The local structure around Er3+ in Er-doped optical fiber (EDFs) was explored by X-ray absorption fine structure (XAFS) measurements. Using several new approaches such as a novel sample preparation, we successfully measured the XAFS. The intensities near the 8.36 keV peaks were observed for the first time using X-ray Absorption Near Edge Structure (XANES) analysis. The intensities increased in the order of Er, Er2O3, and EDF samples, indicating that Er3+ in the EDFs existed as an oxide state. Extend X-ray Absorption Fine Structure (EXAFS) analysis also showed that the oxygen coordination number of Er3+ increased as the Al concentration increased and that the Er-O distances of EDF with Al codoping is longer than that of EDF without Al co-doping.
© 2006 Optical Society of America
Er-doped optical fiber amplifiers (EDFAs) have been applied to signal amplification in conventional bands (C-band) and to longer wavelength bands (L-band) in optical communication systems . In a high-capacity transmission system, an EDFA is one of the most important components because it has a distinctive function, which can directly and simultaneously amplify multiple signals in a wavelength division multiplexing (WDM) system. Generally, to have uniform signal power at each channel of the receiver, an EDFAs is required to perform the wide and flat gains in the C and L bands. To achieve this uniform signal power, various proposals have been made. One common approach is Al co-doping with an Er-doped optical fiber (EDF) [2, 3]. Figure 1 shows the gain profiles with different Al concentrations for the entire C-band (1530-1560 nm) amplification. When EDFA is used to amplify the entire C-band, the pump power is adjusted so that the gain peak near 1530nm is similar to that at near 1560nm. Figure 1 indicates that high Al co-doping can flatten the gain profile, and suppress the reduction in the amplification efficiency. This phenomenon is caused by changing the local structure around Er3+ in the silica glass network. Therefore, numerous papers about the local structures around rare-earth ions in various host glasses have been reported. Some of the host glasses were studied include silicate [4, 5, 6], phosphate [5, 6, 7], borate [5, 6, 8], fluorosilicate [5, 6, 9, 10], and sodium silicate glasses [4, 11]. These reports have shown local structure analysis around Er3+ in multi-component bulk glasses with high Er3+ concentrations. On the other hand, to our knowledge, the local structure around Er3+ in a common EDF with appropriate concentration for amplification has yet to be reported. It is very difficult to obtain Er3+ XAFS spectra in an actual optical fiber due to the small amount of Er3+. Typically, the diameter of the Er-doped core is less than 10 µm and the Er3+ concentration in the doped core area is less than 1500 wt ppm.
This paper aims to clarify the contribution of Al co-doping in the gain profile of an EDF. We successfully used X-ray Absorption Fine Structure (XAFS) measurements to observe the local structure around Er3+ in a silica-based EDF for the first time. It is found that Al codoping influences the Er-O coordination number and the Er-O distance. In addition, the Er-O coordination number increases as the Al concentration increases.
2. EDF samples preparation
The EDF preforms were prepared by the conventional CVD method and were drawn into the fibers, which were used for the silica-based fiber fabrication. Table 1 lists the chemical compositions of the samples. The dopant concentration of each sample was measured by an Electron Probe Microanalyzer (EPMA).
Figure 2 shows the cross section of the EDF samples used. Er3+ was doped into the core, which had a 14 µm diameter, and a concentration of about 1000 wt ppm. If the EDF was ground into a typical powder for an XAFS measurement, the average concentration of Er3+ in the sample was around 10 wt ppm, which is below the detection range in an XAFS measurement.
Thus, to increase the Er3+ concentration in the ground sample, the cladding portion was removed by a fluoric acid solution (HF) up to the region with a 20 µm diameter. The length of the EDF for each sample was 100 m. The EDF was ground into powder, which resulted in an average Er3+ concentration of about 500 wt ppm.
