Optical wavelength conversion (OWC) is expected to be a desirable function in future optical transparent networks. Since high-order quadrature amplitude modulation (QAM) is more sensitive to the phase noise, in the OWC of high-order QAM signals, it is crucial to suppress the extra noise introduced in the OWC subsystem, especially for the scenario with multiple cascaded OWCs. Here, we propose and experimentally demonstrate a pump-linewidth-tolerant OWC scheme suitable for high-order QAM signals using coherent two-tone pumps. Using 3.5-MHz-linewidth distributed feedback (DFB) lasers as pump sources, our scheme enables wavelength conversion of both 16QAM and 64QAM signals with negligible power penalty, in a periodically-poled Lithium Niobate (PPLN) waveguide based OWC. We also demonstrate the performance of pump phase noise cancellation, showing that such coherent two-tone pump schemes can eliminate the need for ultra-narrow linewidth pump lasers and enable practical implementation of low-cost OWC in future dynamic optical networks.
© 2014 Optical Society of America
Optical wavelength conversion (OWC) of high-speed optical data signals has been widely studied as a key functionality to enhance the re-configurability, non-blocking capacity, and wavelength management in future dynamic optical networks . With high-order quadrature amplitude modulation (QAM) formats being more frequently proposed to increase the capacity of such networks, several OWC schemes for 16QAM and 64QAM signals have been demonstrated using nonlinear media including periodically-poled Lithium Niobate (PPLN) waveguides , highly-nonlinear fibers (HNLFs)  and semiconductor optical amplifiers (SOAs) . These OWC schemes are implemented through either the cascaded 2nd-order nonlinearities or third-order nonlinearity. Both the phase and intensity information of the input signal could be preserved in the converted signal, showing the transparency in modulation format. In order to support modulation format transparency and allow multiple cascaded OWC stages in future dynamic networks, it is imperative to minimize the addition of both phase and amplitude noise, particularly for phase-noise sensitive formats, such as high-order QAM signals [4–6]. However, due to the finite linewidth of pump lasers, phase noise from the pumps is transparently transferred together with the original data to the converted signal, causing nonnegligible penalties, particularly in high-order QAM formats, which may limit the achievable performance. To minimize the added noise, costly narrow-linewidth lasers such as external-cavity lasers (ECL) or fiber lasers (FL) are usually deployed as pumps. Moreover, for 64QAM and higher orders, even ECL pumps may be not sufficient to minimize power penalty. For example, power penalties of around 4-dB at 5Gbaud , and 2-dB at 21Gbaud  have been experimentally demonstrated for converted 64QAMs at bit-error rate (BER) of 10−3. It has shown that the use of costly narrow-linewidth FL as pump could help ensure the low power penalty in the OWC of 64QAM .
In this paper, we propose and experimentally demonstrate for the first-time, to our knowledge, a pump-linewidth-tolerant OWC for high-order QAM signals up to 64QAM, using coherent pumps in a pump-phase subtracting configuration. Since the phase of the two pumps is correlated, phase noise from each pump will be canceled out in the converted signal, which becomes independent of the pump linewidth. Hence, this scheme enables the use of low-cost distributed feedback (DFB) lasers instead of costly narrow-linewidth sources, effectively reducing the implementation cost and complexity in networks where multiple OWC units may be used to overcome contention. Previously, a similar concept has been deployed to preserve the linewidth or phase noise in OWC of un-modulated signals  and recently, a similar approach was used in a computer simulated OWC based on non-degenerate FWM in SOA with 16 QAM signals .
