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Abstract
Spatial division multiplexing transmission over few-mode multicore fiber (FM-MCF) recently attracts great interests by simultaneously exploiting two more dimensions than conventional single mode fibers. In this paper, we propose an all-fiber spatial multiplexer (MUX) by cascading mode-selective fiber couplers (MSCs) with a fiber-bundle-type fan-in device, and spatial demultiplexer (DEMUX) by cascading a fiber-bundle-type fan-out device with degenerate-mode-selective fiber couplers and MSCs. Thanks to the low crosstalk of the FM-MCF, spatial MUX/DEMUX and their coupling, weakly-coupled 7-core-2-LP-mode real-time transmission over 1-km of FM-MCF is successfully demonstrated using 10-Gbps commercial enhanced small form-factor pluggable (SFP + ) transceivers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the rapid increase of network users and explosive growth of network traffic, various techniques have been proposed to overcome the capacity limit of optical transmission through conventional single-mode fibers (SMFs), among which spatial division multiplexing (SDM) attracts lots of interests [1]. Typically, SDM transmission can be over multicore fiber (MCF) [2], few-mode fiber (FMF) [3], or few-mode multicore fiber (FM-MCF) [4–8], in which different fiber cores and linearly-polarized (LP) modes can be utilized as parallel spatial channels. Some works have shown that SDM techniques using FM-MCFs can greatly enhance the transmission capacity with more than 100 spatial channels [7,8]. Except for the FM-MCF, spatial multiplexer (MUX) and demultiplexer (DEMUX) consisting of cascaded mode MUX/DEMUX and fan-in/fan-out (FI/FO) devices are also key components in SDM link [9–12].
Crosstalk (XT) among spatial channels has to be solved, which inevitably occurs over the entire SDM link. Generally, the inter-core XT can be significantly suppressed by increasing the core-to-core pitch or setting a low refractive index trench outside each core, so all the signals in multiple cores can achieve independent transmission and reception [2]. But the intra-core modal XT is more difficult to deal with. Firstly, each few-mode core of the FM-MCF should be weakly-coupled so that modal XT between different LP modes along the fiber propagation can be suppressed [13,14]. Secondly, the mode MUX/DEMUX ought to be highly mode selective [9,10]. Thirdly, the spatial MUX/DEMUX and their coupling to/from each few-mode core should also ensure low modal XT [6]. Finally, in circular few-mode cores, the non-circularly-symmetric LPlm (l > 0) modes always have two spatial degenerate modes (LPlma and LPlmb modes) whose spatial orientation may rotate randomly along the propagation [15]. So the mode degeneracy needs to be handled. Due to above strict conditions, most FM-MCF transmission need coherent detection and multiple-input multiple-output (MIMO) digital signal processing (DSP) at the receiver to combat XT [16]. However, it is too complex and expensive for short-reach transmission scenarios such as optical interconnections in datacenter networks where intensity modulation and direct detection (IM/DD) solution is preferred [17,18].
In this paper, we firstly propose an all-fiber low-spatial-XT design of spatial MUX by cascading mode-selective couplers (MSCs) with a FI device, and spatial DEMUX by cascading a FO device with degenerate-mode-selective couplers (DMSCs) and MSCs. Then we fabricate them with fiber-bundle and fused-tapering processes. We also design and fabricate 1-km weakly-coupled 7-core-2-LP-mode FM-MCF. The FM-MCF and spatial MUX/DEMUX are characterized and the results show that low insertion loss (IL), core-to-core XT and intra-core modal XT are achieved for all the spatial channels. Base on the weakly-coupled SDM link, we successfully demonstrate a real-time 7-core-2-LP-mode transmission using 10 Gbps enhanced small form-factor pluggable (SFP + ) transceivers.
