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Abstract
Weakly coupled-mode division multiplexing (MDM) over few-mode fibers (FMF) for short-reach transmission has attracted great interest, which can avoid multiple-input-multiple-output digital signal processing (MIMO-DSP) by greatly suppressing modal crosstalk. In this paper, step-index FMF supporting 4 linearity polarization (LP) modes for MIMO-free transmission is designed and fabricated for the first time, to our knowledge. Modal crosstalk of the fiber is suppressed by increasing the mode effective refractive index differences. The same fabrication method as standard single-mode fiber is adopted so that it is practical and cost-effective. The mode multiplexer/demultiplexer (MUX/DEMUX) consists of cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). The mode MUX and DEMUX achieve very low modal crosstalk not only for the multiplexing/demultiplexing but also for the coupling to/from the FMF. Based on the fabricated FMF and mode MUX/DEMUX, we successfully demonstrate the first simultaneous 4-modes (LP01, LP11, LP21 & LP31) 10-km FMF transmission with 10-Gb/s intensity modulation and MIMO-free direct detection (IM/DD). The modal crosstalk of the whole transmission link is successfully suppressed to less than −16.5 dB. The experimental results indicate that FMF with simple step-index structure supporting 4 weakly-coupled modes is feasible.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, mode division multiplexing (MDM) based on few-mode fiber (FMF) has been widely considered as a promising candidate to overcome the capacity crunch of conventional single-mode fiber (SMF)-based wavelength-division multiplexed (WDM) systems and networks [1-2]. For long-haul MDM transmission systems, coherent detection and multi-input-multi-output digital signal processing (MIMO-DSP) are always required owing to the inevitable modal crosstalk during FMF transmission [3–5]. However, this approach has very high cost and complexity and is not suitable for short-reach transmission scenarios such as passive optical network (PON), datacenter network and other optical interconnections. Hence, weakly-coupled MDM transmission scheme is proposed [6-7], in which modal crosstalk is greatly suppressed and signals in different modes can be transmitted and received independently. In this way, the cost and complexity can be significantly reduced and conventional simple intensity modulation (IM) and MIMO-free direct detection (DD) can be adopted. Different approaches to suppress the modal crosstalk have been proposed. For examples, upstream signals in different modes are transmitted at different time slots to avoid modal crosstalk in a mode-enhanced time division multiplexing PON (TDM-PON) [8]. Bidirectional mode group-interleaved transmission to suppress the modal crosstalk is demonstrated over 2 km of conventional graded-index OM1 multimode fiber with IM/DD [9]. Meanwhile, new fibers such as elliptical-core and rectangular-core FMFs are proposed to suppress the modal crosstalk of degenerate modes [10-11]. The suppression of modal crosstalk for mode multiplexer/demultiplexers (MUX/DEMUX) has been proposed, too [12–14]. In our previous work, we have optimized the design of FMF and mode MUX/DEMUX to experimentally demonstrate weakly-coupled MDM transmission over 55-km 2-mode FMF [15] and bidirectional weakly-coupled MDM-PON transmission over 10-km FMF [16]. Moreover, 7.4-km MDM transmission with 3 independent linearity polarization (LP) modes is demonstrated [17]. However, in these works only 2 or 3 modes are utilized or the transmission distance is confined to less than 10 km. It’s highly desired to realize weakly-coupled MDM transmission with more modes and longer FMF transmission reach.
In this paper, we investigate for the first time the realization of step-index FMF supporting 4 modes for MIMO-free transmission. The same fabrication method as SMF is adopted, which means that the fiber is easy to fabricate with current commercial equipment. The modal crosstalk of the fiber is suppressed by increasing the mode effective refractive index differences (MERIDs) for all the 4 linearity polarization (LP) modes. The mode MUX/DEMUX consists of cascaded mode-selective couplers (MSCs), which are fabricated by fusing and tapering the propose FMF with SMF to achieve maximum compatibility and greatly suppress the modal crosstalk. Based on the weakly-coupled FMF and mode MUX and DEMUX, we experimentally demonstrate the first simultaneous 4-mode 10-km FMF transmission with 10-Gb/s on-off keying (OOK) modulation and MIMO-free direct detection. The experimental results indicate that modal crosstalk is successfully suppressed to less than −16.5 dB and weakly-coupled FMF with simple step-index structure supporting 4 modes is feasible.
2. Design and fabrication of FMF and mode MUX/DEMUX
Generally, all the modes in weakly-guided circular core FMF can be divided into weakly-coupled groups consisting of one circular-symmetric LP mode or two degenerate LP modes. The two degenerate modes have similar propagation constants and their spatial orientation will rotate randomly during transmission because of imperfect fabrication or external perturbation. There are two typical solutions for the demultiplexing of degenerate modes. One is designing non-circularly-symmetric fiber to break the degeneracy and the other is adopting mode-group demultiplexer in which degenerate modes can be simultaneously demultiplexed and detected. For the proposed step-index FMF, we prefer the latter solution and all modes in the same group will be treated as a whole channel.
