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Abstract
Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission is a promising candidate to enhance the capacity of short-reach applications such as optical interconnections, in which low-modal-crosstalk mode multiplexers/demultiplexers (MMUX/MDEMUX) are highly desired. In this paper, we firstly propose an all-fiber low-modal-crosstalk orthogonal combine reception scheme for degenerate linearly-polarized (LP) modes, in which signals in both degenerate modes are firstly demultiplexed into the LP01 mode of single-mode fibers, and then are multiplexed into mutually orthogonal LP01 and LP11 modes of a two-mode fiber for simultaneous detection. Then a pair of 4-LP-mode MMUX/MDEMUX consisting of cascaded mode-selective couplers and orthogonal combiners are fabricated with side-polishing processing, which achieve low back-to-back modal crosstalk of lower than -18.51 dB and insertion loss of lower than 3.81 dB for all the 4 modes. Finally, a stable real-time 4 modes × 4λ × 10 Gb/s MDM-wavelength division multiplexing (WDM) transmission over 20-km few-mode fiber is experimentally demonstrated. The proposed scheme is scalable to support more modes and can pave the way to practical implementation of IM/DD MDM transmission applications.
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1. Introduction
The explosive increasement of network users and various communication services such as internet of things (IoT) and cloud computing have been constantly request the capacity enhancement of optical transmission systems and networks, especially for short-reach optical interconnection applications with low-cost intensity-modulation/direct-detection (IM/DD) transceivers. Recently, mode division multiplexing (MDM) transmission technique utilizing linearly-polarized (LP) modes in few-mode fibers (FMF) has been widely considered as a promising solution to enhance capacity of optical fiber communication systems [1,2]. For short-reach applications, the weakly-coupled MDM approaches are preferred [3,4], in which the modal crosstalk are strictly suppressed so that each LP mode can be utilized as independent channel [5–7]. Weakly-coupled MDM scheme supporting IM/DD transmission based on circular-core weakly-coupled FMF has been proposed [8–10], in which each non-degenerate LP mode or each pair of degenerate LP modes are considered as a whole for single-channel transmission and is compatible with conventional IM/DD transceivers such as small form-factor pluggable (SFP+) optical modules [11,12].
The mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components for IM/DD MDM transmission. Among various MMUX/MDEMUX approaches, those consisting of cascaded all-fiber mode-selective couplers (MSC) are promising for the high modal selectivity, high flexibility and the low coupling crosstalk with transmission FMFs [13,14]. The MSCs with side-polishing processing can achieve low IL and cascading loss because the waveguide structure is not significantly damaged during the fabrication [15,16]. To support stable IM/DD MDM transmission, a degenerate-mode reception structure based on degenerate-mode-selective couplers (DMSC) has been proposed, in which both degenerate LPnm a and LPnm b (n ≥ 1, m ≥ 1) modes can be selectively demultiplexed to the degenerate LP11 modes of an output two-mode fiber (TMF) simultaneously [17]. However, since the phase-matching condition may be satisfied for multiple mode pairs simultaneously, the presence of LP01 mode in the TMF will reduce the modal selectivity.
In this paper, a low-modal-crosstalk orthogonal combine reception scheme for degenerate modes are firstly proposed, in which signals in both degenerate modes are firstly demultiplexed into the LP01 mode of single-mode fibers (SMF), and then are multiplexed into mutually orthogonal LP01 and LP11 modes of a TMF respectively for simultaneous detection. A pair of 4-LP-mode MMUX/MDEMUX consisting of cascaded MSCs and orthogonal combiners are fabricated with side-polishing processing, which achieve low back-to-back (B2B) modal crosstalk of lower than -18.51 dB and IL of lower than 3.81 dB for all the 4 modes. Finally, a stable real-time 4 modes × 4λ × 10 Gb/s MDM-wavelength division multiplexing (WDM) transmission over 20-km triple-ring-core few-mode fiber (TRC-FMF) using on-off keying (OOK) modulation and digital signal processing (DSP)-free detection is experimentally demonstrated.
