Cladding Pumped Multi-core Fiber Amplifiers for Space Division Multiplexing Sophie LaRochelle Center for Optics, Photonics and Lasers (COPL), Department of Electrical and Computer Engineering, Université Laval Québec, Canada [email protected] Cang Jin Specialty Photonics Division OFS Fitel Somerset, NJ, USA [email protected] Abstract—We review the challenges of cladding pumped multi-core optical fiber amplifiers for application to space division multiplexing (SDM). Through numerical simulations, we investigate two fiber designs: the first one with a uniform cladding and the second one with an annular cladding to guide the pump. We compare the multi-core amplifier gain, noise figure and pumping efficiency in a WDM scenario. We present the fabricated fibers and summarize experimentally measured performance that shows gain>20 dB and NF<6 dB over the whole C-band. Finally, we examine scalability of the annular cladding design. Keywords—Erbium-doped fiber amplifier (EDFA), optical amplifier, space division multiplexing (SDM), multi-core amplifier, spatially integrated amplifier, optical fiber, optical communications I. INTRODUCTION Nowadays, optical fiber communication systems are facing an enormous challenge as the needed capacity increases exponentially putting pressure not only on long-haul systems but also on access networks and data communication links. This high bandwidth demand is driven by the unstoppable thirst for high definition video streaming, cloud computing and various big data applications processed in data centers. Wavelength division multiplexing (WDM) and polarization division multiplexing (PDM), as well as advanced modulation formats, were introduced to significantly improve single mode fiber (SMF) capacity. Despite these efforts, it is now forecasted that this capacity will saturate in the forthcoming years [1, 2]. The capacity limits of SMF, estimated from Shannon’s information theory [3], is approximately 35-70 Tb/s for a waveband of 1530 nm to 1625 nm, i.e. C-band plus L-band. The calculation takes into account a spectral efficiency limit of 3-6 bit/Hz due to nonlinear effects [4]. Consequently, there is a growing gap between the data transmission demand and the capacity offered by state-of-the-art technologies, which motivates the development of new multiplexing strategies [1]. Inspired by wireless communications, space division multiplexing (SDM) is now making its way in optical fiber communications as a means to scale its capacity. SDM is implemented using either multi-core fiber (MCF) [5], fewmode fiber (FMF) [6], and even multi-core few-mode fiber [7]. In this paper, we consider single mode multi-core fibers that XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Charles Matte-Breton Center for Optics, Photonics and Lasers (COPL), Department of Electrical and Computer Engineering, Université Laval Québec, Canada [email protected] Younès Messaddeq Center for Optics, Photonics and Lasers (COPL), Department of Physics, Optics and Engineering Physics, Université Laval Québec, Canada [email protected] straightforwardly multiply the capacity by the number of cores provided that inter-core cross-talk is negligible. To this end, transmission fibers are usually designed with a core-to-core spacing of tens of m and dissimilar cores to reduce coupling. Another approach is to use an index trench to further confine the modes. SDM has thus been successfully demonstrated in combination with WDM, PDM and advanced modulation formats to produce impressive aggregate transmission rate exceeding 10 Pb/s in a single fiber strand [8]. However, most transmission results were obtained over short distances corresponding to one span because of the lack of suitable multi-core erbium-doped fiber amplifiers (MC-EDFAs) or by demultiplexing spatial channels before amplification [9]. An efficient in-line MC-EDFA is undoubtedly critically needed in order for SDM to find application in future optical network. The first MC-EDFA, demonstrated by Abedin et al., consisted of seven cores each pumped by a 980 nm laser [10]. Following this work, cladding pumping was introduced in order to reduce the complexity of the amplifier implementation. Figure 1 shows pumping configurations with in-core pumping, edge-coupled cladding pumping and sidecoupled cladding pumping. In Fig. 1a, the multiple lasers could be replaced by a single pump laser with a power splitter to share the pump between all branches. However, only cladding pumping allows the use of a low-cost high-power multimode laser