We report on a modified sol-gel method combined with an innovative high-temperature melting technology for the preparation of Yb3+-doped silica glass rods (∅3−18 mm) with high optical quality and low background loss. We prepared Al-Yb, Al-P-Yb, Al-F-Yb, and Al-P-F-Yb doped silica glass rods for large mode area fibers (LMA) with a high laser power and low core numerical aperture (0.02). We were able to successfully solve the doping homogeneity problem caused by the volatility of P and F. More importantly, we developed a purification technology and successfully reduced the optical attenuation to 0.05 dB/m. An Al-Yb co-doped silica photonic crystal fiber (PCF) with a core diameter of 100 µm was fabricated for laser behaviour characterization. In the continuous wavelength laser measurement, a laser output slope efficiency of 83.3% was obtained from the fabricated PCF. To our knowledge, this is the highest slope efficiency derived from a Yb3+-doped silica PCF prepared by a non-CVD method. In the pulse amplification laser experiment, an average amplified power of 310 W with a peak power of 1.5 MW and a pulse duration of 21 ps were achieved.
© 2017 Optical Society of America
Yb3+-doped silica fibers are highly valued and extensively studied owing to their application in high power lasers, and have become ubiquitous in industrial and scientific applications [1–5]. However, the performance of Yb3+-doped traditional clad fibers (a core diameter smaller than 30 µm), usually prepared by the MCVD method, is mainly limited by the onset of nonlinearities and thermal degradation . To mitigate such detrimental effects, researchers have focused on the development of Yb3+-doped fibers with a large mode area (LMA). Because the large mode area (core diameter between 30 and 200 µm) can greatly reduce the power density at the fiber pump end, the thermal damage threshold during high power pumping enhances. However, with increase in core diameter and mode area, higher-order modes appear, and the laser beam quality worsens, which is an potential reason for the new phenomenon called mode instabilities (MI) during the process of power scaling of Yb3+-doped silica LMA fibers . Researchers around the world have devoted considerable effort to finding the optimum fiber designs with a LMA that can offer effective single-mode operation. Moreover, several types of LMA fibers designs have been proposed, such as all-solid single trench fibers (STF) , PCF based designs such as leakage channel fibers (LCF) , hybrid photonic crystal fibers (H-PCF) , photonic bandgap based fibers such as Bragg fibers , and 2D-all solid photonic bandgap fibers (2D-ASPBGF) . In addition, resonant coupling based designs such as the polygonal chirally coupled core (P-CCC) fibers have also been explored [13,14]. However, all the LMA fibers (except for the P-CCC fiber) require the refractive index of the core-glass to be very close to that of the pure silica cladding, which is a key influencing factor for the laser mode of LMA fibers. However, it is very difficult to be realized because the rare-earth ions and co-dopants in the core-glass are usually the materials that increase its refractive index. Moreover, the size, optical quality and optical attenuation of the core-glass rod are also the key influencing factors for the laser performance of the LMA fibers. Consequently, with the continuing research on the design and fabrication of LMA fibers, scientists have begun to realize that the real big challenge is still the preparation of the fiber core-glass rod.
The abovementioned influencing factors pose huge challenges in the preparation of core-glass rod for LMA fibers. Firstly, the size of the core-glass rod must be sufficiently large. The diameter of the core-glass rod should be larger than 4 and 5 mm for the LMA fibers with core diameters of 50 and 100 μm, respectively. Secondly, the optical quality must be very high, especially the doping homogeneity, and there must be no bubbles and scattering points in the glass. This is because a poor doping homogeneity, bubbles, and other scattering points can cause a large scattering loss in the fibers and damage the laser beam quality. Thirdly, the refractive index of the core-glass rod must be precisely controlled, which is mainly affected by the doping composition and doping homogeneity. To achieve single-mode or approximately single-mode laser output from an LMA fiber to enhance the laser brightness, the refractive index of the core-glass must be very low and close to that of the pure silica glass to obtain a low core numerical aperture (NA). For LMA fibers with core diameters larger than 50 μm, the core NA should be equal to or smaller than 0.02 in order to realize single-mode or approximately single-mode laser output . The last aspect, which is also the most critical factor influencing laser behavior, is the optical attenuation. A lower optical attenuation will result in higher laser output power and slope efficiency.
