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Abstract
In this study, the influence of ytterbium ions (Yb3+) on the fluorescence lifetime of erbium (Er)-doped silica optical fiber (EDF) is investigated. An Er/Yb co-doped fiber is fabricated by modified chemical vapor deposition (MCVD), along with the atomic layer deposition (ALD) method. Moreover, the spectral properties of this fiber, such as absorption, luminescence, excitation and emission spectra, and the fluorescence lifetime, are studied experimentally; the results of the experiments are then compared with those of the EDF. The results revealed the existence of a broadband luminescence spectrum at 800-1300 nm. The fluorescence lifetime of the Er/Yb co-doped fiber at 1531 nm is 11.77 ms, whereas that of the EDF is 10.16 ms. The lifetime of Yb3+ is 415 µs, which is 565 µs less than that of the Yb-doped fiber (980 µs), at 1033 nm. Simultaneously, various models of the Er-doped, Yb-doped, and Er/Yb co-doped fibers in three membered ring (3MR) structures were built, and their excited states were analyzed. The results indicated that an energy transfer is associated with the change in lifetime, and that the doping of Yb3+ significantly improves the fluorescence lifetime of Er3+ at 1533 nm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber amplifier is the key to realizing all-fiber communication. Currently, erbium (Er)-doped fiber amplifiers are widely used for this purpose. Er-doped glass is an important gain material for lasers and amplifiers [1]. The luminous peak of Er occurs at around 1500 nm, and thus, is widely employed in optical communication [2–4]. Er-doped fibers have been rapidly developed as the main laser amplification medium for C-band (1530-1565 nm) and L-band (1565-1625 nm) optical amplifiers [5–7]. Er3+ is generally utilized as a network modifier in silica for Er-doped fibers. However, providing sufficient coordination anions that combine with Er3+ is difficult, which may result in uneven distribution in the silica fiber [8,9]. Furthermore, concentration quenching may occur in the event of high doping concentration; in addition, the pump light is not efficiently absorbed by Er3+. Consequently, pump conversion efficiency, fiber amplifier gain, and the efficiency of output power are reduced [10,11]. To improve the efficiency and signal gain of the system, some sensitizer ytterbium ions (Yb3+) are incorporated within the core of the Er-doped fiber; the Er/Yb co-doped fiber is used as the gain medium. Because of Yb ions larger absorption cross-section, ions in ground state are more efficiently excited to higher energy levels [12]. It must be noted that the incorporation of Yb3+ can inhibit the formation of Er3+ ion clusters and reduce concentration quenching. Furthermore, because the Yb3+ ions possess an approximately 800-1000 nm pump absorption broadband and excitation broadband at 970-1200 nm, they can efficiently absorb the 980-1064 nm pump and quickly transfer the energy to the Er3+ ions to realize a regional light amplification of 1550 nm [13–15].
Thus, it is clear that obtaining the maximum gain and a large emission cross-section are important for the development of optical amplifiers with small component dimensions. The fluorescence lifetime must be as high as possible to allow for a greater pulse power. Accordingly, many reports have indicated that Yb3+ can efficiently transfer energy to other ions, including the Er3+, and that show an impact of Yb3+ co-doping on the photoluminescence decay time attributed to an energy transfer [16]. However, the research of MCVD combined with ALD technology is little. It can precisely control the thickness of the doped materials in the process of fiber preparation, and can fabricate the doped silica optical fiber with good characteristics, uniform materials, strong dispersion, and high doping concentration. ALD alternately introduces the gas phase precursor pulse of the doping source into the heating reactor, and then carries out the chemical reaction process successively, depositing on the surface of the substrate tube until the surface saturation automatically terminates.
In this study, an Er/Yb co-doped silica fiber is fabricated by modified chemical vapor deposition (MCVD), along with atomic layer deposition (ALD). The optical properties of this fiber are analyzed and compared with those of the Er-doped fiber. Furthermore, a local microstructure model of various-ions-doped silica fiber is developed, and its structural characteristics and energy level parameters are studied. This analysis can lead to a better understanding of the energy transfer between Yb3+ and Er3+.
