Mode division multiplexing (MDM) has been widely investigated in optical transmission systems and networks to improve network capacity. However, the MDM receiver is always expensive and complex because coherent detection and multiplex-input-and-multiplex-output (MIMO) digital signal processing (DSP) are required to demultiplex each spatial mode. In this paper, we investigate the application of MDM in short-reach scenarios such as datacenter networking. Two-dimensional MDM and wavelength division multiplexing node structure based on low modal-crosstalk few-mode fiber (FMF) and components is proposed, in which signal in each mode or wavelength can be independently switched. We experimentally demonstrate independent adding, dropping and switching functionalities with two linearly polarized modes and four wavelength channels over a total 11.8-km 2-mode low modal-crosstalk FMFs. The structure is simple without coherent detection or MIMO DSP. Only slight penalties of receiver sensitivity are observed for all switching operations. The influence of modal-crosstalk accumulation for cascaded switching nodes is also investigated.
© 2016 Optical Society of America
With the constant increase of network users and explosive growth of network traffic, the communication capacity of the single-mode fiber (SMF) is approaching the nonlinear Shannon capacity limit . New multiplexing dimensions are expected to be combined to enhance the capacity. Mode division multiplexing (MDM) using few-mode fiber (FMF) instead of SMF has been widely investigated to overcome the capacity crunch in high speed optical transmission systems [1–5]. Because mode coupling is inevitable during long-haul FMF transmission, coherent detection and multiple-input-multiple-output (MIMO) digital signal processing (DSP) are always required to demultiplex each spatial mode [6,7].
In this paper, we focus on the MDM application in short-reach switching scenarios such as datacenter networking. As we know, the introducing of space division multiplexing (SDM) will greatly reduce fiber volume, facilitate integration of optical modules and save energy consumption, which are welcome for datacenter networking. Adopting FMF and multiple-core fiber (MCF) are two typical SDM approaches and they can be combined to further enhance the capacity based on few-mode-multi-core fiber . A mode-selective optical packet switching scheme has been demonstrated , in which only one spatial mode is used for packet transmission at a specific time slot to avoid modal-crosstalk and coherent detection is used for packet detection. However, coherent detection and MIMO DSP will significantly increase system cost and complexity, which is not preferred for short-reach transmission. Another approach is to significantly suppress the mode coupling, so that multiple signals could be independently transmitted and received in different spatial modes. In this way, conventional intensity modulation and direct detection (IM-DD) scheme can be used [10,11]. To achieve this, both the FMF and switching components should have low mode coupling.
Several few-mode compatible components have been proposed to manipulate MDM signals for flexible MDM optical switching networks [12–16]. However, they don’t support independent mode operations or are incompatible with existing switching equipments. In our previous works , we have shown by experiment that two linear polarization (LP) modes can be independently transmitted over 55-km 2-mode low modal-crosstalk FMF. The mode multiplexer/demultiplexer (MUX/DEMUX) also has low modal-crosstalk, which consists of optical fiber mode-selective couplers (MSCs). In this paper, we further investigate the feasibility of independent spatial mode switching for short-reach networks. We propose two-dimensional MDM and wavelength division multiplexing (WDM) node structure, which supports independent spatial mode and wavelength channel switching functionalities. We experimentally demonstrate the proposed node structure without coherent detection or MIMO DSP. Moreover, the modal-crosstalk for cascaded switching nodes is investigated.
2. Node structure
Figure 1 illustrates schematic diagram of the MDM/WDM switching node. The switching node consists of 1 × M mode MUX/DEMUXs, 1 × K wavelength selective switches (WSS) and 1 × K optical couplers (OC). All the components in the node are with SMF connections except the input/output ports. At each input port, the incoming MDM/WDM signal is converted into individual fundamental modes and demultiplexed into SMFs by the mode DEMUX. Then WSSs with K ports are used to switch the WDM signals to destination output ports according to instructions from the control plane. Independent spatial modes and wavelength channels operations such as adding, dropping and inter/intra-mode switching are performed by configuring the WSSs. The OCs combine all the signals. Moreover, the OC could be replaced by the WSS to reduce the node loss. At each output port, the signals are converted into different spatial modes and simultaneously multiplexed into FMF by the mode MUX. We can see that the structure is simple without coherent detection or MIMO DSP, and has good compatibility with conventional single-mode components such as WSSs and OCs.
