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Abstract
A cladding-pumped 4-core erbium-doped fiber (4C-EDF) with a pedestal structure has been firstly, to the best of our knowledge, proposed and fabricated for space division multiplexing (SDM) amplification. The numerical simulation shows that the index-raised pedestal around the fiber core can improve power conversion efficiency (PCE) by enhancing pump power usage. Compared with conventional 4C-EDF, the 4C-EDF with a pedestal has a gain improvement of 4.5 dB and a PCE enhancement of 91.8%, according to the experimental results (pedestal fiber: 9.55%, conventional fiber: 4.98%). For a 6 dBm total input signal power at L-band and a 7.8 W pump power at 976 nm, the pedestal 4C-EDF shows an average gain of 25 dB and an average noise figure (NF) of 6.5 dB over all cores in the wavelength range of 1570.41 nm to 1610.87 nm. The core-to-core gain variation is less than 2 dB.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
SDM has been burgeoning over the last decade [1–3] as a conceivably practical approach to accommodate the mounting data exigency. SDM fibers, including few-mode fibers (FMF) and multicore fibers (MCF), have been applied to SDM transmission links [4–8]. Up to now, the 19-core 6-mode fiber [8] and the 38-core 3-mode fiber [6] have been performed in remarkable experiments, setting a record for data rates of more than 10 Pb/s per fiber and revealing conspicuous capacity expansion capability. However, for long-haul MCF transmission links, multicore erbium-doped fiber amplifiers (MC-EDFA) are indispensable to lengthen the maximum transmission distance for the power loss compensation. Core-pumped [9,10] and cladding-pumped MC-EDFAs [11–19] have been consecutively proposed in the last few years. Compared with the core-pumped scheme, cladding-pumped MC-EDFA is preferable to be applied to practical scenarios, considering sharable pump consumption and component integration [20]. Furthermore, the multimode pump laser diodes (LD) with low cost and high power are resorted to offering uniform pump intensity to the whole inner cladding. However, the low PCE of the cladding-pumped MC-EDFA hinders its practical process.
As for the cladding-pumped regime, the PCE of the fiber is directly affected by the pump absorption, which depends mainly on the core-to-cladding ratio. Therefore enlarging the doping core area [13] and narrowing the silica cladding area with unique designs [21,22] are efficacious in augmenting PCE. However, the PCE of the amplifier cannot be increased infinitely by adjusting merely the fiber geometric parameters under the single-mode operation condition. Moreover, to ensure the amplifier's performance, the core numerical aperture (NA) cannot be decreased at the expense of core composition. Subsequently, other approaches have been reported to increase the pump absorption by recycling the residual pump light [23] and adopting the Er/Yb co-doped fiber to enhance absorption and manifest energy conservation [14]. Nevertheless, turbo cladding pumping calls for a mature preparation process of the residual pump collector, and the Er/Yb co-doped fiber amplifier presents a narrow gain bandwidth and a poor NF. Recently, a Bragg grating written over a large area inside the cladding of the MCF shows a 16% PCE increase by recycling 19% of the output pump power [24]. So far, few groups have successfully improved the pump absorption by adjusting the fiber refractive index profile. Therefore, improving the PCE has become a crucial problem to be solved urgently for cladding-pumped MC-EDFAs.
Pedestal fibers are commonly used as large-mode-area fibers for high-power laser systems to optimize beam quality, thereby increasing the threshold for mode instability [25] and nonlinear effects [26]. To boost the PCE of the MC-EDF, we propose to introduce a pedestal structure around each core of the fiber. The simulation results find that introducing the pedestal can effectively improve the PCE of the 4C-EDF. Additionally, the pedestal can reduce the NA of the fiber core without undermining the core composition, which allows further expansion for the core diameter of the 4C-EDF without shifting the cutoff wavelength onto the operation band. Consequently, the PCE performance of the 4C-EDF can be further improved.
In this paper, we propose and experimentally demonstrate a 4C-EDF with a pedestal structure for SDM amplification. The fiber was fabricated by the modified chemical vapor deposition (MCVD) process in combination with the drilling method. The influence of the pedestal on the pump absorption and gain performance of the 4C-EDF has been analyzed numerically. The amplification performance of this 4C-EDF with a pedestal has been investigated. Compared with the conventional 4C-EDF under the same amplification configuration, the fiber with a pedestal significantly increases the PCE at the L-band.
2. Fiber design
The cladding-pumped MC-EDF appears to have low PCE because of the small core-to-cladding ratio, leading to relatively high pump power to achieve sufficient gain. There are two direct ways to realize high-efficiency EDF design. On the one hand, enlarging the fiber core is an effective strategy to strengthen pump power usage. On the other hand, designing a unique fiber structure to increase the signal output power with injection power kept constant also helps to improve PCE property.
