Open Access
Add to BibSonomy
Abstract
In this study, PbS/Er co-doped fibers (PEDFs) were fabricated by atomic layer deposition (ALD) combined with modified chemical vapor deposition (MCVD). A pumping scheme based on two-photon absorption at 1310 nm of PEDF is proposed for L + band amplification. Through the theoretical analysis, the local environment of Er3+ is changed due to the co-doping of PbS, which improves the two-photon absorption efficiency near 1300 nm. Compared with the 980 nm pump, the PEDFs excited by the 1310 nm pump show better amplification performance in the L + band. And in a bi-directional pumping system, PEDF achieves over 22 dB of gain in the whole L band. In particular, the bandwidth of over 20 dB gain was extended to 1627 nm with a noise figure as low as 4.9 dB. To the best of our knowledge, this is the first time that a high-gain bandwidth of L band amplification has been extended to 1627 nm. The results of unsaturated loss also show that PbS co-doping improves the two-photon absorption efficiency of PEDF to broaden the amplification bandwidth of L + band. These results demonstrate that an effective L + band amplification method is practically provided for future ultra-wideband optical communications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the age of 5 G and extensive big data communication, the demand for optical fiber communication capacity is on the significant rise. The prevalent use of erbium-doped fiber amplifiers (EDFAs) in C and L band optical amplification has been notable [1,2]. Moreover, with the continuous development of gain equalization filter, gain clamping and other technologies, the gain of EDFA is flat and stable, and EDFA is widely used in fiber optic communication systems [3–5]. However, the limited bandwidth of the C band will fail to meet the future communication capacity requirements. As a result, numerous researchers have delved into expanding the gain bandwidth of EDFAs into the L band or even its extended L + band. But a critical challenge emerges due to the signal excited state absorption (SESA) from energy level 4I13/2 to 4I9/2, which precipitates a rapid decrease in the gain of EDFAs after 1610 nm [6]. This directly impedes the bandwidth extension into the L + band. Therefore, minimizing the impact of SESA in the L + band has become a primary focus in the research on broadening the amplification bandwidth. For SESA, there are three main ways to reduce its effect on L + band: red-shift of SESA center wavelength, reduction of SESA bandwidth and intensity.
Researchers have used various methods to minimize the effects of SESA. Among them, tellurite-based and bismuth-based erbium doped fibers (EDFs) can lead to the red-shift of the center wavelength of SESA [7,8]. However, this alteration can pose challenges during fusion splicing when connecting the fiber to other silica-based devices, leading to considerable fusion splicing losses. This has inconvenienced the widespread use of optical fiber network systems. Additionally, researchers have demonstrated that phosphosilicate based EDF offer the capability to expand the gain bandwidth to the L band [9,10]. Therefore, on the basis of increased P content in the fiber core, some researchers have fabricated Er/Yb, Er/Ce and Bi/Er/La co-doped fibers, extending the high gain bandwidth to 1624 nm [11–14]. In addition to co-doping Er3+ with other rare earth elements, some researchers have tried Er/Ba co-doping to red-shift the wavelength of SESA, and achieve broadband L band amplification in the range of 1560-1623 nm [15]. Moreover, a recent EDFA utilized a double-pass amplifier structure to achieve a gain of ≥ 20 dB across the entire L band [16]. This is a breakthrough for the study of L band amplifiers. However, their amplification bandwidth is limited and they cannot extend the high gain bandwidth to the L + band to meet future needs, because these methods cannot more effectively reduce the effect of SESA on the L + band.
Here, to further increase the amplification bandwidth of the L + band, a pump excitation method at 1310 nm based on two-photon absorption of EDF has been proposed. This method reduces the SESA intensity by increasing the number of electrons on the energy level 4I9/2 of Er3+. For two-photon absorption of EDFs, most researchers have focused only on its upconversion property [17,18]. And the efficiency of two-photon absorption is often low, which leads to its limited application, so few people have investigated its amplification characteristics in L + band and the effect of co-doping elements on the two-photon absorption. Some researches have shown that co-doping PbS in EDF can reduce the loss caused by Er3+ clustering, and also affect local environment of Er3+ [19,20], which may influence the efficiency of two-photon absorption. Hence, in this study, we fabricated three erbium-doped fibers with co-doping of different contents of PbS. Their fluorescent properties, gain characteristics in L + band and unsaturated loss for two-photon absorption were investigated. In addition, the effect of PbS co-doping on Er3+ two-photon absorption was analyzed by simulation.