2. Design and fabrication of the spatial MUX/DEMUX and FM-MCF
2.1 Cascaded structure of the all-fiber spatial MUX/DEMUX
Figure 1 shows the schematic diagram of the proposed all-fiber spatial MUX and DEMUX for FM-MCF transmission. In the mode MUX, each MSC can effectively convert signal in fundamental mode of a SMF into a specific LP mode of the FMF with low IL and high modal selectivity relying on the phase-matching condition. Moreover, these MSCs can be cascaded to multiplex multiple LP modes into one FMF [10]. For the spatial MUX, multiple mode MUXs are employed to multiplex signals from different SMFs, and then they are combined by a fiber-bundle-type FI device and fed into the FM-MCF. For the spatial DEMUX, a fiber-bundle-type FO device separates signals from multiple fiber cores into multiple mode DEMUXs, which then demultiplex multiple LP modes of a few-mode core into different output ports. For circularly-symmetric LPlm (l = 0) modes, regular MSCs are effective for demultiplexing and the output fibers are SMFs; while for non-circularly-symmetric LPlm (l > 0) modes, due to random rotation of their spatial orientations, DMSCs are employed and the output fibers are two-mode fibers (TMFs). LPlm DMSC is an asymmetric FMF directional coupler, which can selectively demultiplex LPlma and LPlmb modes of the FMF into LP11a and LP11b modes of a TMF simultaneously [19]. Since the O/E-conversion photodiode of commercial IM/DD transceivers such as SFP + modules are spatially coupled without pigtail fibers, the output signal in LP11 mode from the TMF is compatible with IM/DD transceivers if the numerical aperture of the TMF is similar with that of the SMF. A slight mis-matching of numerical aperture between them will induce extra IL, but will not incapacitate the reception. Therefore, the proposed spatial MUX/DEMUX is highly compatible with current IM/DD transceivers without any hardware or software modifications.
In order to reduce extra IL and modal XT caused by the mode field mismatch at butt coupling, the FMF cores of each single MSC/DMSC, FI/FO devices and the FM-MCF should be identical. Besides, precise central alignment is necessary at each connection point. The maximum number of supported modes may be limited by the accumulated IL and modal XT for the cascaded structure. Except for the all-fiber solution, the proposed spatial MUX/DEMUX can also be realized in one integrated photonic chip such as using ultrafast laser inscription technique [12].
2.2 Weakly-coupled 7-core-2-LP-mode FM-MCF
In this paper, we design and fabricate 1-km 7-core-2-LP-mode FM-MCF for weakly-coupled SDM transmission. Figures 2(a) and 2(b) show the measured cross section image and 2D refractive index profile of the fabricated FM-MCF. The FM-MCF has 7 homogeneous few-mode cores, which are packed on a hexagonal lattice. In order to restrain the inter-core XT as much as possible, the core density of the FM-MCF is decreased and the core-to-core pitch is chosen to be 80 μm. Large pitch could also help the fabrication of the thin cladding fiber for the FI/FO devices and facilitate the butt coupling. Trench-assisted structure is utilized to further suppress the inter-core XT while reducing the bending loss. Concentricity of the central core and the accuracy of the core-to-core pitch are vital for the butt coupling between FM-MCF and FI/FO devices, especially for fiber-bundle-type FI/FO devices. Therefore, drilling process is adopted to fabricate the FM-MCF instead of stack and draw process [20]. Figure 2(c) shows the measured refractive index profile of one of the seven cores at 1550 nm. The rest cores have similar refractive index profiles. The core and cladding radii are 16 and 250 μm respectively. The core-cladding index difference is 1.1%. The measured Δneff between LP01 and LP11 modes is 3.77 × 10−3, which is larger than that of the 2-LP-mode single-core FMF in our previous work [21]. Combined with the wavelength-interleaving scheme, the proposed FM-MCF may support tens of kilometers or even longer weakly-coupled SDM transmission. It should be noted that we have designed a large refractive index margin between the LP11 mode and the cladding to ensure stable transmission of LP11 mode. Unwanted LP21 and LP02 modes appear in the fabricated FM-MCF because of fabrication errors.