It has been proved that modal crosstalk is negatively correlated with the effective refractive index differences among modes [6]. Therefore, the mode effective refractive index differences (MERIDs) among the 4 modes should be large enough to avoid severe modal crosstalk. Typically, the MERID threshold is selected to be larger than 1 × 10−3 in this paper [6]. Figure 1(a) shows the relationship between V and neff for the first 6 LP modes in the FMF, in which V stands for normalized frequency while neff is mode effective refractive index. We can see that the MERID between LP21 and LP02 is always small and there may be large modal crosstalk between them. So we only choose LP21 as the MDM channel while the LP02 will not be used. The situation is similar for the LP31 and LP12 and we choose LP31 instead of LP12.
Fig. 1 (a) The relationship between V and neff for the first 6 modes in FMF, (b) The relationship between Δn and Δneff at the core radius of 8 μm
Figure 1(b) shows the relationship between Δn and Δneff at the core radius of 8-μm, where Δn is the refractive index difference between the core and cladding while Δneff is the MERID between two modes. We can see that Δneff increases with Δn, so a large value of Δn is preferred. However, when it is too large, the difficulty of fiber fabrication will be greatly raised. Here the value of Δn is selected to be 8‰.
Then the core radius r of the fiber is analyzed. In order to support LP31 mode transmission in the fiber, V must be larger than 5.1356, so r must be larger than 6.9 μm correspondingly. According to the results in Fig. 1, the MERIDs among LP01, LP11, LP21 and LP31 modes can be calculated for a Δn of 8‰, and the results are shown in Fig. 2(a). We can see that although Δneff decreases with the increase of r among all the modes, the values of Δneff always keep larger than 1 × 10−3. On the other hand, because the mode MUX and DEMUX in the following transmission experiment are fabricated by fusing and tapering the fabricated FMF, if the value of the core radius is close to the cut-off radius of LP31 mode, the mode MUX and DEMUX will suffer a large insertion loss for LP31 mode. Therefore, a large value of core radius is preferred. However, the bending loss grows rapidly with the increase of the core radius, especially for LP31 mode. Therefore, there will be a trade-off for the core radius here. In our design, the core radius is set to 8-μm and the corresponding V is 5.92. The fiber actually supports 6-mode transmission (LP01, LP11, LP21, LP02, LP31 & LP12), but only 4 of the 6 modes (LP01, LP11, LP21 & LP31) are treated as MDM channels. The effective refractive indexes of them are 1.4543, 1.4524, 1.4498 and 1.4468, respectively. The minimum MERID is 1.9 × 10−3, which is much larger than 1 × 10−3. Figure 2(b) shows the simulated and measured refractive index distribution. The refractive index distribution is measured by a fiber refractive index analyzer (Interfiber Analysis IFA-100). The typical parameters of the fabricated fiber can be found in Table 1. We can see that the refractive index distribution and the measured parameters agree with the design.
Figure 2 (a) The relationship between r and Δneff for the 4 modes used as MDM channels, (b) The measured and simulated refractive index distribution of the designed FMF
Table 1. Typical parameters of the fabricated FMF
Due to easy fabrication, high compatibility with optical fiber and low modal crosstalk, MSC can be used to implement weakly-coupled mode MUX and DEMUX [12–14]. According to the coupling mode theory [18], when phase matching condition is satisfied (Δneff = 0), the two modes will periodically exchange energy with a coupling length equal to Lc = π/2c, where c is the coupling coefficient of the coupled fibers. By controlling the length of tapering, we can realize MSCs converting fundamental mode in SMF to a specific mode in FMF. As shown in Fig. 1, the effective refractive indexes among modes are different, so the FMF must be pre-tapered to satisfy the phase matching condition. In this work, the FMFs are pre-tapered by 0, 2.3, 5.0 and 8.2 mm for LP01, LP11, LP21 and LP31, respectively. The structure of mode DEMUX is the same as the mode MUX, but it operates in a reverse direction. The photograph of the mode MUX and DEMUX is shown in Fig. 3, in which the MSCs are figured out by the red circles.
