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Abstract
In this study, the laser sintering technique combined with the stack-capillary-draw method was used to manufacture Er3+/Yb3+ co-doped silica glass microstructure optical fiber for up-conversion (UC) luminescence for the first time. The obtained fiber, whose structure can be maintained well, shows no obvious element diffusion during the fiber-drawing process. The core glass is amorphous without any crystallization, and Er and Yb elements are uniformly distributed throughout the fiber core. Intense upconverted green and red emissions corresponding to (2H11/2, 4S3/2) → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively, together with an emission corresponding to 2H11/2 → 4I13/2 transition were observed upon the excitation of a 976-nm laser. The intensity of green and red UC luminescence shows a linear dependence on the excitation power which demonstrates that a two-photon absorption process is responsible for the UC process. The results demonstrated the laser sintering technique provides an alternative method to manufacture Er3+/Yb3+ co-doped fiber, which would promote the development of UC luminescent materials.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Up-conversion (UC) luminescence from rare-earth (RE) ions doped optical fibers has attracted extensive attention for domestic and international researchers due to its potential applications in biomedical imaging, multicolor display, sensing, and visible lasers [1–6]. Exploration of a manufacture method or a new host of UC materials has been a pursuit of researchers in recent years.
Much attention has been paid to non-silica glass hosts such as silicate [7], tellurite [8], phosphate [9] and fluoride [10] and so on [11,12] due to their high solubility of RE ions in the manufacture process. However, non-silica optical fibers have some intrinsic disadvantages, such as poor physical and mechanical properties, which make them difficult to fuse and couple with commercial silica optical fiber. Comparing with these fibers, silica glass fibers exhibit unique features including stable thermal performance, high mechanical strength, and low loss in visible light region [13]. It is worth noting that by co-doping appropriate amount of Al$^{3+}$ ions, the ion clusters formed by the narrow solubility of RE ions can be effectively improved, promoting the development of silica glass fibers [14]. Among the trivalent RE ions, Er$^{3+}$ is the most promising UC material to convert near-infrared light into visible light, where blue, green and red emission light can be observed. However, the absorption cross section of single Er$^{3+}$ doped fiber is small, leading to a weak UC luminescence [15]. To improve the UC intensity of Er$^{3+}$, Yb$^{3+}$ is usually introduced as a sensitizer to improve the pump absorption due to its large absorption cross section near 980 nm.
Currently, most of Er$^{3+}$/Yb$^{3+}$ co-doped UC optical fibers are manufactured by MCVD method [16]. Thereinto, the size of doped glass core is limited by deposition in the tube on substrate surface. In sintering process, the volatilization of doped ions results in the formation of central subsidence or peak [17]. The layer-by-layer sintering results in the uneven distribution of refractive index. In order to solve these problems, new techniques for the manufacture of RE doped silica glass have been put forward, including sol-gel method [18], glass phase-separation technology [19], reactive powder sintering of silica (REPUSIL) process [20] and laser additive manufacturing technology [21]. These methods mentioned above have already made important progress. Until now, our group has been devoted to the research of RE doped optical fiber, especially the silica glass microstructure optical fiber manufactured by the laser sintering technique combined with stack-capillary-draw method. For example, Yb$^{3+}$-doped fibers and Tm$^{3+}$-doped silica glass microstructure optical fibers for fiber lasers have been successfully manufactured in our lab [22–24]. The CO$_{2}$ laser was adopted to heat the raw materials at a working wavelength of 10.6 µm, which is the absorption wavelength of silica glass. As an ideal heat source for melting RE ions doped silica glass, CO$_{2}$ laser can flexibly adjust the temperature gradient and thermal field to ensure the homogeneity of doped silica rod in pure oxidation atmosphere. The non-contact laser on the raw material avoids the pollution to the optical fiber core glass and environment.
In this work, we adopted the laser sintering technique combined with stack-capillary-draw method to manufacture the Er$^{3+}$/Yb$^{3+}$ co-doped UC luminescent silica glass microstructure optical fiber. The material structure and elemental distributions of fiber were analyzed. Under the excitation of a 976-nm laser, the UC properties of Er$^{3+}$/Yb$^{3+}$ co-doped silica glass microstructure optical fiber were investigated.
