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Abstract
Cladding-pumped multicore erbium-doped fiber is an important element for future spatial division multiplexing (SDM) amplification. We propose an M-type erbium-doped multicore fiber to achieve high-efficiency SDM amplification. The performance of cladding-pumped erbium-doped fiber with a central refractive index depression has been investigated, and the M-type fiber has better amplification performance than conventional fibers by reducing the signal mode overlap with the doped region. The experiment results show that the M-type 4-core erbium-doped fiber has a gain improvement of 2.8 dB compared with conventional 4-core fiber. The pump conversion efficiency (PCE) has been enhanced from 4.47% to 8.01%. For a 7.0 W pump power at 976 nm, the M-type fiber exhibits an average gain of 20.0 dB and an average noise fiber of 6.8 dB at the L-band. The core-to-core gain variation is less than 1.6 dB.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
SDM technology is supposed to further expand fiber transmission capacity [1–3]. While the multi-input multi-output (MIMO) technology [4] is indispensable for coupled multicore fiber (MCF), uncoupled MCF [5–7] is an attractive candidate for high-capacity SDM networks and requires no sophisticated digital signal processing. Considering the compatibility problems involved with present optical fiber networks and devices [8], as well as the mechanical durability of the optical fiber [9], the multicore fiber with standard cladding diameter is more likely to be implemented in practical applications. In addition, an increase in fiber loss is inevitable in multicore fibers with more spatial division multiplexing channels [8]. The weakly coupled MCF with the lowest loss is the four-core fiber proposed in 2020 [10], whose loss property is compensable with single-core commercial fiber. Thus, low counts weakly coupled multicore fiber communication system has conceivable practical prospects. However, erbium-doped fiber amplifiers (EDFAs) remain essential for compensating the fiber loss in long-distance transmission [11].
There exist two schemes for amplification in optical fiber systems, multipath [12,13] and multicore amplifier [14–17]. The former splits multichannel signals from the MCF into SMFs and adopts multiple SMF amplifiers, leading to high complexity. The other scheme based on multicore erbium-doped fiber amplifiers (MC-EDFAs) shares the same pump source and passive components and makes full use of the advantages of high integration of MCF. Nevertheless, cladding-pumped MC-EDFA shows poor PCE for the low core-to-cladding ratio, which gives rise to low pump absorption. Recent research finds that there is a certain core-to-cladding ratio that maximizes the PCE at the L-band [18]. Many studies are aimed at reducing the power consumption of multicore fiber amplifiers [19,20]. In terms of amplifier structure, turbo cladding pumping boosted the signal output power by reusing the residual pump power [21]. Utilizing bidirectional pumping and adjusting the optimal forward-to-backward power ratio achieved a PCE improvement [22]. Besides, other approaches based on the MCF, such as adopting Er/Yb co-doped fiber [17] to boost output power and writing Bragg grating inside the fiber cladding [23] to recycle pump power, also help to improve PCE. Our previous work designed a pedestal 4-core fiber to enhance the cladding pump absorption and increase the output power for the SDM amplifier [24]. Despite improving gain characteristics, the manufacturing process poses risks during the pedestal deposition step due to the high stress in germanium silicate glass. Our following research focuses on improving the MC-EDFA by designing an easily preparable MCF.
In this paper, we propose an M-type refractive index profile EDF design and apply it to the cladding-pumped 4-core fiber amplifier for lowering power consumption. We numerically investigated the impact of depressed parameters on fiber amplification performance. A series of M-type index profile fibers (MIFs) have been fabricated to experimentally verify the simulation result. At last, we introduced the central depression design to the fabrication of 4C-EDF and compared the fiber with a conventional 4C-EDF. The fiber with a central depression shows an apparent improvement of the PCE at the L-band.
2. Fiber design
Our previous work mentioned that the saturation power would be increased as the signal overlap integral Гs shrunk while other parameters remained constant [24]. We assume the other parameter to be constant when we consider the optimal overlap factor design and calculate the gain spectrums of a series of hypothetical fibers, whose parameters were all the same except for the overlap factors. To numerically illustrate the gain characteristic of these fibers, we calculated the gain spectrums according to the model in Ref. [15]. The simulation parameters such as signal powers, amplified spontaneous emission (ASE) spectrum settings, and erbium ions concentration are the same as those in our previous work. The Гs at different wavelength was calculated from mode field diameter (MFD) [15]. Then the hypothetical fibers with different Гs were constructed by multiplying a constant differing from 0.33 to 0.88. Details about the fiber parameters are listed in Table 1.