Fig. 2 (a) Cross-section of the fabricated FM-MCF. (b) 2D refractive index profile of the FM-MCF. (c) Refractive index profile of one core of the FM-MCF.
2.3 Fabrication of the spatial MUX and DEMUX
We adopt fiber bundle technique for the FI/FO devices fabrication [11]. The fiber-bundle-type FI/FO devices are fabricated by gathering 7 thin-cladding FMFs into closest packed structure. Cores of the 7 thin-cladding FMFs are hexagonally arrayed and their relative positions correspond to each core of the FM-MCF. Due to relatively large core-to-core pitch of the FM-MCF in this paper, we fabricate 80-μm thin cladding FMFs directly without any chemical inching process. Since chemical-inched fibers are quite fragile and must be operated very carefully, this method greatly reduces the difficulty during the following processing. Moreover, the 80-μm thin cladding FMFs have the same core structure as the FM-MCFs and are also utilized to fabricate the mode MUX/DEMUX. The bundled FMFs are fixed in a ferrule with adhesive and the end face is mechanically polished. Figures 3(a) and 3(b) show the photo and the cross-section micrograph of the fabricated FI/FO devices. Both ends of the 1-km FM-MCFs are also fixed into a ferrule and polished. Then they are actively aligned with the FI/FO devices and fixed in a sleeve orderly.
Fig. 3 (a) The fabricated FI/FO devices. (b) End face of the fiber-bundle-type FI/FO devices. (c) The cascaded mode MUX and DEMUX.
Fused tapering process is employed to fabricate each single MSC and DMSC. During the fabrication, weak-fusion is chosen to maintain the geometry of both tapered fibers [22]. The phase matching condition is satisfied by pre-tapering one fiber. Fused-type LP01 and LP11 MSCs are cascaded as the mode MUX. Fused-type LP11 DMSC and LP01 MSC are cascaded as the mode DEMUX. Figure 3(c) shows the fabricated mode MUX and DEMUX.
3. Characteristics of the spatial MUX/DEMUX and FM-MCF
The fabricated mode MUX and DEMUX are firstly assessed in back-to-back (B2B) configuration. The stability for demultiplexing the LP11 mode is evaluated by inserting a mode rotator between the mode MUX and DEMUX [19]. The mode rotator is a 3-paddle polarization controller winding by the FMF. By arbitrarily adjusting the paddles of the mode rotator, the spatial orientation of the LP11 mode will rotate randomly. The optical power of the injected LP11 modes is fixed to 0 dBm over the C-band. The mode rotator is randomly adjusted 50 times at each wavelength and the IL of the LP11 DMSC in the mode DEMUX is measured. Then the LP11 DMSC is replaced by a LP11 MSC whose IL is measured in the same way. The results are shown in Fig. 4(a). It can be seen that the IL of LP11 MSC has a very large variation (> 20 dB) over the C-band. However, the variation is only about 3 dB for the LP11 DMSC. Figure 4(b) shows the detailed results of the measurement at 1550 nm. We can see that the stability for LP11 mode demultiplexing is largely improved by employing LP11 DMSC in the mode DEMUX.
Fig. 4 IL of LP11 DMSC and MSC with 50 randomly orientated LP11 mode injection (a) over the C-band (b) at 1550 nm. (c) IL of LP01 MSC with LP01/LP11 mode injection and IL of LP11 DMSC with LP01 injection over the C-band.
Then the LP11 DMSC is replaced back and the output power of the TMF is measured again by injecting 0 dBm LP01 mode. As shown in the black line in Fig. 4(c), the average IL is 19.1 dB over the C-band. The IL of LP01 MSC in the mode DEMUX is measured by injecting 0 dBm LP01 and LP11 modes, respectively. The results are also shown in Fig. 4(c). The average IL is 1.7 dB with LP01 injection and 21.6 dB with LP11 injection over the C-band, respectively. We can see that the mode MUX/DEMUX in B2B configuration have a low IL and modal XT over the C-band.