2. Theoretical studies
The primary constraint limiting the L band amplification is the SESA of Er3+, which absorbs photons in the L + band to excite electrons of Er3+ in the excited states from the energy level 4I13/2 to 4I9/2 [6,21]. If the number of electrons on the energy level 4I9/2 can be increased, the number of electrons excited from the energy level 4I13/2 to 4I9/2 through the SESA can be reduced, thus decreasing the intensity of the SESA. Therefore, exciting electrons of Er3+ in the ground state from the energy level 4I15/2 to 4I9/2 is feasible, which can be achieved by using an 808 nm laser as a pump for the erbium-doped fiber amplifier. However, the excited state absorption of Er3+ by 808 nm photons [22] leads to an intensive upconversion luminescence, making it unsuitable for L + amplification. Moreover, it has been shown that Er3+ has the capability to excite electrons from the energy level 4I15/2 to 4F9/2 by directly absorbing two photons with the energy about 0.940 eV (1319 nm) [23]. This phenomenon is identified as a two-photon absorption. In EDF, the electrons on excited state energy level 4F9/2 can be relaxed to 4I9/2 by multiphonon relaxation (MPR), which can likewise diminish the intensity of the SESA. According to this, we proposed an excitation scheme pumping at 1310 nm (0.946 eV) to diminish the effect of SESA to the L + band amplification of EDF.
We have built a simplified energy level diagram for Er3+ as shown in Fig. 1. Er3+ can absorb two photons of energy 0.946 eV (1310 nm), exciting the electrons from the ground state energy level 4I15/2 to the excited state energy level 4F9/2 whose energy is equal to 1.892 eV (655 nm), subsequently relaxing to the excited state energy level 4I9/2 by MPR. During this process, the number of electrons on the energy level 4I9/2 continues to increase, which can reduce the number of electrons excited upward by the SESA, since decreases the SESA intensity. After MPR, the electrons finally reach the energy level 4I13/2 and achieve C band luminescence. Then Er3+ absorbs C band photons to realize broadband amplification in the L + band. But under 980 nm pump excitation as shown in the energy level diagram on the right side of Fig. 1, a large number of electrons directly fill the energy level 4I9/2 after MPR. This will lead to a strong SESA and only realize C + band amplification.
However, the low two-photon absorption efficiency can limit the amplification capability in the L + band. In this study, PbS/Er co-doping fiber (PEDF) was used to solve this problem. Accordingly, we investigate the effect of co-doped PbS on Er3+ two-photon absorption by building the local microstructures of EDF and PEDF.
For silica optical fiber, it consists of amorphous state. Its network structure contains a large number of three-membered rings (3MRs, a ring structure formed by three Si and O), 4MRs, 6MRs and hybrid ring structures of silicon [24,25]. In addition, 3MR model structure is widely used in doped silica materials because it can represent the microscopic local properties of silica materials [26–28]. Therefore, the 3MR structure is chosen to build a Er-3MR and a PbS/Er-3MR local structure models. According to the reports [20,29,30], there are possible three structures of Er-3MR in EDF: Er3+ enters into the 3MR structure, Er3+ links to the 3MR through non-bridging oxygen (NBO) and Er3+ forms a new ring structure outside the 3MR with Si in the 3MR. Among them, the structure of Er3+ linked to 3MR via NBO has the highest bonding energy, this structure is the most stable and can be used as a local structural model for EDF. Therefore, we built the Er-3MR model as shown in Fig. 2(a). According to relevant studies [31,32], there are possible three structures of PbS-3MR that are similar to Er-3MR, among them, the structure of PbS linked to 3MR via NBO is the most stable structrure. Based on the optimal structures of Er-3MR and PbS-3MR, we constructed three PbS/Er-3MR structural models as shown in Fig. 2(b-d).
All computational procedures were performed on the Gaussian 09 platform. Then, the models are structurally optimized by using density functional theory (DFT) with Becker-type three-parameter Lee-Yang-Parr (B3LYP) hybridization function to obtain the stable structure with the lowest energy [33]. For H, O, Si and S elements, the 6-31 + G** basis is used, while Pb and Er are replaced by relativistic effective nuclear potentials (RECPs) with 4 and 11 valence electrons, respectively [28,31]. Then, excited state calculations are performed based on time-dependent density functional theory (TDDFT) of the ground state structure to study the energy levels and excitation-emission characteristics [34–36].
The excited state parameters of the Er-3MR are shown in Table 1. There exist six excited states with wavelength of 1564.46, 1283.47, 951.53, 785.02 and 653.03 nm, with corresponding oscillator strengths (f) of 0.0127, 0.0006, 0.0131, 0.0111 and 0.0095, respectively.