2.1 Overlap factor
According to Gile’s model [27], the intrinsic saturation power Psat at signal frequency ν of an EDFA can be expressed as:
Simulations for 4C-EDFs with different pedestal structures were performed. Among the calculation, the core and cladding diameters of the fibers were set to be 6 μm and 125 μm. The simulated pedestal diameters range from 0 μm, which means it is conventional fiber, to 125 μm, which means the index-raised pedestal replaces the inner cladding. The NA for the pedestal and the core was set to be 0.19 and 0.26, respectively. Details about the fiber parameters are listed in Table 1. The signal overlap factor Гs was calculated from the integral of signal mode field diameter (MFD) with erbium ions doping area [11]. Although the erbium ions are not evenly distributed across the core area, we applied the equivalent cross-sectional area to simplify the simulation. And the Гs’s variation with pedestal diameter Dp is illustrated in Fig. 1. The inserted part in Fig. 1 presents the lessening of Гs at the wavelength of 1590nm. When the pedestal diameter comes larger than 25um, the overlap factor decreases very little as the pedestal diameter continues to increase. Given that the index-raised pedestal attenuates the ability of the core to bind the fiber mode, the MFD of the fundamental mode is enlarged, leading to the decrease of signal overlap with the constant doped region. It can be concluded from Eq. (1) that the saturated signal output power will be increased.
Table 1. Fiber parameters
To numerically illustrate the output power variation deriving from the Гs’s reduction, we analyzed the gain characteristic of the 4C-EDFs according to the amplification model in Ref. [11,12]. The following parameters have been selected for the simulation: a multimode forward pump at the wavelength of 976 nm with 5 W power, 20 signal channels ranging from 1570 nm to 1610 nm with a spacing of 2 nm and total power of 0 dBm, 55 amplified spontaneous emission (ASE) spectrum components ranging from 1510 nm to 1620 nm with a spacing of 2 nm, and an erbium ion concentration of 2.76 × 1024 m-3. Initially, the pump distribution was assumed to be uniform over the whole inner cladding region. Thus, the pump overlap factor Гp can be expressed as core-to-cladding area ratio Acore/Acladding. Considering the single core loaded scenario, Fig. 2 shows the gain curves of the loaded core of these 4C-EDFs. Among the calculations, the fiber length used was increased with Гs’s decrease to ensure gain flatness. And the smaller the signal overlap, the longer the fiber length. The gain curve rises integrally as the pedestal gets wider, predominantly caused by the diminution of signal overlap and the increase of fiber length. Signal absorption and emission decrease by the equal coefficient because overlap shrinks. The needed accumulated fiber length for the amplified signal to be saturated tends to be longer, reinforcing the pump radiation absorption. In this case, the impact of the emission cross-section spectrum on the gain spectrum is intensified. On the other side, the increase in fiber length lowers the population density of the upper laser level with fixed pump power. Therefore, the impact of the absorption cross-section spectrum on the gain spectrum is also reinforced and the two spectrums are balanced by optimized fiber length. Furthermore, pump power usage improves resulting from farther absorption of pump light in increased fiber length. Overall, the gain improvement is owing to the enlargement of the pedestal diameter, which leads to the shrink of signal overlap factor and the increase of the fiber length used. However, as the used fiber length is much shorter, this proposed design helps little to improve the usage of pump power in C-band.
Regarding the initial discussion, the core radius of pedestal fiber could be expanded to a larger size than no-pedestal fiber, since the normalized frequency Vcore is calculated from pedestal parameters rather than silica cladding. The red curve in Fig. 2 depicts 1.89 dB gain enhancement due to the enlarged 7.5 μm core diameter and pedestal of 17 μm diameter, while Vcore is still less than 2.405 at 1570 nm. In this case, pump absorption was enhanced to increase the excitation efficiency, and the enhancement factor was directly attached to the core size. Overall, the pedestal will decrease the signal overlap factor and increase the pump absorption longitudinally by enlarging fiber length, which is beneficial to the amplifier performance.
2.2 Pump absorption
With an index-raised pedestal around the core, pump power unabsorbed near the core will be more difficult to leak to the cladding, leading to higher pump density in the pedestal zone. The density distribution of pump lights has been analyzed by ray-tracing [21] in ZEMAX OpticStudio for two fiber structures: uniform cladding and pedestal-inserted cladding. 107 rays were emitted into the input side at 36° for both fibers. After going through the same length, a detector was placed at the end of the fiber to monitor the pump power distribution by counting the number of rays-impacting points at the whole cross-section. As demonstrated in Fig. 3, the pump distribution of the two fibers was calculated from the light numbers. The redder the color, the stronger the pump power intensity. Part of the pump light is confined to the index-raised pedestal, which the doped core will easily absorb. And the pedestals function as pump collectors to attract pump light, resulting in the upswing in pump density of the core area.