2. Fiber manufacture
The specific preparing steps of Er/Yb co-doped silica glass microstructure optical fiber through the laser sintering technique [22] combined with stack-capillary-draw method are described in Fig. 1. First, the powders such as Er$_{2}$O$_{3}$, Yb$_{2}$O$_{3}$, Al$_{2}$O$_{3}$ and SiO$_{2}$ (with a purity of 99.99$\%$), were selected as raw materials, which were calculated according to the specific ratio. Second, the powders were weighed with electronic precision balance and mixed evenly (Fig. 1(a) and (b)). Third, the mixed powder was heated at 800-1100$^{\circ }$C for over 24 hours with a mixture of O$_{2}$ and Cl$_{2}$ gas to eliminate the hydroxyl group (Fig. 1(c)). Fourth, the Er/Yb co-doped silica powder was heated by a CO$_{2}$ laser, and the melting zone temperature was up to 2000$^{\circ }$C. During the heating process, the parent rod rotated downward uniformly and slowly to obtain the uniform Er/Yb co-doped silica glass rod (Fig. 1(d)). Fifth, Er/Yb co-doped silica glass rod was cut and polished into a cylindrical glass rod with a diameter of 3 mm and a length of 15 mm, which was used as the fiber core of the optical fiber preform. Simultaneously, a transparent Er/Yb co-doped glass sample is cut and polished into about 6.2$\times$5.4$\times$4.5 mm$^{3}$ for the optical characterisation (Fig. 1(e)). Sixth, the prepared cylindrical glass rod was wiped with alcohol to remove the contaminated surface layer, and used as the fiber core to prepare the fiber preform by the capillary stacking method (Fig. 1(f)). Seventh, one end of the preform was sealed through an oxy-hydrogen flame ahead of time (Fig. 1(g)). After that, the fiber was successfully drawn from the preform using the special fiber drawing tower at a high temperature of $\sim$1800$^{\circ }$C (Fig. 1(h)).
The amorphous state and crystallisation phase of the core glass were investigated by X-ray powder diffractometer (XRD, D8 Advance, Bruker) with Cu/Ka radiation. The homogeneity of the glass was analysed using an energy dispersive spectrometer (EDS, JSM-6701F, USA). Absorption spectra were obtained using a spectrophotometer (Lambda 900 UV-VIS-NIR, Perkin-Elmer, UK). The fiber loss was measured by the cutback technique using a light source (ZWH3000, Biaoqi, China) and a optical spectrum analyzer (AQ6370D, Yokogawa, Japan). The fiber cross section and elemental distributions were obtained using an EPMA system (EPMA-1600, Shimadzu, Kyoto, Japan). A 976-nm laser diode was used to pump the Er/Yb co-doped silica glass microstructure optical fiber, and then a Maya2000 Pro spectrometers (Ocean Optics, USA) was used to obtain UC fluorescence spectra at a wavelength of 480-800 nm. All the experiments were conducted at room temperature.
3. Optical performance
The XRD pattern was utilized to determine structural performance of the core glass. Fig. 2 presents the XRD diagram of Er/Yb co-doped silica glass prepared by the laser sintering technique. The scanning area of 2$\theta$ was set from 10$^{\circ }$ to 80$^{\circ }$ with a step size of 0.02$^{\circ }$. Only the broad band around a peak of 2$\theta$= 22$^{\circ }$ was observed, indicating the amorphous nature without crystallization, which is a typical diffraction characteristic of glass.
The EDS spectrum in Fig. 3 presents the element composition of the Er/Yb co-doped silica glass, which confirms the presence of erbium (Er), ytterbium (Yb), aluminum (Al), silicon (Si) and oxygen(O). The inset shows the EDS spectral region. To analyse the uniformity of doped elements in the core glass, EDS area scanning and line scanning of glass sample were measured, as shown in Fig. 4. O and Al elements were uniformly distributed in the silica matrix. Due to the small contents of Er and Yb elements, the scanning points were distributed sparsely. As can be seen in Fig. 4(e), the intensity of O, Al, Yb and Er elements changes little along the scanning line, indicating the doped elements in silica matrix exhibited an excellent uniformity.