Table 1. Excited state parameters of the Er-3MR
We optimized three structures of PEDF as shown in Fig. 2(b)(c)(d). The optimized energies for model b, model c and model d are -3174.191, -3174.190 and -3175.193 Hartree, respectively. It can be seen that Ec < Ea < Eb, so most stable structural model c is chosen as the ground state structure of PbS/Er-3MR. Similarly, the excited state parameters of the PbS/Er-3MR model are shown in Table 2. The wavelengths of the excited states are 1549.06, 1504.48, 1306.07, 1074.11, 774.27, 692.73 and 638.88 nm and the corresponding f are 0.0094, 0.0106, 0.0014, 0.0104, 0.0091, 0.0143 and 0.0092.
Table 2. Excited state parameters of the PbS/Er-3MR
In order to obtain information about the emission wavelength, the already obtained Er-3MR and PbS/Er -3MR excited states were calculated again via TDDFT. Calculated emission wavelengths are 1588 nm with f of 0.0027 and 1601 nm with f of 0.0032, respectively. And based on the excitation and emission information of Er-3MR and PbS/Er-3MR models, the energy level diagrams were plotted as shown in Fig. 3. In the Er-3MR, the absorption peak at 1283.47 nm whose f is 0.0006 corresponds to the two-photon absorption at energy level 4F9/2 of Er3+. And this data is often overlooked because no attention has been paid to it in the past. While in PbS/Er-3MR, the f of two-photon absorption at 1306.07 nm is 0.0096, which is much higher than that of Er-3MR. This is due to the addition of PbS that generates more NBO in the silica lattice, leading to a change in the local environment of Er3+ as indicated by the circle labeled in Fig. 2(d). Due to the asymmetric changes in the local environment, the energy level structure of PbS/Er-3MR changes, with the energy level 4F9/2 broadening and the corresponding absorption peak changing from 653 nm to 639-692 nm as shown by the green arrows in the Fig. 3. As a result, the energy level 4F9/2 of PbS/Er-3MR is more capable of absorbing electrons, it has a higher two-photon absorption efficiency. In addition, the doping of PbS caused a change to the energy level 4I13/2 of Er3+, which red-shifted the emission wavelength from 1588 nm to 1616 nm, which is useful for broadband amplification in the L + band.
Fig. 3. Energy level diagram for the local structural models and electron cloud distributions of the Er-3MR and PbS/Er-3MR.
To further analyze the effect of PbS co-doping on the Er3+ local structure, the electron cloud distribution of Er-3MR and PbS/Er-3MR were obtained via Gaussian09 combined with GaussView as shown in Fig. 3. It can be found that the electron cloud of Er-3MR is mainly concentrated on 3MR. And the electron cloud in PbS/Er-3MR is mainly moved from PbS to Er3+, which would enhance the activity of Er3+ and slowly make Er3+ become the more active center in the system. Moreover, in PbS/Er-3MR, the irregular distribution of the electron cloud around Er3+ due to the co-doping of PbS to produce more NBOs that will affect the local environment of Er3+, resulting in the changes in the energy level and red-shifting of the emission wavelength.
The calculated wavelengths and f are broadened by Gaussian broadening to obtain the corresponding absorption/emission spectra as shown in Fig. 4 and Fig. 5. It can be observed from Fig. 4 that the intensity of the absorption peaks of PbS/Er-3MR near 800 and 980 nm is lower compared to that of Er-3MR. Therefore, during the simulation, the emission of PbS/Er-3MR mainly originates from the excitation of the excited state corresponding to 639-666 nm. From the emission spectrum in the Fig. 5, compared with Er-3MR, PbS-3MR showed higher intensity of emission peak. This indicates that the effect of SESA on Er3+ is reduced by exciting the energy level 4F9/2 of Er3+. Therefore, in this study, using two-photon absorption at 1310 nm to excite the energy level 4F9/2 of Er3+ in EDFs is an effective scheme to realize broadband amplification in the L + band. Moreover, co-doping PbS in EDFs could modify the local environment of Er3+ leading to a change in the energy level of Er3+. This will improve the two-photon absorption efficiency of Er3+ and red-shift the emission wavelength to promote the broadband amplification in the L + band.
Fig. 5. Emission spectra of Er-3MR and PbS/Er-3MR (the inset shows a localized view of two-photon absorption).
