1. Introduction

Keeping up with ever increasing demand for bandwidth requires the deployment of one and two Petabit/s submarine cables. The main candidates for achieving these performance targets are two and four-core fiber links [1]. Multicore EDFAs are essential for next-generation submarine space-division-multiplexing (SDM) links. EDFAs are used in submarine links in the C-band even when broadband Raman amplification is utilized [2]. While two-core fiber links [3,4] using conventional single-core amplifiers connected to fan-in and fan-out devices are under consideration, the full potential of SDM requires the development of multicore fiber-integrated (MCF-integrated) amplifiers. Even though both core- and cladding-pumped multicore EDFAs with up to 32 cores have been demonstrated [5], fiber-integrated, low-crosstalk and low-loss solutions for core-pumping are not available. Because of considerations of size, performance, stability, and power handling, a fusion-spliced fiber-based solution is strongly preferred to free-space optical combiners [6]. Devices based on inscribed 3D waveguides, requiring some form of mechanical coupling at the pigtail and MCF ends [7], lack the environmental stability provided by a fusion splicing. For independent core-gain-control, it is necessary to pump the fiber core. Though in-fiber components can be fabricated using a side-polishing approach, at present such devices require cascading four devices to pump all four cores [8,9]. Minimizing the number of components required for pumping, and their associated MCF-MCF splice losses and overall package dimensions, is highly desirable for developing next-generation submarine SDM links. In addition to high pump-power conversion efficiency [10], maintaining low crosstalk in an MCF cable with incorporated EDFAs is critical for maximizing capacity and minimizing cost/capacity of the fiber link [11,12]. Here we present the design, fabrication, and characterization of a fully fiber-integrated and fusion spliced 4-core-WDM (4C-WDM) pump-signal combiner having -63 dB counter-propagating crosstalk, and 70 dB return loss for a core-pumped four-core-EDFA (4C-EDFA).

2. 4C-WDM combiner design and fabrication

The development of the 4C-WDM combiner opens a path to the compact 4C-EDFA shown in Fig. 1 which combines the functionality of four conventional single-core EDFAs or two single- stage amplifier pairs [13]. Use of only two diagonal pump-signal combining channels at each end of the 4C-EDF enables a 4C-EDFA with minimized crosstalk due to co-propagating diagonals and counter-propagating adjacent channels.

 

Fig. 1. Schematic of the 4C-EDFA, in a configuration with co-propagating diagonals and

 counter-propagating neighbors.

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In the design of the 4C-WDM combiner, we utilize the versatile vanishing-core (VC) approach to address diverse applications, including those with stringent submarine requirements [14–16]. The main advantage of the VC approach to coupler design is that excess loss can be reduced by bringing two waveguides close together in the tapering process before the cores are substantially coupled. This has been achieved in alternative approaches by either fiber etching or side polishing. The design concept of the 4C-WDM combiner is illustrated in Fig. 2. Signal crosstalk is minimized with maximal separation between the signal channels in a configuration with pump channels on the inside and signal channels on the outside. The spacing between MCF cores must be substantially larger than the spacing between waveguides carrying the signal and pump. The VC fiber design must, therefore, allow a large draw ratio at both the signal and pump wavelengths. Since the first planned application is a weakly coupled 4C-MCF with core spacing of ∼ 40 µm, we have designed and fabricated a VC fiber allowing for a draw ratio of 9 to 10 with minimized coupling loss to a weakly coupled four-core MCF. This requires that the initial spacing between the waveguides carrying signal at 1550 nm at the untapered end is 360–400 µm, as shown on the left of Fig. 2(c). The large draw ratio allows for freedom of choice in the initial spacing between the signal and pump waveguides to optimize coupling at the tapered end with the reduced diameter shown on the right side of Fig. 2(c). The length of the coupling section, shown in Fig. 2(c), determines the coupling wavelength. Both the coupling length and signal-pump spacing must be optimized in order to achieve minimal signal and pump channel loss over the C-band and at 980 nm. All the components of the device, including the VC fibers and the enclosure are made of silica-based glasses, allowing for conventional fusion splicing for fiber integration. The fabricated 4C-WDM device with four pairs of pump-signal channels, was fusion-spliced to 5 meters of transmission 4C-MCF. The fiber has four homogeneous cores with ∼ 10-µm MFD at 1550 nm, core spacing of 42 µm and core positioning accuracy of better than 0.1 µm. Using the end-view mode of the Fujikura FSM-100P + fusion splicer, pump and signal channels were oriented as shown in Fig. 3(a) with 4C-MCF cores aligned with the “pump” channels of the combiner. After splicing, the combiner was packaged using an extensively tested and submersible-proven, compact package design [16], as shown in Fig. 3(b). The packaged fanouts, utilizing a similar VC-fiber technique, have been tested to meet watt-level power handling, as well as temperature, humidity, and other environmental submarine application requirements. The other end of the 4C-MCF was fusion-spliced to a fanout device. This configuration allowed for the verification of the concept, channel-by-channel optimization and characterization of insertion and polarization-dependent losses, as well as return loss (RL) and crosstalk (XT) measurements.

