Yb/Er co-doped silica glass is the critical core material in the fabrication for Yb/Er co-doped optical fiber. In this paper, we demonstrate the novel laser sintering technology for making the Yb/Er co-doped silica glass rod, and analyze its physical and optical properties. The experimental results show that the fabricated silica glass is amorphous, and Yb3+/Er3+ ions uniformly distribute in the glass matrix. The material presents very good emission property around 1535 nm, and the calculation result of the gain cross section also indicates the good gain property at the emission band. Moreover, the Judd-Ofelt analysis results show that the material has very attractive spectroscopic parameters. These results can be used as good reference for the fabrication of Yb/Er co-doped optical fibers used for high-power fiber lasers in the future.
© 2017 Optical Society of America
The Yb/Er co-doped fibers have many advantages as the gain medium for high power fiber lasers and amplifiers used for eye-safe-based applications and C band optical communication systems [1, 2]. The double cladding large mode area photonic crystal fibers (LMA-PCFs) are believed to be the most attractive selection to overcome the existed issues of nonlinear effect and end face damage in high power fiber laser system [3–5]. Presently, rare-earth doped LMA-PCFs are fabricated mainly based on the Modified Chemical Vapor Deposition (MCVD) technology [6, 7]. However, due to its well-known limitations [8, 9] of low doping concentration, geometry and homogeneity, it is difficult to further enlarge the core size of optical fiber using MCVD. A means of addressing these issues is by utilizing the Non-CVD methods, such as SOL-GEL [10, 11]. But there still exist the unsolved items  of organics contamination, hydroxyl content limitations, valence change of Yb3+ or long processing cycle, which would dramatically affect the background attenuation, laser efficiency and photodarking in high power fiber lasers. As is known, laser manufacturing technology has been wildly used in many areas [12, 13]. The method utilized in this paper is called laser sintering technology (LST) based on CO2 lasers. Due to the strong absorbance of silica glasses to the high energy of CO2 laser operated at the wavelength of 10.6 μm, CO2 laser can be the ideal heat source for melting rare-earth-doped silica glasses. Large size doped silica rod can be easily obtained based on such technology. Compared with the preparation of Yb-doped silica glass , the total doping concentration of the Yb/Er co-doped silica glasses are usually at a much higher level, and it also bring out more serious bubble problem and the accurate controlling of the concentration ratio between Yb and Er. Due to the advantages of the LST, these technical issues can be well solved through the flexible controlling of temperature gradient and thermal field. Meanwhile, benefit from the easy realization of pure oxidizing atmosphere during the preparation process, the LST can also effectively restrain the formation of Yb2+, which can well ensure the effective concentration of Yb3+ and guarantee the energy transfer from Yb3+ to Er3+. Moreover, as the characteristic of non-contact manufacturing procedure, there would exist less contamination of precursory materials.
In this paper, we prepared the Yb/Er co-doped silica glasses utilizing the LST. The properties of material structure, components homogeneity, optical spectrum and other relevant characteristics were measured and analyzed. The Judd-Ofelt analysis was performed, and the relevant spectroscopic parameters were calculated and discussed. The results well indicated the potential applications of the LST for the preparation of multi-rare-earth co-doped gain mediums used for high power fiber lasers. To our knowledge, the research on the properties of the Yb/Er co-doped silica glass prepared by such method has not yet been reported.
2. Experiment procedure
2.1 Fabrication procedure of Yb/Er co-doped silica glass
The mixed powder of SiO2, Yb2O3, Er2O3 and Al2O3 was prepared according to the weight percentage (wt%) of 2.53Yb2O3 - 0.61Er2O3 - 2.46Al2O3 - 94.40SiO2. Then, the mixed powder was sintered to Yb/Er co-doped silica glass by the CO2 laser, and the laser sintering setup is shown in Fig. 1(a). When CO2 lasers radiate on the surface of the pure silica base rod, most laser energy would be absorbed and the heat will transfer in the internal of the silica glass through the processes of phonons and photons thermal conduction. During the process, the surface temperature can reach higher than 2000°C in a very short period of time, and form a temperature gradient along the vertical direction. There would exist a high temperature melting zone as shown. The co-doped powder was delivered to the melting zone using the coaxial powder feeding device by the carrier gas of O2. With the rotation and falling down of the base rod, the melted liquid glass would move bellow the melting zone and the final Yb/Er co-doped silica glass is obtained. The prepared silica glass was cut and roughly polished as shown in Fig. 1(b), the size is of 7.20 mm × 6.18 mm × 52.4 mm.