2. Design principle of the orthogonal combine reception scheme
The structure of the proposed orthogonal combine reception scheme for MDEMUX is shown in Fig. 1, in which the structure of corresponding MMUX is also depicted. In the MMUX, multiple regular MSCs are cascaded to multiplex all the LP modes. During FMF transmission, the degenerate LPnm a and LPnm b modes will experience random coupling and rotation due to imperfect fiber fabrication and external perturbations. But a regular MSC could only demultiplex one degenerate mode, which will induce large received power fluctuation [11]. So in the MDEMUX, two orthogonally cascaded MSCs are firstly utilized to demultiplex the two degenerate modes into the LP01 mode of two SMFs. Then a LP11 MSC is adopt as a power combiner to multiplex the two single-mode (SM) signals into mutually orthogonal LP01 and LP11 modes of a TMF to achieve orthogonal combine reception. It does not matter which SM signal is connected to the TMF pigtail of the LP11 MSC. Then a photo detector (PD) is followed for simultaneous detection. The PD may be spatially coupled or has a few-mode/multimode pigtail fiber. For the other non-degenerate LP modes, regular MSCs are utilized for mode demultiplexing. For a MDM system supporting k non-degenerate modes and l degenerate modes, the MDEMUX consists of (k + 2×l) MSCs for FMF to SMF mode conversion and l LP11 MSCs for SMF to TMF mode conversion. It should be noted that the two branches before the orthogonal combiners should have the same length to avoid temporal broadening of signal. Compared to previous reception scheme utilizing DMSCs, the proposed orthogonal combine reception scheme can avoid the affection of phase matching among multiple mode pairs and can achieve lower modal crosstalk.
3. Fabrication and characterization of the proposed MMUX/MDEMUX
In this section, a pair of MMUX/MDEMUX utilizing orthogonal combine reception for degenerate modes are designed and fabricated. Four cascaded MSCs in the MMUX and 6 cascaded MSCs in the MDEMUX should be fabricated utilizing the 4-mode FMF and SMF, while the 2 combiner MSCs in the MDEMUX should be fabricated utilizing TMF and SMF. Fig. 2(a) depicts the designed (blue line) and measured (orange line) index profiles of the FMF and effective refractive index (n eff) distribution of supported modes. It supports 4 LP modes with a normalized frequency V of 4.8 and a refractive index difference (Δn) between the fiber core and cladding of 0.6% at 1550 nm. Three ring perturbations are applied to increase the n eff spacing among all LP modes and a min|Δn eff| up to 1.89 × 10−3 is achieved. A depressed-index fluorine-doped trench is applied in the cladding to reduce the bending sensitivity of high-order LP modes. A customized step-index SMF with core/cladding radius of 2.48/62.5 µm and Δn of 1.23% at 1550 nm is adopted for the fabrication of LP01 and LP11 MSCs, whose designed (blue line) and measured (orange line) index profiles are depicted in Fig. 2(b). The n eff of the LP01 mode of the customized SMF is 1.453 at 1550 nm. A standard SMF with a core/cladding radius of 4.1/62.5 µm and Δn of 0.36% at 1550 nm is adopted for the fabrication of LP21 and LP02 MSCs. For the MSC acting as orthogonal combiner, a TMF with core/cladding radius of 5/62.5 µm and Δn of 0.688% at 1550 nm is adopted, whose designed (blue line) and measured (orange line) index profiles are depicted in Fig. 2(c).
Fig. 2. Refractive index profile of (a) TRC-FMF, (b) customized SMF and (c) TMF. (d) The n eff of the 4 LP modes in TRC-FMF and the LP01 mode in two kinds of SMFs versus different tapered radii. (e) The n eff of the LP01 and LP11 modes in TRC-FMF and the LP01 and LP11 modes in TMF versus different tapered radii. (f) The n eff of the LP11 mode in TMF and the LP01 mode in standard SMF versus different tapered radii.
To satisfy the phase-matching condition, the fiber with higher mode n eff should be properly pre-tapered. The tapering process could be conducted on a fused biconical taper station and a microscope could be utilized to measure the diameter of pre-tapered fiber to ensure that the radius of the tapering region reaches the designed value. Fig. 2(d) shows the n eff of all the LP modes in the TRC-FMF, the customized SMF and standard SMF as functions of the tapered radius, which are calculated by COMSOL multiphysics and Matlab based on the fabricated index profiles. The red cross points of the blue dotted lines in the tapering curves indicate the proper tapering radii of the fibers with higher mode n eff for the fabrication of different MSCs [17]. We can see that the radius of customized SMF need to be pre-tapered to 58.5/49.2 µm respectively for the LP01 and LP11 MSCs, while the radius of the FMF is supposed to be pre-tapered to 57 µm for the LP21 MSCs. And for phase-matching with LP02 modes, the radius of standard SMF should be pre-tapered to 49 µm. The tapering curves for the fabrication of LP11 DMSC for the TRC-FMF utilizing the TMF are depicted in Fig. 2(e) for comparison. It can be seen that the the radius of the FMF is supposed to be pre-tapered to 42.6 µm for phase-matching with the LP11 mode in TMF. While at this radius, the LP01 mode in FMF may be also phase-matched with the LP01 mode in TMF, which will induce extra modal crosstalk. After the pre-tapering, the FMFs and SMFs are respectively embedded into quartz blocks and polished on the grinding platform (SW-22C, SAMWELL) until only a few microns of cladding remains. The residue cladding thickness of the polished fiber is evaluated by oil drop experiment [17]. Finally, the two half couplers are mated together to form the MSC. The LP11 MSCs acting as orthogonal combiner are fabricated similarly, for which the n eff of the LP11 mode in TMF and the LP01 mode in standard SMF versus different tapered radii are shown in Fig. 2(f). We can see that the radius of the TMF is supposed to be pre-tapered to 57 µm for the fabrication of combiner MSCs.