3. XAFS measurement
The XAFS measurements were conducted using a Synchrotron Radiation beam line [BL16B2] at SPring-8  as shown in Fig. 3. The incident X-ray was monochromated with a Si(111) double-crystal monochrometer. To reduce higher harmonics, the X-ray was reflected with a cylindrical mirror at 3.5 mrad. After that, the sample was irradiated by the incident X-rays, which passed through an ion chamber. The Er LIII-edge X-ray absorption spectra of the samples were measured in the fluorescence mode; Er metal and Er2O3 were compared in the transmission mode. The detection was performed by a KETEK SCARAS system (7-segmented Silicon Drift Detectors system), which had a detection area of 35 mm2 (5 mm2 × 7 segments). The detected signal was amplified by Ortec 673 amplifiers. To receive the fluorescence X-rays effectively, the interval between the sample and the 7-segment SDD was 10 mm. EXAFS analysis and calculations after each measurement were carried out using analytical software such as WinXAS  and FEFF8 .
4. Results and discussion
Figure 4 shows the X-ray Absorption Near Edge Structure (XANES) spectra of all the samples. The peak, which exists around 8.36 keV (a), in all the samples is the Er3+ LIII-edge absorption peak. Er2O3 has three peaks (a), (b), and (c). The EDFs have two peaks, which are denoted as peak (a) and peak (c). The intensities of peak (a) in the EDFs are higher than those in the Er metal and Er2O3, indicating that Er3+ in the EDFs exists as an oxide state and that oxygen is the coordination atom. In addition, the peak (a) in Samples A and B slightly shifts toward the lower energy side compared to that of Er2O3. Hence, Er3+ in the EDF has a different structure from that of Er2O3. The intensity of peak (c) in the EDFs (Samples A, B, C, and D) increases as the Al concentration increases. However, peak (b), which is more prominent in Er2O3, is weakened in the EDFs (Samples A, B, C, and D), and the intensity becomes smaller as the Al concentration increases.
Figure 5 shows the Extended X-ray Absorption Fine Structure (EXAFS) oscillation structures. A clear structure is observed until 8 in k-space, which provides sufficient information to discuss the nearest-neighbor atom structure. Figure 6 shows the Radial Distribution Functions of Er3+ (Er-RDFs) in the samples. Er-O bonding exists at approximately 0.2 nm in Samples A, B, C, D, and Er2O3. However, the Er-O distance of Sample A is shorter than those of the other samples.
Figure 7 show the relationship between the calculated result of the Er-O coordination number and the Al concentration for each sample. This result is derived from a FEFF  fitting. It is clarified that the Er-O coordination number depends on the Al concentration in the EDF. It should be noted that the Er-O coordination number reaches a minimum around an Al concentration of 1.4 wt%. In addition, Samples C and D indicate that the Er-O coordination number increases as the Al concentration increases. Figure 8 also shows the relationship between the Er-O bonding distance and the Al concentration for each sample. As shown in this figure, Sample A, which is not co-doped with Al has the shortest Er-O distance. In addition, the Er-O distances for samples with Al co-doping are nearly constant around 0.232 nm. These results clearly show that Al co-doping strongly influences the local structure around Er3+ in silica-based EDF.
A plausible reason for this strong influence is that Al distorts the Si-O bonding network. This distortion allows Er3+, which have large ionic radius, can enter into the distorted Si-O bonding network. Thus, the Er-Er distance is expanded, which is consistent with the observation that Al doping suppresses the clustering of Er3+ in the optical amplification using an EDF. Consequently, the Er-O bonding distance becomes longer, and the Er-O coordination number increases.
However, to understand the changes in the local structure around Er3+ by Al doping in a silica-based EDF clearly, more detailed measurement and simulations such as a molecular dynamics simulation are necessary.
To clarify the contribution of Al co-doping in the EDF gain profile, the local structures around Er3+ in the core region of the Er doped fiber (EDF) were investigated by XAFS measurement. We successfully applied XAFS using several new approaches such as sample preparation and the fluorescence X-ray detecting system. From this analysis, we observed for the first time that the Er-O coordination number and Er-O distance are changed by varying the Al concentration, and that the Er-O coordination number increases as the Al concentration increases. Future studies will examine the extent of this influence.
The synchrotron radiation experiments were performed in BL16B2 at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. C03B16B2-4003-N, C04A16B2-4030-N, C04B16B2-4030-N, C05A16B2-4030-N).
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