Our proposed scheme can be implemented by either cascaded sum- or difference- frequency generation (cSFG/DFG) in PPLN, or non-degenerate FWM in third-order nonlinear media. Here, we choose a PPLN due to its advantages of compactness, negligible frequency chirp and spontaneous noise, and immunity to stimulated Brillouin scattering . The proposed scheme can also be implemented in a polarization diversity loop , enabling polarization insensitive OWC. The experimental results show that, even with 3.5-MHz-linewidth DFB pump lasers, negligible power penalties (<0.1dB for 16QAM; <0.3dB for 64QAM at BER of 10−3) are achieved for both 16QAM and 64QAM signals at 10Gbaud in a coherent two-tone pump configuration. In contrast, Use of a similar set-up with free-running DFB pumps results in severe phase noise in converted 16/64QAM signals.
2. Operation principle
Figure 1 depicts the operation principle of the pump-linewidth-tolerant OWC. After OWC, the input signal at ω1 is shifted to the frequency ω2, with ω2 = ωp1-ωp2 + ω1, where ωp1, ωp2 and ω1 are the frequencies of pump1, pump2 and the input signal wave, respectively. This configuration is usually used to perform the data exchange between two input wavelengths . To satisfy the phase matching condition and increase the conversion efficiency, ωp1 and ω1 must be arranged symmetrically with respect to the PPLN’s quasi-phase-matching (QPM) wavelength. Under the non-depletion approximation (pump power Pp1, Pp2 » input signal power P1), linear mapping between the input and output complex amplitudes are obtained as , as well as the following phase relationship:
3. Characterization of the PPLN-based wavelength converter
In order to optimize the efficiency and minimize the crosstalk in OWC, we first used non-correlated, continuous wave (CW) signals to characterize the influence of the total pumping power and frequency separation between the interacting waves on the conversion efficiency (CE) and on the generation of spurious nonlinear contributions. The experimental set-up used for CW characterization is depicted in Fig. 2. The light emitted by two ECLs was combined in a 50/50 coupler to generate the pump waves, before optical amplification in a high power erbium-doped fiber amplifier (EDFA). Next, the two pumps were combined with the input signal generated by another ECL in a 10/90 coupler at the input to the PPLN. Optical attenuators (Attn) and polarization controllers (PCs) were used to adjust the input power to the PPLN and to align the polarization states to the optimum axis of the PPLN, respectively. The PPLN waveguide was produced on a lithium niobate substrate, doped with 5% of MgO to reduce the photorefractive damage at high pump powers. The length, poling period, temperature of operation, QPM wavelength and insertion losses of the PPLN device were 6cm, 19.1μm, 30.1°C, 1550.4nm and 3.25dB, respectively. The second harmonic generation (SHG) efficiency of the PPLN, defined as the ratio of the output SHG power to the square of the input power at the QPM wavelength, was 445%/W and the 3dB conversion bandwidth for SFG was 25 GHz. An optical spectrum analyzer (OSA) was inserted after the PPLN for monitoring.
The influence of the total power of the pumps launched into the PPLN, PT, the frequency separation between the SFG and DFG pumps, Δfpumps, and the frequency separation between the input signal and the SFG pump, Δfp-s¸ on the efficiency and generation of spurious nonlinear contributions was evaluated by measuring the CE, signal depletion (SD), and optical spectra after OWC in the PPLN device. Here, the CE is defined as the ratio of the converted signal power to that of the input signal after the PPLN with both pumps switched OFF. The SD is the power ratio of the input signal after the PPLN with both pumps switched OFF and ON, respectively.
The CE and SD are plotted for PT values from 23 to 29 dBm with Δfpumps values of 25, 50, 100 and 200 GHz in Fig. 3(a). As expected, both CE and SD increase with PT, as a result of a stronger power flow from the input signal to the converted wavelength, with the exception of Δfpumps = 25 GHz. In this case, the SD tends to decrease with PT and the CE reaches a maximum value of −10 dB for PT = 27 dBm. This behavior can be explained due to the generation of spurious contributions at frequencies close to the input and converted signals, as shown in Figs. 3(b), 3(c) and 3(d). These result from different combinations of cascaded nonlinear interactions between the pumps, input and converted signals (e.g. ωx1 = ωp2 + ω1-ωp1 and ωx2 = ωp1 + ω2-ωp2). For Δfpumps = 25 GHz, the generation of the spurious contributions is strong, resulting in lower CE and a complex power flow between the interacting waves. By increasing Δfpumps, the spurious contributions become weaker and the CE increases. In addition to the spurious contributions generated in the PPLN waveguide, two other waves (ωxp1 and ωxp2) can be observed in the spectra of Fig. 3, symmetrically displaced around the pumps. These contributions are already observed before the PPLN and they are caused by four-wave mixing between the pumps in the EDFA and patch cords.