2.2 Characteristics measurement
The material structure was analyzed using the x-ray diffraction (XRD, D8 Advance, Bruker). The scanning 2θ range is from 10 to 90 degree with a step of 0.1. The concentration proportion was measured using the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, SPECTRO BLUE SOP). The components homogeneity was analyzed using the Energy Dispersive Spectrometer (EDS, JSM-6701F). The absorption properties in ultraviolet and visible band were measured using the light source operated from 250 nm to 2500 nm. The infrared absorption properties were measured using the Fourier Transform Infrared (FTIR) spectrometer (Nicolet 6700). The emission property was measured via the excitation of the diode laser operated at 976 nm. MAYA2000 Pro spectrograph operated from 200 nm to 1100 nm and the NIRQuest256 spectrograph operated from 900 nm to 2500 nm were employed to record the spectrum. The setup of the absorption and emission properties measurements are shown in Fig. 2. The reference sample as shown in Fig. 2(a) is pure silica glass which is used to deduct the end face scattering and background scattering. The 45° dichroitic mirror (DM) in Fig. 2(b) is of high transmission from 800 nm to 1000 nm and high reflection from 1000 nm to 1800 nm, which is used to separate the pump laser from the fluorescence light. The sample used for the spectrum measurements was cut and polished to the thickness of 2.78 mm. All the measurements are carried out under room temperature (25°C).
3. Experiment results and discussion
3.1 Material structure
Figure 3 is the XRD pattern of the prepared Yb/Er co-doped silica glass. The broadness of the diffraction peak shows the amorphous property. The result well indicates the glassy phase structure of the prepared material and no crystalline phase was formed during the preparation procedure.
3.2 Concentration proportion and components distribution
The measured concentration proportion of the prepared Yb/Er co-doped silica glass is shown in Table 1. The measured proportion is a little different from the original components of mixed powder. This can be attributed to the volatilization of the precursory doped powder, which is resulted from high laser power density. And we didn’t detect the impurity elements in the prepared material.
The components enrichment and the concentration fluctuation can be analyzed through the area scanning and line scanning of the EDS, respectively. The area scanning results are as shown in Fig. 4. The scanned points dispersedly distribute in the whole scanning area and the brightness of the points also show very small difference, which indicate the elements of Er, Yb and Al are uniformly doped in the silica matrix. Moreover, as shown in Fig. 5, the components present very small fluctuation along the scanning line. The experimental results also well indicate the uniform distribution of Yb, Er and Al in the silica matrix.
3.3 FT-IR spectrum
Figure 6 is the measured FT-IR spectrum. The hydroxyl content can be monitored by the absorption peak at 3670 cm−1 and calculated according Ref . The hydroxyl content of the prepared Yb-Er co-doped silica glass is approximately 19.0 ppm. It is a lower value compared with the material prepared by the non-CVD method of Nanoporous glass sintering technology . But it is still a little relative higher value, because no special treatment was undertaken to control the moisture of environment and eliminate the moisture in the protection gas of O2. The existed H2O resulted in the produce of hydroxyl. The dehydroxylation works are still being carried out. We believe the hydroxyl content can be reduced to a very low level, due to the well absorption of hydroxyl radical for the laser energy at 10.6 μm.
3.4 Absorption and fluorescence
The measured absorption spectrum is shown in Fig. 7. The prepared material shows very typical absorption characteristic of Yb3+ and Er3+, the corresponding stark levels are marked in the figure. The main absorption peak is located at 976 nm of Yb3+, which can effectively ensure the well absorption of the pumping power. Moreover, there is no obvious absorption band of Yb2+ observed in the visible wavelength range from 300 to 800 nm.
