A large-size (∅5 mm) Yb3+-doped silica fiber core-glass rod with an Al-P co-doped composition: 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%) was prepared by the sol-gel method combined with high-temperature melting technology. We successfully solved the doping homogeneity problem caused by the volatility of P2O5. The doping homogeneity of the glass rod was very high with the maximum refractive index fluctuation Δn < 2 × 10−4. The refractive index of the glass rod was very low because of the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure. A large-mode-area single cladding fiber (LMA SCF) and photonic crystal fiber (LMA PCF) with core diameters of 35 and 50 µm, respectively, were drawn from this glass rod. Owing to the low refractive index of the core-glass rod and the corresponding low numerical aperture (NA, 0.033) of the core, the LMA SCF exhibited a high laser beam brightness with M2 = 1.3-1.4. The LMA PCF exhibited the maximum output power, which was limited by the available pumping power, of 46 W and the slope efficiency of 61%.
© 2015 Optical Society of America
In the past few years, Yb3+-doped silica fibers, especially large-mode-area (LMA) fibers, have been highly valued and extensively studied owing to their application for high power lasers [1–6]. The main performance limitation of Yb3+-doped traditional clad fibers (with a core diameter usually smaller than 30 μm) is ultimately set by the power density in the core, whose extreme increase leads to the onset of nonlinearities and can cause the damage of fiber end facets . This already occurs in the continuous-wave regime; thus, the amplification of ultrashort laser pulses to high peak powers presents further challenges . The restrictions due to nonlinearity can be overcome by applying a fibers core with large-mode-area. Because the LMA can greatly reduce the power density at the fiber pump end, it also enhances the thermal damage threshold during high power pumping. However, with the increase in the core diameter, the laser transmission mode changes from single-mode to multimode, and the laser-beam quality is greatly deteriorated . Much effort has been put to find optimum fiber designs which can offer effective single-mode operation with LMA. Several LMA fibers have been proposed to meet single-mode operation , such as low numerical aperture step index fiber (low NA-SIF) , all-solid single trench fiber (STF) , PCF based designs such as leakage channel fiber (LCF) , hybrid photonic crystal fiber (H-PCF) , photonic bandgap based fiber such as Bragg fiber  and 2D-all solid photonic bandgap fiber (2D-ASPBGF) , and resonant coupling based design such as polygonal chirally coupled core fiber (P-CCC) [15,16]. However, all the mentioned LMA fibers (except for P-CCC) require the refractive index of the core and cladding is very close. Which is very difficult because that the rare-earth ions and co-dopants in the core-glass are refractive index raising materials. Therefore, the preparation of large-size Yb3+-doped silica glass rod with high doping homogeneity and low refractive index as the fiber core is the critical issue.
The fabrication of LMA fiber challenges the traditional modified chemical vapor deposition (MCVD) method of active fiber preform. The MCVD process combined with solution doping, which has the merits of high purity and ultralow optical loss, has been today’s standard production method of fiber preform for industrial applications [17,18]. However, it is difficult to fabricate large-diameter fiber cores for LMA fibers while sustaining the initial homogeneity [19,20]. Although researchers at the University of Southampton  lately reported that they had prepared the LMA fiber with the core diameter of 40 μm and the NA of the core is reduced to 0.038 by MCVD method combined with solution doping, it is very difficult to control the doping homogeneity especially when the P2O5 is co-doped. Researchers  at the Xi’an Institute of Optics and Precision Mechanics recently reported that they can prepare large size Yb3+-doped silica glass rod (∅ 4.4 mm) using chelate precursor doping technique of MCVD method. However, the doping homogeneity is still not very good. In most cases, the smallest index step that can be precisely obtained by applying MCVD fiber perform fabrication technology is about 0.06 [8, 21]. The NA of the core 0.06 allows for an increase in the single-mode core diameters to ~15 μm in the 1 μm wavelength region, and further enlargement of the core dimensions causes multi-mode laser transmission . Recently, researchers in Jena University and Heraeus company have developed a new powder sinter technology [23,24]. This non-CVD technology can prepare large-size Yb3+-doped silica rod with high doping homogeneity. However, Doping of P2O5 with high homogeneity is not easy for this method and there is no report about P doping using this powder sinter technology. Moderate addition of P has a great effect on suppressing photodarkening (PD) in Yb3+ and Al3+ ion co-doped silica fibers . Meanwhile, the addition of P together with Al can reduce the refractive index of the fiber core. We have investigated the refractive index variation trend with different Al/P mole ratio in Yb silica glass. When Al and P are co-doped in equimolar amounts, the refractive index of the doped-core glass is the lowest because all the Al and P join together to form [APO4] structures. And the refractive index of [APO4] structures is very close to that of pure silica . Here, we report our new research result about Yb3+-doped LMA silica fiber prepared by sol-gel method combined with high-temperature melting technology. Al2O3 and P2O5 were precisely co-doped with a mole ratio of 1 to form [AlPO4] structures and thereby, effectively reduced the core refractive index and the core NA, which decreased to 0.033. More importantly, the core diameter was greater than 5 mm while the doping homogeneity was very high, even in the case of P2O5 co-doping. Finally, we obtained an approximate single-mode laser with the M2 of 1.3-1.4 from the LMA SCF. The LMA PCF exhibited the laser power of 46 W with the slope efficiency of 61%.