The challenges described above make it very difficult to fabricate Yb3+-doped core-glass rods for LMA fibers using the conventional MCVD technology with liquid-doping [1, 15]. Although the MCVD technology has the advantages of high purity and ultra-low optical attenuation, it is difficult to overcome the limitations of the size of the core-glass rod and the low NA. In recent years, considerable efforts have been made for the development of an MCVD process to prepare Yb3+-doped core-glass rods with a large size and low NA [16,17]. The latest advancement offered by the USA involves the realization of a LMA double clad fiber with a core diameter of 52 μm and low NA of 0.025 by the MCVD technology combined with gas-doping . However, the MCVD with gas-doping technology, developed for the preparation of core-glass rods for LMA fibers, has only recently gained attention, and is thus still in the research stage. In addition, the low NA of fiber core obtained by the MCVD technology is mainly achieved by decreasing the doping content of Yb ions. We hope to achieve high doping content of Yb ions in order to decrease the length of optical fibers and suppress the parasitic nonlinear optical effects (e.g., simulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS)) in high power fiber lasers because most nonlinear thresholds scale inversely with the fiber length [18,19]. However, it is not easy to prepared core-glass rod with high doping content of Yb ions and low NA by the MCVD with gas doping technology. Meanwhile, scientists around the word have begun to explore non-CVD methods to prepare core-glass rods for rare earth doped LMA fibers. A well-known non-CVD method is the powder sintering technology developed by Jena University and Heraeus [20,21]. A large core-glass rod with high optical quality and low loss (20−50 dB/km) can be prepared by this method. Using this core-glass rod, LMA (50−150 μm) fibers have been drawn, and a laser power output of several kilowatts from a single LMA fiber has been achieved. However, it is a significant challenge to add P or F to Al-Yb co-doped silica glass using this method to reduce the refractive index of the core-glass rod. The reason of P reducing the refractive index of Al-Yb co-doped silica glass is that P can join Al to form [AlPO4] structures, the refractive index of which is very close to that of pure silica [1, 22, 23]. Thus, it is difficult to reduce the NA of the fiber core to a sufficiently low value to maintain the single-mode laser output . Nonetheless, the kilowatt-class multi-mode laser obtained from the LMA fiber made by Jena University and Heraeus can be applied to industry processing, such as cutting and welding, in which single-mode laser is not necessary.
Here, we introduce the research progress of another non-CVD method called the modified sol-gel method combined with a high-temperature melting and molding technology (hereafter referred to as modified sol-gel technology), which has been studied by us for several years [1, 15, 22, 25, 26]. Using this technology, we have successfully prepared Al-Yb, Al-P-Yb, Al-F-Yb, and Al-P-F-Yb homogeneously doped large-size silica glass rods for LMA fibers. More importantly, we have developed an effective purification technology to reduce the impurities in the Yb3+-doped silica glass and have successfully reduced the optical attenuation to 50 dB/km which is close to the loss level of the fibers prepared by MCVD (20−30 dB/km) or Heraeus (20−50 dB/km) . This is a significant breakthrough for our modified sol-gel method because the optical loss of the fiber prepared by this method was always ~200−500 dB/km until this work. Al-Yb co-doped silica PCF with a core diameter of 100 µm was prepared using this low-optical-loss core-glass rod for laser performance characterization. The detail are discussed in the following sections.