The normalized emission spectrum is shown in Fig. 8. When the material was pumped by the diode laser operated at 976 nm, the main emission peak located around 1530 nm is observed, the main emission band is from 1475 nm to 1700 nm, and the emission of Yb3+ around 1026 nm is very weak. The results indicate obvious energy transition from 2F5/2-Yb3+ level to 4I11/2-Er3+ level.
3.5. Spectroscopic parameters discussion
The measured absorption spectrum is analyzed through the Judd-Ofelt theory . The refractive index and the density of the samples used for analysis are of 1.601 and 2.39 g/cm3 respectively. The absorption bands corresponding to the transitions from the ground state 4I15/2 to the excited states 4G9/2, 4G11/2, 2H9/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2 and 4I9/2 are selected to do the least-squares fitting. The absorption band of 4I11/2 level is not included due to the intense absorption of Yb3+ at the same band. And because of the contribution of magnetic dipole transition of 4I13/2, this absorption band is also excluded. The measured (Sexp) and calculated (Scal) line strengths are shown in Table 2. The fitted J-O intensity parameters Ωk (k = 2, 4, 6) are then used to calculate the line strengths corresponding to the transitions from the upper states. These values are used to calculate the radiative transition probability, branching ratios and radiative lifetime. The most interested upper states are 4I13/2 and 4I11/2 which are of close relationship with the 1530 nm laser under 976 nm excitation. Here, both the contributions of electric dipole transition (Sed) and magnetic dipole transition (Smd) are considered for the transitions of 4I13/2 → 4I15/2 and 4I11/2 → 4I15/2. The results are shown in Table 3.
The local structure and bonding in the vicinity of rare earth ions are usually characterized by the three J-O intensity parameters Ωk. Larger values of Ω2 and Ω4/Ω6 are expected. It can be observed that the Ω2 of our material is of 8.36 × 10−20. This is a relative larger value compared with the other works [18, 19]. Here, Ω2 is is an environment sensitive parameter, and larger value indicates a higher covalency of the chemical bond between the rare earth and oxygen ions and lower symmetry . This means that the asymmetry and covalent environment of Er-O bond in our material is much stronger. Moreover, Ω6 is the vibronic dependent parameter that is related to the rigidity and the viscosity of the host. Smaller value indicates the higher rigidity of the host matrix. The spectroscopic quality of the materials can be characterized by the value of Ω4/Ω6 . As shown in Table 2, this value is of 2.15 for our material which indicates a good matrix for 1530 nm emission. Additionally, as listed in Table 3, the prepared material presents very good lifetime of 8.39 ms at 1530 nm corresponding to the transition from 4I13/2 to 4I15/2. From the attractive spectroscopic parameters above, we can reasonably come to a conclusion that the prepared Yb/Er co-doped silica glass possesses potential applications for the gain medium operated at 1530 nm.
3.6 Gain properties
The gain properties can be qualitatively characterized through the wavelength dependence of net gain as a function of population inversion for the upper state. The gain cross section can be calculated by the equation
where N is the ion density, β is the ratio of population inversion, σemi and σabs are the emission and absorption cross section, respectively, and the two values can be respectively calculated through the McCumber formula and Beer-Lambert equation based on the measured absorption spectrum. The calculated results of the gain coefficient versus wavelength at different population inversion ratio corresponding to Er3+: 4I13/2 →4I15/2 is shown in Fig. 9. It can be observed that when β reaches the value of 0.2~0.4, positive gain can be obtained. Moreover, with increase of β, gain band extends to longer wavelength. When β = 1, the widest gain band can reach a full width at half maximum (FWHM) of 68 nm approximately from 1475 nm to 1620 nm, which can provide a wide selection of the output lasing wavelength in this range.
Yb/Er co-doped silica glass was prepared through the LST. The components of Yb, Er and Al uniformly distributed in the silica matrix. The co-doped silica glass is amorphous, and has attractive spectroscopic parameters and good optical properties. These results would be used as good references for the fabrication of the core materials used for multi-rare-earth co-doped optical fibers used for high-power fiber lasers in the future.
National Natural Science Foundation of China (61575066, 61377100 and 61527822); Guangdong Natural Science Foundation (2014A030313428); and Science and Technology Achievements Transformation Projects of Guangdong Higher Education Institutes (2013B090500025).
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