Fig. 3. Simulated pump power distribution of fibers with uniform cladding (left) and pedestal inserted cladding (right).
To numerically verify the pump absorption increased proportion, two single-core fibers with different structures but the same core composition were fabricated. Fiber A was a conventional double-cladding fiber without a pedestal, and fiber B was the same as fiber A apart from an 11 μm pedestal. The core diameters of the fibers are both 6 μm. The cladding pump absorptions at 976 nm were measured through the cutback method. Due to the pedestal, fiber B exhibited a larger absorption coefficient at the pump wavelength than fiber A. The cladding pump absorption coefficient at 976 nm of A and B is 0.13 dB/m and 0.24 dB/m, respectively. Therefore, the pump overlap factor in the simulation model above should be modified. We empirically assume 1.85-times absorption coefficient enhancement (0.24 dB/m to 0.13 dB/m) as per the experimental data. The modified gain simulation result is shown in Fig. 4. These results were acquired under the same injection condition. On the one hand, the saturated output power gets higher by the decreased signal overlap, and the output power will increase by the lengthened optimal fiber length. The blue line represents the gain curve for the conventional fiber with a total output power of 22.06 dBm. The dashed red line depicts the gain spectrum based on the initial pump overlap factor with an output power of 24.22 dBm. This improvement derives from the overlap factor decrease. On the other hand, the index-raised pedestal, which allows further expansion of the core size in the case of single-mode operation, strengthens the pump absorption. The full red line represents the simulation result in the enhanced pump overlap factor with an output power of 26.6 dBm. The modified model shows a PCE enhancement of almost 2 times, from 3.2% to 9.2%. The fiber with a pedestal structure appears significant gain enhancement compared with conventional fiber.
Fig. 4. Simulated gain spectrum for conventional fiber (blue), pedestal fiber with common absorption (red dotted) and modified absorption (red solid).
3. Experiment
3.1 Fiber preparation
To experimentally investigate the influence of the pedestal on the amplification performance of MC-EDF, we prepared a 4-core Er/Al co-doped fiber with a pedestal (p4CF) via the MCVD process in combination with the solution doping technique (SDT) [25]. By the co-deposition of GeO2 and P2O5 before the soot layer deposition, a pedestal with a diameter of 4.68 mm and a NA of 0.198 was acquired. The refractive index profile of the fiber preform is shown in Fig. 5(a). This was followed by the drawing of the 4-core fiber, carried out through the drilling method. The concentration profile of one core from the p4CF is illustrated in Fig. 5(b), which was measured by an electro-probe microanalyzer (EPMA). The core was doped with 0.18 wt% Er2O3 and 2.9 wt% Al2O3. The pedestal was doped with 4.6 wt% Ge2O3 to reach a NA of 0.198. The fluctuation in the Er2O3 doping distribution derives from the rough precision of the EPMA linear scanning mode, as the background noise interferes with the information.
Fig. 5. (a) Refractive index profile of the preform with a pedestal. (b) Doping element concentration profiles of fiber core and pedestal.
A conventional 4-core erbium-doped fiber (c4CF) without pedestal structure was fabricated using the same method, keeping the same core composition. The geometric parameters of the p4CF and the c4CF in the experiment are listed in Table 2. Due to the pedestal, the p4CF still operates in a single-mode state at the wavelength of 1550 nm, even though the core size is nearly 1.5 times larger than that of c4CF. These two fibers are compared under the condition of single mode operation. It should be noted that there exist higher order modes in the pedestal, but the bending loss is high enough to degenerate these modes and ensure single-mode operation in the fiber core, as shown in Fig. 6.
Table 2. Fabricated fiber geometric parameters
3.2 Experimental setup
According to the discussion above, we built an all-fiber amplifier with a cladding-pumped scheme to characterize the amplification performance of the p4CF and c4CF, as depicted in Fig. 7. Thereinto, the inset on the right side shows the cross-sectional images of these two fibers. The signal light consisted of 25-channels wavelength division multiplexing (WDM) signals ranging from 1570.41 nm to 1610.87 nm, with a spacing of about 1.6 nm. A 1×4 coupler was adopted to split the WDM signals into four parts equally. Thus, the power injection to each core is about 0 dBm. One of the key elements in MC-EDFA, the fan-in/out device, fabricated from a bundle of fibers, was employed to couple signal light into the 4C-EDF. All the input ports were attached to isolators to avoid backward ASE light. A 976nm multimode pump LD was utilized in a forward pumping configuration to offer sufficient inversion particles, and the pump light was coupled into the inner cladding of the fiber by a side pump coupler without any cooling unit. The coupler was fabricated by coiling a piece of tapered coating-stripped pump delivery coreless fiber around the active fiber, as illustrated in Fig. 7, and pump coupling efficiency larger than 95% was achieved. Residual pump light in the inner cladding at the output part was removed through a pump stripper, which was accomplished by exposing the cladding-etched active fiber to air in a glass tube sealed at both ends. The amplified signal spectrum of the core under test was collected by an optical spectrum analyzer (OSA) at the output port of the fan-out attached with an isolator.