The demultiplexing capability for the degenerate LP11 and LP21 modes are firstly investigated with the experimental setup shown in Fig. 3(a). The two orthogonal LP11/LP21 MSCs are cascaded by fusion splicing and the orthogonality is achieved by twisting the FMF between them. Then a combiner MSC is also connected by fusion splicing. The lengths of the two input pigtail fibers of the combiner MSC are precisely measured and controlled to ensure the two branches before the orthogonal combiner have the same length. 0-dBm optical power is launched through a LP11 or LP21 MSC stimulated by a tunable continuous-wave (CW) laser. A mode rotator which is a self-made 3-paddle polarization controller winding by the TRC-FMF is applied before the demultiplexer to adjust the spatial orientations and polarization of input modes, and the output power of the demultiplexer is measured by a power meter. The IL and power stability of regular MSCs for mode demultiplexing are also measured for comparison. 50-time IL measurements are conducted by randomly adjusting the mode rotator and the results are shown in Fig. 3(b) and (c). It can be seen that only slight power fluctuation is observed for the degenerate mode demultiplexers while the power fluctuation for regular MSCs is quite large. Then the modal selectivity is evaluated by exciting different crosstalk modes. That is, the LP01, LP21 and LP02 modes are excited with different MSCs for the modal crosstalk measurement of LP11 demultiplexer, while the LP01, LP11 and LP02 modes are excited for the modal crosstalk measurement of LP21 demultiplexer. The wavelength of probe signal is tuned over the C-band and the results are shown in Fig. 3(d) and (e). We can find that the modal crosstalk of all modes are lower than -21 dB at 1550 nm and lower than -19.6 dB over the C-band.
Fig. 3. (a) Setup for characterization measurements of the LP11/ LP21 demultiplexers. 50-times IL measurement with random input spatial orientations at 1550 nm for (b) LP11 mode and (c) LP21 mode. Modal crosstalk over the C-band for (d) LP11 and (e) LP21 demultiplexers.
The cascading structures of the whole 4-LP-mode MMUX and MDEMUX are shown in Fig. 4(a), in which the LP11 MSC is at the last stage in the MMUX while they are at the first stage in the MDEMUX considering their modal crosstalk is relatively large when passing through coupling regions of other MSCs. The orders for the rest MSCs could be flexibly adjusted. The photo of the MMUX/MDEMUX and the output mode patterns of the MMUX captured by a charge coupled device (CCD) camera (Newport, LBP2-IR2) are shown in Fig. 4(b). We can find that each LP mode is exited with high modal selectivity. The modal crosstalk and IL matrix for B2B case and 20-km FMF transmission at 1550 nm are shown in Table 1 and Table 2, respectively. It can be seen that the worst relative modal crosstalk, which is defined as the power ratio between crosstalk and signal for any two LP modes, is -18.51 dB for the case from LP01 to LP11 modes and the maximum IL is 3.81 dB for the LP21 mode in B2B case. The relative modal crosstalk and IL are lower than -17.5 dB (LP01 to LP11) and 8.55 dB (LP21) for the entire 20-km MDM link.
Fig. 4. (a) The structures of the whole 4-LP-mode MMUX/MDEMUX. (b) The photo of the cascaded 4-LP-mode MMUX/MDEMUX and output mode patterns of the MMUX at 1550 nm.