3. Experimental demonstration
3.1 Experimental setup
We present proof-of-principle experimental demonstrations of the proposed node structure in a three-node test-bed, as shown in Fig. 2. At Node 1, four laser diodes (LDs) with 100-GHz channel spacing at the wavelength of 1549.0, 1549.8, 1550.6 and 1551.4-nm (λ1~λ4) are generated by four tunable lasers and coupled by polarization maintaining optical couplers (PM-OCs). The pulse pattern generator (PPG) generates 10-Gb/s Pseudo Random Binary Sequence (PRBS) electrical signal with the word length of 215-1. WDM on-off-keying (OOK) signal is generated by an optical Mach-Zehnder modulator (MZM) and then is split into two branches by a 50:50 OC. The lower branch is delayed by a 10-km SMF to eliminate the correlation. On the other hand, an erbium doped fiber amplifier (EDFA) is used to amplify the signal. The two branches are multiplexed by the mode MUX and converted into MDM/WDM signal. The LP11 mode is a mode group which consists of two degenerate modes (LP11a and LP11b). If both LP11 modes are excited, the degeneracy between them may cause high modal-crosstalk. In this experiment, we excite only one LP11 mode. The relative angle of the connectors at the input port of mode DEMUX is manually adjusted to demultiplex the excited LP11 mode. Adopting elliptical core FMF can break the degeneracy and realize independent transmission for LP11a and LP11b modes . The insertion loss of the mode MUX for LP01 and LP11 modes are about 0.3~1 dB and 3~5 dB, respectively. The insertion loss of the mode DEMUX for the LP01 and LP11 modes are about 1~2 dB and 5~7 dB, respectively. The insertion loss of the mode MUX/DEMUX will not significantly influence OSNR if they are compensated by DEFAs and they could be further reduced by optimizing the design and fabrication parameters of the mode MUX/DEMUX. The modal-crosstalk of the mode MUX/DEMUX is about −16 dB from LP01 mode to LP11 mode and −27 dB from LP11 mode to LP01 mode. The fabrication parameters of the low modal-crosstalk FMFs is the same as .
At the input port of switching node (Node 2), the MDM/WDM signal is firstly demultiplexed into SMFs by mode DEMUX. Single-mode 1 × 4 WSSs and 1 × 4 OCs are utilized for WDM signal switching. The WSSs are configured to realize adding, dropping and switching operations, which are described as follows:
- 1) λ1 and λ2 in the LP01 mode and LP11 mode pass through the switching node without mode switching (MS) operations.
- 2) λ3 in the LP01 mode and LP11 mode is dropped through the output 3 of WSS1 and WSS2, respectively. Then another signal at λ3 is generated for adding operation through the input port 3 of OC1 and OC2. The optical switches are used to route the added or dropped signal to its destination input or output port. When one mode is dropped and re-added, λ3 in another mode passes through the switching node with λ1 and λ2 together. The dropped signal is detected by the receiver (Rx).
- 3) The LP01 mode is converted into LP11 mode and LP11 mode is converted into LP01 mode. Specifically, λ4 in the LP01 mode passes to the input port1 of OC2 through the output port 2 of WSS1, while λ4 in the LP11 mode passes to the input port 2 of OC1 through the output port 1 of WSS2.
Single-mode EDFAs are used at the output ports of the OCs to amplify the signals in order to compensate the loss of the mode MUX/DEMUX, WSSs, and OCs. After switching operations, the combined signals are multiplexed into independent spatial modes by the mode MUX.
