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A high-power Yb/Ce co-doped double-clad fiber with low optical loss was successfully fabricated by an optimized chelate gas phase deposition technique. It exhibits a nearly homogenous distribution of Al, Ce and Yb ions in the fiber core region, which reduce the clustering. The core attenuation at 1080 nm and 1383 nm are 12 dB/km and 46 dB/km, respectively, indicating high optical performance with a low optical loss. The amplifier stage with this fiber delivers 4.62 kW excellent beam quality (M2 = 1.67) laser output with a slope efficiency of 80.3%. The experimental results show that the chelate gas phase deposition technique is a prospective method to fabricate a Yb/Ce co-doped fiber with low optical loss, which is beneficial for acquiring multi-kilowatt continuous-wave fiber laser with excellent beam quality.
© 2017 Optical Society of America
1. Introduction
Compared with the other conventional lasers, all-fiber laser has more potential in combining high laser output and high beam quality together [1–7]. The rapid power scaling of high power fiber lasers mainly benefits from the cladding-pumped fiber technology. This structure of the rare-earth doped fiber leads to high photon conversion efficiency, as well as the large surface area allowing the thermal load to dissipate quickly [8–11]. Owing to the rapid development of the fiber manufacture technology [12–15], the output power of all-fiber laser has scaling up to 20 kW level with single mode and 500 kW level with multi-mode at present [16].
However, with the promotion of output power form high-power fiber laser sources, the active fiber suffers more thermal burden and photodarkening (PD) which have become the main obstacles for further advance of fiber laser [17–21]. The heat generated in the active fiber mainly derives from the quantum defect between ground state and exited state of Yb3+, background loss of the quartz glass, micro cracks and contaminates on the fiber surface [22–24]. One of the most effective methods is to reduce optical loss in the core of active fiber by developing novel strategies or improving traditional techniques to fabricate the active fiber [25–30]. Recently, the conventional modified chemical vapor deposition (MCVD) in conjunction with chelate gas phase deposition (CGPD) technique has great potential to fabricate an ytterbium-doped fiber with low optical loss [31,32]. Preparation of high purity raw material and process gas is the foremost section in fabrication of high quality optical fibers and the reactions take place in a closed environment to promote removal or destruction of contaminates and water in air [33,34]. Moreover, the composition of the preform can be accurately controlled by adjusting the gas flow so that a proper doped concentration and homogenous distribution can be obtained. In the charge transfer (CT) model, Engholm group stated that the valence instability of Yb-ion in the silica glass matrix results in color center formation with induced absorption observed at visible and near-IR wavelengths, which was the origin of PD. Cerium with 3 + and 4 + valence state can trap both holes and electrons in the silica glass and thus reduce the number of color center, therefore Ce co-doping, which successfully have been apply to radiation protection of silicate glass, is used to improve the PD resistivity [35].
In this work, a Yb/Ce co-doped double-clad fiber with low optical loss was fabricated by CGPD technique. Using this fiber as the amplifier stage in a master oscillator power amplifier (MOPA) configuration laser setup, an excellent beam quality laser output (M2 = 1.67) with an output power of 4.62 kW was obtained. The surface temperature of the active fiber at the top power was monitored and was less than 40.0 °C. It implies that the CGPD technique is an effective manner to fabricate Yb-doped fiber with low optical loss, so that multi-kilowatt continuous-wave (CW) fiber laser output can be expected for low thermal burden.
2. Experiment
The Yb/Ce co-doped preform was fabricated by CGPD technique. The precursor materials include Yb(thd)3, Ce(thd)3, AlCl3, SiCl4 and SiF4. Yb(thd)3, Ce(thd)3, AlCl3 were heated at their corresponding sublimating temperature (100-250°C) and formed oxide particles such as Yb2O3, CeO2 and Al2O3. Doping Yb2O3, CeO2 and Al2O3 into silica glass can increase refractive index, which is not good to realize high power laser with an excellent beam quality. Therefore, SiF4 as fluorine dopant was introduced to reduce the refractive index increment to keep core a low numerical aperture (NA) or the acceptable level. On the other hand, the fluorine can influence the properties of silica glass through changing glass network structure [32].