The impact of Δfp-s on the CE and SD is depicted in Fig. 4, for Δfpumps = 50 GHz. According to Fig. 4(a), the CE and SD values are almost independent of Δfp-s. However, for Δfp-s = 125 GHz, some of the spurious contributions are very close to the QPM wavelength of the PPLN (Fig. 4(b)) and non-negligible secondary nonlinear interactions between them occur, including both cSFG/DFG and cascaded SHG and DFG interactions. Such secondary interactions are the reason why the SD values decrease for PT higher than 28 dBm in Fig. 4(a). In addition, we also verified that the CE and SD are independent of the power of the input signal, at least for signal power values up to 7 dBm, and that best performance in terms of both CE and SD is achieved for SFG and DFG pumps with equal power.
4. Pump-linewidth-tolerant OWC of 16 and 64 QAM signals
4.1 Experimental set-up
After evaluating the influence of the pump power and the frequency separation between the interacting waves on the efficiency and crosstalk in OWC, the experimental set-up depicted in Fig. 2 was upgraded to that shown in Fig. 5, in order to perform OWC of 16 and 64 QAM signals. To minimize the phase noise from the input signal, a 5kHz linewidth FL emitting at 1552.52 nm was deployed as the light source, and modulated by an in-phase/quadrature (IQ) modulator. Two de-correlated 4- or 8-level driving electronics originating from 10-Gbaud PRBS streams with length of 215-1 were generated from an arbitrary waveform generator (AWG) to drive the IQ modulator, which has a Vπ of 3.5V and an optical bandwidth of around 25GHz. For comparison, two different pump configurations were adopted. In the coherent pump configuration, the two pumps were generated from a single laser source at 1548.08 nm using a TTG, which consisted of a high-extinction-ratio optical modulator  driven by a 25-GHz clock. The high-ER modulator was fabricated on the x-cut LiNbO3 substrate with two embedded active trimmers in each arm and has an extinction ratio of up to 60dB. The two phase-correlated coherent pumps were obtained with a 50-GHz frequency separation and a >40-dB spurious suppression ratio. In the free-running pumps configuration, two independent free-running lasers emitting at 1547.88 and 1548.28 nm were used as pumps with 50-GHz spacing. For each configuration, either 500-kHz linewidth ECLs or 3.5-MHz linewidth DFBs were deployed as laser sources for pumps. The wavelength of the pump waves was selected to achieve high Δfp-s (more than 500 GHz) in order to reduce crosstalk after OWC, as described in the previous section, and to satisfy the QPM condition of the PPLN device at low temperature of operation. In order to maximize both CE and SD, the total pump power launched into the PPLN was set to the maximum value of about 28.8 dBm (25.8 dBm for each pump), obtaining CE of −6.5 dB and SD of 25 dB, with input signal power of 6 dBm.
The converted signal, generated at 1552.92 nm, was detected by a digital coherent receiver after an optical filter. The receiver included another FL acting as a local oscillator (LO), an optical 90-degree hybrid and two balanced photo-detectors (PDs). After detection by the PDs, the data was digitized at 50 GSamples/s using a digital storage oscilloscope with a 12.5-GHz analog bandwidth. The captured data was then processed off-line through digital signal processing (DSP), including compensation of skew, power and IQ imbalance, data resampling, linear equalization by finite impulse response filtering, carrier phase recovery and final hard-decision circuits. In order to ensure a fair comparison, the same digital equalization was deployed for the BER measurements in all configurations.
