration for WDM signals [12] or for PDM signals by firstly performing the polarization demultiplexing and then processing separately [13–15]. The simultaneous regeneration based on a single device for the signals utilizing multiple multiplexing techniques without demultiplexing is quite desirable.
In this paper, we propose and experimentally demonstrate the all-optical amplitude regeneration for the WDM-PDM return-to-zero phase-shift keying (RZ-PSK) signals using a single semiconductor optical amplifier (SOA). The regeneration is based on the cross phase modulation (XPM) effect and the subsequent filtering, which had been proved to be an effective way to eliminate the intensity noise while preserving the phase information [16]. The capability for multi-channel and polarization orthogonality preserving operations using the single SOA is discussed and analyzed. Although the SOA is inherent polarization sensitive, the simultaneous regeneration for the multiple wavelengths each with dual polarization states can be still achieved without introducing any demultiplexing processes, by optimizing the working parameters. The phase information can be preserved without introducing any obvious distortion. 4-wavelength PDM RZ-PSK signals at 20G Baud rate can be amplitude regenerated with an average power penalty improvement of 1.75dB at 10−9.
2. Experimental setup and operation principle
The experimental setup is shown in Fig. 1 . Four WDM channels (wavelength from 1547 to 1560 nm with spacing of 3.2nm, 1554 exclusive) are coupled into the modulation unit with an array waveguide grating (AWG) to obtain the 4-channel RZ-PSK signals. The 4 CW lights are driven by the RF data (PRBS 231-1) from a pattern generator at 20G Baud rate. The polarization states for the 4 channels are optimized by the polarization controllers (PCs). A span of 1km single mode fiber (SMF) is utilized to achieve the de-correlation. A WDM synchronization scheme consisting of two AWGs and four tunable optical delay lines (ODLs) are used to compensate the differences of the wavelength dependent group velocity through the transmission link, by performing the de-multiplexing, the delaying and the re-multiplexing respectively. Then the WDM-PDM signals are obtained by employing a polarization multiplexing scheme that consists of a PC, a polarization beam splitter (PBS), a variable optical attenuator (ATT), an ODL and a polarization beam combiner (PBC). The ODL is used to de-correlate the data stream of the two polarization tributaries. The ATT is used to balance the optical power of two polarization states. The signals are then launched into the regenerator for processing. The SOA used in the experiment is a CIP nonlinear device (CIP SOA-NL-1550) with a polarization dependent gain (PDG) of ~1dB. It is operating at bias current of 180mA in our experiment. Another CW beam (1553 nm) is fed into a phase modulator (PM), which is driven by the synchronous 10 GHz sinusoidal RF clock signal, to obtain a CW signal with sinusoidal phase modulation. A subsequent delay interferometer (DI 3) with 40 GHz free spectral range (FSR) is used to obtain a 20 GHz optical clock signal using the destructive interference. The delay of the RF clock can be adjusted to synchronize the RZ-PSK signals. The input power of the clock and the WDM-PDM signals are set to 10 and 0dBm respectively. A multi-channel filter, which is another DI (DI 1) with FSR of 100 GHz, is used to perform the filtering for all the wavelengths. For observation and analysis, the regenerated signals are then demodulated by a fiber based multi-channel polarization insensitive demodulator (DI 2), following by the wavelength and polarization demultiplexers (band pass filter and PBS). The results can be analyzed by the error analyzer (EA) and the polarization analyzer (PA).
The amplitude regeneration for the WDM phase modulated signals had been analyzed and successfully demonstrated in previous work [17]. By using a clock signal with large power, the SOA can work at deep saturation without cross gain modulation while the XPM can be applied to all the input channels. Furthermore, the self phase modulation can be mitigated due to the small line width enhancement factor of the quantum well SOA, under the deep saturation [18]. No obvious phase distortion will be introduced during the regeneration.
In our scheme, the incoming multi-channel signals need to be synchronous with the clock signal. For parallel all-optical signal processing utilizing the XPM induced by an additional clock, the synchronization is essential to ensure the good performance for a simultaneous processing to all the channels [19–21]. On the other hand, if an optical clock signal with very small duty cycle is utilized, the requirement for synchronization can be released significantly and the synchronization tolerance will be much larger, as demonstrated in [22].