Table 1. Fiber parameters
At a cladding pump power of 5 W and a signal power of 0 dBm, we got the output spectrums of these fibers. As shown in Fig. 1(a), the gain spectrums arise with the decrease of the signal overlap factor. The output power decreases by almost 2.5 dB when the overlap factor increases from 0.3 to 0.9. The residual pump power of these hypothetical fibers decreases from 3.7394 W to 1.7249 W as the used fiber length increases from 104 m to 385 m. Excessive extension of the used fiber length will introduce inevitable background loss. Figure 1(b) shows the output signal power at different Гs for a constant pump power of 8 W in the ideal model. Output power decreases by almost half while the overlap factor increases from 0.3 to 0.9. The pump and signal absorption are determined by cross-section and overlap factors. In the cladding-pumped EDF, the absorption cross-section behaves far larger at the signal wavelength compared with the pump wavelength at 976 nm, and the pump overlap factor Гp is two orders of magnitude smaller than the signal overlap factor Гs. When the Гs is relatively large, the signal power gets saturated easily and at this moment the pump power remains at a high level. Decreasing the Гs helps to slow down the absorption of the signal so that the saturation length increases and the pump light continues to be absorbed. Thus, more of the pump light energy is converted to signal light, inducing the improvement of the PCE.
Fig. 1. (a) Simulated gain spectrums of the hypothetical fibers, (b) output power variation with different overlap factors.
M-type fiber, which contains a central index depression, exhibits different mode distributions compared with step-index fiber (SIF). With the introduction of a depressed layer in the refractive index profile, the binding ability of the core to the fundamental mode field is weakened, thus increasing the MFD. The overlap factor is determined by the integration of the signal mode field in the doping region, which means that the smaller the distribution of the mode field in the doping region, the smaller the overlap factor. Thus, the MIF exhibits smaller Гs than the SIF. To illustrate the variation of the overlap factor specifically derived from the central depression depth and width, we presented the variation of the Гs at 1590 nm, shown in Fig. 2(a). In a fiber with a fixed core size, the overlap minimizes along with the deepening and widening of the depression. The modal power distribution evolves from a Gaussian shape to a flat-top-shape with the deepening of the depression and becomes a depressed shape eventually [25]. That means the proportion of the mode distribution in the doping region decreases with the deepening and widening of the depression, leading to the decrease of the Гs. Moreover, the MIF has a shorter cutoff wavelength λc than the SIF at the same core size and refractive index difference [26]. However, the bending loss property deteriorates with the excessive expansion of the depression layer. Taking an MIF with a central depression index difference of 0.0087 and a depression ratio, the ratio between the depression width and the core diameter, of 0.65 as an example, as shown in Fig. 2(b), the core radius while keeping the cutoff wavelength at 1550 nm of the MIF and the SIF are 3.68 µm and 2.83 µm, respectively. The bending loss of the fiber is less than 0.1 dB/km while the bending radii is 80 mm. To illustrate the advantage of MIF design in amplifiers, we simulated the gain characteristic of three different fibers: SIF, MIF with the same core size, and MIF with the same cutoff wavelength. Details about the fibers are listed in Table 2.
Fig. 2. (a) Overlap factors in different depression widths and depths. (b) The cutoff wavelength for SIF and MIF in different core radii. (c) The simulated gain and NF spectrums at the C-band and (d) the spectrums at the L-band.
Table 2. Simulated parameters for three fibers
Figure 2(c) and Fig. 2(d) show the gain spectrums of these fibers at C-band and L-band, respectively. When it comes to operating at the C-band, the large absorption cross-section determines the shorter used fiber length for the amplifier. At the pump power of 25 W and the signal power of 0 dBm, we simulated the gain spectrums. The signal transferred to a 23-channel C-band signal ranging from 1525 nm to 1569 nm. The fiber lengths are optimized to get a flattened gain spectrum in the simulation for three fibers, which are 9.7 m, 15 m, and 14 m for SIF, MIF-a and MIF-λc. The SIF gets the smallest length for the large signal absorption. The MIF-λc gets a shorter length compared with the MIF-a because the enlarged core increases the pump absorption. In the case of the same core size, the cladding absorption coefficients of these fibers are equal. However, the MIF-a has a lower overlap factor, which lengthens the fiber saturation length, bringing amplification efficiency improvement by 1.8 dB. In the case of the same cutoff wavelength, the MIF-λc has a larger cladding absorption coefficient and a longer saturation fiber length. So the gain spectrum of the MIF is augmented significantly by 3.0 dB overall compared with the SIF. The MIF exhibits the same advantage in amplification efficiency while operating in L-band, as shown in Fig. 2(d). At a pump power of 10 W, the lengths for these fibers are 129 m, 178 m, and 152 m. The MIF-λc also performs the best in gain at the L-band.
