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Abstract
The extended L-band 4-core Er/Yb co-doped fiber and amplifier (MC-EYDFA) is first proposed and demonstrated, to the best of our knowledge, for space division multiplexing combined with wavelength division multiplexing application. The fiber core co-doped with Er/Yb/P is adopted for bandwidth expansion, and the long wavelength extends to 1625 nm. Numerical simulations further show that efficient amplification and higher saturation power are achieved with the 1018 nm cladding pumping. Based on the integrated 4-core fiber amplifier, an average gain of ∼22 dB covering 1575-1625 nm is experimentally obtained with a 4 W pump power and a 3 dBm total signal power, and the max core-dependent gain (CDG) variation is measured to be 1.7 dB.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Data traffic demand in backbone networks has grown exponentially over the last decades [1]. Current rapid development in space division multiplexing (SDM) technology based on multi-core fiber (MCF) or few-mode fiber (FMF) is occurring because of strong desires to satisfy the capacity demand of transmission systems [2]. In particular, deploying MCF has attracted considerable attention for long-haul transmission links, without the requirement for complex digital signal processing techniques [3,4]. Multi-core erbium-doped fiber amplifier (MC-EDFA) are envisaged as necessary devices for lengthening transmission distance and realizing cost-effective systems, adopting core-pumped and cladding-pumped configurations. Cladding-pumped MC-EDFA utilizing multimode laser diode (LD) with high power and low cost are considered to be the key for both power saving and component integration, as opposed to using single-mode LDs for the direct pumping of individual cores in core-pumped MC-EDFA [5].
Up to date, cladding-pumped MC-EDFA combined with dense wavelength division multiplexing has been exploited for providing more bandwidth over multiple spatial paths, which is the most straightforward and effective solution to increase capacity by orders of magnitude [1,5]. Nevertheless, the bandwidth of MC-EDFA is only developed to 1610 nm to date [6]. Ascribing to signal excited state absorption (SESA), gain bandwidth extension becomes intractable over 1600 nm at L-band [7]. Although some available strategies were reported to suppress SESA [8–10], the drawbacks of low stability and poor compatibility with silica fiber communication systems make it impractical to use. Recently, a cladding-pumped multi-element amplifier comprising 4-Er/Yb-doped signal fibers was cascaded to extend the bandwidth to 1615 nm [11]. Er/Yb/P doped-fiber has shown the potential to obtain broad bandwidth in the extended L-band, profiting from SESA suppression and clustering avoidance [12,13]. Furthermore, introducing Yb3+ helps not only to increase pump absorption but also broaden the selection of pump wavelengths in the cladding-pumped amplifier, where the decreased overlap between the pump and doped cores results in low pump efficiency [14,15].
It is to be noted however that one drawback of EYDFA is the energy transfer saturation between Yb3+ and Er3+, accompanied by Yb-band amplified spontaneous emission (ASE) [16]. Abundant ASE accumulated in the fiber leads to insufficient gain and efficiency, especially in L-band because of longer fiber lengths [17,18]. Several approaches were proposed to recycle ASE in the single-core EDFA such as introducing a fiber Bragg grating [19] and designing a double-pass configuration [20]. Such attempts also provide inspiration for EYDFA, however, they put forward higher requirements on passive components in multi-core systems. Using the Yb-band core-pumping scheme to improve the advantage of L-band signal over ASE in the gain competition is effective in suppressing ASE, similar to C-band core-pumped EDFA [18,21], but comes at the cost of sacrificing multi-core system integration. Cladding-pumped schemes with Yb-band pump wavelength were effectively employed to mitigate abundant ASE for further power scaling in fiber laser [22,23]. As a well-developing pumping method, the use of cladding-pumped configuration combined with the Yb-band pump wavelength may exhibit good potential in addressing the drawbacks of MC-EYDFA.
In this paper, the first 4C-EYDFA for the extended L-band operation is demonstrated, employing 1018 nm cladding pumping. First, we investigated and verified the effect of Er/Yb/P co-doped fiber for extended L-band amplification. Theoretically, the stimulated model confirmed the power evolution in EYDFA and optimized the Yb-band pump wavelength. Comparisons between different pump wavelengths were conducted, and the results confirm the conjecture of pump-to-ASE loss reduction and saturation power increase with Yb-band pumping. In section 3, the optimized double cladding 4C-EYDF was fabricated by the modified chemical vapor deposition (MCVD) process in combination with solution doping technology (SDT). Experimentally, the integrated 4C-EYDFA with homemade active fiber was established, and experimental results show the gain properties are enhanced by using the optimized 1018 nm cladding pumping. Amplification performance covering the C + L band was measured, and the results are in good agreement with the simulated ones.