2. Experimental process
An Er/Yb co-doped silica fiber was fabricated by a novel method—an MCVD and ALD combination [17,18]. The composition of the fiber preform material was then determined using a scanning electron microscope (SEM) (JSM-7500F, JEOL LTD, Japan). As provided in Table 1, which concentration ratio was better in our experiment, but not the best. The percentages of Er and Yb were 1.66 wt% and 0.7 wt%, respectively. A small quantity of Al doping—1.39 wt%, can improve the characteristics of the Er and Yb ions. The refractive index difference (RID) was measured using an optical fiber analyzer (S14, Photon Kinetics Inc., USA). The luminescence spectrum was recorded in a backward pump setup, in which a 980 nm laser (as the pump source) and an optical spectrum analyzer (OSA) (Yokogawa AQ-6315A) were used. The excitation and emission spectra and fluorescence lifetime of the optical fiber were obtained using a fluorescence spectrometer (FLS980, Edinburgh Instruments Inc., English) equipped with a red sensitive single photon counting photomultiplier (Hamamatsu R928P), placed in a Peltier air-cooled housing. The test details were as follows: the optical fiber samples were stripped off the coating materials, cut to about 12 cm, and about ten optical fibers were bundled and placed on the fixture, and then it was placed on the sample stage. Firstly, the emission spectra were measured and obtained according to the absorption spectrum characteristics of the doped optical fibers, then based on the intensity of different emission peaks, the corresponding excitation spectra were found. Finally the excitation spectra were measured.
Table 1. Weight percentage list of the various silica fibers
The core and cladding diameters are 16.2 µm and 122.8 µm, respectively, whose cross-sections are shown in Fig. 1. As shown in the figure, the RID between core and cladding is 0.005.
2.1 Absorption and luminescence spectra
To study the optical properties of the fiber further, the cut-back method was used to analyze the spectral absorption, which is shown in Fig. 2. The figure clearly shows four absorption peaks of the Er-doped fiber in the band range 600-1700 nm, which occur at 646 nm (9.1 dB/m), 783 nm (5.1 dB/m), 968 nm (12.2 dB/m), and 1525 nm (28.2 dB/m). The absorption of the Er/Yb co-doped fiber is enhanced, due to the increase in ions concentration. And five absorption peaks are seen in the band range 600-1700 nm, which occur at 642 nm (19.6 dB/m), 790 nm (7.1 dB/m), 900 nm (11.6 dB/m), 966 nm (20.4 dB/m), and 1525 nm (21.3 dB/m). Further, the absorption at 980 nm is significantly enhanced because of the incorporation of Yb3+, forming a new absorption peak at 900 nm. Yb3+ has a significant absorption broadband at 980 nm in 2F7/2→2F5/2 [19]. It is further seen that the absorption of Yb3+ at 980 nm influences the energy level emission of Er3+ at 4I15/2→4I11/2 [20]. Moreover, the absorption at 642 nm is enhanced, which belongs to Er3+ at 4F9/2→4I15/2, which indicates that the Yb ions can enhance the up-conversion absorption efficiency of the Er ions.
Fig. 2. Absorption and luminescence spectra of different silica optical fibers: (a) Absorption spectra and (b) Luminescence spectra.
To analyze the emission characteristics of the Yb3+ ions doped with Er3+ further, we measured the luminescence characteristics of the optical fiber (1.0 m) by the backward pumping method, under excitation at 980 nm with 400 mW, which is shown in Fig. 2(b). The up-conversion luminescence peaks of 523, 542, and 648 nm are mainly attributed to the electronic transitions of Er3+, corresponding to 4H11/2, 4S3/2, and 4F9/2→4I15/2 [21]. The luminescence peaks at 800 nm and 1300 nm bands are clearly seen, which belong to typical emission peaks of the Yb ions. The luminescence spectrum at 648 nm of the Er/Yb co-doped silica fiber is apparently stronger than that of the Er-doped fiber. The observed emissions of the Er3+ ions are due to the transition from red at 650-700 nm to the 4F9/2→4I15/2 transition [22]. It is seen that, in addition to increasing the absorption, the Yb ions enhance the up-conversion fluorescence of Er ions. This results in transfer of energy from the excited Yb3+ ions to the excited Er3+ ions. The photons are stacked and the Er3+ ions may be excited to the 4F7/2 level.
2.2 Excitation and emission spectra
To further understand the characteristics of optical fiber, we analyzed the excitation and emission spectra. The excitation and emission spectra of the Er-doped silica fiber are shown in Fig. 3(a). Experimental results indicated a noticeable excitation peak at 980 nm in the excitation spectrum. The sample was excited with 980 nm, and the corresponding emission peak was located at 1530 nm; simultaneously, the transition of Er3+ ions from 4I11/2 to 4I15/2 occurs.