In this experiment, the purity of all raw materials and process gas is required above 5N level. Firstly, the chemical reactions between the precursor materials and O2 took place in a Heraeus F-300 silica tube at a temperature of 1850 °C. Then the doped silica tube was collapsed into a solid preform at a temperature up to 2000 °C. After that, according to the core/cladding ratio of fiber, the preform was processed form to octagon one with suitable jacket and mechanical treatment. Finally, the preform was drawn to 30/600 μm fiber at 2100 °C. As well as a low index polymer was used as the outer-clad to achieve the wave-guide of pump light and a protect polymer was coated as the second coating to prevent mechanical and chemical damage, otherwise it could weaken the fiber and compromise its performance.
Compared with circular optical fiber, the octagonal fiber has an additional freedom degree, so that two laser measuring heads (WSYS_ODAC18XY, Zumbach) placed with an angle of 22.5 degrees were employed during the fiber drawing process. According to the collected data, the cladding diameter (flat-to-flat) can be calculated and transferred to the central computer through an analog interface. And then the velocity parameter of fiber drawing was adjusted to the designed values in real time through proportional-integral-differential (PID) algorithm, improving the stability of fiber diameter along the longitudinal direction.
The refractive index profile (RIP) of preform was measured by the Photon Kinetics 2600 preform analyzer. Meanwhile, the composition distribution of Yb/Ce co-doped fiber in the radial direction was characterized using electron probe micro-analysis (EPMA, JXA-8230, Japan). In order to analyze vertical homogeneity, the diameter of fiber was obtained from the cladding diameter monitoring system in drawing tower. In addition, the core attenuation coefficient was measured by traditional cut-back method Photon Kinetics 2500 optical fiber analysis system at the wavelength range of 600nm-1600nm.
As shown in Fig. 1, the laser performance of a Yb/Ce co-doped fiber was tested in a MOPA configuration. Six 900 W diode lasers (LDs) at 976 nm are applied as the pump sources which are coupled into the 30/600 ytterbium-doped double-cladding fiber (DCF) via the (6 + 1) × 1 pump coupler (MPC). The values of cladding absorption are 0.48 dB/m at 914.6 nm and 1.22 dB/m at 976.4 nm respectively. In order to absorb 15 dB pump power and suppress the stimulated Raman scattering (SRS), the required length of active fiber is 15 m. Two cladding power strippers (CPS) were used to enhance the beam quality by wiping off the residual pump power and prevent seed laser from the reflected power [36–39]. To avoid any end-face reflection, the end cap coating for anti-reflection (AR) was spliced to deliver the output signal power. In addition, a mode field adapter (MFA) was adopted to decrease the splice loss between the seed laser and combiner. The output power was recorded by a power meter (PM, OPHIR, 5kW-ROHS), and the spectrum was measured by optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C). Also, the beam quality under high power condition was obtained by a high-power laser quality monitor (HP-LQM, PRIMS GmbH).
3. Results and discussion
The RIP of preform core is determined by the concentration of doped elements. As shown in Fig. 2, the refractive index difference (RID) between the core and cladding is about 1.2 × 10−3 corresponding to the numerical aperture (NA) of 0.06. The fluctuation of the core refractive index along the radial direction is about 0.16 × 10−3, implying the homogenous distribution of the dopants.
The fluctuations of cladding diameter before and after applying two laser measuring heads are shown in Fig. 3. After applying the controlling units, the fluctuation and error of the cladding diameter are 0.3% and ± 2.0 μm, respectively, less than before of that (4.2%, and ± 25.0 μm). The significant improvement on the stability of the cladding diameter is advantageous to decrease the propagation loss along the longitudinal direction and optimizing the beam quality of laser output.
Composition distributions of the homemade Yb/Ce co-doped fiber in the core region are presented in Fig. 4. It can be seen from microscopic analysis graphs that Al, Ce and Yb ions uniformly distributed in fiber core region, which indicates the concentration of Al, Ce and Yb ions in the fiber core area presents a small change. Moreover, the dopant concentrations of Al, Ce and Yb ions are estimated to be about 6750 weight ppm, 3200 weight ppm and 8545 weight ppm, respectively. The homogenous distributions of Al, Ce and Yb ions contribute to improving optical wave-guide and reducing the clustering in the active fiber, which are both helpful for the reduction of background loss and up-conversion effects [40,41].