2. Modified sol-gel technology for the preparation of Yb3+-doped silica glass rods
The modified sol-gel technology developed by us starts from a molecular mix in the sol-gel process. Tetraethoxysilane (TEOS), C2H5OH, AlCl3∙6H2O, H3PO4, YbCl3∙6H2O, and NH4F or (NH4)2SiF6 were used as precursors. Hydrochloric acid or ammonium hydroxide and de-ionized water were added to sustain the hydrolysis reaction. The above-mentioned analytically pure chemical reagents were mixed and stirred at 30−50°C to form a homogeneous and clear doping sol. The sol was then heated from 80 °C to 1100 °C to achieve a dried gel powder in which the hydroxyls and organics were almost decomposed. The gel powder was melted into a bulk silica glass at 1750 °C in vacuum. The bulk glass can be further molded into a core-glass rod as the preform core of an LMA fiber. This basic preparation process has also been discussed in our previous works [15, 23, 25, 26]. In this work, we explored the gel-powder purification technology and new melting and molding technologies. The dried gel powder was purified by CCl4 or POCl3 in a He and O2 atmosphere at 600−900 °C for 2−4 h. After purified by CCl4 or POCl3, the gel-powder was kept in O2 atmosphere for 1 h to remove residual Cl2. The powder was then melted at 1750−1780 °C in a N2 atmosphere under a pressure of 1−2 MPa to obtain bulk silica glass with no bubbles and scattering points and with a low optical loss. After the bulk glass was molded into a core-glass rod, the LMA cladding fibers and LMA PCFs were drawn by the rod-in-tube and stack-capillary-draw techniques, respectively, at 2200°C. The scheme of the entire preparation process is shown in Fig. 1.
3. Results and discussion
3.1 Optical quality of the Yb3+-doped silica glass rod
For rare-earth doped silica glass and fibers, optical quality and optical loss are always the most important influencing factors, and the top technical challenges. The optical quality of a Yb3+-doped silica glass rod mainly refers to the doping homogeneity and scattering points in the silica glass, including bubbles, stripes, un-melted microcrystals, and other impurities. The doping homogeneity is usually estimated by refractive index fluctuation Δn of the core-glass rod measured using the PK2600 Refractive Index Profiler (Photon Kinetics). Figure 2 shows the refractive index profiles of Al-Yb and Al-P-F-Yb co-doped silica glass rods. For the rare-earth doped silica glass, higher doping contents (including rare-earth elements and other co-doping agents, e.g., Al, P, and F) result in a poor doping homogeneity. Although the Al-Yb co-doped core glass rod shown in Fig. 2(a) has a high doping content (1.0Al2O3-0.1Yb2O3 mol%), the doping homogeneity is very good with a refractive index fluctuation Δn of 2 × 10−4. When F or P is introduced into the silica glass, the doping homogeneity becomes a significant problem due to the strong volatility of F and P. In particular, when the NA of the LMA fiber core is required to be 0.02 for a good laser beam quality, the silica glass should be doped with a higher F content to make the refractive index of the core glass very close to that of the pure silica glass. In this case, the high doping homogeneity is a great challenge. Figure 2(b) shows a very good homogeneity of Al-P-F-Yb co-doped silica glass rods with a NA of ~0.02. The refractive index fluctuation Δn is about 2 × 10−4, which is difficult to achieve, and shows the advantages of the modified sol-gel technology. Details of the fabrication and laser properties of the Al-P-F-Yb co-doped silica glass rods and LMA fibers will be reported in another paper.
EPMA-mapping analysis of the Al-Yb co-doped core-glass rod was performed using an electron probe micro-analyzer (EPMA, Shimadzu, 1720H) to clearly distinguish the doping homogeneity and distribution of Yb3+ in the silica glass (Fig. 3) It can be observed visually that the Yb3+ ions are distributed randomly and uniformly in the silica glass without any aggregation.
