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Abstract
A Yb:YAG-derived silica fiber was fabricated by a molten-core fabrication method, in which a Yb:YAG crystal was used as the core material and a silica tube was used as the cladding material. The fiber’s transmission loss was measured to be 0.49 dB/m at 1.55 µm, using a cut-back method. The fiber microstructure image showed that the cladding region was a uniform glass structure, while the core structure was amorphous. An all-fiber-integrated cladding-pumped laser based on Yb:YAG-derived silica fiber was demonstrated. With an incident pump power of 28 W, an output power of 6 W was obtained at 1.06 μm, with a slope efficiency of 21.7%.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent decades, fiber lasers have seen rapid development because of their high efficiency, high beam quality, compactness, and convenient thermal management. Particularly in the field of high-power lasers, fiber lasers have been one of the most important kinds of solid-state lasers [1–6]. At present, multiple kilowatts of power with diffraction-limited beam quality can be generated in a single fiber [4,5]. These fiber lasers have been based on traditional silica glass fibers. Further power scaling is limited by the thermal and stimulated Brillouin scattering (SBS) properties of silica glass [6,7]. One of the solutions to this problem is to use new optical fiber materials with better thermal and optical properties.
Recently, YAG-derived silica fibers have attracted much attention for their novel properties in fiber lasers [8–12]. They have been fabricated with a “molten-core fabrication” approach, in which a YAG crystal was used as the core material and a silica tube was used as the cladding material. Using this approach, the fiber combines some characteristics of YAG crystals and silica glass fibers, and possesses characteristics including high rare-earth doping potential, high thermal conductivity [13], low photodarkening effect [14], and high SBS threshold [15]. These properties indicate that these kinds of fibers have the potential to realize high-power lasers.
Currently, research and development regarding Yb:YAG-derived silica fibers (YDSFs) is still in the early stages. There are many difficulties involved in fabricating a high-quality YDSF, such as the diffusion between cladding and core and the change in the crystal structure. It is therefore necessary to further study the fiber fabrication process and the microstructure of the fiber. Most YDSF lasers are free-space-pumped because of the difficulty involved in fusion welding with low splice loss between a conventional fiber and a YDSF [16]. The free-space-pumped structure is complex and instable. Compared with this structure, an all-fiber-integrated cladding-pumped structure is more compact, robust, and reliable [17]. Also, cladding-pumped fiber lasers can deliver much higher power. However, there have been no reports of an all-fiber-integrated YDSF laser generating watt-level output power, to our knowledge.
In this paper, a YDSF with a core diameter of 6.3 μm was fabricated using a 10 at.% Yb:YAG crystal as the core material and a high-purity silica tube as the cladding material. The composition was determined by energy disperse spectroscopy (EDS). Elemental analysis indicated that the SiO2 concentration in the core region was 75.3 wt%, and the Yb2O3 concentration in the core region was 4.8 wt%. Using a cut-back method, the transmission loss of the fiber was measured to be 1.79 dB/m at 1.06 μm and 0.49 dB/m at 1.55 µm, respectively. A transmission electron microscope was used to analyze the microstructure of the fiber. The results indicated that the cladding region was a uniform glass structure, while the core structure was amorphous. By optimizing the splice parameters including the heating position and duration, a splice loss (10/125DC silica fiber-YDSF) was measured to be 0.18 dB. An all-fiber-integrated cladding-pumped laser based on this YDSF was demonstrated for the first time. At an incident pump power of 28 W, the maximum output power at 1.06 μm was 6 W. This is the highest power achieved in similar YDSF lasers, to our knowledge. The corresponding slope efficiency was 21.7%.
2. Fiber fabrication
The fiber preform was composed of a commercially Yb:YAG crystal and a high-purity silica glass tube (99.99%, Heraeus). The inner and external diameters of silica tube were 2.8 mm and 10 mm, respectively. One end of the silica tube was sealed to form a cone shape. The doping concentration of the Yb:YAG crystal was 10-at.%, and the diameter was 2.6 mm. The Yb:YAG crystal was inserted into the silica tube after cleaning. The fiber can be produced using a standard fiber drawing tower with a graphite heating furnace. The length and diameter of the hot zone were ~5 cm and ~4 cm, respectively. The drawing temperature was controlled at ~2000 °C, matching the melting point of the YAG crystal. Using a molten-core fabrication approach, a rod fiber with the diameter of 1.8 mm was obtained. Then the rod fiber was inserted into a silica tube (Dinner = 2 mm, Dexternal = 10 mm) to constitute a new preform. During the second drawing process, the drawing speed was controlled at ~3 m/min with the tension of ~15 g. Fibers were fabricated with a cladding diameter of ~125 µm and a core diameter of ~6.3 µm. The measured core diameter of the fibers was slightly larger than the calculated value of 5.9 µm. This is likely due to the interdiffusion of the materials between the core and cladding.