2. Theoretical analysis and numerical simulation
2.1. Doped fiber for the extended L-band operation
The energy level schematic of the Er-Yb co-doped system is shown in Fig. 1(a). Pump photons can be absorbed by the 2F7/2 level of Yb3+ and/or the 4I15/2 level of Er3+, which are then excited to the 2F5/2 and 4I11/2 levels, respectively. The excited Yb3+ in the 2F5/2 level is de-excited to the 2F7/2 level and the Er3+ in the ground state is promoted to the 4I11/2 level, due to the energy transfer between Yb3+ and Er3+. The excited Er3+ will rapidly non-radiative relax to the metastable level 4I13/2 attributed to the extremely short lifetimes of the 4I11/2. The signal amplification occurs through the stimulated transitions between the 4I13/2 and 4I15/2 levels. Meanwhile, the ASE occurs simultaneously between the Yb3+ levels 2F5/2→2F7/2 and Er3+ levels 4I13/2→4I15/2. However, the SESA phenomenon, which is known as one part of Er3+ in 4I13/2 upward transitions to 4I9/2 after absorbing the energy of signal photons, is considered a key factor hindering L-band expansion over 1600 nm [24]. Moreover, cooperative up-conversion process between Er3+ in the clusters limits the efficiency of high-concentration EDFs.
Fig. 1. (a) Energy-level diagram of the Er-Yb co-doped system. (b) Diagrammatic sketch with the Er/Yb/P co-doped system.
The clusters in EDF are formed not only consist of Er3+ but also by other types of rare earth ions co-doping in the fiber core [13]. The phosphosilicate network in the EYDF provides a harmonious structure, as revealed in Fig. 1(b). Yb3+ can prevent the up-conversion process by facilitating the separation of Er3+ inside the clusters. Meanwhile, the formation of [PO4]3- and [PO3O1/2]2- disrupts the ligand field of Er3+ and compresses the electron cloud [12]. Varied Stark-splitting resulting from the external disturbance of the coordination can minor the energy level difference between 4I15/2 and 4I13/2, leading to the red-shifting and narrowing of the emission spectrum to reduce the influence of SESA. Thus, it provides a flat emission spectrum over 1580 nm, which benefits stretch gain bandwidth to longer wavelengths.
To further verify the effect of Er/Yb/P co-doped fiber for the extended L-band operation, a wideband output power spectrum was measured, as shown in Fig. 2, with homemade EDF and EYDF under 976 nm pumping. The fibers were fabricated using the same procedure and dopant concentration of Er3+. Although EDF shows better gain flatness profiting from co-doping with aluminum, EYDF achieves a wider gain bandwidth covering 1565-1630 nm. The full width at half maximum (FWHM) of the output power spectrum in EYDF is 60 nm from 1565 nm to 1625 nm. By contrast, EDF shows a poor extended ability only to 1617 nm with 52 nm. The addition of Yb/P into the EDF does contribute to suppressing the influence of SESA and avoiding concentration quenching, which is crucial for the extended L-band operation.
2.2. Theoretical analysis of power propagation of EYDFA
To investigate the performance of EYDFA, the simulation model was conducted based on the power propagation and rate equations as described in [25,26], with a 976 nm forward cladding-pumped configuration. The steady-state propagation of the pump Pp and signal Ps in the forward ($+ $) and backward ($- $) directions are governed by the following:
The power evolution could be divided into four stages. Yb-ASE enhances more rapidly than the emission of Er3+ and achieves the maximum power during stage α and β, which is attributed to more pump energy being delivered to the Yb3+. In stage γ, Er-ASE ramps up in an increasing slope through both the energy transfer and pump absorption. Here, Er-ASE mainly exists in the C-band because the cross section has an order of magnitude difference than that of the L-band. Also, since the fiber length used in L-band is usually long and the pump almost exhausts at the latter part of the fiber, Yb-ASE is gradually reabsorbed acting as a secondary pump for the Er-band signal [26]. Ultimately, the signal power ascends and gradually saturates by extracting the stored energy of Er-ASE and Yb-ASE in stage δ. At the same time, the abundant counter-propagating ASE consumes the pump energy and population inversion, then dramatically accumulates in the front of the fiber, profiting from the high pump intensity and competitive advantage. With injected pump power of 4 W, the backward Yb-ASE power and Er-ASE power at the input end of the fiber are about ∼30 dBm and ∼29 dBm, respectively. Moreover, the maximum forward Yb-ASE and Er-ASE during the evolution reach ∼24 dBm and ∼21 dBm, respectively. However, the output power of the fiber is only ∼19 dBm, indicating that a significant amount of pump power is lost due to pump-to-ASE loss.