diode, and only side-coupled cladding pumping allow inline amplification without spatial demultiplexers/multiplexers at the MC-EDFA input/output. Since these components typically introduce extra cross-talk and degrade noise figure, side-coupled cladding pumping therefore appears as the preferred solution. Fig. 1 Pumping configurations of multi-core erbium-doped fiber amplifiers showing a) in-core pumping, b) edge-coupled cladding pumping, and c) sidecoupled cladding pumping. A well-known limitation of cladding pumped amplifiers is the low pumping efficiency resulting from the small pump mode field overlap with the doped cores. Many studies on cladding pumped amplifiers are therefore presently directed at improving pump utilization while simultaneously optimizing amplifier configuration to reduce complexity and related devices costs [12]. Low pump absorption results from the small aggregate core area with respect to the large pump cladding area (assuming a uniformly distributed pump power). Fig. 2a shows such a configuration with a uniform cladding and sixdoped cores. In Fig. 2b, we show the proposed configuration that has an annular cladding to confine the pump in a smaller area and in which signal cores are also enlarged. Alternate proposal to improve pump efficiency include, for example, the introduction of erbium-ytterbium which does however limit the gain in the short wavelength side of the C-band [13] spontaneous emission and assuming no input signal, the inversion of the erbium doped fiber, , can be estimated from Pp Aclad N2 Pp Aclad In this paper, we discuss MC-EDFA designs optimized through numerical simulations and compare the performance of three cladding configurations. We summarize previously published characterization results of gain and noise figure (NF) across the C-band. Finally, we discuss the scalability in terms of number of cores in the proposed annular cladding configuration. II. MODELING AND SIMULATIONS The MC-EDFA, with co-propagation cladding pumping at 980 nm, is modeled using standard rate and propagation equations as described in [11, 14, 16]. Signal cores are assumed to be independent and, in comparison to a single core amplifier, the only modification is the pump propagation equation: dPp z N1 dz with p a,p p Pp z N core Acore . Aclad In (1), Pp is the pump power, z the propagation distance, N1 the erbium ion population of the ground state, and a,p the absorption cross-section at 980 nm. The pump overlap factor, p, takes into account the absorption by all cores and, therefore, is function of the number of cores Ncore, the area of the individual cores Acore and the area of the cladding, Aclad, in which the pump is confined. When neglecting amplified p a,p where N2 is the erbium ion population of the metastable level, is its lifetime, and h p the energy of a pump photon. With the values shown in Table 1 and a uniform cladding having a 110 m diameter, requires Pp 1.0 W while requires Pp 8.5 W. Both (1) and (2) therefore show the importance to reduce Aclad and increase Acore in order to limit pump power requirements. TABLE I. FIBER AND SIMULATIONS PARAMETERS Description Fig. 2 Multi-core amplifier, with six-doped cores, cladding pumped in a) a standard uniform double cladding and b) an annular cladding with a depressed refractive index central region (inner cladding) and enlarged signal cores. The outer cladding is made of a low index polymer. h Core-to-core pitch Number of cores Refractive index of the pump cladding (silica) Refractive index of the double cladding (polymer) Numerical aperture of the pump waveguide Pump absorption cross-section Lifetime of metastable level Pump wavelength Central signal wavelength Number of channels Frequency spacing of channels Signal power Total signal power Symbol Value Ncore nclad 40 m 6 1.444 npoly 1.37 NApump 0.46 a,p p s N s Ps Ps,tot 2.18 10-25 m2 10 ms 980 nm 1550 nm 8 500 GHz -12 dBm -3 dBm A. Signal core optimization Table 1 shows the parameters used for the MC-EDFA optimization. The geometry is a double cladding fiber with six cores placed along a ring with a 40 m pitch. The silica cladding is surrounded by a low index polymer resulting in a pump waveguide with 0.46 numerical aperture (NApump). The fiber parameters that need to be determined are the core radius (rcore), and numerical aperture (NAcore), as well as the erbium ion concentration ( ) and fiber length (L). The erbium ion doping is assumed to be uniform in the core and the absorption and emission cross-sections are presented in Fig. 3. The MCEDFA