3. Experiments and discussions
3.1 Fiber characteristic
PEDFs and EDF were fabricated by atomic layer deposition (ALD) combined with modified chemical vapor deposition (MCVD). The basic parameters of PEDF1, PEDF2 and EDF are listed in Table 3. The cross-sections of fibers were measured using an optical microscope (Olympus Bx43, Japan). Measured core and cladding diameters of PEDF1, PEDF2 and EDF are 6.72/125.96 µm, 8.37/127.05 µm, and 9.74/128.01 µm. Fiber core compositions were tested by an electron probe microanalyzer (Shimadzu EPMA-8050 G, Japan). The content of Er3+ in PEDF1, PEDF2 and EDF was close with each other, and PEDF2 has a higher PbS content than PEDF1. The background loss at 1200 nm of PEDF1, PEDF2 and EDF obtained by the truncation method are 0.05 dB/m, 0.04 dB/m and 0.15 dB/m, respectively. The background loss of PEDFs is much lower than that of EDF.
Table 3. Basic characteristics of EDFs in this work
And the absorption coefficients at different wavelength were measured as listed in the Table 3. Compared to PEDFs, EDF has a lower absorption coefficient at 655 nm but a higher absorption coefficient at 1310 nm. This is due to the high background loss of the EDF, superimposed on the actual 1310 nm absorption coefficient resulting in a high 1310 nm absorption coefficient from the test. Therefore, other means are needed to characterize the two-photon absorption efficiency of doped fibers. But the absorption coefficients of PEDF2 at 655 and 1310 nm were higher than those of PEDF1 with increasing PbS content. This indicates that the increase of PbS co-doping content can improve the two-photon absorption coefficient.
3.2 Fluorescence properties and two-photon absorption
The luminescence properties of the EDFs were measured by forward pumping configuration under excitation at 1310 nm with pump power of 310 mW as shown in Fig. 6(a). It can be found that the EDFs show broadband fluorescence in the L + band by the excitation at 1310 nm. Moreover, the fluorescence intensity of PEDF2 is higher than that of PEDF1 and EDF. Combined with simulations, PbS co-doping clearly improves Er3+ two-photon absorption efficiency, suppresses SESA and enhances L + band luminescence.
Fig. 6. (a) Fluorescence properties with 1310 nm pump excitation. (b) Logarithmic relationship between the fluorescence power at 1610 nm and the pump power at 1310 nm of EDF, PEDF1 and PEDF2.
In order to verify that the 1310 nm pump excitation of PEDF1, PEDF2 and EDF is two-photon absorption, fluorescence power variation was recorded while changing the pump power. The number of pump photons to be absorbed for each excited photon can be determined according to Eq. (1) [23,37].
The fluorescence power “I” at 1627 nm is directly proportional to Pn. The “P” denotes the 1310 nm pump power and “n” denotes the number of pump photons absorbed by the fiber. Fig. 6(b) shows the logarithmic of the fluorescence power at 1627 nm (log I) as the function of the logarithmic of 1310 nm pump power (log P), and the slope is “n”. The curves in Fig. 6(b) can well linearly be fitted and the slopes of PEDF1, PEDF2 and EDF are about 1.56, 1.88 and 1.42, respectively. The values of the slopes are between 1.5 and 2, indicating that the fluorescence at 1627 nm is attributed by the two-photon absorption at 1310 nm of Er3+ [23]. Divide both sides of Eq. (1) by P as shown in Eq. (2). The ratio of I to P can be used to express the conversion efficiency from two-photon absorption at 1310 nm to 1627 nm. The ratio of I to P is proportional to Pn-1, therefore Pn-1 can be used to represent the conversion efficiency from two-photon absorption at 1310 nm to 1627 nm. Among these three EDFs, PEDF2 has the highest conversion efficiency of P0.88, due to the increase of PbS co-doping content.
3.3 Amplification characteristics
To explore the effect of two-photon absorption at 1310 nm on the L + band amplification of EDFs. We constructed a forward amplification system to compare the gain characteristics of EDFs pumped at 980 and 1310 nm. Their pump powers were 1 W and 746 mW, respectively. And the optical signal power is -20 dBm.
Figure 7 illustrates the forward gain spectra of EDF under pump excitation at 980 and 1310 nm. The lengths of the EDF used were 20 and 40 meters, respectively. The large difference in length is due to the different absorption coefficients of the EDF between 980 and 1310 nm as shown in Table 3. Compared with 980 nm pump, the gain excited by 1310 nm pump is significantly higher in the L + band. In addition, the gain coefficients, defined as the ratio of the gain to the pump power [38], were calculated for excitation by different pumps at 1610 nm. As shown in the inset in Fig. 7, the gain coefficient for EDF pumped at 1310 nm is 0.012 dB/m, which is significantly higher than that pumped at 980 nm. This suggests that two-photon absorption at 1310 nm could enhance the amplification efficiency of EDF in the L + band.