Fig. 3. Excitation and emission spectra of: (a) Er-doped silica optical fiber and (b) Er/Yb co-doped silica optical fiber.
Because of the obvious energy conversion between Yb3+ and Er3+, the excitation and emission spectra of the Er/Yb co-doped silica fiber material were analyzed, and the results are shown in Fig. 3(b). In the excitation spectrum, there are obvious excitation peaks at 918 nm and 980 nm which vary from that of the Er-doped fiber [23]. Further, 918 nm occurs in the Yb3+ ion transition absorption band, which indicates the likely existence of energy transfer in the Er3+ and Yb3+ ions. The Er/Yb co-doped fiber sample was excited with 918 nm, and the corresponding emission spectrum was observed to be composed of two parts: the first part comprises two emission peaks at 976 nm and 1030 nm, caused by the transition of Yb3+ from 2F5/2 to 2F7/2; the second part is composed of one emission peak at 1533 nm, caused by the transition of Er3+ from 4I13/2 to 4I15/2. Furthermore, the excitation peak of Yb ion occurred at 918 nm, But the characteristic peak of Er ion was still observed in the emission spectrum, which indicates the possibility of energy transfer.
Then the Er/Yb co-doped fiber sample was excited with 980 nm, and the corresponding emission peaks were observed: the emission peak of 1033 nm is caused by the transition of Yb3+ from 2F5/2 to 2F7/2, and that of 1531 nm is caused by the transition of Er3+ from 4I13/2 to 4I15/2. In addition, because of the absorption of 980 nm in Yb3+ during the transition from 2F7/2 to 2F5/2, the excitation peak at 980 nm is obviously stronger than that at 918 nm, indicating that Yb3+ absorbed a part of the excitation light and enhanced the emission intensity of Er3+ at 1533 nm, which indicates an energy conversion between Yb3+ and Er3+ [24].
2.3 Fluorescence lifetime decay curve
According to the Forster-Dexter resonance energy transfer theory [25], if a resonance energy transfer exists in a material, the fluorescence lifetime of the donor will reduce with the increase in energy transfer efficiency, whereas that of the receptor will increase [16]. The fluorescence lifetime of the Er-doped fiber was further measured, as shown in Fig. 4. Based on the excitation emission spectrum results, the excitation light of Er-doped silica fiber was set at 980 nm and the emission light was set at 1530 nm, and then the fluorescence lifetime was measured to be of 10.16 ms. Further, we tested the Yb-doped silica fiber (700 ppm) under an excitation of 980 nm and emission of 1018 nm, under which condition, the fluorescence lifetime was measured as 980 µs, which is similar to the typical lifetime of Yb3+ (840 µs) in silica fibers [19].
Fig. 4. Luminescence decay curves of: (a) Er-doped silica optical fiber and (b) Yb-doped silica optical fiber.
We further measured the fluorescence lifetime of Er/Yb co-doped fiber, which is shown in Fig. 5. The excitation light of the Er/Yb co-doped silica fiber was set at 980 nm and the emission light at 1531 nm. Under this condition, the fluorescence lifetime was measured to be as high as 11.77 ms. It must be noted that when the emission light was set at 1033 nm, the fluorescence lifetime of Yb3+ was measured as 415 µs.
Fig. 5. Luminescence decay curves of Er/Yb co-doped silica optical fiber at: (a) 1033 nm and (b) 1531 nm.
Based on the comparison of fluorescence lifetime, the lifetime of Er3+ at 1531 nm in the Er/Yb co-doped silica fiber is 11.77 ms, whereas that in the Er-doped fiber at 1530 nm is 10.16 ms—an increase of 1.61 ms. Moreover, the lifetime of Yb3+ at 1030 nm in the Er/Yb co-doped silica fiber is 415 µs, and that in the Yb-doped fiber is 980 µs—a reduction of 565 µs. These results indicate the possibility of energy transfer between the Er and Yb ions, particularly from Yb3+ to Er3+. The material absorbs the excitation light energy, excites the Yb3+ ions, and then transfers the energy from Yb3+ to Er3+, where Er3+ is stimulated to emit light.
