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Simultaneous all-optical RZ-OOK to NRZ-OOK format conversion for two tributaries in PDM signal is demonstrated utilizing a single section of highly nonlinear fiber through polarization nonlinear loop mirror configuration. Less than 1-dB power penalty is achieved in a 2 × 12.5-Gb/s PDM system, and only 1.4-dB SNR penalty is obtained in a 2 × 40-Gb/s PDM system.
©2012 Optical Society of America
1. Introduction
All-optical format conversion that is one of the key all-optical signal processing techniques may play an important role in future optical networks [1–3]. Over the past few years, numerous format conversion schemes have been proposed and demonstrated. Among those, return-to-zero to non-return-to zero (RZ-to-NRZ) format conversion may find applications in transparent interconnections of future optical links or nodes. Most demonstrated RZ-to-NRZ format conversion schemes utilized nonlinear effects, such as cross-phase modulation (XPM), cross-gain modulation (XGM), and four-wave mixing (FWM). These effects are generated in different nonlinear media, including nonlinear optical fiber [4–8], semiconductor optical amplifier (SOA) [9, 10], photonic crystal fiber [1, 11], waveguide or others [2, 12–15].
Hitherto, most format conversion approaches are done for the signal with a fixed polarization state. However, polarization-division-multiplexing (PDM) technique is becoming a promising candidate for high-speed and high capacity optical communication systems as it can double the spectral efficiency directly by combing two polarization channels of the same bit rate and same wavelength [16–18]. Therefore, all optical format conversion in such systems may be desired.
RZ-to-NRZ format conversion based on XPM effect for the signal with a fixed polarization state has been well-studied before [1, 3–5, 14], and polarization insensitive conversion for such signal has been achieved through a polarization-diversity loop configuration [1]. In this paper, with a similar setup to the polarization-diversity loop [1, 19], we demonstrate all-optical RZ-to-NRZ format conversion for PDM systems based on XPM in a polarization nonlinear loop mirror. The loop configuration can convert two polarization tributaries of the RZ signal into a NRZ signal simultaneously and reassemble the converted NRZ signals back to the PDM signal automatically. The effectiveness of this scheme is experimentally demonstrated in a 2 × 12.5-Gb/s and a 2 × 40-Gb/s PDM system. We also investigate the SPM induced spectral broadening of the input RZ signal and walk-off effect between the signal and CW probe light on the converted NRZ signal by adjusting the wavelength separation between the signal and the CW probe light. Less than 1-dB power penalty is achieved measured at 10−9 BER in the 2 × 12.5-Gb/s PDM system when the wavelength separation is set ~3-nm. In the 2 × 40-Gb/s PDM system, 1.4-dB SNR penalty is obtained at the wavelength separation of ~7-nm.
2. Configuration and principle
The operation principle for the RZ-to-NRZ format conversion based on XPM is shown in Fig. 1 . When a RZ input signal and continuous-wave (CW) probe light are launched into a nonlinear medium, nonlinear phase modulation between them occurs due to the optical Kerr effect. For the CW probe light, the XPM induced phase modulation is governed by the rate of power change of the input RZ signal. The red chirp is caused by the rising edge of the RZ signal, while the blue chirp is generated due to the falling edge of the RZ signal. In addition, the instantaneous wavelength remains unchanged when the rate of power change is zero, namely the chirp-free wavelength components. Therefore, an inverted NRZ signal can be obtained by filtering out the chirped-free components of the input CW probe light.
Figure 2 illustrates the CW probe light and PDM signals propagating inside the polarization nonlinear loop mirror. The input light (i.e. CW probe light and PDM signals) from the circulator and polarization beam splitter (PBS1) is separated into two orthogonal polarizations at the output of PBS and counter-propagate inside the loop. The state of polarization (SOP) of the CW probe light is aligned to be 45° with respect to the principle of the PBS1 (by adjusting PC1) to equally divide the probe light power into two orthogonal components, while the amplified RZ-OOK PDM signals are demultiplexed by the PC2 and PBS1, and counter-propagate through the loop. A PC (PC3) inside the loop is used to achieve a round trip polarization rotation of 90°, therefore the two counter-propagating orthogonal components of the input signal can recombine at the input port (port3) of the PBS1. Finally, the signal is guided out of the loop by the circulator. At the output of the circulator (port3), a tunable optical band pass filter (OBPF) is used to extract the chirp-free component of the input CW probe light for obtaining the NRZ signals.
Fig. 2 Conceptual diagram of all-optical PDM format convertor. PC: polarization controller, PBS: polarization beam splitter, HNLF: highly nonlinear fiber, OBPF: optical band-pass filter.
3. Experiment results and discussion
Firstly, we investigate the proposed scheme in a 2 × 12.5-Gb/s RZ-OOK PDM system. The experimental setup is shown in Fig. 3 . It consists of a 2 × 12.5-Gb/s RZ-OOK PDM transmitter (Fig. 3(a)), an all-optical RZ to NRZ format conversion block based on polarization nonlinear loop mirror (Fig. 2), a RZ-OOK PDM receiver, as well as related test equipments. In the transmitter, the light from distribute-feed-back (DFB) laser oscillating at ~1556 nm is modulated at 12.5-Gb/s by two cascaded Mach-Zehnder modulators (MZM) with 231-1 pseudorandom bit sequences (PRBS) to generate RZ-OOK signal with the full-width at half-maximum (FWHM) of ~38-ps. Then the 2 × 12.5-Gb/s RZ-OOK PDM signals are obtained employing an interleave scheme that is composed of a coupler, two polarization controllers (PC6 & PC7), a variable optical attenuator(VOA), 1-km single mode fiber (SMF) and a PBC. The SMF and the VOA are used to decorrelate the data streams and balance the optical power between two channels, respectively. The format conversion block is similar to Fig. 2. The tunable filter (OBPF) has a 3-dB bandwidth of ~0.2 nm. The zero dispersion wavelength, dispersion slope, nonlinear coefficient and polarization mode dispersion (PMD) of the 2-km highly nonlinear fiber are 1556 nm, 0.02 ps/nm2/km, 30 (W·km)−1, and < 0.2 ps/km1/2, respectively.