To achieve better amplification performance based on MIF design, we investigated the influence of depression parameters and core size on the gain characteristic. Suppose a prepared fiber preform with a fixed refractive index profile, the ratio between the depressed layer to the whole core is constant. When the manufactured preform is drawn as spans of large core fiber, the fiber gets a high absorption coefficient for the large core-to-cladding ratio, and this is beneficial to pump light conversion. However, the overlap Гs also gets enlarged with the increased core, which is opposed to the design purpose according to the discussion above. To evaluate the influence of those two aspects, we definite a quality parameter Qcol by the following equation:
a is the core radius and Γ is the signal overlap factor. The larger the quality parameter, the better the fiber performance. With the increase of the core size, the cladding pump absorption coefficient changes quadratically while the overlap factor changes polynomially-like. As shown in Fig. 3(a), the Qcol decreases first and then increases as the core diameter increases. Because the gradient of the core area is lower than the gradient of the overlap factor while the core size is relatively short, the Qcol in region I decreases as the core diameter increases from 3 µm to 4 µm. The amplification efficiency is lower in this region when the core size is larger. As the core diameter continues to increase, the gradient of the core area reaches the gradient of the overlap factor and then surpasses it. Thus, in region II the Qcol augments with the increase of the core diameter. Then, we calculated the gain spectrum to directly show the influence of the core size on the amplification performance according to the simulation model above. Figure 3(b) shows the simulated gain spectrums of MIFs with different core diameters but the same depression ratio under the pump power of 15 W. Consistent with the Qcol variation in Fig. 3(a), the MIF gets the worst amplification performance at the core diameter of 4 µm. As the core diameter increases from 4 µm to 8 µm, the gain spectrum rises gradually. The fiber exhibits the best gain performance with an average gain of 31 dB at the core diameter of 8.5 µm, 3.5 dB better than that at the core diameter of 4.0 µm. It is worth mentioning that the MIF gets worse bending loss performance when the core diameter is extremely small. The fiber with a 3 µm core diameter is supposed to get worse performance in the practical situation. Thus, we can increase the core diameter as much as possible when designing the MIFs for better PCE performance.Fig. 3. (a) Quality factor variation with the core diameter and (b) simulated gain spectrums of different MIFs.
3. Single-core M-type erbium-doped fiber
To verify the conclusion above, we experimentally investigate the amplification performance of the MIF. MIFs with different core diameters and the same depression ratio were fabricated. These fibers, MIF1, MIF2, and MIF3 were drawn from the same preform to ensure the same doping composition. The core diameters of these fibers are 7.0 µm, 7.5 µm, and 8.0 µm, respectively. The cladding diameters of these fibers are all 125 µm. The preform was manufactured by the modified chemical vapor deposition (MCVD) process and the central index depression was formed by reducing the doping amount of germanium dioxide. The preforms manufactured in this way maintain a high degree of consistency and hardly risk fragmentation. The refractive index profile is illustrated in Fig. 4(a), which was measured by a preform analyzer (PK2650). A refractive index depression is formed in the center of the preform and the depression ratio is close to 0.65. The concentration distribution of one fiber is shown in Fig. 4(b), which was measured by an electro-probe microanalyzer (EPMA). Owing to the low doping amount of germanium dioxide in the middle of the core, a low refractive index depression formed, and other dopant elements were distributed evenly across the cross-section. Due to the test accuracy problems of the EPMA, the distribution of the low-doping erbium ions fluctuates heavily.
Fig. 4. (a) Refractive index profile of the preform and (b) doping element concentration profiles of the drawn fiber.
Next, as shown in Fig. 5, we built up a cladding-pumped structure to measure the amplification characteristic of these 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 1.6 nm. A combiner was adopted to couple the pump light emitted by a 976 nm multimode pump LD into the EDF through the signal arm. The input and the output ends were attached to isolators to avoid backward ASE light. A cladding pump stripper was employed at the end of the active fiber to remove the residual pump light. An optical spectrum analyzer collected the light at the output end to analyze the gain and NF spectrums.