Fig. 7. Amplification characteristics of EDF with different pump excitations. (the inset shows the gain at 1610 nm variations with different pump excitations of EDF).
And Fig. 8(a) shows the forward gain spectra of PEDFs. The lengths of the fibers used in the 980 and 1310 nm forward amplification systems are 15 m and 60 m for PEDF1, 15 m and 55 m for PEDF2, respectively. Similar to EDF, 1310 nm pumping also excites broadband amplification of PEDFs in the L + band. And a gain of 28 dB at 1610 nm is obtained by 1310 nm pumping, much higher than the 15 dB excited at 980 nm. Especially, the gain at 1627 nm can even reach up to 17 dB, which has the possibility to further broaden to longer L + band. Similar to the simulation results, the L + band broadband amplification characteristics of PEDF2 are better than those of PEDF1 and EDF as the PbS co-doping content increases.
Fig. 8. (a) Amplification characteristics of PEDFs with different pump excitations. (b) The gain at 1610 nm variations with different pump excitations of PEDFs.
In addition, the gain coefficients at 1610 nm are shown in Fig. 8(b). The gain coefficients of PEDF1 and PEDF2 excited by 980 nm pump are 0.008 dB/m and 0.010 dB/m, which are close to the EDF’s. This indicates that the amplification efficiency in the L + band is close with each other when PEDF1, PEDF2 and EDF are excited by 980 nm pump. But at 1310 nm excitation, the gain coefficients of PEDF1 and PEDF2 are 0.020 dB/mW and 0.029 dB/mW, which are much higher than that excited at 980 nm. It's noteworthy that the difference of gain coefficients between PEDF1, PEDF2 and EDF is close to 0.01 dB/mW. Combined with the analytical results from the simulation, this suggests that the increase of PbS co-doping content improves the two-photon absorption efficiency of Er3+ to suppress the intensity of SESA, and then extend the amplification bandwidth of the L + band.
Comparing with 980 nm pumping, it can be found that PEDF1, PEDF2 and EDF excited at 1310 nm have higher gain and wider bandwidth in L + band. And the performances of PEDFs are much better than that of EDF. Therefore, we constructed a bidirectional amplification system pumped at 1310 nm by PEDF1 and PEDF2 to further explore their amplification performance in L + band. The configuration structure of the amplifier is shown in Fig. 9. One tunable laser source (TLS550, Santec, Japan) was used to generate optical signals with the power of -20 dBm and the bandwidth covering 1560 nm-1635 nm. Two 1310 nm laser diodes were used as pump sources for bidirectional pumping with the maximum output powers of 1036 mW and 746 mW, respectively. And two L + band wavelength division multiplexers (WDMs) were used to couple or parallel-separate the pump light and the optical signals. An isolator (ISO) was used to isolate the reverse fluorescence. The amplified signals were detected with the optical spectrum analyzer (OSA).
Figure 10 shows the gain and noise figure (NF) of PEDF1 and PEDF2 using the fiber length with optimized maximum gain in the L + band, which is 90 m for PEDF1 and 75 m for PEDF2. The forward and backward pump power is 1036 and 746 mW. It is noteworthy that both PEDFs have fairly high broadband amplification characteristics in the L + band. Among them, PEDF1 exhibits a gain surpassing 21 dB across the complete L band, with the gain of 21.7 dB and NF of 6.8 dB at 1625 nm. Especially, PEDF2 demonstrates a gain exceeding 22 dB throughout the entire L band, with the gain of 25.9 dB and the NF of 6.3 dB at 1625 nm, which is better than the previous study of only 20 dB at 1625 nm [16]. Beyond the 1600 nm range, the amplification performance of PEDF2 notably surpasses that of PEDF1, indicating that a higher PbS content enhances the conversion efficiency under 1310 nm two-photon absorption in the L + band. And the PEDF2 expands the gain bandwidth to 1631 nm with the gain of 3.7 dB. Especially, the gain over 20 dB can even cover the wavelength range of 1564-1627 nm, which is the broadest bandwidth of over 20 dB gain to the best of our knowledge among existing L band-related studies.
