1. Introduction

Space division multiplexing (SDM) using multi-core fibers (MCFs) or few mode fibers is an indispensable technology for >100 Tb/s capacity transmission [1–7], and multi-core erbium doped fiber amplifiers (MC-EDFA) for use in SDM transmission have been intensively studied. Significant progress has been made on the performance of core-pumped 7 core MC-EDFAs [8–11], and cladding-pumped 6, 7 or 12 core MC-EDFAs have been also investigated [12–14]. However, core pumping schemes of reported MC-EDFAs were based on conventional single-core components, and may not result in the expected cost advantage by integration. Cladding-pumped EDFAs are difficult to optimize and allow no independent control over the gain performance of each core. With the aim of exploiting the strong points of both pumping methods, we developed a free-space pumped 19-core MC-EDFA [15]. Core pumping and signal isolation was realized via shared optical components, and the feasibility of the MC-EDFA system for 19 x 80 Gb/s SDM transmission over 900 km in conjunction with a new low-crosstalk 19-core fiber was demonstrated. Here we report on the details of our 19-core transmission system, and in particular focus on the gain characteristic of the 19-core EDFA which was obtained with an anti-reflection (AR) coated, spatial multiplexer to reduce the impact of upstream terminal reflections [16]. Our latest transmission results show the system feasibility for 19 x 100 Gb/s SDM transmission over 1200 km.

2. 19-core MCF for long-haul SDM transmission and EDF

Figure 1(a) shows the cross-sectional view of the new 19-core fiber with core identification numbers. To exploit the cladding area effectively, both outer 12 cores (#8-#19) and inner 6 cores (#2-#7) were not arranged hexagonally but circularly with a 15-degree offset against each other. The inner-core circle radius of 39.0 μm was chosen to be slightly larger than the half of the outer-core circle radius, of 37.6 μm. The inner-core circle radius was optimized to obtain the same aggregated crosstalk (XT) for the center core (with 6 neighboring cores) and inner cores (with 5 neighboring cores each). This new arrangement reduced the aggregated XT by 4-5 dB for each core compared to the case of a 37.6 μm pitched hexagonal arrangement. The cladding diameter was 220 μm and each core was surrounded by an index trench as shown in Fig. 1(b). The average optical properties of outer, inner and center cores are shown in Table 1.The average attenuation loss and effective area (A_eff) at λ = 1550 nm are 0.285 dB/km and 85.0 μm², respectively. The average cable cut-off wavelength (λ_cc) was measured to be 1356 nm. One thing should be noted that though the averaged attenuation losses in Table 1 are close with each other, core-to-core difference of attenuation losses is fairly large and well exceeds 0.1 dB/km (standard deviation = 0.075 dB/km).

 

Fig. 1 19-core MCF structure. (a) Cross-sectional view, (b) index profile of trench-assisted cores. (c) Aggregate XT of 30 km spliced MCF link at 1550 nm.

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Table 1. Average MCF properties at 1550 nm.

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To simulate a realistic link with multiple splice connections, we divided the 30 km long MCF into four segments and rotated the cores before fusion splicing them together. The splice loss between each core-rotated MCF was about 0.3 dB on average. The aggregated XT from neighboring cores at λ = 1550 nm and bending radius of 55 mm was measured to be about −42 dB for each core as shown in Fig. 1(c). The total loss of the spliced 30 km link will be shown later in Table 2 together with the EDFA properties.

Table 2. Properties of SDM components and span for each core. Gain values are for 1550 nm.

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We fabricated a 19-core EDF with the same core arrangement as that of the 19-core MCF. Figure 2 shows the cross-sectional view of the EDF. The cladding diameter was also set to 220 μm. The mode field diameter at 1580 nm is estimated to be 6.5 μm. λ_cc for a 2 m sample ranged from 940 to 965 nm. The attenuation coefficients of the erbium-doped cores are within 6.0 to 6.6 dB/m at 1530 nm. Root-mean-square difference of MCF core positions and corresponding EDF core positions was about 1.5 μm. Splice losses between EDF and MCFs ranged from 0.5 to 1.3 dB for each core and at each end.

 

Fig. 2 19-core EDF structure

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3. 19-core WDM coupler, isolator, and inline 19-core EDFA prototype

Figure 3(a) shows the configuration of the 19-core WDM coupler. In the WDM coupler, the SDM signals from the input MCF are emitted into free space, collimated by a lens, and then passed through a free-space pump combiner, where the signals are combined with the pump beams at 980 nm. Both signal and pump beams are focused on the facet of an MCF by an achromatic lens, which has the same focal length for both 1550 nm and 980 nm beams. The MCF facets in the WDM coupler module are AR coated, but polished at an angle of 0° to facilitate beam coupling. Thus, these facets are estimated to have reflection of about −30 dB each. Signal and pump loss was measured to be less than 0.4 dB as summarized in Table 2. The WDM coupler is housed in a metal box of 70 × 70 × 100 mm³, to which a custom-designed holder is attached that positions the SMF collimators according to the core layout of the MCF.

 

Fig. 3 SDM devices with shared optics. (a) WDM coupler configuration, (b) isolator configuration, (c) outlook of the isolator, (d) inline 19-core EDFA and characterization setup.

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Figure 3(b) shows the configuration of the 19-core isolator. Signal beams are emitted into free space and pass through a custom-made polarization independent free-space isolator with large aperture before being coupled to the output MCF. Optics sharing enabled us to compactly assemble the isolator module as shown in Fig. 3(c). The MCF facets in the isolator module are AR-coated and polished at a 6° angle to suppress the reflections to less than −50 dB of the signal power.