Table 1. Modal crosstalk and IL matrix for B2B case (Unit: dB)
Table 2. Modal crosstalk and IL matrix for 20-km transmission (Unit: dB)
4. Experimental setup and results for IM/DD MDM-WDM transmission
The 4-mode × 4λ × 10 Gb/s MDM-WDM transmission is carried out to evaluate the performance of the MMUX/MDEMUX with the experimental setup shown in Fig. 5. At the transmitter, two bit-error-ratio testers (BERT, Sinolink, BERT34N) generate 8-channel 10-Gbps pseudo-random-binary-sequences (PRBS, 27-1, 29-1, 211-1, 215-1 by each BERT) electric signals simultaneously. The electric signals modulate eight SFP + (Sharetop) transmitters (Tx) by SFP + driver boards. The wavelength-interleaving (WI) scheme is adopted to further suppress signal-to-crosstalk beating interference [18]. The central wavelengths λ1, λ3, λ5, λ7 are from 1549.71 to 1552.14 nm with a WDM spacing of 100 GHz, while the central wavelengths λ2, λ4, λ6, λ8 are from 1550.13 to 1552.55 nm with a WDM spacing of 100 GHz. The odd and even wavelengths are multiplexed by SM wavelength multiplexers (WMUX), respectively, and then is divided by 1 × 2 optical couplers (OC). Three optical delay lines (ODL) are adopted for decorrelation. Four SM variable optical attenuators (VOA) are used before the MMUX for power adjustment. Then the interleaved wavelengths are multiplexed by the MMUX. After 20-km transmission, the received signal is demultiplexed by the MDEMUX firstly. Then two few-mode wavelength demultiplexers (FM-WDEMUX) which consist of customized dielectric multilayer thin-film filters and two-mode pigtail fibers are followed to demultiplex each wavelength channel [19]. Finally, each demultiplexed signal is detected by the SFP + receivers (Rx) and BERTs for real-time BER calculation.
The experimental results are shown Fig. 6. The transmitted and received normalized optical spectra of the four LP modes after 20-km MDM transmission are shown in Fig. 6(a). The modal crosstalk of adjacent modal channels is all lower than -17.5 dB. The BER performances of MDM transmission for B2B case at λ5 (LP01 and LP21) and λ6 (LP11 and LP02) are then measured by directly connecting MMUX and MDEMUX, and the results are shown in Fig. 6(b). The SM B2B performance for the SFP + module is also plotted for reference. We can find that the worst (LP11) receiver sensitivity penalty of the 4 LP modes is less than 1.4 dB compared to SM B2B thanks to the low modal crosstalk of the MMUX/MDEMUX. Then one-by-one 20-km FMF transmission is conducted and the results are shown in Fig. 6(c). We can observe that both LP01 and LP02 modes have about 1.2 dB receiver sensitivity penalty compared to SM B2B case, which mainly comes from chromatic dispersion during FMF transmission. Compared to LP01 and LP02 modes, an extra receiver sensitivity penalty of about 0.3 dB is observed for both LP11 and LP21 modes because of intra-LP-mode dispersion (ILMD) during transmission [20]. The BER curves after 20-km MDM transmission are shown in Fig. 6(d). Compared to the results of one-by-one 20-km transmission shown in Fig. 6(c), a maximum additional penalty of 1.6 dB is observed, which is induced by modal crosstalk in FMF transmission and MMUX/MDEMUX.
Fig. 6. (a) Transmitted and received normalized optical spectra of the 4 LP modes. Measured BER curves of (b) MDM transmission for B2B case, (c) one-by-one 20-km transmission and (d) MDM 20-km transmission. (e) Q2 factors of each mode and wavelength under 20-km MDM-WDM transmission. (f) Q2 factors of each mode versus duration time. (g) Eye diagrams of LP21 mode for 20-km MDM-WDM transmission after working 2, 6, 10 hours.
The performance for 20-km MDM-WDM transmission case is shown in Fig. 6(e). It can be seen that the Q2 factors of all modes and wavelengths are above the forward error correction (FEC) limit (9.8 dB at the BER of 1 × 10−3). Finally, a twelve-hour continuous transmission test is conducted to evaluate the stability of the whole system. The Q2 factors of each LP mode at λ5 or λ6 versus duration time are shown in Fig. 6(f). We can find that the Q2 factors are all above the FEC limit and the variation for each LP mode is less than 1.3 dB during the 12-hour continuous transmission test. The eye diagrams of LP21 mode for the case of 20-km MDM-WDM transmission after 2, 6, 10-hours continuous working are depicted in Fig. 6(g).
5. Conclusion
In this paper, a low-modal-crosstalk orthogonal combine reception scheme for degenerate modes demultiplexing is proposed. A pair of 4-LP-mode MMUX/MDEMUX consisting of cascaded all-fiber MSCs are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low B2B modal crosstalk of lower than -18.51 dB and low IL of lower than 3.81 dB for all the 4 modes. Based on the low-modal-crosstalk MMUX/MDEMUX and weakly-coupled TRC-FMF, a stable 4 LP modes × 4λ × 10 Gb/s MDM-WDM 20-km transmission utilizing commercial SFP + optical modules is experimentally demonstrated.
Funding
National Natural Science Foundation of China (62101009, U20A20160); Pengcheng Self-Approval Funding (PCL2021A04).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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