In addition, we have compared the gain performance of the PEDF excited by two-photon absorption with the existing L band amplifiers listed in Table 4. The reason that the maximum gain of PEDF is lower than that of EDF3, EDF4 and EDF5 is that the low two-photon absorption efficiency results in too few electrons accumulated in the excited state compared to the 980 or 1480 nm pumps. Moreover, the signal power used is -20 dBm higher than the signal power used by the other EDFs. If we use lower signal power then higher gain will be obtained. Notably, the amplifier system used in EDF3 and EDF5 are two-stage amplification system and double-pass amplification system, respectively [14,16]. These systems are more effective in improving L band gain, but overly complex systems can lead to deterioration of the NF. To explore the amplification performance of these EDFs in the L + band, their gain and NF at 1625 nm is recorded. Due to the two-photon absorption suppression of SESA, the PEDF in this study achieves a gain of 25.9 dB and the NF of 6.3 dB at 1625 nm, which is much better than that of other EDFs. At present, research has only succeeded in extending the L band where the gain exceeds 20 dB up to 1625 nm [16]. In particularly, the PEDF in this study achieves broadband amplification in the L + band by extending over 20 dB gain from 1564 to 1627 nm.
Table 4. Research results of various research institutions on L band broadband amplifiers
3.4 Unsaturated loss
Furthermore, in order to experimentally investigate the effect of PbS co-doping on the two-photon absorption efficiency of Er3+ in EDFs, we tested the unsaturated loss (UL) at 1310 nm using the truncation method. And the absorption efficiency at 1310 nm of the fiber is quantified and measured by using the quality factor Mα defined in Eq. (3) [39]. Here, αus denotes the unsaturated loss of the fiber and αs denotes the saturated loss. And the UL of PEDF1, PEDF2 and EDF at 1310 nm are shown in Fig. 11. The Mα of PEDF1, PEDF2 and EDF are 21.5, 31.4 and 12.6%, respectively. It can be seen that Mα increases with the increase of PbS content in the EDFs. This indicates that the co-doping of PbS could improve the absorption efficiency at 1310 nm of PEDFs. This result is in agreement with previous simulations. Since PbS co-doping affects the local environment of Er3+ in EDFs, it enhances the capability of Er3+ on the energy level 4F9/2 to absorb electrons, thus improving the two-photon absorption efficiency at 1310 nm. Moreover, compared with PEDF1 and EDF, PEDF2 has better amplification characteristics in the L + band with the improvement of two-photon absorption efficiency.
4. Conclusion
In this study, we fabricated three erbium-doped fibers with different co-doped contents of PbS by ALD combined with MCVD. And local structural simulation models of EDF and PEDF are established. By simulation, it was found that the local environment of Er3+ was changed with the addition of PbS, resulting in higher two-photon absorption efficiency of PEDF near 1300 nm than that of EDF. Experimentally, the difference in L + band amplification of the EDFs excited at 1310 and 980 nm were compared. At 1610 nm, the gain coefficient of PEDF2 excited at 1310 nm is 0.029 dB/mW, much higher than that excited at 980 nm, due to two-photon absorption at 1310 nm can effectively suppress SESA. With a bidirectional pumping system excited at 1310 nm, the PEDF2 achieves a gain of over 22 dB in the L band, and specifically, over 20 dB of gain extension to 1627 nm. To the best of our knowledge, this is the first L + band broadband amplifier that has been reported to extend 20 dB to 1627 nm. In order to investigate the reason that PEDF combined with two-photon absorption can realize broadband amplification in the L + band. UL was studied and it was found that the co-doping of PbS improves the two-photon absorption efficiency of PEDF at 1310 nm thereby further suppressing the SESA, which is similar to the simulation results. The results obtained in this study have great potential in expanding the data capacity of fiber optic communications.
Funding
National Key Research and Development Program of China (2020YFB1805800).
Acknowledgments
We appreciate the High Performance Computing Center of Shanghai University, and Shanghai Engineering Research Center of Intelligent Computing System. (No. 19DZ2252600)
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.