2. Core-glass rod and LMA fibers preparation and characterization
Starting from sol-gel method, we fabricated a fiber preform core-glass rod ∅5 × 100 mm in size with a composition of 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%). The preparation processing was described in our previous work [18, 27]. By further improvement and optimization of the preparation technology, the doping content of the volatile component (e.g., P2O5) was precisely controlled, and the doping homogeneity was greatly improved. Table 1 lists the content of the compositions of the theoretical glass sample, experimental glass samples prepared early and in this work for comparison. The Al, P and Yb content of the experimental glass sample was determined by a Thermo iCAP 6300 radial view Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) after complete dissolution of the glass sample in HF solution. From Table 1, we can see that P2O5 volatized strongly in experimental glass sample prepared early and the content of P2O5 is lower than that of Al2O3. For the experimental sample in this work, although still a bit of the Al, P and Yb content volatized during the high-temperature melting process, the important point is that we prevented the strong volatility of P2O5 and made the content of Al2O3 and P2O5 almost equivalent. It is very helpful for the management of refractive index and doping homogeneity. The core-glass rod was then drawn into LMA SCF by rod-in-tube technique and LMA PCF by the stack-capillary-draw technique at 2200 °C. Figure 1 shows an image of the prepared core-glass rod.
To study the spectroscopic properties of core glass, it was cut and polished to a 2-mm-thick glass chip. The absorption spectrum was recorded using a spectrophotometer (Lambda 900 UV-VIS-NIR, Perkin-Elmer) in the range of 850-1100 nm. The fluorescence spectra were detected (FLSP 920, Edingburg Co., UK) under 896 nm excitation. The fluorescence lifetime was measured under pulsed 980-nm LD excitation using the instrument FLSP 920.
To assess the homogeneity of the dopant distribution in the silica glass rod, the radial refractive index distribution of the glass rod was measured (PK2600 Refractive Index Profiler, Photon Kinetics). The homogeneity distributions of the elements Al, P, Yb, and Si were characterized by the electron probe micro analyzer (EPMA, Shimadzu, 1720H). The laser-beam quality M2 and laser behavior of the prepared LMA SCF and PCF were tested on the fiber laser experimental setup shown in Fig. 2, as described in detail in the following sections.
3. Results and discussion
3.1 Doping homogeneity of the fiber core-glass rod
To assess the doping homogeneity of the fiber core-glass rod, the radial refractive index distribution of the glass rod was measured using the PK2600 refractive index profiler, and the result is shown in Fig. 3(a). The maximum refractive index fluctuation Δn in the core-glass rod was less than 2 × 10−4, which features fine refractive index homogeneity. The increase in the refractive index fluctuation at the center of the core-glass is a test error caused by the profile calculation algorithm. By contrast, Fig. 3(b) and 3(c) show the refractive index profiles of the Al-Yb co-doped silica fiber core prepared by sol-gel method in our early work (Δn = 0.85 × 10−3)  and fiber preform prepared by MCVD technology (Δn = 1 × 10−2) . Their refractive index homogeneity is not very good due to the volatilization of P2O5 to form “central dip”. Figure 3(a) also shows that the core-glass rod is ~3 mm in size. Actually, we can control the size from 2 to 10 mm. The refractive index step between the core and cladding measured in the profile in Fig. 3(a) corresponds to an NA of 0.033.
To further investigate the doping homogeneity, the homogeneity distributions of the elements Al, P, Yb, and Si were characterized using the EPMA. The EPMA radial line scan of the core-glass slice (cut from the core-glass rod) prepared in this work is shown in Fig. 4(b)-4(e). Figure 4(a) is the radial line distribution of P element of the core-glass slice prepared early listed here for comparison. Although the Y-axis don’t show the content of the doping elements, the radial line distributions of the elements can still give us a good assessment of the doping homogeneity. We can see that the P distribution of the core-glass slice prepared in our early work is not uniform with some “dips” due to the strong volatilization of P2O5. But the doping homogeneity of the core-glass slice prepared in this work is very good, as there are no significant radial variations or gradients in the doping levels of Yb, P and Al. The doping homogeneity level is better than that of MCVD fiber core-glass and is comparable to that of Heraeus  core-glass rod.
3.2 Spectral properties of the Yb3+, Al3+ and P5+co-doped silica core-glass
Figure 5(a) shows the typical absorption and fluorescence spectra of Yb3+ ions measured from a 2-mm-thick Yb3+, Al3+ and P5+ co-doped silica core-glass chip. Their spectral properties had been described in detail in our early works [18, 26, 28]. The maximum absorption cross section (σabs) at 976 nm is 6 × 10−21 cm2 calculated according to the measured absorption spectrum. The absorption cross section is low due to the very low absorption intensity of Yb3+The very low absorption intensity may be caused by the change of the coordination environment of Yb3+ at the condition of low ion doping concentration (Yb2O3 0.035mol%, 1.52´1019 ions/cm3). The fluorescence lifetime at 1020 nm is 896 μs by single exponent fitting of the measured photoluminescence decay curve (not shown here). Figure 5(b) shows the FTIR spectrum of the core-glass chip. There is just a minor absorption peak is observed at 3670 cm−1 because of the low hydroxyl (OH) content. By the FTIR spectrum, the hydroxyl content of the core-glass slice can be calculated to be smaller than 2 ppm according to the method in reference .