The MDM/WDM signal is received at the receiver node (Node 3) after 1.8-km FMF transmission. The MDM/WDM signal is demultiplexed by the mode DEMUX and the followed WSS acts as an optical filter with the bandwidth of 50-GHz. The signal is detected by the photodiode (PD) and the bit-error-ratio (BER) is calculated by the BER tester (BERT). The received power in the following paper is marked as the optical power after variable optical attenuator (VOA).
3.2 Experimental results
We measure far-field mode patterns of the signals before and after passing through switching node. The insets in Fig. 2 show the mode patterns at the point A after 10-km FMF transmission and at point B after adding, dropping, express and switching operations and 1.8-km FMF transmission. From the mode patterns at the point B, we can see that pure mode patterns are obtained for the signals (λ1 and λ2) after passing two pairs of mode MUX/ DEMUXs and 11.8-km FMF transmission without MS operations; and that LP01 mode can be successfully converted to LP11 mode for the added signals (λ3); and that LP01 mode and LP11 mode (λ4) could be successfully inter-converted each other, indicating the good performance of the mode conversion using the mode MUX/DEMUX. The optical spectra of the signals at the points a~f in Fig. 2 are shown in Fig. 3.
Figure 4(a) and 4(b) depict the BER performance and eye diagrams of the signals without MS operations. The λ1 and λ2 have similar performance and we only plot λ2 (1549.8-nm). The BER performance in back-to-back (BTB) case is also shown as reference. The BER performance of the signals in LP01 and LP11 modes with BTB transmission are measured by connecting a pair of mode MUX/DEMUX together without FMF transmission, which are similar to the OOK BTB transmission. We can see that the eye diagrams after passing through the Node 2 and 11.8-km FMF transmission show slight signal degradation for both modes. The shape difference between OOK BTB, LP01 and LP11 comes from the chromatic dispersion (CD) of the FMF and the random mode coupling between the LP01 and LP11 modes. The BTB receiver sensitivity at the BER of 10−3 is around −33.7 dBm. After directly passing through the Node 2 and 11.8-km FMF transmission, the receiver sensitivity for LP01 mode and LP11 mode are around −31.0 dBm and −29.6 dBm, respectively. Compared with OOK BTB transmission at the BER of 10−3, the LP01 and LP11 modes experience receiver sensitivity penalties of 2.7 dB and 4.1 dB, respectively. For LP01 mode, the penalty derives from the CD and modal-crosstalk from other modes. For LP11 mode, besides these reasons, the phase mismatching between the LP01 and LP11 modes in the mode MUX/DEMUX also contributes the penalty.
Figures 5(a) and 5(b) illustrate the BER curves and eye diagrams of the signals at λ3 after mode adding and dropping operations. The signals without MS operations are also shown as reference. From the eye diagrams in Fig. 5(b), we can see the signal in LP01 and LP11 modes is slightly degraded after the adding and dropping operations. Compared with the BTB case at the BER of 10−3, the LP01 mode experiences receiver sensitivity penalties of 0.8 dB and 1.2 dB after adding and dropping operations. The receiver sensitivity penalties of LP11 mode after adding and dropping operations are 1.4 dB and 1.8 dB, respectively. We can see that the signals without MS operations have a little worse performance than the added/dropped signals, which results from the modal-crosstalk accumulation after passing through two pairs of mode MUX/DEMUXs. Compared with the added/dropped signals, the extra receiver sensitivity penalties for the signals without MS are about 1.9 dB and 1.3 dB for LP01 mode and about 2.7 dB and 2.1 dB for the LP11 mode.
