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We propose a reconfigurable Tbit/s network switching element using double-pass liquid crystal on silicon (LCoS) technology accompanied by bidirectional degenerate four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF). We demonstrate the LCoS + HNLF-based 2.3-Tbit/s multi-functional grooming switch which performs simultaneous selective add/drop, switchable data exchange, and power equalization, for 23-channel 100-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) signals. Less than 1.5-dB power penalty is observed for power equalization at a bit-error rate (BER) of 10−9. Selective single-/two-channel add/drop are achieved with power penalties less than 1.2 dB. Switchable two-channel data exchange and simultaneous six-channel data exchange are implemented with power penalties less than 5 dB.
©2011 Optical Society of America
1. Introduction
Towards the unabated exponential growth of network capacity and traffic rates [1], data traffic grooming becomes of increasing importance to enhance the efficiency and flexibility of networks. At network switching nodes, grooming switch will likely be required to simultaneously perform multiple signal processing functions on high-speed large-capacity data channels. In wavelength-division-multiplexed (WDM) systems beyond Tbit/s [2,3], it might be advantageous for a reconfigurable network node to achieve: (i) wavelength add/drop [4], (ii) data exchange between different wavelengths [5], and (iii) power equalization across all wavelengths [6,7]. It is also desirable that these functions be accomplished in a data-format-transparent fashion such that spectrally-efficient differential quadrature phase-shift keying (DQPSK) [8] data can be accommodated.
Previous research work has been reported to perform the separate, individual functions of wavelength add/drop [4,9], data exchange [5,10,11], and power equalization [6,7]. For instance, 640-Gbit/s (16x40-Gbit/s non-return-to-zero (NRZ)) throughput, packet-selective reconfigurable optical add/drop multiplexers were implemented using wide pass-band acoustooptic wavelength-tunable filter [9]. We demonstrated two-channel data exchange of 100-Gbit/s return-to-zero DQPSK (RZ-DQPSK) signals using parametric depletion of non-degenerate four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) [10]. More recently we achieved four-channel data exchange of 100-Gbit/s DQPSK signals using bidirectional degenerate FWM in an HNLF [11]. Although these functions have been accomplished individually, to the best of our knowledge, there has been little research on aggregating all these three functions together at a network switching node. A laudable goal would be to perform all these functions for multi-channel DQPSK signals in a high-speed, large-capacity, reconfigurable network element.
In this paper, we propose and demonstrate a reconfigurable 2.3-Tbit/s network grooming switch element that performs simultaneous add/drop, data exchange, and power equalization of 23x100-Gbit/s RZ-DQPSK channels using a double-pass liquid crystal on silicon (LCoS) technology and bidirectional single-pump FWM inside a single highly nonlinear fiber (HNLF) [12]. Less than 1.5-dB penalty at a bit-error rate (BER) of 10−9 is observed for add/drop and power equalization. Switchable two-channel data exchange and simultaneous six-channel data exchange are demonstrated with penalties less than 5 dB at a BER of 10−9.
2. Concept and principle of operation
Figure 1 illustrates the expected multiple functions at network switching nodes, including add/drop, data exchange, and power equalization. Simultaneous implementation of all these functions can potentially enhance the efficiency and flexibility of network management.
Figure 2 shows the concept and principle of multi-functional grooming switch, which relies on the double-pass LCoS technology and bidirectional nonlinear interactions inside a single HNLF. For input unequalized multi-channel WDM signals, the available signal manipulations through the multi-functional grooming switch unit include: (i) local selective dropping of one or more channels and adding of the corresponding channels carrying new data information, (ii) switchable data exchange between multiple channels of interest, and (iii) power equalization across all the WDM channels. An example of 7-channel WDM signals is presented in Fig. 2. The core part of the setup is a wavelength selective switch (WSS) based on a two-dimensional (2-D) array of LCoS pixels [13,14]. The phase retardance of each pixel is set by adjusting the voltages applied to the LCoS. The 2-D LCoS array can be described with two axes, i.e., horizontal wavelength axis and vertical displacement axis. Unequalized 7-channel 100-Gbit/s DQPSK signals are sent from an input/output fiber array (port A) to a diffraction grating, which angularly disperses each wavelength channel to a different portion of the LCoS along the horizontal wavelength axis. Vertically, the light diverges to overlap a large number of pixels (typically about 400). Independent attenuation control of optical power and spatial switch of individual wavelength channels to the desired fiber array ports (S1 to port B, S4/S5 to port C, S2/S3 port D, S6/S7 to port E) are achieved by manipulating the phase front of the 2-D array of LCoS pixels along the vertical axis. Multiple grooming functions on the channels of interest are then applied to different fiber array ports. As shown in Fig. 2, in addition to the power equalization of all channels enabled by attenuation control, wavelength add/drop at port B and data exchange between port D and port E are adopted. In order to perform simultaneous multi-channel data exchange between S2 and S7 as well as S3 and S6, bidirectional degenerate FWM in a single HNLF is employed [11]. Compared with the non-degenerate FWM-based data exchange using two pumps [10], only single pump is required here. After data exchange, the information carried by different channels is swapped. In order to deliver the newly added channels and exchanged channels together, the LCoS device is utilized in a double-pass fashion such that fiber array port A not only delivers input unequalized signals but also exports output signals after grooming switch. In addition to the channels undergoing add/drop (S1) and data exchange (S2, S3, S6, S7), other channels (S4, S5) are also kept and sent back by a fiber loop mirror through port C. The dropped channel (S1) is achieved at port B. Optical circulators are adopted to assist the double-pass operation. Considering the dashed boxes in Fig. 2 as an LCoS + HNLF-based switch unit, reconfigurable multi-functional grooming switch (simultaneous add/drop, data exchange and power equalization) is enabled by the double-pass programmable LCoS and bidirectional nonlinear interactions in a single HNLF.