4.2 Experimental results and discussions
The optical spectra after the PPLN with or without pumps for wavelength conversion of 64QAM signals are shown in Fig. 6. For both free-running pumps (ECL/DFB) and coherent two-tone pumps (ECL/DFB), similar CE and SD were obtained. To investigate the impact of the phase noise of the converted signal in the OWC process under different configurations, the corresponding carrier phase, recovered after coherent reception, is plotted in Fig. 7. To avoid additional phase noise from LO, in the coherent receiver, another FL with linewidth of around 5kHz was used as LO. Figure 7(a) shows the recovered carrier phase of the input 16QAM signal from a FL laser to serve as a reference. Figures 7(b) and 7(c) show the recovered carrier phase with free-running DFB pumps, and with coherent two-tone DFB pumps, respectively. As shown in Fig. 7(b), since the phase noise from DFB pumps were transparently transferred to the converted signal in the free-running configuration, a distinct phase variation emerged in the recovered carrier phase. For coherent two-tone pumps, shown in Fig. 7(c), even for DFB pumps, the phase noise from pumps was effectively cancelled out, resulting in stable carrier phase similar to that of a 16QAM signal with a narrow linewidth FL as a laser source. These results indirectly verify the operation of pump phase noise cancellation in the proposed OWC with coherent pumps. Further evidence was obtained by measuring the linewidth of an unmodulated signal after wavelength conversion with coherent DFB two-tone pump. It shows the similar linewidth as the input light from FL, i.e. around 5kHz. This is consistent with the theoretical analysis in , and also directly verifies the cancellation effect of pump phase noise in the proposed scheme.
As shown in Fig. 8, the constellations of the converted 16/64QAMs signals were re-constructed and measured with different pump lasers and pump configurations. With coherent two-tone pumps, for either ECL or DFB pump laser, clear constellations are observed. However, with ECL pump lasers in free-running configuration, symbol rotation in phase starts to become evident in the 64QAM constellation due to additional phase noise from the free-running ECL pumps. With DFB free-running pumps, the presence of even greater pump phase noise causes clear spreading of the symbols around the unit circle for both formats, which is more severe for the higher amplitude symbols. The results can also be confirmed from the measured BER curves as a function of optical signal-to-noise ratio (OSNR) at 0.1 nm for both input and converted 16/64QAM signals, shown in Fig. 9. With coherent pump configuration, for both ECL and DFB pump lasers, negligible power penalty (<0.1dB for 16QAM; <0.3dB for 64QAM at BER of 10−3) is observed compared with the input signal at 10Gbaud. However, for the case of free-running pumps, although we can get negligible power penalty (<0.3 dB at BER of 10−3) for 16QAM using ECL as pump laser, when increasing the modulation level to 64QAM, a 0.5-dB penalty at BER of 10−3 and an error floor at around 3x10−5 is observed. With free-running DFB pumps, due to the strong phase noise, even at >30-dB OSNR, a BER of around 10−2 was observed for 16QAM and it was not possible to demodulate the 64QAM signal for any noise level. The BER, recovered carrier phase and constellation results verify the effectiveness of the elimination of the pump phase noise in the OWC for high-order QAM with coherent two-tone pumps.
We propose and experimentally demonstrate a pump-linewidth-tolerant OWC scheme, suitable for high-order QAM signals. The experimental results show that, even using DFB laser as pump source, negligible power penalty is achieved for converted 16QAM and 64QAM, which can be further extended to OWC of higher-order QAM signals. The proposed scheme eases the linewidth requirement for the pump lasers, simplifies the configuration, and enables practical implementation of low-cost OWC in future dynamic optical networks.
The work was supported in part by Grant-in-Aid for Young Scientist (A) (25709031) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Fundação para a Ciência e Tecnologia (PhD grant SFRH/BD/78425/2011).
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