Figures 6(a) and 6(b) show the BER curves and eye diagrams of the LP01 and LP11 modes at λ4. After inter-mode switching, the LP01 and LP11 modes are swapped. Similar eye diagrams are obtained after inter-mode switching for the LP01 and LP11 modes. The OOK BTB receiver sensitivity is around −33.9 dBm. The receiver sensitivities for LP01 mode and LP11 mode after inter-mode switching and FMF transmission are around −30.5 dBm and −31.0 dBm, respectively. Compared with the OOK BTB transmission at the BER of 10−3, the receiver sensitivity penalties of the LP01 and LP11 modes after inter-mode switching and 11.8-km FMF transmission are about 3.4 dB and 2.9 dB, respectively. In MDM networks, the signals in different spatial modes have different performance and the performance difference will increase if they are always transmitted in the same mode across the entire switching system. This performance difference comes from the unequal crosstalk of mode MUX/DEMUX between different spatial modes and phase mismatching between the high-order modes and LP01 mode. It could be mitigated by optimizing the fabrication parameters of the all fiber mode MUX/DEMUX or adopting inter-mode switching operation.
3.3 Investigation on the impact of cascaded switching
In short-reach MDM optical networks, the modal-crosstalk coming from mode MUX/DEMUX may be the dominant factor to degrade the signal performance. The cascading of the switching nodes will result in the modal-crosstalk accumulation, which would limit the network scale. Therefore, it is necessary to investigate the influence of modal-crosstalk accumulation. Figure 7 shows the experimental setup of the MDM signal passing through different number of cascaded switching nodes. The parameters of transmitter and receiver are the same as that in Fig. 2. N pairs of mode MUX/DEMUXs are cascaded together to emulate N switching nodes. When N is modified, the Q-factors (dB) are observed, and the results are shown in Fig. 8. Both LP01 and LP11 modes achieve good Q-factors when they are transmitted separately. After passing through four nodes, the Q-factors are 26.3 dB and 24.3 dB for LP01 and LP11 modes, respectively. Figure 9(a) shows the eye diagrams of LP01 and LP11 modes after separately passing four cascaded switching nodes. When the two spatial modes are transmitted simultaneously, the Q-factors are decreased for the two modes and the Q-factor difference between the LP01 and LP11 modes (marked as the bidirectional arrow and figures in Fig. 8) becomes larger as the number of cascaded nodes increases, which comes from the modal-crosstalk accumulation in the mode MUX/DEMUX. Compared with the without inter-MS scenario, we can see that the Q-factor difference between the two modes could be mitigated in the inter-MS configuration. The eye diagrams of LP01 mode and LP11 mode after passing through four switching nodes in the case of inter-MS and without inter-MS are shown in Figs. 9(b) and 9(c), respectively.
4. Discussion and conclusion
In this paper, only two mode operations are demonstrated, which can be extended to more modes if the FMF and the mode MUX/DEMUX can support more modes. To accommodate more modes with low modal-crosstalk, the effective index difference between the core and cladding should be increased for the FMF, which may increase the difficulty during fiber fabrication and the insertion loss may be significantly increased. Moreover, adopting asymmetrical fiber core to break mode degeneracy is necessary to increase the total number of independent modes. The maximum number of independent modes for practical implementation should be studied.
In conclusion, we propose and experimentally demonstrate all-optical two-dimensional MDM/WDM switching node for short-reach networks. Owing to the low modal-crosstalk of the FMF and mode MUX/DEMUX, OOK modulation and direct detection are adopted to avoid coherent detection and MIMO DSP. We successfully demonstrate error-free performance of 10-Gb/s optical OOK signal independent adding, dropping and switching functionalities with two LP modes and four wavelength channels, showing the feasibility of the proposed node structure. Furthermore, the influence of modal-crosstalk accumulation for cascaded switching nodes is investigated. The entire switching system supports the cascading of at least four switching nodes while keeping the signals performance above the FEC limit.
Program 973 (2014CB340105 and 2014CB340101); National Natural Science Foundation of China (NSFC) (61377072, 61275071 and 61505002).
Part of this work is published in OFC 2016, Paper W2A.47 .
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