3. Theoretical calculation and discussion
For the silica optical fiber, it is made of amorphous structure. In an amorphous silica optical fiber material, a large number of 3-membered rings (3MRs), and similarly 4MR, 6MR, and hybrid ring structure units exist [26]. The doping microstructures around the silica rings are useful in forming stable binary silica structures among hybrid rings and network interstices, as described in detail in our previous study [27]. For the Er-doped silica optical fibers, Er ions are doped into the ring structure. Then a doping microstructure model was built and calculated. The calculation results show that the optical properties of the doped materials mainly come from the doped ions role. Then the silica network structure has a small effect on the optical properties. Besides, the 3MR ring is also considered in economic calculations. A 3MR structure model widely accepted for doped silica materials was also reported in [28]. In addition, ALD is atomic scale, nanometer-level particles doping method. It can make the ions better dope into the soot body of SiO2 on the inner surface of the substrate tube. What is more, Er or Yb atoms are easier to embed into the inner network structure, and replace the site of the original silicon atom, and then form a binary system. However, other preparation techniques, such as, Outside Vapor Deposition (OVD), MCVD, Solution-liquid or high temperature doping techniques, etc., are micrometer-level particles, and even several hundred micrometers-level particles doping. Therefore, for ALD technique, the doping microstructure model is more suitable representative local structure. At the same time, this description of network structure is only used to visualize the structure model. Therefore, here a 3MR structure is chosen. Furthermore, Raman and IR spectroscopy can be utilized to identify the Si-O-Si or Si-O-Er vibrations. The Er-doped silicon dioxide binary system is homogenous with Si-O-Si and Si-O-Er bonds. X. Wang et al. showed that the Yb3+ ions predominantly form Yb-O-Al and Yb-O-Si linkages in the Yb-doped high-silica lanthanum alumino-silicate glass [29]. A large number of bridging oxygen with T-O-T (T:Er3+ or Yb3+) bond linkages result in the formation of a network structure [30].
The calculations were performed using the Gaussian 09 software, and the structures were optimized using the DFT with Becke-type three-parameter Lee-Yang-Parr (B3LYP) hybrid function [31]. For oxygen (O), hydrogen (H), and silicon (Si) elements, the 6-31 + G** basis sets were used. For Er and Yb elements, 11 valence electron effective core potentials (ECPs) for trivalent state was used [32]. The optical properties of the absorption and emission spectra were calculated based on the TD-DFT [33]. First, each model was optimized to obtain the most stable model. Then the harmonic frequency of the system was calculated based on the optimized structure model. Finally, the energy level of the models was obtained and the influence of Yb3+ on Er/Yb co-doped fiber was observed. In addition, the effect of Al ions is not considered because of the economic calculation [34]. After explain the energy transfer between Yb and Er, it will be taken.
In this study,we established the models of Er3+ based on the 3MR structure; the 3MR structure then connected the O and Er atoms. We simulated three different doping models to obtain the relevant computational data, which are shown in Table 2. Based on the calculated excited states of these models, the last model, that had the highest bond energy, was found to be more stable. We applied the same models to the Yb atoms, based on which the basis for further calculations on Er/Yb co-doped local structure model was obtained.
Table 2. Excited state parameters of the Er-3MR local structure model
Based on the optimized structure of ground state, the excitation energy and oscillator strength corresponding to each transition can be computed. The calculated results of Er-3MR are listed in Table 2. Seven excited states associated with energies of 0.7697, 0.9233, 1.4398, 1.6148, 1.8193, 2.0907, and 2.1587 eV, corresponding to the oscillator strengths of 0.0438, 0.0691, 0.0008, 0.0044, 0.0004, 0.0051, and 0.0027, respectively, were present. The calculated absorption bands were located approximately at 1510, 972, 861, 767, 681, 593, and 574 nm, respectively. Comparison with the experimental data indicated that the whole trend was mostly consistent.
In addition, we calculated the excitation state parameters of the Yb-doped quartz microstructure, which are listed in Table 3. Two excited states were present, that were associated with energies of 0.9293, 1.3681 eV, corresponding to the oscillator strengths of 0.0086, 0.0264, respectively. The calculated absorption bands at 1034 nm and 976 nm were found to be in good agreement with the experimental data.