In addition to doping homogeneity, scattering points (bubbles, un-melted microcrystals, and other impurities) also critically affect the optical quality of the Yb3+-doped silica glass. If the fiber is pumped at high power, the scattering points in the fiber core glass can cause significant laser scattering loss and decrease the laser slope efficiency. Thus, the scattering points must be removed if the Yb3+-doped silica fiber is expected to perform well during the laser operation. Figure 4 shows the pictures of the Yb3+-doped silica glass rods for visual inspection. Figure 4(a) is the photograph of a polished core-glass rod with a diameter of 5 mm. Figure 4(b) shows the glass rod immersed in kerosene, with a refractive index very similar to that of silica glass. At the same time, the glass rod was irradiated from one end by a the green laser to clearly show the scattering points, if any, in the core-glass rod. The scattering points seen in the kerosene in Fig. 4(b) indicate the presence of tiny impurities in kerosene. However, we can see that there are no scattering points in the body of the core-glass rod. Figure 4 (c) shows a very simple but effective method to determine the optical quality of the core-glass rod. The core-glass rods were irradiated with light, the source of which was above and perpendicular to the core-glass rod. Thus, shadows are visible and shown in Fig. 4 (c). We can see that the lower rod, which was prepared earlier, is poor and contains too many stripes and bubbles, while the upper rod is very clear, which indicates a very good optical quality of the core-glass rod.
3.2 Spectral properties of the Yb3+-doped silica glass rod
In order to study the spectroscopic properties of the 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. A fluorescence spectrum was detected (FLSP 920, Edingburg Co., UK) at the 896 nm excitation. The fluorescence lifetime was measured via pulsed 980-nm LD excitation using an FLSP 920 instrument. The spectral properties of Yb3+ in laser glass and fibers were determined by the Yb3+ ions and laser glass matrix, wherein the Yb3+ ions play a leading role. The spectral shape of Yb3+ in the silica glass matrix is almost unchanged when co-doped with Al, P, or F, except for a tiny shift of 2 nm of the absorption and fluorescence peaks at 975 nm. Figure 5(a, b, c) shows the typical absorption, fluorescence spectra and the fluorescence decay curves of the Yb3+ ions measured from a 2-mm-thick Al-Yb co-doped silica core-glass chip. Figure 5(d) shows the energy level diagram of Yb3+ ions. The absorption peak at about 975 nm is associated with the transition of Yb3+ 4f electrons from the ground state 1 to the excited state 5 (1→5). The coordinate environment significantly affects the absorption and fluorescence intensities and the lifetime of Yb3+. In particular, if the second-nearest neighboring ions of Yb3+ are the glass network former ions, then the type of the network former ion has a large influence on the spectral intensity and lifetime. Table 1 lists the absorption (σabs) and emission (σem) cross sections as well as the fluorescence lifetimes (τm) at 1020 nm of the Yb3+-doped silica glasses with different co-dopants. In samples 1#, 2#, 3#, and 5#, either there is no P (1# and 5#), or the P content is not more than the Al content. In this case, the second-nearest neighbor ions of Yb3+ are mostly Al, therefore, these samples have similar absorption (σabs) and emission (σem) cross sections, as well as fluorescence lifetimes (τm). However, for sample 4#, the P content is larger than the Al content, and the second-nearest neighbor ions of Yb3+ are mostly P. In this case, the absorption (σabs) and emission (σem) cross sections of Yb3+ are greatly reduced, and the lifetime increases to 1077 µs. Some of these spectral properties have been described in detail in our early works [15, 22, 26].
3.3 Purification of Yb3+-doped silica glass rods and optical attenuation
For rare-earth doped laser glasses and fibers, optical attenuation is the key factor that influences the laser output performance. Ultra-low optical loss allows the fiber to achieve high laser output slope efficiency and laser power. In Yb3+-doped silica fibers, optical scattering and impurity absorption are the main sources of optical loss. Optical scattering is caused by doping inhomogeneity and scattering points, such as bubbles, which has been greatly improved. Impurity absorption mainly originates from Fe and Cu, which can seriously damage the laser performance of the Yb3+-doped silica fibers owing to their strong absorption around 1 µm. As is known, the MCVD technology has the remarkable merit of ultra-low optical loss due to its closed deposition system. In contrast, the sol-gel technology is a relatively open system, therefore, Yb3+-doped silica powder and glass usually have more impurities, and the optical attenuation is generally above 0.1 dB/m. To solve this problem, a purification treatment technology was developed. After purification, the contents of Fe and Cu impurities were less than 1 ppm in Yb3+-doped silica glass rods. Using the purified core-glass rod, a single clad fiber was drawn for optical attenuation measurements. Figure 6 shows the optical loss spectrum of the single clad fiber. We can see that the background attenuation at 1200 nm is greatly reduced to 0.05 dB/m, which is close to the loss level of the fibers prepared by MCVD (20-30 dB/m) or Heraeus (20-50 dB/m) . The hydroxyl absorption at 1385 nm is about 0.3 dB/m.