The absorption spectra was measured at 320-1650 nm, as shown in Fig. 5. The absorption bands of Er$^{3+}$ and Yb$^{3+}$ ions, peaked at 366, 380, 408, 452, 488, 522, 544, 654, 796, 922, 976 and 1530 nm, were observed. Thereinto, the absorption intensity at 976 nm is strongest, which is generally chosen as the pumping source of Er/Yb co-doped fibers for UC luminescence experiments. As can be seen from the inset of Fig. 5, the transmission spectrum shows that Er/Yb co-doped glass sample maintains a good transparency, and the transparency is up to 83$\%$ in the visible region and 90$\%$ in the near-infrared region.
Using Er/Yb co-doped silica glass as the core, a fiber preform was fabricated through stack-capillary-draw method and drawn into the Er/Yb co-doped microstructure optical fiber through a special fiber drawing tower. The end profile of the optical fiber is shown in Fig. 6(a). The diameter of fiber core, inner cladding and outer cladding are 35 µm, 106 µm and 136 µm, respectively. The fiber loss at 1310 nm was measured to be 0.50 dB/m by the cutback technique, which is much lower than that of highly Er$^{3+}$/Yb$^{3+}$ co-doped phosphate core multimaterial fibers (7.5 dB/m) by the melt-quenching method [25], and Er$^{3+}$ doped phosphate glass fiber ($\sim$10 dB/m) by the rod-in-tube technique [26]. To have an insight into the uniformity of rare earth ion doping in the fiber, we provided the EPMA pattern of Er/Yb co-doped microstructured optical fiber. Fig. 6(b-d) shows the EPMA area scanning of Al, Er, Yb and Si elements in the fiber ends. It was observed that the bright spots in the core area are relatively uniform. The scattered spots in cladding area are caused by the detection noise of the instrument. The boundary structure of core and cladding in Fig. 6(b-d) is similar to that in Fig. 6(a), which proves that the core-cladding structure of Er/Yb co-doped microstructure optical fiber is completely preserved. The line scanning in Fig. 6 shows that the doped elements in the fiber core fluctuates slightly, further confirming that the distribution of elements in the fiber core area is fairly uniform. The range of intensity fluctuation is 120-180, 190-330 and 330-560 for Er$^{3+}$, Yb$^{3+}$ and Al$^{3+}$ ions, respectively.
Fig. 6. (a) The end profile of Er/Yb co-doped silica microstructure optical fiber; (b) The EPMA area scanning of Al, Er, Yb and Si elements; (c) The EPMA line scanning of Al, Er and Yb elements.
4. UC luminescence experiments
Fig. 7 provides the experimental setup of UC luminescence in Er/Yb co-doped silica microstructure optical fiber. The pumping source of 976 nm laser was injected into the optical fiber in a spatially coupled way through a collimating lens L1 (f=20 mm) and a focusing lens (f=12.7 mm). After the 976 nm residual pump light was filtered out by a filter (Thorlabs FESH900), the UC spectra were collected by the spectrometer. The inset provides the fiber photo captured during the UC process excited by a 976-nm laser. The fiber bahaves a very bright UC luminescence to the naked eyes.
Fig. 7. The experimental setup of UC luminescence in Er/Yb co-doped silica microstructure optical fiber. MMSF: multimode silica fibre. L1: collimating lens f=20 mm. L2: focus lens f=12.7 mm. F1: Filter (THORLABS FESH900). Inset is the fiber photo captured during the UC process.
Fig. 8 shows the UC luminescence spectrum of Er/Yb co-doped silica microstructure optical fiber under the 976-nm fiber laser excitation. Four UC emission bands peaking at 530, 546, 661, and 788 nm are visible, which was attributed to the transitions of $^{2}$H$_{11/2}$ $\rightarrow$ $^{4}$I$_{15/2}$, $^{4}$S$_{3/2}$ $\rightarrow$ $^{4}$I$_{15/2}$, $^{4}$F$_{9/2}$ $\rightarrow$ $^{4}$I$_{15/2}$, and $^{2}$H$_{11/2}$ $\rightarrow$ $^{4}$I$_{13/2}$, respectively. Therein, the upconverted green emission intensity at 546 nm, and the upconverted red emission intensity at 661 nm are much stronger than other emission bands. Fig. 9 shows the UC luminescence spectra of 0.7-m fiber under different 976-nm laser powers. The spectra were gradually enhanced with pump power increasing but the emission peak did not shift. When the pump power of laser was higher than 1.88 W, no luminescence saturation was observed. The UC results indicate that this fiber material by the laser sintering technique is promising in application of short wavelength laser generation.