Table 3. Excited state parameters of the Yb-3MR local structure model
3.1 Er/Yb co-doped structural models
Based on previous studies, to understand the influence of Yb ions on optical properties of the Er ions better, the Er/Yb co-doping structure models were built up, as shown in Fig. 6. The red, grey, shallow green, and dark green structures represent O, Si, Er, and Yb, respectively. We chose four kinds of models and calculated the E(UB3LYP) energy.
Table 4 lists the energy parameters of different doping sites in the hybrid 3MR local structure model. From the table, it is seen that Ec ≅ Ed < Eb < Ea. The C and D microstructures doped on the 3MR model were found to be the most stable configurations of the hybrid rings network structure; in addition, these models were calculated using the DFT.
Table 4. Energy parameters of different doping sites in the 3MR microstructure model
We then calculated the four excitation state parameters, which are listed in Table 5. Nine excited states associated with energies of 1.0149, 1.1512, 1.3622, 1.3733, 1.4377, 1.6049, 1.7834, 1.9372, and 2.1585 eV were present, with corresponding oscillator strengths of 0.0002, 0.0064, 0.0288, 0.0038, 0.0067, 0.0010, 0.0011, 0.0001, and 0.0003, respectively. The calculated absorption bands were located at 1521, 1076, 970, 890, 887, 875, 745, 622, and 592 nm.
Table 5. Excited state parameters of the Er/Yb-3MR local structure model
Comparison of the energy levels shows that the absorption peak of the Yb ions occurs at 976 nm, corresponding to an excitation energy of 1.368 eV. Similarly, the absorption peak of Er ions occurs at 972 nm, corresponding to an energy of 0.9233 eV. The energy levels of Yb3+ and Er3+ are similar, and thus, an energy transfer occurs between them; this process can increase the number of particles on 4I11/2. The Er/Yb co-doped model structures are for further study.
From the excited states parameters, compared with Er-doped model, the excitation state of Er/Yb co-doped model was increased by the excitation state at 1076 nm, and in addition, new absorption peaks were generated at 970, 890, 887, and 875 nm. It is important to note that the absorption range the Er/Yb co-doped model was wider, from 875nm to 970 nm, compared with that of the Er-doped model. This indicates that the absorption efficiency of the Er/Yb co-doped fiber is higher than that of the Er-doped model at approximately 980 nm, and that the Yb ions can efficiently transfer their excitation energy to the Er ions. The energy level structures of the Er-doped and Yb-doped fibers are shown in Fig. 7. In the figure, the absorption energy is shown on the left, and f is the intensity of the oscillator strength. This figure clearly illustrates the process of energy transfer between Er3+ and Yb3+, and the increase in energy level at 980 nm; this condition is conducive to electron excitation, which increases the fluorescence lifetime of the Er ions. Therefore, it is clear that this process helps increase the fluorescence lifetime of the optical fiber, and further has a greater significance in enhancing the properties of the Er/Yb co-doped fiber lasers.
4. Conclusions
In this study, we fabricated an Er/Yb co-doped silica fiber using a combination of the ALD and MCVD methods, and investigated its optical properties. The Er/Yb co-doped silica fiber clearly exhibited a broadband luminescence at 800-1300 nm. The fluorescence lifetime of the Er ions was increased by 1.61 ms whereas that of the Yb ions was reduced by 565 µs. Further, the Er-3MR, Yb-3MR, and Er/Yb-3MR local structure models were built, and their structure parameters were calculated using the Gaussian-09 software by DFT and TD-DFT theories. The energy levels of Yb-3MR and Er-3MR were found to be similar-approximately at 970 nm. Furthermore, the Er/Yb-3MR model had a wider absorption range (875-970 nm) on comparison with the Er3+ model. These results sufficiently prove that the doping of Yb3+ significantly improves the absorption cross section of Er3+, which in turn, increases the number of ions in the 4I13/2 energy level, thereby increasing the fluorescence lifetime of Er3+ at 1533 nm. Here, comparing with different kinds of optical fibers via the MCVD combining with ALD fabrication processes, we found the Yb3+ positively influences the fluorescence lifetime of the Er3+ in silica optical fiber. Then the effect of other fabrication technologies on fluorescence lifetime is not discussed. Next we would further focus on the effect of different fabrication technologies on fluorescence lifetime.
Funding
National Natural Science Foundation of China (61520106014, 61975113, 61635006, 61935002).
Acknowledgment
We acknowledge Haijun Wang of Jiangnan University for computer resources
Disclosures
The authors declare no conflicts of interest.
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