References
1. P. J. Winzer, D. T. Neilson, and A. R. Chraplyvy, “Fiber-optic transmission and networking: the previous 20 and the next 20 years [Invited],” Opt. Express 26(18), 24190–24239 (2018). [CrossRef]
2. M. Pal, M. C. Paul, A. Dhar, et al., “Investigation of the optical gain and noise figure for multi-channel amplification in EDFA under optimized pump condition,” Opt. Commun. 273(2), 407–412 (2007). [CrossRef]
3. C. A. F. Marques, R. A. Oliveira, A. A. P. Pohl, et al., “Adjustable EDFA gain equalization filter for DWDM channels based on a single LPG excited by flexural acoustic waves,” Opt. Commun. 285(18), 3770–3774 (2012). [CrossRef]
4. M. Karasek, A. Bononi, L. A. Rusch, et al., “Gain Stabilization in Gain Clamped EDFA Cascades Fed by WDM Burst-Mode Packet Traffic,” J. Lightwave Technol. 18(3), 308–313 (2000). [CrossRef]
5. Z. Zhai, A. Halder, and J. K. Sahu, “Flat-gain L-band amplifier containing AlPO4 units in aluminophosphosilicate erbium-doped fibers,” Opt. Lett. 48(21), 5579–5582 (2023). [CrossRef]
6. M. Bolshtyansky, I. Mandelbaum, and F. Pan, “Signal excited-state absorption in the L-band EDFA: Simulation and measurements,” J. Lightwave Technol. 23(9), 2796–2799 (2005). [CrossRef]
7. Y. Ohishi, A. Mori, M. Yamada, et al., “Gain characteristics of tellurite-based erbium-doped fiber amplifiers for 1.5-µm broadband amplification,” Opt. Lett. 23(4), 274–276 (1998). [CrossRef]
8. J. H. Shin and J. H. Lee, “Investigation of signal excited-state absorption in bismuth-based erbium-doped fiber amplifier,” J. Opt. Soc. Am. B 27(7), 1452–1457 (2010). [CrossRef]
9. Z. W. Zhai, A. Halder, M. Núñez-Velázquez, et al., “Temperature-Dependent Study on L-Band EDFA Characteristics Pumped at 980 nm and 1480 nm in Phosphorus and Aluminum-Rich Erbium-Doped Silica Fibers,” J. Lightwave Technol. 40(14), 4819–4824 (2022). [CrossRef]
10. Y. Sun, Q. Yang, Y. Wang, et al., “Extending laser wavelengths to 1630 nm in centimeter-scale Er-phosphate fiber,” Opt. Lett. 48(2), 456–459 (2023). [CrossRef]
11. Y. Chen, Y. Lou, Z. M. Gu, et al., “Extending the L-band amplification to 1623 nm using Er/Yb/P co-doped phosphosilicate fiber,” Opt. Lett. 46(23), 5834–5837 (2021). [CrossRef]
12. Z. Zhai and J. K. Sahu, “1480 nm Diode-Pumped Er3+:Yb3+ Co-Doped Phospho-Alumino-Silicate Fiber for Extending the L-Band Gain Up to 1625 nm,” J. Lightwave Technol. 41(11), 3432–3437 (2023). [CrossRef]
13. Y. Lou, Y. Chen, Z. Gu, et al., “Er3+/Ce3+ Co-doped Phosphosilicate Fiber for Extend the L-band Amplification,” J. Lightwave Technol. 39(18), 5933–5938 (2021). [CrossRef]
14. L. Zeng, J. Wen, Y. Wu, et al., “Exceeding 25 dB Gain Broad-Spectrum Amplification in L-Band Based on a Bi/Er/La Co-Doped Silica Fiber,” IEEE Photon. Technol. Lett. 35(18), 990–993 (2023). [CrossRef]
15. S. Jalilpiran, V. Fuertes, J. Lefebvre, et al., “Baria-Silica Erbium-Doped Fibers for Extended L-Band Amplification,” J. Lightwave Technol. 41(14), 4806–4814 (2023). [CrossRef]
16. Z. W. Zhai, A. Halder, J. K. Sahu, et al., “Erbium -Doped Fiber Amplifier with Extended L -Band Gain to 1625 nm,” in Optical Fiber Communications Conference and Exhibition (OFC), 2023).