Figure 6 shows the results of these fibers, whose lengths were optimized for gain flatness. At the pump power of 6 W and the signal power of 0 dBm, we compared the amplification characteristics of these fibers. The lengths were 57 m, 50 m, and 48 m for MIF1, MIF2 and MIF3. Thanks to the larger core diameter, MIF3 shows an average gain that is 1.6 dB higher than that of MIF1. The MIF3 presents a 23 dB average gain and 4.2 dB NF at the L-band. These MIFs indicate that a fiber with a larger core diameter exhibits a superior gain spectrum while maintaining an undeteriorated NF spectrum. This trend is consistent with the simulation results shown in region II from Fig. 3(a), where the core diameter dominates rather than the overlap factor. However, the trend in region I could not be verified among these fibers because the small core size fiber drawn from the same preform behaved poorly in terms of bending loss property. Thus, we can draw the fiber into a core diameter as large as possible to achieve an improvement in PCE.
4. 4-core M-type erbium-doped fiber
To increase the PCE of the cladding-pumped multicore fiber amplifier, we apply the designed MIF to the construction of the amplifier. A 4-core erbium-doped fiber with a central depression amid the refractive index profile was fabricated via the MCVD process in combination with the drilling method. The preform used to make the 4-core fiber was the same as in section 3. A conventional 4-core erbium-doped fiber (c4CF) with a common refractive index profile was fabricated using the same method for comparison. The geometric parameters of the m4CF and the c4CF are listed in Table 3. To fully demonstrate the advantage in PCE improvement of the m4CF, the two fibers are compared in the situation of the same cutoff wavelength, which enables the core diameter of the m4CF to be larger than the c4CF.
Table 3. Fabricated 4-core fiber geometric parameters
According to the discussion above, we built up an all-fiber amplifier with a cladding-pumped scheme. As depicted in Fig. 7, the four-way signal lights entered the 4-core fiber from the SMFs through the fan-in, whose input ports were attached with isolators to avoid backward ASE light. At the output end of the fan-in, a 4-core fiber isolator was used to block the reflected light to prevent parasitic oscillation. A 976 nm multimode pump LD was employed to form particle numbers inversion and entered the active fiber through a side-pump combiner. A cladding pump light stripper was used to remove the residual lights in case of damage to other devices. The amplified spectrums were collected by OSA at the output end of the fan out to analyze the gain spectrum.
Figure 8 shows the gain performances of these fibers. The used fiber lengths for m4CF and c4CF are 45 m and 50 m respectively. Even though the m4CF possesses a longer saturation length, the used m4CF length is still shorter than the c4CF. Because the higher cladding pump absorption coefficient deriving from the enlarged core size shrinks the used length for the m4CF. The bending radii are both 80 mm for these fibers. Considering that the insertion losses of the fan in/out vary from 1.2 dB to 3.2 dB, we measured the internal gain in the experiment to evaluate the performance of the erbium-doped fiber only. Under the cladding pump power of 7W, the average internal gain of m4CF and c4CF was about 23 dB and 20 dB from 1570 nm to 1610 nm, while each core's input signal power was 0 dBm. The gain difference of each core is less than 1.6 dB in the m4CF and 1.8 dB in the c4CF, revealing excellent uniformity in the fabrication. The m4CF presents a higher gain of 2.8 dB than the c4CF, consistent with the simulation result. The total output signal powers of all m4CF and c4CF cores are 27.49 dBm and 24.96 dBm, respectively. The corresponding PCEs for the fibers are 8.01% and 4.47%. Because of the different pump power, the PCE of the c4CF is a little smaller than that in our previous work which is 4.99%.
Fig. 8. Gain ranges and NF of the m4CF (red zone and line) and the c4CF (blue zone and line) versus wavelength at the L-band.
5. Conclusion
In conclusion, M-type erbium-doped fiber was first applied to the cladding-pumped multicore fiber amplifier. The amplification characteristic variation with the central depression parameters was simulated to select the appropriate fiber design. A series of single-core M-type EDFs with different core diameters have been fabricated and tested to verify the simulation results. Two multicore fibers, m4CF and c4CF, were prepared and their amplification performance was compared. The experimental results show that the m4CF exhibits a gain enhancement of 2.8 dB, and the PCE increases from 4.47% to 8.01%. The gain variation in the m4CF is less than 1.6 dB. This novel refractive index profile design is easy to employ and of great benefit to lower the power consumption of the cladding-pumped MC-EDFA.
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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