Fig. 3. (a) Forward power propagation and (b) backward power propagation evolution along the fiber length in EYDFA with 976 nm cladding-pumped.
In the case of the conventional L-band EDFA, the pump-to-ASE loss is one of the main factors resulting in poor efficiency [17]. The massive Yb-band emission of EYDFA in both directions is the manifestation of the pump rate larger than the rate of Yb-Er-transfer [29]. Hence, the observed power growth slope of the Yb-ASE is always larger than that of Er-ASE. Avoiding unwanted Yb-ASE will improve the advantage of signal in the gain competition, considering that, taking advantage of Yb-band pumping to reduce pump-to-ASE loss may show a good prospect of gain enhancement for the extended L-band EYDFA.
2.3. Optimization of Yb-band pump wavelength
Using the modeling introduced above, numerical simulations to investigate the impact of Yb-band pumping for EYDFA were conducted. The influence of concentration quenching is also considered in the simulation, where Cup with a value of 3.0 × 10−24 m3/s represents the cross-relaxation coefficient of erbium ions. Additionally, the typical cross section of the fiber refers to [26], and the parameters used in the simulation are shown in Table 1 (parameters not mentioned in the table were chosen the same as [25]). Optical fiber amplifier usually operates with a flat gain in current communication networks by using a gain flattening filter, and the flat gain is limited by the channel with the minimum gain of the doped fiber [15]. Meanwhile, considering that the simulation was aimed at evaluating the PCE performance rather than providing analysis of detailed modal characteristics, the inter-core crosstalk and assumed a uniform distribution of pump power across the cross-section were neglected. Consequently, the comparison was done for the highest flat gain at the optimal fiber length of the loaded core of the MC-EYDF under a single core loaded scenario.
Table 1. Simulation parameters
Considering that amplification characteristic varies with the pump wavelength, different Yb-band pump wavelengths were optimized first. The pump wavelengths chosen in the simulations were 1000 nm, 1018 nm, 1030 nm, 1040 nm, and 1064 nm. Figure. 4(a) depicts the curves of output power variation with fiber length. The output power with all pump wavelengths increases rapidly and then becomes saturated with the increasing fiber length. Moreover, the saturated output power is increased with the increasing pump wavelength (∼1018 nm), as the small pump absorption coefficient (PAC) lengthens the fiber length used to ensure the pumping radiation absorption. After reaching a maximum, the saturated output power begins to decrease as the pump wavelength continues to increase, which is attributed to the high value of absorption/emission affecting the energy transfer from the pump to the L-band signal, especially for 1064 nm.
Fig. 4. (a) The output power variation with fiber length for different Yb-band pump wavelengths. (b) Simulated gain and NF spectrum at the optimal fiber length. (c) Calculated PCE as a function of pump power for different pump wavelengths.
Figure. 4(b) further compares gain and noise figure (NF) spectrum with pump wavelengths of 1000 nm, 1018 nm, 1030 nm, 1040 nm, and 1064 nm at the optimal fiber lengths of 28 m, 31 m, 30 m, 28 m, and 23 m, respectively. The results show that saturation power limits the maximum flat gain that could be obtained for pump wavelength < 1018 nm. At the same time, a higher flat gain is achieved for 1018 nm with a negligible impact on the NF that remains below ∼6 dB. While using a wavelength > 1018 nm, however, the gain curve is decreased integrally. The variation of PCE with increasing pump power in Fig. 4(c) also proves that pump power is converted to the L-band signal efficiently with 1018 nm pumping, compared to other pump wavelengths. While a pump power of 6 W is used, the obtained PCE of 1018 nm reaches 26.5%. By contrast, the PCE reduce to 20.4%, 19.4%, 10.8% and 1.9%, respectively, with wavelengths of 1000 nm, 1018 nm, 1030 nm, 1040 nm and 1064 nm. The small PAC at 1018 nm leads to the possibility of large saturation power and enough pump absorption by enlarging fiber length. However, the gain amplification is inefficient for the smaller PAC such as at 1000 nm. For those reasons, the optimization should take a tradeoff between the PAC and the value of absorption/emission into account. Therefore, the simulations were then executed to give a characteristic comparison in this paper between the two types of pumping schemes, ie. traditional 976 nm pumping vs well-chosen 1018 nm pumping.
