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Abstract
Multiple-input-multiple-output digital signal processing (DSP) has become a severe bottleneck for mode division multiplexing (MDM) because of its huge computational complexity. In this paper, we propose a novel scheme for real-time DSP-free intensity-modulation/direct-detection (IM/DD) MDM transmission, in which the transmission few-mode fiber (FMF) is characterized by multiple-ring-core structure to suppress modal crosstalk among each LP mode, while each pair of non-circularly-symmetric degenerate modes is simultaneously demultiplexed by a degenerate-mode-selective fiber coupler for DSP-free reception. Based on a 10 km ultralow-modal-crosstalk double-ring-core FMF and a pair of all-fiber 4-LP-mode MUX/DEMUX, we demonstrate the first IM/DD MDM prototype system using commercial single-mode (SM) 10 Gbps SFP + modules and 4K video transceivers without any hardware modifications. The temperature and wavelength dependence are evaluated. The stable Q2-factor performance proves that it can be a smooth evolution scheme from conventional SM IM/DD systems. Moreover, the scheme can be further extended to support more modes with improved FMF design adopting more ring areas.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, mode division multiplexing (MDM) has been widely investigated as a promising solution to enhance the capacity of optical communication systems and networks [1,2]. The modal crosstalk has been widely considered to be hard to suppress and strongly-coupled MDM approaches have been proposed using coherent detection and multiple-input multiple-output (MIMO) digital signal processing (DSP) to demultiplex all the MDM channels [3]. However, the computational complexity and cost for strongly-coupled MDM approaches will increase dramatically with longer FMF link and more linearly-polarized (LP) modes, which prevents the real-time hardware implementation of MIMO receiver [4,5]. Weakly-coupled approaches suppressing the modal crosstalk as much as possible to avoid MIMO processing are expected to be compatible with conventional intensity modulation/direct detection (IM/DD) transceivers [6–8].
There are two kinds of modal crosstalk in circular-core FMF we have to deal with to realize IM/DD MDM transmission. The first one is inter-LP-mode crosstalk. According to mode coupling theory, the modal crosstalk between two LP modes is inversely related to their modal effective refractive index difference (|Δneff|) and the overall performance of MDM system will be mainly determined by the minimum |Δneff| (min|Δneff|) among all the LP modes [9]. For a step-index circular-core (SI-CC) FMF, we can increase the core-cladding index difference (Δ) and/or decrease the core radius to enlarge the min|Δneff| [10]. However, this will yield high fiber nonlinearities, large attenuation and severe modal birefringence [11]. Recent studies have shown that multiple-ring core structure can be a more effective approach to enlarge the min|Δneff| by adjusting all the LP modes towards equal spacing as much as possible [12–14]. The second modal crosstalk is the intra-LP-mode crosstalk for each pair of non-circularly-symmetric degenerate modes (LPlma and LPlmb, l ≥ 1). Due to imperfect fiber fabrication and external perturbations, their modal field may rotate randomly along the FMF propagation and strong coupling will occur. In order to avoid MIMO processing, it is reasonable to use the two degenerate modes as a whole LP mode. However, regular non-circularly-symmetric LP mode DEMUXs can only convert one degenerate mode with a certain spatial orientation to the fundamental mode of a single-mode fiber (SMF) [15]. Signal power at other spatial orientations will be abandoned at the receiver, which will induce large power fluctuation. In our previous work, we have proposed a degenerate-mode-selective coupler to demultiplex both degenerate modes simultaneously and detect without any DSP [16]. The combination of multiple-ring-core FMF and degenerate-mode-selective reception makes it possible to realize real-time IM/DD MDM transmissions.
In this paper, we firstly introduce the operating principles of the DSP-free IM/DD MDM scheme based on multiple-ring-core FMF and degenerate-mode-selective reception, which can realize smooth evolution from conventional single-mode (SM) transceivers. Then we demonstrate the first IM/DD MDM prototype system using commercial SM 10 Gbps SFP + modules and 4 K video transceivers based on a 10 km double-ring-core FMF and an all-fiber 4-LP-mode MUX/DEMUX. The temperature and wavelength dependence are investigated. The scheme can be further extended to support more LP modes with improved design of multiple-ring-core FMF.