The fiber end surface features were measured using a scanning electron microscope (SEM, S-4800 Hitachi), as shown in Fig. 1(a). The SEM image illustrates that the core shape was circular, and the boundary between the core and the cladding was clear. The composition was determined by EDS. Figure 1(b) shows the composition distribution profiles of the fiber cross-section. Elemental analysis indicated that the SiO2 concentration in the core region was 75.3 wt%, and the Yb2O3 concentration in the core region was 4.8 wt%. As expected, interdiffusion occurred between the Yb:YAG core and the silica cladding during the drawing process. At the center region of the heating furnace, the silica tube was very soft, and the bottom of the Yb:YAG crystal was in a liquid state when the temperature reached ~2000 °C. Other parts of the crystal were still in solid state, which would cause the liquid crystal and the softened cladding to be extruded by gravity. This would make the diffusion more prominent. A negative pressure control on the Yb:YAG crystal could help to reduce the diffusion and generate a better interface between core and cladding [18,19].
The microstructure of the fiber was characterized using a high-resolution transmission electron microscope (TEM). A sample with a thickness of 46 nm was prepared using focused ion beam technology (FIB, FEI Helios Nanolab 600i), as shown in Fig. 2(a). The green lines indicate the interface between core and cladding. Figure 2(b) shows a TEM image under a resolution of 100 nm. The core-cladding interface can be easily distinguished. The sample was further tested under a resolution of 50 nm. Figures 2(c) and 2(d) are TEM images of the cladding region and the core region, respectively. The cladding region still maintains a uniform glass structure. The core region no longer has a crystalline structure, but is instead a mixed state of silica and yttrium aluminosilicate (YAS). As shown in Fig. 2(d), there was a background of silica and YAS nanoparticles. The bright region is the silica background, and the dark region is nano-sized YAS particles with irregular morphology. The core region was analyzed by electron diffraction to further determine the core structure. As shown in Fig. 2(e), electron diffraction results indicate that the core structure is amorphous. The heating method is an important factor influencing fiber quality. Inside the furnace, the length of the high-temperature region is several centimeters. When the preform reached the high-temperature region, significant amounts of SiO2 diffused into the core. While the fiber was out of the high-temperature region, YAS glass gradually formed in the core [13,20]. Reducing the length of the high-temperature region using a CO2 laser heating method could help to maintain the crystal structure [21,22].
Fig. 2 TEM and electron diffraction images of optical fibers. (a) Fiber TEM sample made by FIB. (b) TEM image of junction area between core and cladding. (c) TEM image of cladding region. (d) TEM image of core region. (e) Electron diffraction image of fiber.
The transmission losses of these fibers including absorption and scattering loss were measured using a cut-back method. The transmission loss at 1.06 μm was unexpectedly high, at 1.79 dB/m, likely due to reabsorption of the Yb-doped YAS core [16,24]. To examine the background loss (distinct from the Yb absorption) of the host material, a 1.55 µm single-frequency laser was used as the light source. As shown in Fig. 3(a), the slope of the fitting line indicated the fiber transmission loss, which was 0.49 dB/m. Furthermore, a continuously tunable single-frequency laser (TSL-210VF, Santec) was used to test the loss from 1520 nm to 1564 nm. Figure 3(b) gives the results at 2 nm intervals; it can be seen that the transmission loss of the fiber was ~0.5 dB/m. This transmission loss is comparable to that of similar YDSFs [14,16]. However, it is still higher than traditional silica fibers because of the impact of the background loss, the diffusion between core and cladding, and the change in the crystal structure. Further improvements are needed with respect to fiber fabrication technology.
Fig. 3 The optical fiber loss at 1.5 µm measured by cut-back method. (a) The transmission loss at 1.55 µm. (b) The transmission loss from 1520 nm to 1564 nm.
3. YDSF laser performance
The laser performance of the YDSF was studied with both core-pumping and cladding-pumping methods. The experimental setup for the fiber laser under core pumping is shown in Fig. 4. The pump source was an 800 mW single-mode-fiber (SMF) coupled 976 nm laser diode with a core diameter of 5.8 μm. The laser oscillator contained a gain fiber and a pair of SMF fiber Bragg gratings (FBG). The high reflectivity (HR) and low reflectivity (LR) FBG were inscribed with a 3-dB bandwidth of 1 nm (99.9% reflectivity) and 0.3 nm at 1064 nm, respectively. A filter coated for high transmission (HT) at 1064 nm (T>99.0%) and HR at 976 nm (R>99.0%) was used to separate the 1064 nm laser from the residual pump laser. The output power was measured by a PD300-IR detector connected to NOVAII power meter (Ophir).