2.4. Comparisons of the pumping scheme
For different fiber lengths, the variations of gain and NF spectrums with traditional 976 nm pumping and well-chosen 1018 nm pumping are plotted in Fig. 5(a). It can be seen that the gain is enhanced across the entire simulated wavelengths while using 1018 nm pumping, compared to 976 nm. Moreover, for 1018 nm pumping, the gain at the long-wavelength direction is enhanced as the fiber length increased to 35 m, which exceeds ∼30 dB. At the same time, the gain at the short-wavelength direction drops, and the NF deteriorates more drastically, which is attributed to the cross section being larger than that of the long-wavelength resulting in the fast-supersaturated state. However, the gain at the long-wavelength direction obtained by 976 nm pumping reaches only ∼20 dB under the same conditions, which can be observed from the dashed line.
Fig. 5. Gain and NF spectrums for different fiber lengths with 976 nm pumping (dash) and 1018 nm pumping (solid) at a signal power of (a) −3 dBm and (b) −15 dBm.
To prove the characteristics with different scenarios, amplification performance with the signal input power set to −15 dBm is depicted in Fig. 5(b). The small signal gain shows the gain difference at the long-wavelength direction between the two types of pumping schemes rises from ∼10 dB to ∼14 dB. The gain competitive advantage of ASE prevents the signal from acquiring an efficient pump energy extraction, in case of insufficient signal power, which makes the effect on gain improvement by reducing pump-to-ASE loss easier to obtain. Besides, a low population inversion due to long fiber length leads to higher NF, especially at the short-wavelength direction, which reaches ∼6 dB and ∼8 dB for a signal power of −3 dBm and −15 dBm, respectively. Generally, an NF of approximately 8 dB is adequate for transmission experiments [30]. So, the deterioration is acceptable and the issue can be alleviated by optimizing for the core refractive index, and the balance of gain/NF properties [31,32].
The Yb-ASE spectral power evolution was completed for verification, with the fiber length of 18 m and 31 m for 976 nm and 1018 nm pumping, respectively. It can be seen in Fig. 6(a) and 6(b) that the position of forward Yb-ASE amplification is shifted from the former to the latter part of the fiber and the wavelength of peak power changes from ∼1060 nm to ∼1090 nm, indicating that forward Yb-ASE is significantly suppressed with 1018 nm pumping injected. On the other hand, backward Yb-ASE accumulated near the input end cannot be absorbed and the power will increase further as the fiber length is longer. As shown in Fig. 6(c) and 6(d), the existence of 1018 nm pumping suppresses the generation of unwanted backward Yb-ASE and avoids the waste of pump energy.
Fig. 6. Yb-ASE spectral power variation along the fiber length: forward ASE with (a) 976 nm and (b) 1018 nm pumping; backward ASE with (c) 976 nm and (d) 1018 nm pumping.
Further, the results plotted in Fig. 7(a) demonstrate that the sharp drop of pump power generated by Yb-ASE at the input section is alleviated with 1018 nm pumping injected. At the same time, the proper PAC helps the pump power be absorbed more evenly and converted to output power efficiently. The population density in Fig. 7(b) further illustrates that 1018 nm pumping avoids the accumulation of Yb-inverse population. Meanwhile, instead of being weakened distinctly, the inversion level of Er3+ becomes more uniform along the fiber length. Overall, suppressing unwanted forward and backward Yb-ASE to reduce pump-to-ASE loss does contribute to an improvement in gain characteristics in the extended L-band EYDFA.
Fig. 7. Variation of (a) pump power and (b) inverted population density along the fiber length with 976 nm pumping (dash) and 1018 nm pumping (solid).