2. Principles
Generally, all the modes in a SI-CC FMF can be divided into weakly-coupled LP modes, each of which consists of one circularly-symmetric LP0m mode or a pair of non-circularly-symmetric degenerate modes. A ring-core FMF can be treated as a SI-CC FMF with perturbed refractive index ring areas. Previous works have shown that the core structure with multiple concentrated ring areas can effectively suppress the inter-LP-mode crosstalk among each LP mode [12–14]. As shown in Fig. 1(a), conventional SI-CC FMF may support different LP modes with non-equally effective index spacing. The design principle of multiple-ring-core FMF is to adjust the effective refractive index (neff) of all LPlm modes to approach equal spacing as much as possible. Since different LP modes have different power spatial distributions, only one ring structure cannot satisfy the neff adjustment for all LP modes. Therefore, multiple-ring-core structure, considering the power distributions of all LP modes, can be utilized to achieve a large min|Δneff| and the inter-LP-mode crosstalk can be effectively suppressed, as shown in Fig. 1(b). Currently, the design of multiple-ring-core FMF can be realized by parameter sweeping and the computational complexity will increase dramatically with the number of rings. New methods should be developed to simplify the design. These multiple-ring-core FMFs should be fabricated using commercial plasma chemical vapor deposition (PCVD) technique, so they have higher fabrication cost than that of simple step-index SMFs. However, since they always have much smaller core diameters than conventional multimode fiber (MMF), their fabrication cost may be much lower than that of the MMF.
Fig. 1. Schematic diagram of the LP modes distributions of (a) step-index circular-core FMF and (b) multiple-ring-core FMF
After separating each LP mode as much as possible with the multiple-ring-core structure, the intra-LP-mode crosstalk in each pair of degenerate modes can be resolved by degenerate-mode-selective couplers (DMSCs) [16]. DMSC is an asymmetric FMF directional coupler consisting of an input FMF and an output two-mode fiber (TMF). It can simultaneously convert each pair of high-order LPlma and LPlmb modes of the input FMF into LP11a and LP11b modes of the output TMF regardless of the modal-field rotation and coupling. Then an IM/DD MDM transmission link can be realized by the combination of multiple-ring-core FMF and degenerate-mode-selective reception, as shown in Fig. 2. At the transmitter side, signals from SM intensity-modulation transmitters are converted into each LP mode and then launched into the multiple-ring-core FMF. After fiber transmission, a mode DEMUX is applied consisting of cascaded mode-selective-couplers (MSCs) for LP0m mode reception and DMSCs for degenerate-mode reception. The output TMF of the DMSC is coupled to photodetector for optoelectronic conversion. The photodetector of commercial IM/DD transceivers such as SFP + modules are always spatially coupled without pigtail fibers. So the optical signal in LP11 mode of the TMF can be directly detected by these IM/DD transceivers. Parameters of the TMF can be optimized so that the mode effective area of the output LP11 mode is similar with that of the fundamental mode of a SMF. Different mode effective areas may bring slightly extra insertion loss (IL), but will not incapacitate the reception. We can see that the proposed IM/DD MDM scheme is highly compatible with commercial IM/DD transceivers without any hardware or software modifications.
Fig. 2. IM/DD MDM transmission link with degenerate mode reception scheme. TMF, two-mode fiber; IM/DD, intensity modulation/direct detection; Tx, transmitter; Rx, receiver; DMSC, degenerate-mode-selective coupler; MSC, mode-selective coupler.