Figure 5 shows the output power of the 1064 nm laser with respect to the 976 nm pump power with various output end reflectivities. The optimized fiber length was 20 cm. As shown in Fig. 5, higher output power can be generated with a 10% reflectivity output FBG. The FBG has a core diameter of 5.3 μm with the numerical aperture (NA) of 0.14. At an incident pump power of 580 mW, the maximum output power of 245 mW was obtained. The corresponding slope efficiency was 45.4%. This value was twice that in [9], which was the highest for similar structures in previous reports.
To obtain higher output power, the cladding-pumping method was used in the following experiment. The setup was similar to the scheme for core pumping. A multimode fiber (Dcore = 120 µm, NA = 0.22) coupled 976 nm LD was used as the pumping source with a maximum output power of 60 W. The resonant cavity was formed by a pair of double-cladding (DC) FBGs. The FBGs were based on commercial silica optical fiber (Passive-10/125DC, LIEKKI) with core diameter of 10 μm and NA of 0.08. The output power was measured by a PM10 detector connected to a FieldMaxII–TO power meter (Coherent).
To achieve cladding-pumped absorption, the YDSF is placed in a box filled with the water (n = 1.33), producing an inner-cladding NA of 0.58. The bending radius of the YDSF is 8 cm. To investigate the influence of the FBG, two different FBGs, with reflectivities of 10% and 20%, were used in the following experiments. Figure 6 gives the output power of the 1064 nm laser with respect to the incident pump power. As shown in Fig. 6(a), better output performance was obtained with 10% reflectivity of FBGs under the YDSF length of 90 cm. At an input pump power of 19 W, an output power of 3.5 W was obtained. The conversion efficiency was 18.4%. To further optimize the output power, fibers with lengths of 90 cm, 100 cm, and 110 cm were used. Results are shown in Fig. 6 (b); it can be seen that a maximum output power of 6 W was achieved with a 100-cm YDSF under an incident pump power of 28 W. The corresponding slope efficiency and conversion efficiency were 21.7% and 21.4%, respectively. To prevent the gratings from being damaged, we did not increase the pump power further. The spectral information was recorded using an optical spectrum analyzer (AQ6370D, YOKOGAWA). As shown in Fig. 7, the central wavelength of the output laser was 1063.2 nm with a resolution of 0.02 nm, and the linewidth was 0.08 nm (FWHM). In the experiment, a pair of gratings was used to realize the laser output. Therefore, the obtained linewidth with the all-fiber structure was narrower than that with a free-space coupling pumped structure, which also resulted in low efficiency [16]. The fiber parameters, such as the numerical aperture and core diameter mismatch between YDSFs (Dcore = 6.3 µm, NA = 0.42) and double-cladding fibers (Dcore = 10 µm, NA = 0.08), were a major factor causing the low efficiency [23]. Further improvement can be realized by reducing the mismatch.
As mentioned above, the YDSF may be a promising gain fiber for high-power lasers. It possesses higher rare-earth doping potential, higher thermal conductivity, lower photodarkening effect, and a higher SBS threshold than conventional silica fibers. However, there are still some difficulties during the fiber drawing process, such as the background loss, the diffusion between core and cladding, and the change in the crystal structure. Therefore, some improvements are needed regarding fiber fabrication technology. The laser performance of the YDSF was investigated with two pumping methods. Because the fiber was not double-cladding fiber, the cladding-pumping scheme was realized by placing the fiber in water. The maximum output power of 6 W was obtained, but with this method, the inner-cladding NA of 0.58 could not match the DC-FBG (NA = 0.48) better. Moreover, the NA is so high that it induced severe mode degradation and bending loss. A portion of the incident pumping (high order mode) could leak from the cladding of the fiber, reducing the laser pumping efficiency. A double-cladding structure for the YDSF could be designed to achieve a higher efficiency.
4. Summary
We have fabricated a YDSF with a molten-core fabrication method using a 10 at.% Yb:YAG crystal as the core material and a silica tube as the cladding material. With a SEM and EDS, the SiO2 concentration in the core region was measured to be 75.3 wt%, and the Yb2O3 concentration in the core region was measured to be 4.8 wt%. Using a continuously tunable single-frequency laser as the light source, the transmission loss of the fiber was measured to be ~0.5 dB/m from 1520 nm to 1564 nm. The microstructure of the fiber was then analyzed. The TEM analysis indicates that the cladding region is a uniform glass structure, while the core structure is amorphous. The laser performance of the YDSF was studied with both core-pumped and cladding-pumped methods. The maximum output power of 6 W was obtained at 1.06 μm when the incident pump power was 28 W. The corresponding slope efficiency was 21.7%. This is the highest output power achieved in similar YDSF lasers, to our knowledge.
Funding
National Natural Science Foundation of China (61527823, 61475086, 118041292); Joint Foundation of the Ministry of Education (6141A02022413, 6141A02022421); Shandong Province Science and Technology Research Projects (2015GGX101039); Young Scholars Program of Shandong University (2016WLJH13); Fundamental Research Funds of Shandong University (2018JCG02).
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