17. J. M. Hickmann, E. A. Gouveia, A. S. Gouveia-Neto, et al., “Two-photon-resonant photoinduced second-harmonic generation in Er3+-doped germano-aluminosilicate optical fibers pumped at 1.319 µm,” Opt. Lett. 19(21), 1726–1728 (1994). [CrossRef]
18. D. L. Nicacio, E. A. Gouveia, A. M. Reis, et al., “Generation of intense green light through amplified spontaneous emission in Er3+-doped germanosilicate single-mode optical fiber pumped at 1.319 µm,” IEEE J. Quantum Electron. 30(11), 2634–2638 (1994). [CrossRef]
19. T. Wang, X. Chen, X. Pan, et al., “Efficient structural manipulation of PbS in Er-doped silica optical fibers for enhanced amplification systems,” J. Lumin. 257, 119689 (2023). [CrossRef]
20. X. Pan, Y. Dong, J. Wen, et al., “Improved Fluorescence and Gain Characteristics of Er-Doped Optical Fiber with PbS Nanomaterials Co-Doping,” Materials 15(17), 6090 (2022). [CrossRef]
21. P. R. Morkel and R. I. Laming, “Theoretical modeling of erbium-doped fiber amplifiers with excited-state absorption,” Opt. Lett. 14(19), 1062–1064 (1989). [CrossRef]
22. L. Tian, Z. Xu, S. Zhao, et al., “The Upconversion Luminescence of Er3+/Yb3+/Nd3+ Triply-Doped β-NaYF4 Nanocrystals under 808-nm Excitation,” Materials 7(11), 7289–7303 (2014). [CrossRef]
23. J. M. Hickmann, E. A. Gouveia, A. S. Gouveia-Neto, et al., “Enhancement of third-harmonic blue-violet light at 440 nm by erbium ions in Er3+–GeO2-doped silica monomode optical fibers pumped at 1.319 µm,” Opt. Lett. 20(16), 1692–1694 (1995). [CrossRef]
24. S. K. Nayak, B. K. Rao, S. N. Khanna, et al., “Atomic and electronic structure of neutral and charged SinOm clusters,” J. Chem. Phys. 109(4), 1245–1250 (1998). [CrossRef]
25. G. S. Henderson, D. R. Neuville, B. Cochain, et al., “The structure of GeO2–SiO2 glasses and melts: A Raman spectroscopy study,” J. Non-Cryst. Solids 355(8), 468–474 (2009). [CrossRef]
26. Y. Chen, W. Wang, Y. Yang, et al., “Near 0.5 dB gain per unit length in O-band based on a Bi/P co-doped silica fiber via atomic layer deposition,” Opt. Express 31(9), 14862–14872 (2023). [CrossRef]
27. T. Y. Wang, J. X. Wen, W. Y. Luo, et al., “Influences of irradiation on network microstructure of low water peak optical fiber material,” J. Non-Cryst. Solids 356(25-27), 1332–1336 (2010). [CrossRef]
28. X. Sun, J. Wen, Q. Guo, et al., “Fluorescence properties and energy level structure of Ce-doped silica fiber materials,” Opt. Mater. Express 7(3), 751–759 (2017). [CrossRef]
29. Q. Wang, J. Wen, Y. Luo, et al., “Enhancement of lifetime in Er-doped silica optical fiber by doping Yb ions via atomic layer deposition,” Opt. Mater. Express 10(2), 397–407 (2020). [CrossRef]
30. T. Li, J. Wen, Y. Wu, et al., “Influences of Bi and Yb ions on the emission efficiency of an Er-doped silica optical fiber,” Opt. Mater. Express 12(10), 3918–3929 (2022). [CrossRef]
31. Y. Dong, J. Wen, F. Pang, et al., “Optical properties of PbS-doped silica optical fiber materials based on atomic layer deposition,” Appl. Surf. Sci. 320, 372–378 (2014). [CrossRef]
32. Y. Dong, M. Zhang, H. Zhang, et al., “Enhanced fluorescence and gain characteristics of PbS doped silica fiber with PbSe nano-semiconductor co-doping,” Opt. Fiber Technol. 80, 103370 (2023). [CrossRef]
33. J. L. F. Da Silva, M. V. Ganduglia-Pirovano, J. Sauer, et al., “Hybrid functionals applied to rare-earth oxides: The example of ceria,” Phys. Rev. B: Condens. Matter Mater. Phys. 75(4), 045121 (2007). [CrossRef]
34. M. Atanasov, C. Daul, H. U. Gudel, et al., “Ground states, excited states, and metal-ligand bonding in rare earth hexachloro complexes: A DFT-based ligand field study,” Inorg. Chem. 44(8), 2954–2963 (2005). [CrossRef]
35. K. Raghavachari, “Perspective on “Density functional thermochemistry. III. The role of exact exchange”,” Theor. Chem. Acc. 103(3-4), 361–363 (2000). [CrossRef]
36. C. Peng and H. Bernhard Schlegel, “Combining Synchronous Transit and Quasi-Newton Methods to Find Transition States,” Isr. J. Chem. 33(4), 449–454 (1993). [CrossRef]
37. H. T. Amorim, M. T. de Araujo, E. A. Gouveia, et al., “Infrared to visible up-conversion fluorescence spectroscopy in Er3+-doped chalcogenide glass,” J. Lumin. 78(4), 271–277 (1998). [CrossRef]
38. Y. Wang, N. K. Thipparapu, D. J. Richardson, et al., “Ultra-Broadband Bismuth-Doped Fiber Amplifier Covering a 115-nm Bandwidth in the O and E Bands,” J. Lightwave Technol. 39(3), 795–800 (2021). [CrossRef]
39. D. Sporea, L. Mihai, D. Negut, et al., “γ irradiation induced effects on bismuth active centres and related photoluminescence properties of Bi/Er co-doped optical fibres,” Sci. Rep. 6(1), 29827 (2016). [CrossRef]