3. Prototype system setup and characterization measurements
We establish a 4-LP-mode real-time IM/DD MDM prototype system for 4K video transmission. Its schematic diagram is shown in Fig. 3(a). At the transmitter side, four 4K video sources (Xiaomi, Set-up Box 4th) generate high definition multimedia interface (HDMI) video signals simultaneously. Then four 4K video transmitters (HDMI fiber extenders, Meintercom, ML231-T) generate SM 10 Gbps optical On-Off Keying (OOK) signals utilizing SFP + transmitters (Tx, Afalight, AFASP11) at 1550 nm. The 4 SM optical signals are multiplexed by a 4-LP-mode MUX and converted to LP01, LP11, LP21 and LP02 modes, respectively. After the propagation over 10 km double-ring-core FMF, the MDM signals are demultiplexed by a 4-LP-mode DEMUX, which converts the LP01 and LP02 modes into LP01 mode of SMFs, and the LP11 and LP21 modes into LP11 mode of TMFs, respectively. Then the signals are detected by SFP + receivers (Rx, Afalight, AFASP11) in 4K video receivers (HDMI fiber extenders, Meintercom, ML231-R). The output 4K video electrical signals are sent to displays through HDMI cables. No Erbium-doped fiber amplifiers (EDFA) or variable optical attenuators (VOA) are used. The whole prototype system is shown in Fig. 3(b).
Fig. 3. (a) Schematic diagram of the prototype system of 4K video MDM transmission. (b) Prototype system of 4K video MDM transmission (see Visualization 1).
Figure 4(a) shows the designed (dashed red line) and measured (solid blue line) refractive index profile of the double-ring-core FMF employed in the prototype system [14]. The FMF supports 6 LP modes and the first 4 LP modes are used in this work. The min|Δneff| among all LP modes is up to 1.49×10−3 under a Δ of 0.828%. Figure 4(b) shows the photo of the fabricated 4-LP-mode MUX/DEMUX. The mode MUX consists of cascaded LP01, LP11, LP02, and LP21 MSCs. The mode DEMUX is composed by cascading LP21 DMSC, LP02 MSC, LP11 DMSC and LP01 MSC. The MSCs and DMSCs are fabricated in the form of fused-type fiber couplers using the double-ring-core FMFs. Employing same FMFs can effectively suppress the modal crosstalk at the coupling points between FMFs and mode MUX/DEMUX. It should be noted that all 6 LP modes can be used for weakly-coupled MDM transmission providing corresponding MSCs and DMSCs, and the accumulated IL and modal crosstalk should be further evaluated with the cascading MUX/DEMUX structure.
Fig. 4. (a) Refractive index profile of the double-ring-core FMF. (b) Fabricated 4-LP-mode MUX/DEMUX. See the text for explanation.
The mode patterns out of the fabricated 4-LP-mode MUX are captured by a charge coupled device (CCD) camera (Newport, LBP2-IR2) and shown in Fig. 5(a). The impulse responses of the 4-LP-mode MUX combined with 10 km double-ring-core FMF are measured and the results are shown in Fig. 5(b). A short SM optical pulse at 1550-nm is launched into each input port of the mode MUX one by one to selectively excite each LP mode in the FMF. The pulses arrive at different times in the photodetector due to different group delays. We can see each LP mode is exited with a high modal selectivity. The modal crosstalk matrix of the entire MDM link (the 10 km double-ring-core FMF combined with the 4-LP-mode MUX/DEMUX) at 1550 nm is measured and the results are shown in Table 1. The modal crosstalk between any two LP modes can be suppressed to less than -12.4 dB. Table 2 shows the mean modal crosstalk, worst modal crosstalk and IL of the 4 LP modes of the MDM link at 1530, 1550 and 1570 nm, respectively. The IL of each of the 4 LP modes is less than 13.9 dB over the C band.
Fig. 5. (a) Mode patterns after the mode MUX. (b) Impulse responses of the mode MUX combined with 10 km FMF.