Fig. 2 Concept and principle of LCoS + HNLF-based multi-functional grooming switch (add/drop, data exchange, power equalization).
3. Experimental setup
Shown in Fig. 3 is the experimental setup for the LCoS + HNLF-based grooming switch. The employed C-band LCoS device has an insertion loss of ~4 dB, a variable channel/filter bandwidth of 15 GHz-4 THz (0.1-32 nm), a frequency setting resolution of 1 GHz, and a attenuation control range of 0-15 dB with a resolution of 0.1 dB. The achievable attenuation implies the maximum power equalization ability of 15 dB. By changing the phase pattern on the LCoS, reconfigurable functionality is available on a time scale of less than 100 ms via software control [13]. ITU-grid-compatible 23 wavelength channels (from S1: 1531.12 nm to S23: 1566.31 nm) with a channel spacing of 200 GHz are combined together via an arrayed waveguide grating (AWG). 2.3-Tbit/s (23x100-Gbit/s) pseudo-random binary sequence (PRBS) RZ-DQPSK signals are prepared by sending 23 wavelength channels to a 100-Gbit/s (50-Gsymbol/s) DQPSK transmitter followed by a 50% 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. The obtained 23-channel 100-Gbit/s RZ-DQPSK signals with unequalized power levels are fed into the LCoS + HNLF-based grooming switch unit. A tap from an optical coupler (OC) at the input is used to provide the signal for the add operation. The fiber array port A of LCoS device is the input/output of the grooming switch. Different channels of input WDM RZ-DQPSK signals are delivered to different fiber array ports B-D according to the corresponding operations to be applied. As shown in Fig. 3, selective add/drop is performed at port B and switchable data exchange is implemented between port D and port E. Other channels are guided back through port C. A 520-m piece of HNLF with a zero-dispersion wavelength (ZDW) of ~1555 nm and a nonlinear coefficient (γ) of 20 W−1·km−1 is incorporated between port D and port E to enable the data exchange based on the bidirectional single-pump (1555.75 nm) degenerate FWM. As a consequence, the double-pass programmable LCoS accompanied by the bidirectional degenerate FWM in an HNLF makes it possible to implement the reconfigurable multi-functional grooming switch of 2.3-Tbit/s (23x100-Gbit/s) RZ-DQPSK signals.
Fig. 3 Experimental setup for LCoS + HNLF-based multi-functional grooming switch of 23-channel 100-Gbit/s RZ-DQPSK signals. PC: polarization controller; AWG: arrayed waveguide grating; Tx: transmitter; OC: optical coupler; CIR: circulator; BPF: band-pass filter; EDFA: erbium-doped fiber amplifier.
4. Experimental results and discussions
Figure 4 depicts the measured spectrum of the input unequalized 23-channel 100-Gbit/s RZ-DQPSK signals with a power fluctuation of ~9.1 dB. Shown in the insets are typical balanced eyes for the in-phase (Ch. I) and quadrature (Ch. Q) components.
Fig. 4 Measured spectrum and balanced eyes for input unequalized 23-channel 100-Gbit/s RZ-DQPSK signals.