Figure 3(d) shows the configuration of the inline 19-core EDFA (and SDM MUX / DEMUX for characterization). The WDM coupler, 7 m EDF and the isolator are connected with each other through fusion splices. Fusion splices of two MCFs without core rotation typically introduced less than 0.1 dB losses. 10 separate Fabry-Perot laser diodes (LDs) and 3 dB couplers are used to generate 19 SDM pump beams in total (1 or 2 pump beams for each LD), and it is envisaged that these lasers may be integrated into a single device in the future.

When preparing our initial report [15], we were aware that EDFA performance has been significantly degraded by reflections of the amplified spontaneous emission at the input side [16]. At that time however we could only prepare a SDM MUX (and DEMUX) without AR-coating for characterization. Subsequently, we were able to use AR-coated MCF, polished at an angle of 4° in the MUX/DEMUX with a return loss between 38 dB to 44 dB for each channel, and such fibers were then fusion spliced to the input and output of the MC-EDFA for characterization. Figure 4 summarizes the measured gain characteristics. Figures 4(a)–4(d) and Figs. 4(e)–4(h) show the gain and NF spectra with different pump injection powers and signal input powers, respectively. Note that the indicated input signal power values were measured at the SDM MUX input, and are different from the actual input power values to the EDFA by SDM MUX losses (0.7~1.1 dB). A comparison of Figs. 4(a) and 4(e) with previous results (Figs. 4(a) and 4(b) of ref [15].) clearly shows that both NF and gain were improved by using the AR coated MCF. A small signal gain of 19.6 to 23.3 dB and an NF of 6.0 dB to 7.0 dB were obtained at 1550 nm with maximum pumping of 400 to 480 mW for each core (Fig. 4(b)). The NF at 1530 nm was still degraded with maximum pump powers and −15 dBm signal powers (Fig. 4(f)), but improved when the signal input power was increased to −5 dBm (Fig. 4(h)). Figures 4(i) and 4(j) shows the gain saturation and NF dependence on the input power at 1550 nm. Moreover, Figs. 4(k)–4(m) show the gain and NF dependence on the pump power. Each figure shows that the gain profile at 1550 nm is similar to those of conventional EDFAs. Further performance improvements at 1530 nm are expected when reflections inside the WDM coupler are reduced by angled polishing.

 

Fig. 4 Gain and NF of inline 19-core EDFA. (a)-(d) gain spectra, (e)-(h) NF spectra, (i) gain saturation, (j) NF dependence on the input power, (k)-(m) dependence on the pump power. Colors allocated to each core indicate the pairing of pump-LD sharing.

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4. SDM recirculating transmission

Figure 5(a) shows the setup of the recirculating SDM transmission. We used 100 Gb/s PDM-QPSK signals (λ = 1550.92 nm) and split them into two paths. One path led to an acousto-optic modulator (AOM) gate and a recirculating loop, which consisted of an SDM channel under test. The signals in the other path were further split by a factor of 18 and used as dummy channels in the remaining SDM channels. The peak launch power for the recirculating channel was adjusted to be same as the launch power of the non-recirculating channels (−12 dBm), and the loop gain was adjusted to be 0 dB using a variable optical attenuator (VOA). It is believed that each recirculating signal experiences the same amount of XT as in the case of simultaneous recirculation. Hence, it was possible to simulate simultaneous recirculation of all the SDM channels. The signals were launched into the 19-core EDFA through the SDM MUX, amplified, and transmitted over the 30 km spliced MCF. After passing through the SDM link, the recirculating signals were demultiplexed back to the SMFs. We adjusted the MC-EDFA pump powers to the level that the net gains of all the SDM channels exceeded 0 dB and employed an additional single-core EDFA in the loop to compensate for losses of single-core devices, such as the loop AOM. A −10 dB branch of the recirculating signals was routed to a coherent receiver and recorded by a 23 GHz electrical bandwidth, 50 GSample/s digital storage oscilloscope. Three sets of 4-million bits were processed off-line with C++ digital signal processing for bit error ratio (BER) calculation. Signal processing included optical frontend correction, chromatic dispersion compensation, retiming, polarization demultiplexing with 11-tap constant modulus algorithm, and frequency/phase recovery with 4th-power phase estimation. Short-term linewidth of the lasers used for transmitter and local oscillator was specified to about 100 kHz.

 

Fig. 5 Recirculating SDM transmission (a) setup and (b) results.

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Figure 5(b) shows a BER dependence on the transmission distance for each SDM channel. A transmission distance of more than 1200 km was shown to be achievable for all the channels assuming the threshold of 20% forward error correction (BER = 2.7 × 10⁻²) [17].

5. Summary

We developed an SDM transmission system suitable for 19-channel SDM transmission over 1200 km with 100 Gb/s PDM-QPSK signals. This system was realized with various new SDM technologies. Among these are (1) a 19-core MCF with a new core arrangement resulting in an improved aggregated crosstalk of −42 dB over 30 km, (2) a 19-core EDF, (3) a novel free-space 19-core pumping scheme with optics sharing, and (4) a compact 19-channel polarization-independent isolator. An MC-EDFA was realized with gain ranging from 19.6 dB to 23.3 dB and NF ranging from 6.0 dB to 7.0 dB for each SDM channel at 1550 nm. We believe that these new technologies are indispensable for realizing cost-effective SDM optical networks in the future.