The experimental setup of the entire SDM link is shown in Fig. 5(a). We utilize only one mode DEMUX which is manually connected to each measured core. Firstly, the LP01 and LP11 mode patterns out of the spatial MUX are captured by a charge coupled device (CCD) camera (Newport, LBP2-IR2) and shown in Fig. 5(b). Then IL of the FI device combined with 1-km FM-MCF is measured by injecting LP01 or LP11 mode into each fiber core one by one at 1550 nm. IL of the FO device is also measured. The results are shown in Table 1. Except for the transmission attenuation of the 1-km FM-MCF, the extra IL is caused by the misalignment between FM-MCF and fiber-bundle-type FI/FO devices. Moreover, the misalignment may also induce inter-core XT and intra-core modal XT. The inter-core XT is evaluated by injecting 0-dBm LP01 or LP11 mode into each core one by one and measuring the output power of other 6 cores. The inter-core XT for all 7 cores is very weak and cannot be detected by a power meter (EXFO FPM-300) with a sensitivity of −60 dBm.
Table 1. Insertion Loss of the FI/FO devices (Unit: dB)
Finally, the IL and modal XT matrix of each core of the entire SDM link are measured. The input power of each input port of the mode MUX is fixed to 0 dBm and the output power of the mode DEMUX are measured respectively. For core 1, the measurements are at 1550 nm, 1530 nm and 1570 nm respectively. For core 2-7, the measurement is at 1550 nm. The results are shown in Table 2. The value of the modal XT is defined as the power of the XT relative to the power of the signal. The largest IL is 5.6 dB (LP11 mode of core 1 at 1570 nm). The largest modal XT is −10.21 dB (LP11 mode of core 5 at 1550 nm).
Table 2. IL and Modal XT Matrix of the entire SDM link (Unit: dB)
4. Experimental setup and results of real-time SDM transmission
Figure 6 shows the experimental setup of the real-time SDM transmission. A bit error rate tester (BERT, Sinolink BERT34N) is used for generating data patterns and error detection. Four-channel 10-Gbps electric signals of 29-1, 215-1, 223-1 and 231-1 pseudo-random binary sequence (PRBS) data patterns modulate onto 4 commercial 10-Gbps SFP + transceivers at 1550 nm (Afalight, 10GBASE-ER-CWDM-1550) by SFP + driver boards (Youthton, YXT-SFP + TEST BOARD). Electro-absorption modulated lasers (EML) and PIN diodes are adopted in these SFP + transceivers. The output optical power of each SFP + transmitter (Tx) is about 0 dBm. The single-mode (SM) optical signals out of SFP + #3 Tx and #4 Tx are converted to LP01 and LP11 modes of FMF respectively using a mode MUX. The MDM signals are launched into the measured core of FM-MCF through FI device. For the remaining 6 cores, dummy SDM channels are generated as follows. The optical signal of SFP + #1 Tx is split by a 1 × 8 SM optical coupler, in which 6 branches are connected to the LP01 input ports of 6 mode MUXs. Similarly, the optical signal of SFP + #2 Tx is split and connected to the LP11 input ports of the mode MUXs. The 6 MDM signals are launched into the remaining 6 cores by FI device as the dummy SDM channels. 4 SM variable optical attenuators (VOA, EXFO FVA 600) are placed after each Tx to balance the optical power of each channel and enable the adjustment of the detected power at the SFP + receiver (Rx). Note that SFP + #3 Tx and #4 Tx should have larger attenuations to ensure identical power injection to each core. Erbium-doped fiber amplifiers (EDFAs) are not employed in this experiment. After 1-km FM-MCF transmission, the SDM signals are firstly demultiplexed to 7 single-core MDM signals by a FO device. Then a mode DEMUX is connected to the measured core to demultiplex the 2-LP-mode MDM signals into SMF and TMF pigtails. After detection by the SFP + Rx, bit error rates (BER) are measured by the BERT. It should be noted that forward error correction (FEC) is not used in these commercial SFP + transceivers.