The polarization orthogonality preserving of the proposed signal processing based on the SOA is analyzed as follow. The SOA used in the experiment is not polarization independent, resulting in the different polarization rotations for the two polarization tributaries when suffering the XPM effect in the SOA. This will destroy the polarization orthogonality. Here in our scheme, we propose and demonstrate the regeneration with no obvious polarization orthogonality distortion, by optimizing the working condition of the scheme. The PDM signal is launched into the SOA, assisting by a PC, which is used to keep the X-Pol signal α linearly polarized with respect to the SOA TE mode axis. Apart from the indirect interaction through the carrier dynamics in the device, these two polarizations propagate independently. The angle α is determined by the SOA induced gain and phase modulation for the X-pol and Y-pol respectively. Given that the gain and phase modulation for the orthogonal components are real and constant numbers for a fixed input, an optimal angle α can be always obtained by adjusting the PC before the SOA and the polarization state of the clock signal to ensure a polarization insensitive operation. In other words, the X-pol signal can be kept α linearly polarized with respect to the TE mode axis to ensure the two output signals are still orthogonal after the regeneration, as the schematic diagram in Fig. 2 shown. As a result, the processing for the PDM signals can be expected.
In the experiment, the amplitude distortion of the signal is emulated by detuning of the driving voltage of the first MZM from the optimum value [8], resulting in the amplitude is different for signal “π” and “0” while the phase information remains unchanged. As a result, the amplitude noise can be emulated without introducing any phase noise. Please note that the amplitude noise generated by this method only affects one output port of the demodulator (DI) while demodulating [23]. In other words, only one output side of the DI has the amplitude noise.
3. Results and discussions
Results show that the proposed scheme works well for the WDM-PDM signals simultaneously. The measured spectra are presented in Figs. 3(a) to 3(d), showing the input spectra of the distorted signals, the spectra after the SOA, one of the detailed input signals and one of the regenerated signals, respectively.
Fig. 3 Measured spectra of (a) Before the SOA (b) After the SOA (c) One of the input RZ-PSK (d) One of the regenerated RZ-PSK (Res: 0.05nm)
In order to verify the polarization orthogonality during the regeneration, the polarization states are measured for one of the 4-channels using the PA. Taking channel 1 (1547nm) as the representative, Figs. 4(a) and 4(c) show the polarization states on the Poincare sphere before and after the regeneration, for the two polarization tributaries respectively. The initial states for the two tributaries are orthogonal. The original polarization state of the clock is shown in Fig. 4(b). Although there shows rotations for the two tributaries after the regeneration, it is clear that the output signals are still orthogonal despite of the polarization dependence and the nonlinear processing in the SOA. Similar performance can be observed for the other channels.
The eye diagrams for the four channels (each includes two polarization tributaries) are measured and presented in Fig. 5 . The measurements are performed for the RZ-PSK signals before and after the regeneration, the demodulated duobinary (DB) part and the alternate mark inversion (AMI) part respectively. Due to the available facility, no balanced detection is utilized. As we analyzed, the amplitude noise will only present at the DB port of the demodulated signal, and it can be significantly reduced for the both polarization tributaries.
The bit error rate (BER) measurements are performed for all the distorted and regenerated DB parts (both for the X- and Y-pol) respectively. For simplification, the average curves forthe distorted and the regenerated signals are plotted in Fig. 6 , and a power penalty improvement of 1.75dB is achieved. The penalty difference between the two polarization tributaries is less than 0.1dB, indicating a good polarization insensitive operation.
As mentioned previously, the noise in the experiment is emulated by adjusting the modulator bias, which is different from the real noise. In order to validate the universality of the proposed scheme, a simulation is performed using the white Gaussian noise. The parameters and details of the simulation is same as that of Ref [23], and the noise improvements (the amplitude noise difference between the input and output) versus the input optical signal to noise ratio (OSNR) are calculated in Fig. 7 . The detail constellations for the input and output signals corresponding to an input OSNR of 20 dB are presented as the insets.
4. Conclusion
We have proposed and demonstrated an amplitude regeneration scheme for WDM-PDM RZ-PSK signals. The simultaneous regeneration utilized a single SOA and subsequent filtering, and no wavelength and polarization demultiplexing is needed. By optimizing the working parameters, 4-channel dual polarization signals at 20G Baud rate can be simultaneously amplitude regenerated with an average power penalty improvement of 1.75dB.
Acknowledgments
This work was partially supported by the National Basic Research Program of China (Grant No. 2011CB301704), National Natural Science Foundation of China (Grant No. 61007042 and 61275072), National Science Fund for Distinguished Young Scholars (No. 61125501) and the Fundamental Research Funds for the Central Universities (Grant No. 2012QN104).
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