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A Yb/Ce codoped aluminosilicate fiber was successfully fabricated by the chelate gas phase deposition technique. Using the homemade fiber as the amplifier stage in a master oscillator power amplifier configuration laser setup, a near single mode laser output (M2 = 1.55) with an output power of 1026 W and slope efficiency of 84.8% is obtained. In the 100 hour photodarkening experiment, the entire reduced power is less than 14 W, and the ratio, compared to the initial 1026 W output power, is less than 1.4%. The investigation of optical properties indicates that Yb/Ce codoped aluminosilicate fiber fabricated by the chelate gas phase deposition technique shows homogenous distribution, appropriate absorption coefficient, low background attenuation, high optical-to-optical efficiency and a rather low photodarkening loss, making it a promising candidate as an active fiber for a reliable and efficient fiber laser in high-power applications.
© 2016 Optical Society of America
1. Introduction
Compared with the competing laser system, fiber lasers show unique advantages such as low maintenance, compact structure, good thermal management, high optical efficiency and excellent beam quality [1–3]. For these reasons, fiber lasers, especially Yb-doped fiber laser are widely used in laser processing such as printing, marking, metal processing and some military applications [4–7].
The performance of the Yb-doped high power fiber laser strongly depends on the optical properties of the active fiber. As a key component, several characteristics are required for the Yb-doped fiber such as homogenous refractive index profile along the radial and longitude directions, appropriate absorption coefficient, low background attenuation and low photodarkening (PD) loss. To make such a quality active fiber, different manufacture methods are developed such as plasma chemical vapor deposition (PCVD) process [8], atomic layer deposition method [9], outside vapor deposition (OVD) [10,11], powder sinter technology [12], direct nanoparticle deposition (DND) [13], and modified chemical vapor deposition [14–16]. Currently, the modified chemical vapor deposition in conjunction with solution doping technique (SDT) is the most common method to fabricate a Yb-doped fiber for commercial use. However, the efforts to achieve higher laser output with the Yb-doped fiber obtained by conventional SDT approach was hindered for the intrinsic limitations concerning inhomogeneous distribution along radial and longitudinal direction, low RE doping concentration, imprecise control of composition and refractive index profile, and high background loss [17,18]. Since the first reported in 1990, the chelate gas phase deposition (CGPD) technique have been attracting more and more attentions and expected to solve the problems bothering the SDT approach [19]. In the CGPD method, the RE-dopants are designed as chelates, and all precursors are maintained in the gas phase in a closed environment, separated from contaminants and water. The precise control of the doped composition by adjusting the gas flow ensures the homogeneity along with the radial and longitudinal directions of the preform.
In our previous work, we have achieved an output power of 3592 W with 72.5% slope efficiency through a Yb/Ce codoped aluminosilicate fiber fabricated by the CGPD technique [18]. In the present work, the optical properties of the Yb/Ce codoped fiber, fabricated by the CGPD technique was studied furtherly. Specifically, the homogeneity of the refractive index, cladding absorption, core background attenuation, and laser performance were estimated. In addition, a 1026 W × 100 hour PD experiment was conducted. To the best of our knowledge, this represents the highest PD power demonstrated for a Yb/Ce codoped aluminosilicate fiber by the CGPD technique. The investigation of optical properties indicates that the Yb/Ce codoped aluminosilicate fiber fabricated by the CGPD technique can be a promising candidate as an active fiber for a reliable and efficient fiber laser in high-power applications.
2. Fiber fabrication and measurements
The Yb/Ce codped aluminosilicate fiber was mainly prepared though three processes: preform fabrication, preform machining and fiber drawing. The Yb/Ce codoped preform was produced by MCVD with chelate gas phase deposition technique. A Heraeus F300 silica tube was adopted as the substrate tube, heated by a H2/O2 flame and keeping rotating to ensure a same thermal environment around the tube. The raw materials used were SiCl4, AlCl3, Yb(thd)3, Ce(thd)3, and SiF4. The chelates were vaporized in separate evaporators at the temperature of 210°C.The reaction gas was O2 and transporting gas was He. It should be noted that all raw materials and processing gas were with high purity (above 5N). The reactions of raw materials and O2 were taken place in the tube at the temperature of 1850°C. After more than ten passes of deposition, the hollow tube was collapsed to a solid preform with an atmosphere of Cl2 and O2 at a higher temperature (about 2000°C). Subsequently, the preform was jacketed with a suitable silica tube and then processed to octagonal shape with the designed core/cladding ratio. Then the preform was drawn to 20/400 μm fiber at the temperature of 2000-2100°C.
The homogeneity of Yb/Ce codoped fiber along the radial and longitudinal directions was illustrated by the refractive index profile (RIP) of the preform, which was tested by the Photon Kinetics 2600 preform analyzer every 20 mm interval. The cladding absorption and core attenuation were measured by traditional cut-back method. Figure 1 shows the schematic setup of master oscillator power amplifier (MOPA) configuration applied to investigate the laser performance of the homemade Yb/Ce codoped fiber. Eight 200 W laser diodes (LD) with a center wavelength at 976 nm were employed in the setup: two of them were used as the pump source of the seed light (about 170W), and other six of them were used as the pump source of the amplifier stage. The combiners were taken to couple the pump light from 976 nm LD together into a fiber. 20m length Nufern Yb-doped fiber was used as the gain media in the seed light, and 25m length homemade Yb/Ce codoped fiber was used as the gain media in the amplifier stage and to be tested. The striper was to enhance the beam quality of the seed light by wiping off the cladding modes. The power was recorded by an Ophir 1.5 kW-ROHS power meter. The spectrum was measured by an optical spectrum analyzer (Yokogawa AQ-6370D). The beam quality was tested by PRIMES GmbH LQM-HP beam quality analyzer at a power of 500 W. With the laser setup, the PD experiment was conducted with a top power of 1026W for 100 hours as well.