gain is calculated using the WDM scenario detailed in Table 1: eight CW wavelength channels, spaced by 500 GHz and with input power of -12 dBm/channels. Figure 4 shows the average gain of the wavelength channels as a function of rcore and NAcore. The calculations are done for a = 2.5x1025 m-3, L=2 m and Pp=15 W. The single-mode region is limited by the dotted red line labeled 2.405, which is the cut-off condition of the LP11 mode. The star corresponds to the chosen design with rcore=5 m and NAcore=0.105. Fig. 5 shows the amplifier gain as a function of and L where it can be shown that shorter fibers require higher erbium concentrations to maximize gain. A concentration of = 2.5x1025 m-3 was chosen to keep fiber length short and minimize cross-talk in future amplifier designs that would have a large number of cores and reduced core-tocore pitch. Fig. 3 Absorption and emission cross-sections in the C-band of an erbiumdoped fiber used for the MC-EDFA simulations. Fig. 5 Simulated average gain of the eight channels plotted for different fiber lengths as a function of the erbium ion concentration, for rcore=5 m, NAcore=0.105 and Pp=15 W. Fig. 4 Simulated average gain of the eight channels as a function of core radius and core numerical aperture for a fiber length of 2 m, an erbium ion concentration of 2.5x1025 m-3, and an injected pump power in the cladding of 15 W. B. Cladding design In this work, we consider three different fibers: Fiber 1 with a uniform cladding (Fig. 1a), Fiber 2 that has an inner cladding of a lower refractive index than the silica cladding and Fiber 3 that has air as the inner cladding. As represented in Fig. 2b, both Fiber 2 and Fiber 3 create an annular cladding that increases p. A key parameter in the design of these annular cladding is the distance, dmin, between the core boundary and the silica cladding-to-polymer interface (outer cladding in Fig. 2). If the cores are too close to this interface, there can be confinement or leakage loss to the polymer coating. This problem was investigated in [15] for multi-core transmission fibers. In the present case, since the doped fiber is only a few tens of meters long and the outer polymer coating has a lower index than the silica cladding, this loss is expected to be negligible. Nonetheless, the proximity of the cladding can also cause the waveguide to become birefringent. Figure 6 shows the birefringence of the fundamental mode (calculated using COMSOL) as a function of the core position in a 110 m diameter silica cladding of for Fiber 1, and with an inner cladding diameter of 50 m for Fiber 2 and Fiber 3. Considering that fabrication constraints limit dmin to 10 m, the calculated birefringence is lower than 1 10-6 in all cases. Fig. 6 Fiber birefringence calculated as a function of the core position in the silica cladding assuming a uniform cladding (Fiber 1, black), a solid glass annular cladding (Fiber 2, red) and an annular cladding with a hollow center (Fiber 3, blue). The x-axis is the radial distance from the fiber axis. TABLE II. Description Core radius Core numerical aperture Pump cladding diameter Core-cladding distance Inner cladding diameter Erbium ion concentration Erbium doped fiber length FIBER DESIGN symbol value rcore NAcore dclad dmin dinner 5 m 0.105 110 m 10 m 50 m 2.5x1025 m-3 2m L The gain and noise figure (NF) of the amplifier were calculated using the design summarized in Table 2 and the initial parameters indicated in Table 1. The pump power is assumed to be uniformly distributed in the whole cladding area for Fiber 1, and limited to the annular cladding region for Fiber 2 and Fiber 3. The results, shown in Figure 7, indicate that the average signal gain and NF of the eight WDM channels are respectively >24 dB and <3.5 dB when a pump power greater than 8.5 W is injected in the cladding. Fig. 8 a) Stack assembly for the fabrication of Fiber 2 and scanning electron microscope image of b) Fiber 1, b) Fiber 2, and c) Fiber 3. 