Fig. 3 Experimental setup: (a) PDM transmitter, (b) PDM format conversion; MZM: Mach-Zehnder modulator, VOA: variable optical attenuator, PBC: polarization beam combiner, ECL: external cavity laser, BERT: BER tester.
In our experiment, the CW probe light emitted by the external-cavity-laser (ECL) is combined with the amplified RZ-OOK PDM signals by a coupler and then injected to the polarization nonlinear loop mirror through the circulator and a PBS. As mentioned before, the SOP of the CW pump is aligned to be 45° with respect to the principle state of the PBS1 to equally divide the pump power into two orthogonal components, while the amplified RZ-OOK PDM signals are demultiplexed by the PC2 and PBS1 and counter-propagate through the loop. After propagating through the loop, the spectrally broadened two orthogonal components of the CW probe light are recombined at port1 of PBS1 and guided out of the loop by the circulator. The OBPF at port3 of the circulator is used to exclude the original signals and slice the chirp-free component of the broadened CW probe light spectrum for obtaining the inverted NRZ signals.
Figure 4 shows the spectra of the CW probe light, the RZ signal before and after the HNLF, and the converted NRZ signal. The wavelength of CW probe light and the original RZ-PDM signal are 1559 nm and 1556 nm, respectively. The power of CW probe light and original RZ-PDM signal injecting into the polarization nonlinear loop mirror are ~10 dBm and 17.36 dBm (~14.36 dBm for each polarization tributary), respectively. It is clear that the spectrum of the input CW probe light is significantly broadened due to the XPM effect inside the loop. Under the same condition, the bit error rate (BER) measurements are taken for the original RZ signal and the converted NRZ signals (both polarization tributaries), as shown in Fig. 5 . Some typical eye diagrams and pulse patterns are inserted as well (i.e. the input RZ signal and converted NRZ signal, note here patterns are compared at 215-1 PRBS). The converted NRZ signal is logically inverted compared to the original RZ signal, and less than 1-dB power penalty (measured at 10−9 BER) is achieved for both polarization tributaries.
Fig. 4 Spectra of the CW probe light, the RZ signal before and after the HNLF, and converted NRZ signal in the 2 × 12.5-Gb/s PDM system (one polarization tributary). The wavelengths of the CW light and the original RZ-PDM signal are 1559 nm and 1556 nm, respectively.
Fig. 5 BER curves of the 2 × 12.5-Gb/s PDM system with typical eye diagrams and pulse pattern inserted. The channel with 1-km decorrelating SMF is referred as CH1 and the other channel referred as CH2.
Figure 6 shows the power penalties (measured at BER = 10−9) of the output NRZ signals as we vary the wavelength of the CW probe light (both polarization tributaries results are plotted), while the wavelength of the input RZ-PDM signals is fixed at 1556 nm. When the wavelength separation between the CW probe light and input RZ-PDM signals is beyond a certain range (i.e. ~5nm), larger power penalty (i.e. >2-dB) is observed. For shorter wavelengths, the additional power penalty is caused by the SPM induced spectral broadening of the input RZ signal. While for longer wavelengths, the power penalty arises from the walk-off between the signal and the probe light. Therefore, the probe wavelength should be chosen properly to optimize the quality of the converted NRZ signal. Note that the performance of output NRZ signals depends on the pulse width of the input RZ signal, the shape and the bandwidth of the optical filter as well.
Fig. 6 Power penalties for the converted NRZ signals vs. wavelength of the CW probe light in the 2 × 12.5-Gb/s PDM system. The wavelength of the input RZ-PDM signal is fixed at 1556 nm.
We further investigate our scheme in a 2 × 40-Gb/s RZ-OOK PDM system (pulses with FWHM of ~12-ps). Figure 7 shows the measured signal-to-noise ratio (SNR) and corresponding eye diagrams of the original 40-Gb/s RZ signal and two polarization tributaries of converted NRZ signal using 86100C high speed oscilloscope (the received power is fixed at −2.0 dBm). About 1.4 dB SNR penalty is obtained for both polarization tributaries when we set the wavelength spacing between the RZ signal and the probe light to ~7-nm. Here the SNR is defined as:
where, I1 and I0 are the histogram means at one and zero levels, respectively. σ1 and σ0 are the standard deviation of the histogram at the one and zero levels, respectively. The measurements are made over the middle 5% of the eye-diagram.Fig. 7 SNR and corresponding eye diagrams of the 2 × 40-Gb/s PDM system measured by 86100C high speed oscilloscope (optical bandwidth up to 65 GHz). (a) Input RZ signal and converted NRZ signal for (b) CH1 and (c) CH2.
4. Conclusion
We have experimentally investigated a scheme of all optical RZ-OOK-PDM to NRZ-OOK-PDM format conversion, and demonstrated in both 2 × 12.5-Gb/s and 2 × 40-Gb/s PDM systems. Less than 1-dB power penalty is achieved measured at 10−9 BER in the 2 × 12.5-Gb/s PDM system, and 1.4-dB SNR penalty is obtained in the 2 × 40-Gb/s PDM system.
Acknowledgment
The work was supported by National Basic Research Program of China (2012CB315704) and the Program for New Century Excellent Talents in University (NCET-08-0821).
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