3.3 Background attenuation of the LMA fibers
In order to assess the background attenuation of our fibers core-glass rod prepared by sol-gel method combined with high-temperature melting technology, The LMA SCF was drawn for optical loss test. Figure 6 shows the optical loss spectrum of the LMA SCF. The background attenuation measured at 1200 nm is 0.24 dB/m and the OH-Absorption at 1385 nm is 0.72 dB/m. The relatively high background attenuation is mainly caused by the light-scattering point in Yb3+-doped silica core-glass rod.
3.4 Laser behavior of LMA SCF
Using the sol-gel method combined with high-temperature melting technology, Al2O3 and P2O5 were precisely co-doped in equimolar amounts in Yb3+-doped silica glass. Our preparation technology effectively suppressed the volatility of P2O5 and significantly improved the doping homogeneity. Owing to the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure, the refractive index of the Yb3+-doped core-glass rod was dramatically reduced. This gave rise to a low NA of the core-glass. In order to assess the laser-beam quality, the LMA SCF was drawn without plastic cladding and the basic optical parameters and sizes of the LMA SCF were shown in Table 2.
The laser experiment was carried on a 15-m-long LMA SCF using a 970-nm pump light with a spot diameter of 200 μm and an NA of 0.22. The pump light was collimated and focused by a pair of aspherical lenses (Fig. 2) and then entered into the cladding of the LMA SCF. A dichroic mirror (transmission (T) 99% @ 970 nm, reflectivity (R) 99% @ 1040 nm) was butt-coupled to the Yb-doped fiber. The fiber output end face with 5% Fresnel reflection worked as another cavity mirror. A filter with a transmission less than 0.5% at 970 nm was inserted before a spectrum meter or a power meter to make sure that only the emission light was detected. The overall absorption coefficient (including background loss) of the fiber at the pump wavelength (970 nm) was 1.1 dB/m. The laser spectrum and laser power input-output curve with a micrograph of the fiber cross section were shown in Fig. 7. The laser spectrum ranges from 1040 to 1070 nm. 3.2 W laser power with the slope efficiency 33% was obtained from the LMA SCF. Compared with our laser result of LMA PCF (slope efficiency 70%) reported in reference , the low slope efficiency is mainly caused by the leak of pump light at the conditions of low NA of fiber core, bending state of the fiber when measuring the laser power and without outer cladding (the air acts as the outer cladding).
The laser-beam quality was measured using the laser experimental setup shown in Fig. 2. The laser-beam profiles in the far field and the measured beam quality factors are shown in Fig. 8. The laser maintains a near diffraction-limited beam quality with an M2 factor of 1.3-1.4 at different output powers, conforming the stable laser performance of the fiber.
3.5 Laser behavior of LMA PCF
LMA PCF was drawn using the same Al-P-Yb co-doped core-glass rod. The micrograph of the LMA PCF cross section is shown in Fig. 9(a). The core diameter is 50 μm; the inner cladding is 254 μm; and the outer diameter is 442 μm, without the plastic cladding. However, the PCF structure is not ideal, the air holes vary in diameter, and there are four bigger air holes glued to the fiber core. The laser experiment was carried on a 650-cm-long LMA PCF using the same laser test condition as that of LMA SCF. The overall absorption coefficient (including background loss) of the fiber at the pump wavelength (970 nm) was 1.7 dB/m. Figure 9(b) indicates the laser input-output curve. The slope efficiency is 61%, and the maximum output power is limited to 46 W by the available pumping power. However, the laser-beam quality is not as good as that of the LMA SCF. The laser-beam quality worsens, perhaps because of the larger core diameter of 50 μm and the defects of the PCF air holes. Further study is needed.
In this study, a large-size (∅5 mm) Yb3+-doped silica fiber core-glass rod with Al and P co-doped in equimolar amounts was prepared using the sol-gel method combined with high-temperature melting technology. The volatility of the P2O5 was effectively controlled, which greatly improved the doping homogeneity and allowed the doping-content mole ratio of Al to P to be maintained as 1, yielding an [AlPO4] structure. The formation of [AlPO4] structures greatly reduced the refractive index of the core glass, yielding a corresponding low NA of 0.033. Using this core-glass rod, we prepared LMA SCF and LMA PCF. Finally, we obtained an approximate single-mode laser with an M2 of 1.3-1.4 from the LMA SCF. The LMA PCF exhibited the laser power of 46 W with the slope efficiency of 61%.
This research is financially supported by National Natural Science Foundation of China (NSFC) (Grant No. 61505232 and No. 61405215).
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