3. Results and discussion
As can be determined from the RIP of the Yb/Ce codoped preform displayed in Fig. 2, the difference between the refractive index of the core and the cladding is in a range of 1.24~1.60 × 10−3, and thus the numerical aperture (NA) varies from 0.060 to 0.068, along the length of the preform. Because the change of the refractive index is linear with the addition of the doped elements (Al, F, Yb, and Ce) [20], the homogeneity of the compositional distribution can be illustrated by the RIP of the preform. The slight change of the core refractive index (approximately 0.36 × 10−3) implies that the dopants were distributed homogenously along the radial direction of the preform. For a better observation of the homogeneity along the longitudinal direction, the fluctuations of core diameter and difference of refractive index along the length of the preform are depicted in Fig. 3. The fluctuations of the core diameter and the difference of the refractive index are less than 0.6% and 8% respectively, indicating a quite homogenous distribution along the longitudinal direction of the preform. This demonstrates that a homogenous preform along both the radial and longitudinal directions can be obtained by the chelate gas phase deposition technique.
The absorption spectra of the homemade Yb/Ce codoped fiber and the Nufern fiber are shown in Fig. 4. The measured absorption coefficients of the test fiber are 1.66 dB/m at 976 nm and 0.64 dB/m at 915 nm, respectively. Comparison with the spectrum of the Nufern fiber shows that the OH-attenuation at 1390 nm of the test fiber is much smaller than that of the Nufern fiber, which indicates the advantage of CGPD in reducing OH-content. During the chelate gas phase deposition, the inside of the hollow silica tube is dried by Cl2 repeatedly prior to the deposition, and the isolated environment ensures that little OH can mix into the core of the preform during the entire fabrication process. However, the Nufern fiber is prepared through the solution doping technique that inevitably brings the –OH into the soot preform during ion doping process.
The core attenuation spectrum of the Yb/Ce codoped fiber is presented in Fig. 5. Due to the intense absorption of Yb3+ in the core of fiber, the attenuation between 800 and 1100 nm cannot be obtained for the low signal-to-noise ratio. To estimate the core background attenuation at the laser wavelength of 1080 nm, we fitted the slope of the core attenuation spectrum between 700 and 1200 nm with a Rayleigh scattering term λ−4 and determined it to be 18 dB/km. The values of the core background attenuation at 1200 and 1300 nm are 7.2 and 6.4 dB/km, respectively. The quite low values of core background attenuation are beneficial in high power fiber lasers for the low thermal burden and high efficiency.
The laser performance of the homemade Yb/Ce codoped fiber was tested in a MOPA configuration laser setup shown in Fig. 1. An output power of 1026 W at 1080 nm with slope efficiency of 84.8% was obtained as shown in Fig. 6. Considering that the temperature of all fiber splices is below 30°C and the input power is restricted by the pump source, the fiber has the potential of bearing higher input power in future. Figure 7 shows the M2 value of our fiber at the output power of 500 W. The M2 factor is 1.55; Mx2 = 1.46 and My2 = 1.64. The tested results indicate that a near single mode laser output is obtained with our homemade Yb/Ce codoped fiber.
The PD resistance is one of the most key properties of a fiber, especially when used in an efficient fiber laser for a long time. Ongoing efforts have been devoted to the measurement, simulation and mitigation of PD induced loss [21–23]. To evaluate the PD resistant ability of the homemade Yb/Ce codoped fiber under the practical operating conditions, a 100 hour PD test was conducted with the same laser setup in the continuous wave operation mode. As shown in Fig. 8, the output power is 1026 W initially, and then drops to 1020 W exponentially in the first 5 hours. Finally, the entire reduced power is less than 14 W, and the ratio, compared to the initial 1026 W output power, was less than 1.4%. Compared with the near-kW PD experiment of Yb-doped fiber fabricated by chelate gas phase deposition [24], the PD loss was decreased from 2.2% to 0.8% in the first 10 hours. The very low PD loss indicates that the Yb/Ce codoped fiber fabricated by the chelate gas phase deposition technique shows good PD resistance, and a PD experiment under higher laser power will be conducted in future. The inset of Fig. 8 shows the fluctuation of the output power in 10 hours. The fluctuation of the power is less than 5 W, and the ratio of fluctuation to the average value is less than 0.5%. In summary, the rather low PD loss and power fluctuation demonstrates the power stability of the laser setup, in which the Yb/Ce codoped fiber was adopted as an active fiber.
4. Conclusion
A Yb/Ce codoped aluminosilicate fiber was successfully fabricated by the chelate gas phase deposition technique. The RIP of the preform demonstrates homogeneity along both the radial and longitudinal directions. The cladding absorption and core attenuation of the fiber were obtained by the traditional cut-back method. The absorption coefficients are 1.66 dB/m at 976 nm and 0.64 dB/m at 915 nm, respectively. The core attenuation at the laser wavelength of 1080 nm is deduced to be 18 dB/km, and those at 1200 and 1300 nm are 7.2 and 6.4 dB/km, respectively. The laser performance of the Yb/Ce codoped fiber was tested in a MOPA configuration laser setup. A near single mode laser output (M2 = 1.55) with an output power of 1026 W and slope efficiency of 84.8% is obtained. In the 100 hour PD experiment, the entire reduced power is less than 14 W, and the ratio, compared to the initial 1026 W output power, is less than 1.4%. The fluctuation of power during 10 hours of observation is less than 5 W, with a ratio of 0.5%.
Funding
National Natural Science Foundation of China (NSFC, No. 61205039, No. 61138007, No. 61675229); and the National Key Research and Development Program of China (No. 2016YFB0303804).
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