3. Experimental setup and results
The experimental setup of 4-mode MDM transmission is shown in Fig. 4. At the transmitter, 10-Gb/s pseudo random binary sequence (PRBS) with a period of 215-1 is generated by pulse pattern generator of an electric bit-error-rate tester (BERT). The laser diode (LD) has a wavelength of 1550-nm. The optical intensity modulator (IM) is driven by the PRBS code to generate OOK signal. In this experiment, 10-Gb/s OOK modulation is adopted because it is the most common modulation format in short-reach transmission and its direct detection is always performed without any DSP. It is more suitable to evaluate the influence of modal crosstalk in the FMF and mode MUX/DEMUX because it’s more sensitive to transmission impairments than advanced modulation formats such as polarization-division-multiplexed differential quadrature phase shift keying (DQPSK) with coherent detection and powerful DSP. Then the signal is amplified by an erbium doped fiber amplifier (EDFA). An optical coupler (OC) is followed to split the signal into four branches. Delay lines are used to eliminate correlation among the branches while variable optical attenuators (VOAs) are used to adjust the launched optical power. Then signals from all the branches are converted to corresponding modes and multiplexed by the mode MUX. After 10-km FMF transmission, the mode DEMUX converts and demultiplexers signals in all the modes into fundamental mode of different SMFs. Then photodiodes (PD) are applied to detect signals. The BER is calculated by the receiver of the BERT.
We firstly measure the modal crosstalk by launching continuous-wave light into each mode one by one with a fixed input power of 0 dBm and measuring the power at the output port of each mode. The results are shown in Table 2. The modal crosstalk of all the 4 modes is less than −19.4 dB for B2B, while it is less than −16.5 dB after 10-km FMF transmission.
Table 2. The measured modal crosstalk among LP01, LP11, LP21 and LP31 for B2B and 10 km transmission
Table 3 shows the insertion losses of the 4 modes for a pair of mode MUX/DEMUX and 10 km FMF. We can see that the MDL mainly comes from the mode MUX/DEMUX, which is attributed to the fabrication method and cascading structure. The mode-selective couplers are fabricated by fusing and tapering the FMF with SMF, during which the fiber core of FMF will become smaller and generate different attenuations for all the modes. Typically, higher-order LP modes will suffer larger insertion loss. Moreover, for the cascading structure, signals in different modes will travel through different quantities of tapering areas, which further leads to larger MDL.
Table 3. Insertion Losses for the Mode MUX, DEMUX and FMF
The impulse responses of each mode are measured and the results are shown in Fig. 5. Subfigures in a row show the impulse responses of the same exciting mode at the output ports of each mode, while subfigures in a column show the impulse responses at the same output port for each exciting mode. Although the Δneff between LP02 and LP21 is less than the threshold of 10−3, no significant modal crosstalk is observed for all these 4 modes. It should be pointed out that for the excited LP21 mode, the impulse response measured at the output of LP21 mode contains a small pulse of crosstalk from LP02 mode (at the time of about 128 ns), as shown in subfigure (k). This result indicates that although the Δneff between LP02 and LP21 is less than the threshold of 10−3, the unused LP02 mode has only very slight influence to its adjacent LP21 mode.
Figure 6 shows the mode patterns before and after FMF transmission at point A and B in Fig. 4. Each mode is excited one by one to measure their mode patterns respectively and their hybrid mode patterns are measured by exciting them simultaneously. We can see that signals successfully preserve their mode patterns after 10-km FMF transmission. Figure 7 shows the eyediagrams of 4-mode simultaneous launching for back-to-back (B2B) and after 10-km FMF transmission. The results indicate that the transmission reach can be easily extended if fiber with longer length is available.
The BER performance is shown in Fig. 8. All the 4 modes are launched simultaneously into the FMF and the BER performance for 200-m and 10-km FMF transmission are compared. For a given BER of 10−3, the B2B receiver sensitivity is about −29.3 dBm, while they are about −28.1, −27.0, −25.0 and −23.2 dBm for LP01, LP11, LP21 and LP31 mode after 10-km transmission, respectively. The B2B performance is measured as a reference. The power penalties of the 4-mode transmission are about 1.2, 2.3, 4.3 and 6.1 dB, respectively. We can see that the BER performances agree with the eyediagrams. The performance of 10-km transmission gets slightly worsen compared with that of 200-m transmission and the differences of the receiver sensitivity are less than 0.5 dB for all the 4 modes. This means that the transmission impairment for the 10-km FMF is very low and the transmission.
4. Conclusions and discussions
FMF with simple step-index structure is designed and fabricated, whose parameters are optimized by increasing the minimum MERID for 4 weakly-coupled LP modes. Mode MUX/DEMUX with low modal crosstalk is fabricated based on cascaded mode-selective couplers. Then we experimentally demonstrate 4-mode weakly-coupled MDM transmission over 10-km FMF with simple direct detection instead of complex coherent detection and MIMO DSP. The experimental results show that the modal-crosstalk among the 4 modes of the FMF is very low. Limited by available fiber length, the transmission performance is only investigated for 10-km FMF in this paper. Further investigation for longer fiber reach will be performed by establishing re-circulating loop experiment setup.
Funding
973 Program (No. 2014CB340105 and No. 2014CB340101); National Natural Science Foundation of China (NSFC) (No. 61771024, 61627814, 61505002 and 61690194); Fundamental Research Project of Shenzhen Science and Technology Foundation (JCYJ 20170412153729436, 20170307172513653).
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