We propose a simple approach to performing simultaneous multi-channel data exchange using bidirectional degenerate four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF) assisted by optical filtering. Data exchange between two-channel 100-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) signals with a variable channel spacing is implemented with a power penalty of less than 4.2 dB at a bit-error rate (BER) of 10−9. Moreover, simultaneous ITU-grid-compatible four-channel 100-Gbit/s RZ-DQPSK data exchange is demonstrated with a power penalty of less than 4.7 dB at a BER of 10−9.
©2011 Optical Society of America
The rapid growth in network capacity and traffic rates raises the significance of data traffic grooming exchange, which is considered to be a promising technique for enhancing the efficiency and flexibility of networks . In a wavelength-division-multiplexed (WDM) network, data exchange, which is also known as wavelength interchange or wavelength exchange, would require the swapping of data from one wavelength with the data from another wavelength. Typically, a WDM network has many data channels on different wavelengths. In this scenario, it would be advantageous and desirable to achieve simultaneous multi-channel data exchange.
Optical nonlinearities are potentially suitable candidates to enable data exchange between two different wavelengths. Previous reports of data exchange include the use of degenerate four-wave mixing (FWM) and interference effect of an optical parametric loop mirror (PALM) [2, 3], parametric depletion effect of non-degenerate FWM in a highly nonlinear fiber (HNLF) [4–8], and cascaded sum- and difference-frequency generation (cSFG/DFG) in a periodically poled lithium niobate (PPLN) waveguide . Actually, the physics behind these data exchange schemes are wavelength conversions based on χ(2) or χ(3) nonlinear interactions. In the PALM-based data exchange [2, 3], in addition to degenerate FWM-based wavelength conversions, an incorporated dispersive element plays an important role to induce phase imbalance (an odd multiple of π) between the counterpropagating signals, resulting in the data exchange via the interference effect of the loop mirror. PALM-based two-channel wavelength interchange for on-off keying (OOK)  and 10-Gbit/s differential phase-shift keying (DPSK) signals  were demonstrated, showing impressive performance. In the parametric depletion-based data exchange [4–9], two pumps are employed in the non-degenerate FWM or cSFG/DFG nonlinear interactions which result in the simultaneous depletion of original signals and generation of newly converted signals. Parametric depletion-based two-channel wavelength exchange for 10-Gbit/s non-return-to-zero (NRZ) signals  and byte-level exchange for 10-Gbit/s RZ and 40-Gbit/s NRZ signals [6, 9] were implemented. Besides the OOK formats, we recently investigated non-degenerate FWM-based phase-transparent two-channel data exchange for 40-Gbit/s DPSK signals  and 100-Gbit/s differential quadrature phase-shift keying (DQPSK) signals . So far as we know, the reported schemes enable the two-channel data exchange. However, the extended applications to simultaneous multi-channel data exchange might be limited. A laudable goal would be to explore the data exchange between multi-channel signals with spectrally-efficient DQPSK modulation format.
In this paper, we propose a simple alternative approach to performing data exchange between multi-channel DQPSK signals using bidirectional degenerate FWM in a single HNLF accompanied by optical filtering . Single pump is employed and four-channel signals are symmetric with respect to the pump. Simultaneous four-channel 100-Gbit/s return-to-zero DQPSK (RZ-DQPSK) data exchange is demonstrated with a power penalty of less than 4.7 dB at a bit-error-rate (BER) of 10−9.