We first perform 2.3-Tbit/s grooming switch with single-channel add/drop and two-channel data exchange. Figure 5 plots the measured BER performance for simultaneous multi-functional grooming switch, including add/drop for S18, data exchange between S12 and S21, and power equalization across all the channels (S12-S23). As shown in Fig. 5(a1) and 5(a2), less than 1.5-dB power penalty at a BER of 10−9 is achieved for equalization. The penalty fluctuations for Ch. I and Ch. Q among different wavelength channels are ~0.8 and ~0.7 dB, respectively. As shown in Fig. 5(b1) and 5(b2), the power penalties for add and drop (S18) are measured to be less than 1 and 0.5 dB, respectively. As shown in Fig. 5(c1) and 5(c2), less than 4.4-dB power penalty at a BER of 10−9 is observed for data exchange between S12 and S21.
Fig. 5 BER curves for (a1)(a2) power equalization (S1-S11, S13-S17, S19, S20, S22, S23), (b1)(b2) single-channel add/drop (S18), and (c1)(c2) two-channel data exchange (S12 and S21).
Figure 6 depicts the measured spectrum and typical balanced eyes after multi-functional grooming switch, in which power levels of all 23 channels are equalized with a fluctuation less than 1 dB (input unequalization: ~9.1 dB), the data information carried by S12 and S21 is exchanged, the original S18 is dropped as depicted in the inset and new data information is added to S18. Figure 7 shows the power penalties at a BER of 10−9 for the multi-channel grooming switch corresponding to Figs. 5 and 6.
Fig. 6 Measured spectrum and balanced eyes after multi-functional grooming switch (S18: add/drop; S12&S21: data exchange; S1-S23: power equalization).
Fig. 7 Power penalties for the multi-channel grooming switch (S18: add/drop; S12&S21: data exchange; S1-S23: power equalization).
Remarkably, reconfigurable multi-channel grooming switch is also available thanks to the programmable LCoS. For example, by simply varying the channels of interest delivered to port D and port E, it is possible to implement switchable data exchange. Figure 8 depicts the BER performance for switchable data exchange between S10 and S23, S13 and S20, S14 and S19. Power penalties less than 4.5 dB at a BER of 10−9 are observed for the switchable data exchange. In addition, selective add/drop operation is also enabled simply by delivering the channel of interest to port B and the measured power penalty is less than 1.2 dB for add and 0.5 dB for drop at a BER of 10−9.
Fig. 8 BER curves for switchable two-channel data exchange between (a1)(a2) S10 and S23, (b1)(b2) S13 and S20, (c1)(c2) S14 and S19.
We further demonstrate 2.3-Tbit/s grooming switch with two-channel add/drop and six-channel data exchange. Shown in Fig. 9 is the measured spectrum and typical balanced eyes after grooming switch with power equalization (<1 dB) for all 23 channels (input unequalization: ~9.1 dB), two-channel add/drop for S6 and S7, and simultaneous six-channel data exchange (S10, S11, S12, S21, S22, S23). The inset of Fig. 9 depicts the spectrum of dropped S6 and S7. The BER performance is plotted in Fig. 10 and power penalties less than 1.2 dB for two-channel add, 0.5 dB for two-channel drop, and 5 dB for six-channel data exchange are observed at a BER of 10−9.
Fig. 9 Measured spectrum and balanced eyes after multi-functional grooming switch (S6, S7: add/drop; S10, S11, S12, S21, S22, S23: data exchange; S1-S23: power equalization).
Fig. 10 BER curves for (a1)(a2) two-channel add/drop (S6, S7) and (b1)(b2) simultaneous six-channel data exchange (S10, S11, S12, S21, S22, S23).
The penalties of add/drop and equalization are mainly due to the degraded optical signal-to-noise ratio (OSNR). Taking equalization as an example, the equalization process more or less reduces the signal power, i.e., more attenuation for high power channel while less attenuation for low power channel. The reduced signal power causes the degradation of OSNR and the resultant degraded BER performance. The relatively large penalties of data exchange are mainly caused by the beating effects of in-band interference between the newly converted signal and the original residual signal [10,11]. As shown in Figs. 6 and 9, the observed small spike between S16 and S17 is the residual pump which is not completely suppressed after data exchange. Such small pump spike can be further effectively suppressed by employing band-pass filters in Fig. 3 with sharper rising and falling edges. Considering the principles of add/drop, data exchange and power equalization, the proposed multi-functional grooming switch is expected to be available at even higher speed and transparent to the modulation format.
Acknowledgments
We acknowledge Omer F. Yilmaz, Xiaoxia Wu, and Scott R. Nuccio for the helpful discussions, and the generous support of the National Natural Science Foundation of China (NSFC) under grant 61077051, the Fundamental Research Funds for the Central Universities, Huazhong University of Science and Technology (HUST; 2010MS035), the Defense Advanced Research Projects Agency (DARPA) under contract FA8650-08-1-7820, and the NSF-funded Center for the Integrated Access Networks (CIAN).
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