Using Er/Yb/P co-doped fiber offers the advantage of using passively cooled pump LDs, which can further reduce the overall power consumption [14]. When temperature control is not mandatory, the output stability of EDFA is susceptible to thermal drift of the pump wavelength, which is typically in the range of ±1 nm [33]. Figure. 8(a) compares the gain spectrum variation and gain fluctuations to the pump wavelength shifting ±2 nm relative to 976 nm and 1018 nm, respectively. Within this range, the corresponding extreme relative gain variations are ∼3.5 dB and ∼0.9 dB when using 976 nm pumping. However, for the 1018 nm pumping, the relative gain variations are reduced to ∼0.2 dB and ∼0.06 dB. In this range, the relative output power variation is shown in Fig. 8(b). It can be seen that the power variation is much larger with 976 nm than that of 1018 nm, which reduces from 11.0% to 0.4%. In practice, when considering multicore fibers, the core-dependent gain (CDG) becomes a crucial factor affecting transmission distance, particularly in amplifiers pumped at the 976 nm absorption peak, where the impact of core-to-core variation in dopant concentration is prominent. As for the 1018 nm pumping scheme, it helps minimize variation in core-to-core absorption to decrease CDG by turning the pump wavelength away from the peak absorption wavelength to the flat absorption wavelength range. The output stability of 1018 nm pumping is certainly an attractive feature for MCF-EYDFA to deal with the thermal drift of the pump wavelength.
Fig. 8. Comparison of (a) gain spectrum variation and gain fluctuations, and (b) output power variation induced by the pump wavelength shift.
3. Experimental results
3.1. Fabrication and parameters characterization of the 4C-EYDF
The MC-EYDF was prepared via the MCVD process with the SDT, and the Er/Yb co-doped preforms were fabricated with the same condition. The typical refractive index profile is shown in Fig. 9(a). With the introduction of P5+ to prevent the back-energy transfer between Er3+ and Yb3+, the numerical aperture (NA) of EYDF is usually higher than 0.2 [34]. Hence, the profile was intended to have an index-raised pedestal with a diameter of 2 mm, which enables core size expansion in case of single-mode operation and strengthens pump absorption with an increasing core-to-cladding ratio.
Fig. 9. (a) Refractive index profile of the preform. (b) Elements distribution of the fiber core. (c) The cladding and (d) the core absorption coefficient for the fabricated MC-EYDF.
Next, the 4C-EYDF was drawn and the elements distribution of one core was recorded by the Electron Probe Micro Analyzer (EPMA) as depicted in Fig. 9(b). The core was co-doped with Er3+/Yb3+ and the pedestal was doped with Ge4+, which makes the fiber reach a NA of 0.18 and a cutoff wavelength of 1350 nm. The cladding and core absorption coefficient of one core were recorded by PK2500 measurement system (Photon Kinetics, Inc.) through the cutback method as plotted in Fig. 9(c) and 9(d). Compared with Er3+ doped fiber, co-doping of Yb3+ gives access to increase pump absorption and broaden pump absorption between 900-1020 nm, which extends the selection of pump wavelengths. On the other hand, the introduction of P5+ needed for EYDF makes the core absorption peak wavelength redshift to 1535 nm and thus helps gain bandwidth stretch to the L-band as discussed in section 2.1. The detailed fiber specifications are listed in Table 2.
Table 2. Parameters characterization of the MC-EYDF
3.2. Experimental setup
Guided by the simulation and discussion above, an integrated experimental setup of 4C-EYDFA was constructed, as illustrated in Fig. 10. The signal with 59 channels covering 1575.8 nm to 1625.3 nm with ∼0.8 nm spacing was chosen as the original signal. The Fan-in/out couplers to connect the multiple spatial channels to multiple single-core input and output fibers were first characterized, with an average loss of ∼2 dB per core and core-to-core loss variation of ∼±1 dB. After being split, the signals were launched into the variable optical attenuator (VOA) to compensate for the core-to-core loss of Fan-in, which ensures a uniform input signal power of −3 dBm across individual cores. The forward pump light was coupled into the inner cladding of 4C-EYDF with the power of 4 W injected by a side pump coupler (SPC) with a coupling efficiency of ∼65%. With the signal always bound to the core and the pump light in pump fiber leakage into the inner cladding of 4C-EYDF, a configuration with cladding and core hybrid pumping was constituted. The cladding pump light acts as the forward-pumping source and the amplified co-propagating ASE serves as an in-band core-pumping source [35]. The multicore isolator (MC-ISO) followed the other end of the active fiber to prevent parasitic oscillation from the back reflections of the fiber end in the Fan-in/out couplers. Ultimately, the amplified signal spectrum was collected by an optical spectrum analyzer (OSA) at the output port of the Fan-out. Noticed that, the gain and NF measurement was completed by simultaneously amplifying all the spatial channels.