4. Results and discussion
The gain performances of these two fibers have been investigated based on the amplification configuration in Fig. 7. Lengths of the p4CF and c4CF were optimized to be 134 m and 96 m, respectively, for gain flatness at L band operation. And the fibers were wrapped around disks with a radius of 80 mm. It should be noted that the insertion loss of the fan in/out varied from 1.2 dB to 3.2 dB, leading to an uneven performance of the fully-loaded 4 cores. Thus the internal gain was measured in the experiment, which was defined as the ratio of the output signal power to input signal power directly at both ends of the active fiber [16]. The insertion loss difference of the fan-in was compensated by a variable optical attenuator to maintain equal signal injection power for all cores. Figure 8 shows the internal gain range of these two fibers. The average internal gain of both p4CF and c4CF was about 24.5 dB and 20 dB from 1570 nm to 1605 nm in all cores under a cladding pump power of 7.8 W (8.0 W of the LD), while the total input signal power was 6 dBm. The p4CF amplifier presents a higher gain of about 4.5 dB than the c4CF amplifier, which is consistent with the simulation results. The gain enhancement is mainly attributed to further absorption for pump light, deriving from the pump collecting effect of the pedestal and the increased active fiber length as the shrink of signal overlaps.
Then, to evaluate the power consumption of the p4CF amplifier, the total output power and residual pump power were measured. Due to the pedestal, the effective mode area and the core diameter increase without exciting higher-order modes in the core, while pump radiation remaining in the cladding decreased by 2 W compared to the c4CF. As shown in Fig. 9, the maximum and minimum power in all cores was 23.09 dBm and 22.45 dBm, respectively, and the average output power was 22.7 dBm, which could be further improved by increasing the input signal power or pump power. Under the pump density of 0.54 mW/μm2, which is the lowest to the best of our knowledge, we achieve amplification performance satisfied for preamplifier stage application by the pedestal fiber amplifier. However, even though 134 m p4CF fiber was used, 39.9% (∼3.2W) of the pump power remained in the inner cladding after amplification. The result showed an enhancement of 91.8% compared to the c4CF, whose average output power was 19.92 dBm. There exist improvement measures that to increase the core number to enlarge efficiency moreover [17] and to apply turbo pumping to reuse the residual pump power [28]. It should be noted that the output power for these two fibers excludes the loss of the fan out.
Fig. 9. Total output power (solid) and resident pump power (dotted) change as a function of injected pump power for c4CF (blue) and p4CF (red).
We also measured the gain and NF characteristics of the p4CF amplifier over the C band under the same cladding-pumped configuration. The signal turned to 31-channels WDM signals ranging from 1524.11nm to 1572.06nm with a total power of 0 dBm per core. Due to the higher absorption and emission cross-section at the short wavelength, the fiber length was reduced to 14m for gain flatness, limiting the power to transfer to a longer wavelength. Figure 10 shows the averaged amplification performance of the amplifier with the pump power of 7.8 W for the L band and 26 W for the C band. Gain as large as 18 dB was obtained across the whole C band, and the average NF was about 6 dB, decreasing as the wavelength increased. The core-to-core gain and NF variation was less than 2 dB and 1.5 dB for both operation band. The crosstalk was measured to be lower than -45 dB at the wavelength of 1600 nm for all cores.
5. Conclusion
In conclusion, a cladding-pumped 4C-EDF with a pedestal has been first, to the best of our knowledge, proposed and experimentally demonstrated. The influence from the pedestal and the core size enlargement were simulated to investigate the pump absorption and gain performance. An all-fiber amplifier with a cladding-pumped scheme was established to characterize the amplification performance of the p4CF and c4CF. The experimental results show that the 4C-EDF fiber with a pedestal exhibits a gain enhancement of 4.5 dB and a PCE improvement of 91.8%. Additionally, ∼25 dB internal gain and ∼6.5 dB NF was observed in the wavelength range of 1570.41 nm to 1610.87 nm for a pump density of 0.54 mW/μm2. The core-to-core gain variation was measured to be ∼2 dB, and the NF variation of ∼1.5dB. This novel pedestal design of the multicore erbium fiber is of great significance in improving the PCE and gain of the MC-EDF.
Funding
National Natural Science Foundation of China (11875139, 61975061).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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