BER performance for MDM transmission in B2B configuration is firstly evaluated by connecting the output of mode MUX and the input of mode DEMUX. SFP + #3 (for LP01 mode) and #4 (for LP11 mode) at 1550 nm are used in this measurement. The SM B2B performance of both SFP + #3 and #4 are measured for reference by directly connecting the output port of Tx with the input port of Rx. The results are shown in Fig. 7(a). Compared to SM B2B case, the power penalties for LP01 only B2B, LP11 only B2B, LP01 B2B MDM and LP11 B2B MDM cases are about 0.2, 1.0, 0.6 and 1.8 dB at the BER of 10−4, respectively. The penalty of LP11 only B2B case is 0.8 dB larger than the LP01 only B2B case. This extra penalty may come from the lower responsivity of the PIN diode in SFP + Rx for LP11 mode reception.
Fig. 7 BER curves of (a) B2B MDM transmission at 1550 nm. (b) MDM transmission over core 1 at 1550 nm. (c) MDM transmission over core 1 at 1530 nm. (d) MDM transmission over core 1 at 1570 nm.
SDM transmission over 1 km of FM-MCF is subsequently performed when core 1 is measured. 4 SFP + transceivers at 1550 nm are all switched on and 4 VOAs are attenuated simultaneously to ensure identical input power of each mode to all 7 mode MUXs. Figure 7 (b) shows the measured BER performances. Compared to the SM B2B case, the penalty of both modes in one by one and MDM transmission are all less than 3.3 dB at the BER of 10−4. It should be noted that the penalty of LP11 only transmission is 2 dB larger than the LP01 only transmission. However, the extra penalty is 0.8 dB at B2B case. We infer the reason is not only the lower responsivity for LP11 mode reception, but also the modal dispersion among the eigenmodes of LP11 mode during FM-MCF transmission [23].
Then SDM transmissions at 1530 nm and 1570 nm are evaluated respectively when core 1 is measured. The SFP + transceiver #3 at 1550 nm is replaced by a 1530-nm SFP + transceiver (Afalight, 10GBASE-ER-CWDM-1530) whose SM B2B performance is firstly measured. Due to lack of more 1530-nm SFP + transceivers, after the evaluation of LP01 one by one and LP01 MDM transmission at 1530 nm, two input SMFs of the mode MUX for core 1 are exchanged. Then the LP11 transmission at 1530 nm is measured. We also evaluate the SDM transmission using a 1570-nm SFP + transceiver (Afalight, 10GBASE-ER-CWDM-1570) in the same way. Figures 7(c) and 7(d) show the experimental results. We can see that the BER performances are similar with different wavelength, which demonstrates the whole system is insensitive to wavelength over the C-band.
Finally, the SDM transmission performances of the other 6 cores are measured at 1550 nm. The measured BER curves are shown in Figs. 8(a) – 8(f). The penalties for both modes in core 2-7 are less than 2.0, 3.2, 2.1, 3.7, 2.6 and 3.1 dB respectively, among which core 5 is the worst. Larger penalties for LP11 mode are also observed. Since the inter-core XT is negligible compared to the intra-core modal XT, the performances are mainly determined by the latter.
5. Summary
An all-fiber low-XT spatial MUX and DEMUX consisting of FI/FO devices and MSCs/DMSCs are firstly proposed and then fabricated with fiber bundle and fusion tapering processes. Based on the spatial MUX and DEMUX, we experimentally demonstrate real-time 7 × 2 × 10 Gbps SDM transmission over 1 km of weakly-coupled FM-MCFs using commercial SFP + transceivers. The digital-signal-processing-free scheme can be extended to FM-MCF transmission supporting more fiber cores and LP modes and is promising for capacity enhancement of short-reach applications.
Funding
National Natural Science Foundation of China (61771024, 61627814, 61505002, 61690194 and 61605004), and Shenzhen Science and Technology Plan (JCYJ 20170412153729436, 20170307172513653, 20170817113844300). Projects Foundation of YOFC (No. SKLD1708).
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