3.4 Laser performance of an LMA PCF
An LMA PCF was drawn using the prepared and purified Al-Yb co-doped core-glass rod (1#: 1.0Al2O3-0.1Yb2O3). The laser behavior of the prepared LMA PCF was tested on the fiber laser experimental setup shown in Fig. 7.
The micrograph of the LMA PCF cross section is given in the inset of Fig. 8. The core diameter is 100 µm with a NA of 0.08, the inner cladding is 400 µm with the NA of 0.5, and the outer diameter is 550 µm without plastic cladding. The overall absorption coefficient (including the background loss) of the fiber at the pump wavelength (976 nm) is 24 dB/m. The laser experiment was carried on a 150-cm-long LMA PCF using 976-nm pump light with a spot diameter of 400 µm and NA of 0.22. The laser experimental setup is shown in Fig. 7. The laser output power versus the incident pump power curve is shown in Fig. 8. Benefiting from the low optical loss (0.05 dB/m) and high absorption coefficient of the pump laser (24 dB/m), a laser output slope efficiency of 83.3% was obtained from this PCF. To the best of our knowledge, this is the highest slope efficiency for Yb3+-doped silica PCFs prepared by non-CVD methods.
To further investigate the laser pulse amplification property of the Al-Yb co-doped LMA PCF, a fiber laser amplification experimental setup was built and is shown in Fig. 9(a). A seed source at 1030 nm with a repetition rate of 10 MHz and pulse duration of 21 ps was used in this system. The signal, after passing through a single-mode (SM) fiber, had an average power of 18 W with a near-Gaussian beam. The pump source was a 976 nm laser diode (pump 2). The measured average output power as a function of the pumped power is shown in Fig. 9(b). An average amplified power of 310 W with a peak power of 1.5 MW was acquired from the LMA PCF due to the limitation of the available pumped power. The average light-light efficiency is about 63%. The inset in Fig. 9(b) shows the beam profile in the far field. The laser beam quality factor M2 is about 5. And it is still the multimode transmission due to the high refractive index of the fiber core with only Al3+ and Yb3+ co-doping. We can see from the beam profile that the laser spot is asymmetrical, and the diameter of the laser spot ranges from 85 to 94 μm in our test process. The single-mode laser output result from the Al, P, F, and Yb co-doped LMA fiber will be reported soon in another work.
Using the modified sol-gel method combined with innovative high-temperature melting and molding technology, we prepared Al-Yb, Al-P-Yb, Al-F-Yb and Al-P-F-Yb doped silica glass rods for LMA fibers with high laser power and low core NA (0.02). We successfully solved the doping homogeneity problem caused by the volatility of P and F, and achieved Yb3+-doped silica glasses with excellent optical quality. More importantly, a purification technology was developed, and the impurities in Yb3+-doped silica glass and fiber were effectively removed. The optical attenuation was then successfully reduced to 0.05 dB/m. Using the low-optical-loss Al-Yb doped silica core glass rod, we prepared an LMA PCF with a core diameter of 100 µm and obtained a laser output slope efficiency of 83.3% in the continuous wavelength laser measurement. To the best of our knowledge, this is the highest slope efficiency accorded by a Yb3+-doped silica PCF prepared by a non-CVD method. We also achieved a 310 W average amplified power with a peak power of 1.5 MW and 63% of average light-light efficiency from the LMA PCF. The modified sol-gel method combined with the innovative high-temperature melting and molding technology developed by us proved very effective and economical for the fabrication of rare-earth (Yb, Tm, Er, Nd, etc.) doped large-size silica glass rods and LMA fibers.
National Natural Science Foundation of China (Grant No. 61505232 and No. 61405215).
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