3.3. Characterization of the 4C-EYDFA
The pump wavelengths chosen for comparison in the experiment were 976 nm and 1018 nm according to the simulated optimization, with the optimal fiber lengths of 9.5 m and 19.5 m for a higher flat gain. 940 nm pumping, as a conventional pumping scheme to increase the efficiency of EYDF, was also taken into comparison with a fiber length of 13.5 m. The core-to-core pitch of 43 µm and used fiber lengths of a few tens of meters make the crosstalk in this fiber much less of an issue. The mean crosstalk between neighboring cores for the amplifier was evaluated to be less than −45 dB while signal light (@1625 nm) was launched in one core and pump light was injected across all cores. This is the overall amplifier crosstalk including its from multicore passive devices and the inherent crosstalk from fiber should be much smaller.
Figure. 11(a) depicts the gain and NF spectrum with different pumping schemes. The average gain at short-wavelength and long-wavelength is up to ∼17.7 dB and ∼14.7 dB for 1018 nm, respectively. However, for 976 nm pumping, the average gain is ∼12.3 dB and ∼7.9 dB. Also, the gain is enhanced over the entire measured wavelengths while using 1018 nm pumping, which the trend is consistent with the theoretical results in Fig. 5(a), although the error between gain measurement and simulation exists due to the relatively strong relations of the fiber parameters, and the insertion loss of the components and splices. Furthermore, saturation in output limits the maximum gain that can be extracted. A maximum gain of 27.1 dB is observed with 1018 nm pumping, compared to 21.3 dB and 15.8 dB for 940 nm and 976 nm pumping, respectively. With the same gain value at the short-wavelength direction, the 17-dB gain range is extended to ∼1624 nm for 1018 nm pumping, while that of ∼1618 nm for 940 nm pumping. Moreover, the NF performance manifests the same as the prediction by the simulation model. The advantages of output stability and being away from peak absorption wavelength make the max CDG reduce to ∼1.7 dB as plotted in Fig. 11(b), compared to ∼2.1 dB for 976 nm and ∼2.0 dB for 940 nm pumping. Overall, a good agreement is observed between measurements and simulations, which indicates that the Yb-band pumping configuration provides an approach for performance enhancement in the extended L-band.
With the optimized pumping scheme, the amplification characterization of 4C-EYDFA over the C + L-band was measured under the same configuration. Figure. 12 demonstrates the averaged amplification performance of 4C-EYDFA with 940 nm pumping for the C-band and 1018 nm pumping for the L-band, respectively. The injection power of the signal and pump was consistent in both cases. With signals ranging from 1544 nm to 1572 nm, the fiber length was optimized to 4.5 m to reach an average gain of ∼20 dB. Table 3 summarizes the latest MC-EYDFA reports so far. The proposed 4C-EYDFA shows excellent amplification characteristics and the performance is also envisaged to be further improved by adjusting the Yb/Er atomic ratio [36] and core-dependent loss of components [40].
Table 3. Advances research for the MC-EYDFA (*: This work)
4. Conclusion
In conclusion, we theoretically and experimentally propose and demonstrate an integrated 4C-EYDFA for the extended L-band with 1018 nm cladding pumping for the first time, to the best of our knowledge. We first compared cladding-pumped Er and Er/Yb/P doped fiber and found that the latter shows potential benefits over the spectral range of 1565-1625 nm, profiting from SESA suppression. After optimizing the Yb-band pump wavelength and comparing the proposed pumping configuration with conventional ones, the results show that well-chosen 1018 nm pumping provides superior performance owing to the attractive features of reducing pump-to-ASE loss and obtaining higher saturation power. Experimentally, the average gain of ∼22 dB was obtained covering 1575-1625 nm based on the integrated 4C-EYDFA with homemade Er/Yb doped fiber and 1018 nm cladding pumping. Meanwhile, the max CDG was reduced to ∼1.7 dB due to stability output and off-peak pumping properties of Yb-band pumping. It is suggested that this novel amplifier can be employed as a potential approach to achieve a 50-nm bandwidth and enhance performance, especially for SDM applications.
Funding
National Natural Science Foundation of China (11875139).
Acknowledgments
The authors thank the China National Natural Science Foundation for helping identify collaborators for this work. The authors wish to thank CJphotonics Ltd. for providing fibers.
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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