Table 1. Modal crosstalk matrix of the MDM link at 1550 nm (Unit: dB)
Table 2. Modal crosstalk and IL of the MDM link over the C band (Unit: dB)
4. Performance evaluation of the prototype system
In order to evaluate the prototype system, we replace the 4K video sources and 4K displays with a bit error rate tester (BERT, Sinolink, BERT34N). The HDMI fiber extenders are correspondingly replaced by SFP + driver boards (Youthton, YXT-SFP+ TEST BOARD). The BERT generates four-channel 10 Gbps PRBS (211-1, 215-1, 223-1, 225-1) electric signals simultaneously. The electric signals modulate the SFP + Tx by the SFP + driver boards. The output optical power of each SFP + Tx is about 0 dBm. 4 SM variable optical attenuators (VOA, EXFO FVA 600) are utilized after each Tx to balance the optical power of each channel and enable the adjustment of detected power at the SFP + Rx. In these SFP + modules, electro-absorption modulated laser (EML) and PIN diode are adopted and forward error correction (FEC) is not used. The detected signals by the SFP + Rx are sent back to the BERT for real-time BER calculation.
The BER performance of one-by-one transmission for each mode in B2B configuration is firstly measured by directly connecting mode MUX and DEMUX. The results are shown in Fig. 6(a). We can see that the LP11/LP21 B2B transmission has about 1.5 dB penalty compared to LP01/LP02 B2B transmission at the BER of 10−6. The penalty mainly comes from the lower responsivity of the SFP + Rx for LP11 mode reception because of slight modal field mismatching. The BER performance of one by one 10 km transmission for each LP mode is then measured. The results are shown in Fig. 6(b). LP01 mode B2B transmission is plotted for reference. We can see that the LP11/LP21 mode only 10 km transmission also has about 1.5 dB penalty compared to LP01/LP02 mode. Figure 6(c) shows the BER curves of the 4-LP-mode 10 km MDM transmission. The penalty for each of the 4 LP modes is less than 3.6 dB compared to LP01 B2B transmission. The receiver sensitivity penalty mainly comes from the modal crosstalk of the FMF transmission and mode MUX/DEMUX. Eye diagrams of LP01 mode only 10 km transmission and LP01 MDM 10 km transmission are shown in Fig. 6(d).
Fig. 6. BER curves of (a) one-by-one B2B transmission, (b) one-by-one 10 km transmission, (c) MDM 10 km transmission. (d) Eye diagrams of LP01 only 10 km transmission and LP01 MDM 10 km transmission.
In order to evaluate the temperature stability of the proposed MDM system, we place the MDM link (FMFs and mode MUX/DEMUX) into a thermal cycling chamber (GWS, HLS-10). Then we measure the Q2 factors of 10 km MDM transmission at detected power of -17 dBm per mode from -20°C to 80°C. The results are illustrated in Fig. 7(a). We can see that the Q2-factor fluctuations are smaller than 3 dB for each of the 4 LP modes, proving the proposed MDM link has a wide operating temperature range. Finally, Q2 factors of 10 km MDM transmission are measured at 1530, 1550 and 1570 nm, respectively by using SFP + modules with corresponding wavelength. The measured Q2 factors at detected power of -17 dBm per mode are shown in Fig. 7(b). Compared to the performance at 1550 nm, the Q2 factors drop less than 1 dB at both 1530 and 1570 nm for each of the 4 LP modes, which demonstrates the wavelength insensitivity of the proposed MDM system over the C band.
Fig. 7. (a) Q2 factors for 4-LP-mode MDM transmission over -20°C to 80°C. (b) Q2 factors for 4-LP-mode MDM transmission at 1530 nm, 1550 nm and 1570 nm.
5. Summary
Real-time IM/DD MDM transmission has been highly desired for years to enhance capacity of optical communication systems. We propose a novel scheme based on multiple-ring-core FMF and degenerate-mode reception and demonstrate the first prototype system of DSP-free IM/DD MDM transmission with 4 LP modes over 10 km double-ring-core ultralow-modal-crosstalk FMF. The scheme is highly compatible with current IM/DD transceivers without any hardware or software modifications and can be extended to support more LP modes with improved design of multiple-ring-core FMF.
Funding
National Natural Science Foundation of China (61505002, 61605004, 61627814, 61690194, 61771024, 61901009); Science and Technology Planning Project of Shenzhen Municipality (20170307172513653, 20170817113844300, JCYJ 20170412153729436); Projects Foundation of YOFC (SKLD1708); Postdoctoral Research Foundation of China (2018M641086).
Disclosures
The authors declare no conflicts of interest.
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