2. Concept and principle
Figure 1 illustrates the concept and principle of multi-channel data exchange. Degenerate FWM with a single continuous-wave (CW) pump is utilized. Four-channel DQPSK signals (S1-S4) are symmetric with respect to the CW pump. Simultaneous data exchange between S1 and S4 as well as S2 and S3 is expected. In general, such exchange function is not applicable with the unidirectional degenerate FWM in a single HNLF since the newly converted signals cannot be separated from the original signals. A potential solution is to explore the bidirectional degenerate FWM in a single HNLF assisted by optical filtering. As shown in Fig. 1, for the input four-channel signals (S1-S4), the filtered S1, S2 and CW pump are sent to HNLF from the left side, yielding S4 and S3 via degenerate FWM. The newly generated S4 and S3 are selected at the right side of HNLF while the original S1, S2 and CW pump are blocked. Meanwhile, the filtered S3, S4 and CW pump are fed into HNLF from the right side, producing S2 and S1 by degenerate FWM. The newly converted S2 and S1 are selected at the left side of HNLF while the original S3, S4 and CW pump are removed. As a consequence, simultaneous four-channel data exchange (S1&S4, S2&S3) can be achieved using bidirectional FWM in a single HNLF assisted by optical filtering. The combined S1-S4 from both sides of HNLF are the output four-channel signals after data exchange. Note that the in-phase (Ch. I) and quadrature (Ch. Q) components of DQPSK signals are swapped after data exchange due to the phase-conjugation characteristic of degenerate FWM.
3. Experimental setup
Figure 2 depicts the experimental setup for multi-channel data exchange. Four-channel 100-Gbit/s 27-1 pseudo-random binary sequence (PRBS) RZ-DQPSK signals (S1-S4) are obtained by sending four tunable CW lasers to a 100-Gbit/s (50-Gsymbol/s) DQPSK transmitter (Tx) followed by a RZ pulse carver. The 100-Gbit/s DQPSK transmitter (SHF 46214A) is a thermally stable Lithium Niobate Mach-Zehnder modulator (MZM) with a nested Mach-Zehnder interferometer (MZI) structure. Four-channel RZ-DQPSK signals are then separated, relatively delayed by integral symbols, recombined, and sent to a fiber loop mirror incorporating a 460-m piece of HNLF, two band-pass filters (BPF1, BPF2), and optical couplers (OCs). The HNLF has a nonlinear coefficient of 20 W−1·km−1, a zero-dispersion wavelength (ZDW) of ~1556 nm, and a dispersion slope of ~0.026 ps/nm2/km. A CW pump is coupled into the fiber loop mirror from both sides of the HNLF to enable bidirectional degenerate FWM in a single HNLF. Four-channel signals are arranged symmetrically relative to the CW pump. Note that BPF1 (BPF2) passes S1 and S2 (S3 and S4) while blocks S3, S4 and pump (S1, S2 and pump), resulting in simultaneous multi-channel data exchange between S1 and S4 as well as S2 and S3 via bidirectional degenerate FWM. At the output of the fiber loop mirror, the collected four-channel signals after data exchange are sent to a pre-amplified receiver (Rx) for BER measurements. A 50-GHz delay line interferometer (DLI) is used to demodulate the in-phase and quadrature components of 100-Gbit/s DQPSK signals.
4. Experimental results and discussions
Compared to the non-degenerate FWM-based data exchange with two pumps , single pump with its wavelength (1554.94 nm) close to the ZDW of HNLF, is employed in the bidirectional degenerate FWM-based data exchange. The pump power fed into HNLF from left (clockwise) and right (counter-clockwise) sides is about 14.4 and 15.4 dBm, respectively.
We first demonstrate two-channel (SA, SB) data exchange. BPF1 and BPF2 as illustrated in Fig. 2 pass only SA and SB, respectively. Shown in Fig. 3 (a1) is the spectrum of ITU-grid-compatible input two-channel 100-Gbit/s RZ-DQPSK signals (SA: 1550.12 nm, SB: 1559.79 nm) with a channel spacing of 9.67 nm, measured at point A in Fig. 2. The power of SA and SB coupled into HNLF from left and right sides is about 9 dBm. Figure 3(b1) shows the spectrum after two-channel data exchange in the presence of CW pump (solid curve), measured at point B in Fig. 2. Note that some residual signals are also observed in the absence of CW pump (dashed curve), which we believe mainly come from the Rayleigh scattering inside the HNLF . The extinction ratio (ER) of the newly exchanged signal to the residual signal of SA and SB is measured to be 18.1 dB. Furthermore, data exchange between two variable channels is also applicable as long as SA and SB are arranged to be symmetric with respect to the CW pump. Figure 3(a2)(b2) and (a3)(b3) show two more examples of data exchange with variable channel spacing of 4.84 nm (SA: 1552.52 nm, SB: 1557.36 nm) and 14.51 nm (SA: 1547.72 nm, SB: 1562.23 nm). The extinction ratio of exchanged signals (SA, SB) is measured to be 17.9 and 17.9 dB in Fig. 3(b2) and 17.2 and 18.1 dB in Fig. 3(b3).