Fig. 8. The obtained UC luminescence spectrum of Er/Yb co-doped silica microstructure optical fiber.
Fig. 9. The UC luminescence spectra of Er/Yb co-doped silica microstructure optical fiber upon the excitation of a 976-nm laser at different pump powers.
The UC emission intensities I have a relationship with the pump powers P, namely ${I} \varpropto {P}^{n}$, where n represents the photon numbers of 976-nm wavelength to emit a photon in upconverted luminescence [5]. The relationships between emission intensities and pump powers are plotted on a double-log scale, where the slope coefficient of the linear fitting line determines the number of photons n. As shown in Fig. 10, the slope coefficients of the double-log plots are fitted to be 1.99, 1.82, 1.41. and 1.21 for the UC emission luminescence at 530 nm (green), 546 nm (green), 661 nm (red) and 788 nm, respectively. These values are all close to 2, indicating that a two-photon absorption process was involved in the UC emission [15].
Fig. 10. The double-log plots of UC luminescence in the Er/Yb co-doped silica microstructure optical fiber upon the excitation of a 976-nm laser.
Fig. 11(a) shows the UC luminescence spectra of Er/Yb co-doped silica microstructure optical fiber upon the excitation of a 976-nm laser at different lengths. When the length of the fiber increases from 0.7 m to 1.5 m, the peak of red emission (661 nm) was shifted by 8 nm to long wavelength, derived from the re-absorption of Er$^{3+}$: $^{4}$F$_{9/2}$ $\rightarrow$ $^{4}$I$_{15/2}$ transition. The red emission was re-absorption by the Er$^{3+}$ along the direction of light transmission, which is enhanced with the increasing of the fiber length. Meanwhile, with the increase of fiber length, no red shift is observed in the peak of green emission (530 nm and 546 nm). This may be due to the fact that Er$^{3+}$: $^{4}$H$_{11/2}$ and $^{4}$S$_{3/2}$ energy levels are farther away from the ground state Er$^{3+}$: $^{4}$I$_{15/2}$ energy level, so the radiant fluorescence is no longer absorbed. Fig. 11(b) shows the green/red intensity ratio as a function of fiber length. It can be observed that the ratio fluctuation is very small as the length increased from 0.7 m to 1.5 m, indicating that the green and red UC luminescence is very stable.
Fig. 11. (a) The UC luminescence spectra and (b) The green/red intensity ratio at different lengths of Er/Yb co-doped silica microstructure optical fiber upon the excitation of a 976-nm laser.
5. Conclusion
In this study, we exhibited how a Er/Yb co-doped silica glass microstructure optical fiber was manufactured through the laser sintering technique combined with stack-capillary-draw method, and demonstrated UC luminescence performance of the fiber. The core glass is amorphous without any crystallization, and exhibits an excellent uniformity, maintaining a good transparency of 83 $\%$ in the visible region and 90 $\%$ in the near-infrared region. The structure of the corresponding Er/Yb co-doped fiber is completely preserved in the drawing process. Er$^{3+}$ and Yb$^{3+}$ ions are distributed throughout the fiber core with an excellent uniformity. Upon the excitation of a 976-nm laser, the strong green and red UC luminescence was observed. It can be concluded that a two-photon absorption process was involved in the UC emission. These experimental results show that the technique is likely to be an effective complement for the method to obtain UC luminescent materials.
Funding
National Natural Science Foundation of China (61935007, 61935010, 61735005, 61527822, 62005081); National Key Research and Development Program of China (2018YFB0407400, 2019YFB2204002); Research and Development Program in Key Areas of Guang dong Province (2018B010114002); Guangdong Basic and Applied Basic Research Foundation (2019A1515111120); The startup fund from Southern University of Science and Technology; Project support by Future Greater-Bay Area Network Facilities for Large-scale Experiments and Applications (LZC0019); The Verification Platform of Multi-tier Coverage Communication Network for Oceans (LZC0020).
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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