4 nm and covering the C-band. Isolators were placed at the fiber input and output. One core was characterized at the time using translation stages to butt coupled the comb source at the input and send the output to an optical spectrum analyzer. Due to the large pump power remaining after the 2 m amplifier length, a short section of the cladding, near the fiber output, was placed in an etching paste to extract the pump from the cladding and to send it to a pump dump. The pump input and output coupling schemes resulted in short sections of the multicore erbium-doped fibers being left unpumped, which most probably degraded the performance. Fig. 7 Simulated a) gain and b) noise figure. Values are the average of the eight channels as a function of the injected pump power in the cladding for Fiber 1 (uniform cladding, red), as well as Fiber 2 and Fiber 3 (annular cladding, black). III. EXPERIMENTAL RESULTS For each multi-core erbium doped fibers, three preforms were fabricated by modified chemical vapor deposition (MCVD). The erbium ions were inserted by solution doping. Etching of the cladding was then performed and the resulting thinned rods were assembled in a stack (Fig. 8a) that was later drawn into an optical fiber. Before assembly, each preform was cut in half to make two cores. Scanning electron microscope images of Fiber 1, Fiber 2 and Fiber 3 are shown in Fig. 8b-d. Fiber characterization showed a good correspondence with design values, except for the inner cladding diameter that was slightly larger at 60 m. It is apparent in Fig. 8c and Fig. 8d that the cores are closer to the inner cladding boundary. Also, the erbium ion concentration was slightly higher at 2.8x1025 m3. The pump overlap was p=0.05 for Fiber 1 and p=0.07 for Fiber 3. For Fiber 2, p is difficult to evaluate as some pump power is leaking into the inner cladding. Following fiber fabrication, MC-EDFAs were assembled and characterized by injecting the pump light in the cladding through a tapered coreless fiber wrapped around the cladding [17]. The gain and noise figure were measured by injecting eight CW wavelength channels spaced by approximately by Fig. 9 Gain and noise figure measurement for the eight channels in the cores of the three fibers. Measurements are performed one core at the time and the data shows the average gain (symbol) and the core-to-core variations (minimum and maximum values defined by the error bars). Figure 9 summarizes the characterization results while the detailed measurements can be found in [18,19]. For characterization of Fiber 1, the signal power was -11 dBm/channel (total input power of -2 dBm), while for Fiber 2 and 3 it was – 12 dBm/channel (-3 dBm total) (due to experimental constraints as these experiments were performed several months apart). The pump power was also slightly different at 13 W of pump power injected in the cladding for Fiber 1 and 15 watt for Fiber 2 and Fiber 3. In Figure 9, the data indicate the average gain and NF of the six cores of each fiber, while the error bars indicate the core-to-core variations (minimum to maximum values). Results show a 3 dB gain flatness over the C-band with less than 2 dB core-to-core gain variations. The core-to-core variation of the NF is less than 1 dB and ranges from 5.5 dB to 3.5 dB over the C-band. IV. DISCUSSION AND CONCLUSION Multi-core cladding-pumped erbium doped fibers can be designed to meet system requirements. Cladding pumping reduces the component count and allow the use of low-cost high power laser source. However, efficient use of pump power requires careful cladding and core design to maximize pump overlap. In core-pumped erbium-doped fiber designs, the core area is typically reduced to increase signal and pump overlap with the doped region. However, in the present case of cladding pumping, the signal core area should be enlarged as much as possible while remaining in the single mode regime. Large cores contribute to pumping absorption and reduced amplifier saturation by lowering signal intensity. Erbium doping could further be placed at the edge of the core, or even in the cladding in order to reduce saturation [20, 21]. Annular cladding is a solution that reduces cladding area while managing cross-talk since each cores only has two neighbors. Although in this design we eliminate the central core, that is present in many designs, such an annular cladding with a 170 m external diameter would allow the placement of 16 cores along a ring with core-to-core pitch close to 30 m. With a 25 m annular cladding thickness, the pump overlap factor would be approximately p=0.1. Each of these cores would have only two next nearest neighbor. The worst case cross-talk between the cores can be calculated using coupled-mode equations describing coupling between identical cores. With identical cores and 30 m core pitch, there could be significant cross talk. However, the coupling is easily reduced using dissimilar cores without penalties to the amplification properties. Finally, the amplifier shows good NF and gain across the C-band. 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