 

Fig. 2. 4C-WDM device concept. (a, b) Microscope images of the cross sections of the combiner and MCF. s1 – s2 and p1 – p4 correspond to signal and pump channels respectively. (c) Schematic of the side view. The length of the device is 36 mm, including a 20 mm taper and a 4 mm coupling section. Core pitches are 42 and 374 and pump-signal spacings are 14 and 130 µm at the tapered and untampered ends, respectively.

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Fig. 3. (a) End-view alignment mode pictures prior to 4C-WDM combiner-MCF splice. (b) Fusion-spliced packaged 4C-WDM.

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3. Performance characterization

Measurements of the 4C-WDM fabricated with a draw ratio of 9.1, initial signal channel separation of 374 µm, pump-signal separation of 130 µm, and coupling length of 4 mm are shown in Fig. 4. For the insertion loss measurements shown in Fig. 4(a), a broadband light source was connected one-by-one to the “pump” and “signal” single-core combiner pigtails and an optical spectrum analyzer was connected via a fanout device to the corresponding MCF core aligned with the pump channel. As seen in Fig. 4(a), the dips of all pump channels at ∼ 1550 nm are within a range of +/- 8 nm, which is much smaller than the width of the signal channel transmission band. This close match of spectral positions allows for low C-band loss in all 4 channels, as seen in Fig. 4(b). Using the same combiner-fanout configuration and a narrow-band light source, the XT was measured at 980 nm for the pump channels with the results shown in Fig. 4(c). The average XT in the C-band for signal channels is shown in Fig.4d. Return loss of better than 70 dB for all the combiner channels was measured with the use of a LUNA 6415 OFDR. Since the most likely EDFA configuration of the fabricated 4C-WDM combiner is the counter-propagating geometry for adjacent cores, as illustrated in Fig. 1, we conducted additional high-resolution tunable laser measurements of the signal and pump channels corresponding to diagonal cores 2 and 4. The insertion loss, PDL, and XT, in the C-band for these channels are shown in Fig. 5. The insertion losses at 980 nm are below 0.4 dB for both pump channels. The C-band counter-propagating XT for pairs of adjacent channels, 2-1, 2-3, 4-1, and 4-3 was measured to be below -70 dB.

 

Fig. 4. Performance of four pump and four signal channels of the 4C-WDM fusion spliced to 5 meters of a 4C-MCF and a fanout. (a) Total insertion loss, (b) C-band signal channel loss, (c) 980 nm XT, (d) C-band average XT.

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Fig. 5. Performance of the two diagonal signal channels, 2 and 4, of the 4C-WDM combiner.

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For the pumping configuration shown in Fig. 1, the combiner may be co-packaged with a fanout device within a compact enclosure with dimensions of 58 × 25 × 5 mm, as shown in Fig. 6. The size of the enclosure shown is limited by the long-term reliability of the bent pigtail fibers. The package constructed provides a failure probability of less than 1 ppm over a lifetime of 25 years [17]. Since there is already access to single-core fibers between the combiner and the fanout, as shown inside the dashed boxes in Fig. 1, standard EDFA components such as gain-flattening filters and isolators can be incorporated into the package with the same footprint, though with increased height. In addition, instead of four-core transmission MCFs, as shown in Fig. 1, four single-core or two dual-core transmission fibers may be used with this 4C-EDFA configuration.

 

Fig. 6. Four-core combiner co-packaged with a fanout as schematically illustrated in the dashed boxes shown in Fig. 1.

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4. Conclusion

A fiber-integrated multicore pump-signal combiner is fabricated with 0.4 dB signal insertion loss for two diagonal channels over the C-band, 0.4 dB pump loss at 980 nm, -63 dB counter-propagating crosstalk, and 70 dB return loss. The device is packaged in a compact, robust, submersible-proved package. This 4C-WDM combiner makes possible bi-directional, ultra-low XT multicore EDFAs.