Figure 5 shows the core attenuation spectrum of the Yb/Ce co-doped fiber. At shorter wavelengths, impurities introducing absorption bands, such as hydroxyl (OH−1), H2, Cl−1 (from reagents AlCl3, SiCl4), make remarkable increase of the fiber loss. The OH−1 is not caused only by contamination from reagents, but can diffuse from substrate tube. The loss increases rapidly towards shorter wavelengths. Due to the presence of Yb ions, the signal light is absorbed strongly in the core region. As a result, the attenuation cannot be observed in the range of 800-1100 nm for the low signal-to-noise ratio [42]. Using a Rayleigh scattering term λ−4, the attenuation spectrum between 700 nm and 1200 nm in the core region was fitted. The value of the core background attenuation at 1080 nm is 12 dB/km, which represents that the background loss is brought from the silica matrix. Also, it can be seen that the core attenuation at 1383 nm is about 46 dB/km, indicating a relative low OH-content. The quite low background attenuation at the wavelength of laser output and low OH-content result from the high pure raw material, the separated deposition environment and the introduction of Cl2 prior to the deposition process in the preform fabrication.
The optical performance of Yb/Ce co-doped fiber was tested in a MOPA configuration shown in Fig. 1. An output power of 4.62 kW at about 1080nm was obtained when the seed laser power of 0.8 kW and pump power of 4.76 kW were launched into the power amplifier. As shown in Fig. 6 (a), the output laser power of Yb/Ce co-doped fiber power amplifier reaches to 3.82 kW corresponding to a slope efficiency of 80.3%. As shown in Fig. 6 (b), the thermal image of the active fiber was obtained under the condition of maximum power in laser performance experiment (i.e. 4.62 kW). It becomes apparent that the surface temperature of the fiber is less than 40.0 °C, owing to the low optical loss of the active fiber fabricated by CGPD technique. In addition, the coating polymer of the active fiber can suffer high temperature of ~80 °C, indicating that the active fiber is within the safety criterion. Taking into account the limitation of pump power, a higher laser output can be achieved with higher pump source in the future.
Fig. 6 (a) Output power of the Yb/Ce co-doped fiber power amplifier with the increase of pump power; (b) thermal image of the active fiber at 4.62 kW.
The beam quality of the laser output at maximum power point is depicted in Fig. 7. The excellent beam quality with a M2 factor of 1.67 is obtained with our homemade Yb/Ce co-doped fiber. The tested results show that the CGPD deposition technique is a promising approach to fabricate high-performance optical fiber.
The laser spectrum was measured using high-precision OSA with a minimum resolution of 0.02 nm, as shown in Fig. 8. The central wavelength is 1079.90 nm and the full width at half maximum (FWHM) is 2.4 nm. Besides, a little band at 976 nm and 1130 nm can be observed, indicating the presence of residual pump light and SRS in the output power. Often the high-index ultraviolet adhesive is used to remove the unwanted light from DCF. The CPS have to be able to withstand about 100 W and most polymers are not capable of exceeding temperatures more than 80 °C, so we reduce the amount of polymer to ensure system security. Thus a little band at 976 nm can be observed. For the central wavelength of 1080 nm laser, the Stokes wavelength should be around 1134 nm, corresponding to the frequency shift of 13.2 THz in the silica fiber. By integrating the spectral curve, the ratios of pump power and SRS power to the total output power are 0.72% and 0.45%, respectively. Therefore, the influences of residual pump light and SRS on the laser output power can be ignored.
4. Conclusion
A Yb/Ce co-doped double-clad fiber was successfully fabricated by CGPD technique. After applying two laser measuring heads, the fluctuation of octagonal fiber diameter is decreased from ± 25.0 μm to ± 2.0 μm, indicating an obvious diameter stability improvement along the longitudinal direction. The homogenous composition distribution is demonstrated by the EPMA area scanning of Yb and Al ions. Through the cut-back method, the core attenuation fitted with Rayleigh scattering term are 12 dB/km at 1080 nm and 46 dB/km at 1383 nm, respectively, indicating high optical performance with a low optical loss. Using the Yb/Ce co-doped fiber as the amplifier stage, 4.62 kW laser output with a M2 factor of 1.67 was obtained.
Funding
This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 61675229, No. 61138007).
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