To further verify the 100-Gbit/s DQPSK data exchange, we observe temporal waveforms and balanced eyes of demodulated in-phase (Ch. I) and quadrature (Ch. Q) components of 100-Gbit/s RZ-DQPSK signals corresponding to Fig. 3(a1)(b1). As shown in Fig. 4 , it can be clearly seen that the data information carried by two-channel 100-Gbit/s RZ-DQPSK signals is mutually converted after the bidirectional degenerate FWM, resulting in the successful implementation of 100-Gbit/s DQPSK data exchange. In addition, the in-phase and quadrature components of DQPSK signals are swapped after data exchange, which we believe is due to the phase-conjugation characteristic of degenerate FWM.
Figure 5 depicts BER curves for two-channel 100-Gbit/s RZ-DQPSK data exchange with variable channel spacing, which correspond to Fig. 3(b1), (b2) and (b3), respectively. Less than 4.2-dB power penalty at a BER of 10−9 is observed, which could be ascribed to the beating effect between the newly converted signal and the original residual signal.
We further demonstrate simultaneous multi-channel data exchange. ITU-grid-compatible four-channel 100-Gbit/s RZ-DQPSK signals (S1: 1546.12 nm, S2: 1547.72 nm, S3: 1562.23 nm, S4: 1563.86 nm) are employed for multi-channel data exchange. S1 and S2 pass through BPF1 while S3 and S4 go through BPF2 as shown in Fig. 2. Figure 6(a) depicts the spectrum of input four-channel 100-Gbit/s RZ-DQPSK signals (measured at point A in Fig. 2). S1(S2) and S4(S3) are symmetric with respect to the CW pump. The power of S1-S4 launched into the HNLF from left (S1, S2) and right (S3, S4) sides is about 9.9, 8.4, 8.3, and 10.0 dBm, respectively. Figure 6(b) shows the spectrum after four-channel data exchange (measured at point B in Fig. 2) in the presence of CW pump (solid curve). The spectrum of residual signals (i.e., resulting from Rayleigh scattering in HNLF) in the absence of CW pump (dashed curve) is also depicted in Fig. 6(b). The extinction ratio of the newly exchanged signals to the residual signals of S1-S4 is measured to be 18.4, 19.5, 17 and 17 dB, respectively.
Figure 7 further displays temporal waveforms and balanced eyes of demodulated in-phase (Ch. I) and quadrature (Ch. Q) components of 100-Gbit/s RZ-DQPSK signals before and after data exchange. It is verified that four-channel 100-Gbit/s RZ-DQPSK data exchange (S1&S4, S2&S3) is successfully implemented. Also, it is noted that Ch. I and Ch. Q of DQPSK signals are swapped after data exchange as a result of the phase-conjugated degenerate FWM.
Figure 8 plots the BER curves for four-channel 100-Gbit/s RZ-DQPSK data exchange. Less than 4.7-dB power penalty is observed at a BER of 10−9, which could be caused by the beating effect between the newly exchanged signals and the original residual signals.
Differing from previous PALM-assisted data exchange employing a dispersive element and HNLF-based data exchange using parametric depletion effect of non-degenerate FWM process, we propose an alternative simple approach to implementing simultaneous multi-channel data exchange by exploiting bidirectional degenerate FWM inside a single HNLF accompanied by optical filtering. Two-channel data exchange with a variable channel spacing as well as simultaneous four-channel data exchange are demonstrated with less than 4.2- and 4.7-dB power penalties, respectively, at a BER of 10−9.
We acknowledge the generous support of the DARPA under the contract number FA8650-08-1-7820 and the NSF-funded Center for the Integrated Access Networks (CIAN).
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