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Abstract
In this work, Na+ ions doping into the Yb-doped fiber is proposed to improve the photo-darkening resistance. The results show that the photo-darkening induced excess loss at equilibrium state at 633nm, 702 nm, 810 nm, and 1041 nm is 115.54dB/m, 86.87dB/m, 25.51 dB/m, 2.92 dB/m, respectively, when co-doping with Na+ ions. More than 30% excess loss is reduced comparing to the Yb-doped fiber without Na+ ions. The mechanism of Na+ ions doping to mitigate the PD excess loss is discussed. Besides, we measured the laser efficiency of Yb/Al/Na co-doped fiber to be 76.1%. This result remains almost the same with Yb-doped fiber and proves that the addition of Na+ ions do not deteriorate the fiber slope efficiency. The background loss of the two fibers also stays close. The results indicate this method is promising in high power fiber laser development.
© 2017 Optical Society of America
1. Introduction
In recent years, Yb-doped fiber lasers have attracted more and more attention due to their compactness, ruggedness, high beam quality, and flexibility, compared to traditional solid-state lasers [1–3]. They have been rapidly developed and widely applied in industrial processing, military, medical treatment, etc [4–6]. Until now, Yb-doped fiber lasers with kW continuous wave and MW pulsed wave output power have been reported. However, caused by photodarkening effect (PD) [7, 8], the output power declines when operating at high power level for a long time. The PD effect introduces background loss in visible wavelengths and the loss extends to the near-infrared range, covering the pumping and lasing wavelengths, therefore deteriorating the fiber laser property. It is generally convinced to be one of the main bottlenecks of laser power increasing.
The mechanism of PD effect is still under exploration. Color centers in Yb-doped fibers are considered to be the reason to induce PD effect. Oxygen deficiency center (ODC) [9], charge transfer band [10], and Tm impurities [11] are three widely accepted explanations for the color centers formation mechanism. To overcome the PD effect, various methods have been proposed, including photo-bleaching [12–16], thermal-bleaching [17], H2 [18] or O2 [19] loading, and co-doping with other ions (Al, Ce, P, etc.) [20–22]. Among these methods above, doping with ions is the most convenient approach to realize, which can be easily done during fiber fabrication process. Engholm et al. reported that Ce ions suppress PD effect well because they have two roles of hole- and electron-traps [21]. Sakaguchi et al. found that co-doping of alkaline earth metals (Mg2+/Ca2+/Sr2+/Ba2+) could stabilize the Yb3+ valence state and therefore effectively reduce the PD effect [23]. Anirban Dhar et al. proposed that the multielement (P-Yb-Zr-Ce-Al-Ca) Yb doped fiber is a promising candidate for laser applications with enhanced PD resistivity [24]. However, the addition of phosphorus and cerium will raise the fiber numerical aperture (NA), induce fiber background loss, and further reduce the slope efficiency of fiber lasers [25]. Besides, phosphorus dopant also decreases the absorption and emission cross section of Yb ions [26]. The multielement doping method requests a balance between the doping ions and the fiber properties and also introduces more complexity and difficulties for fiber fabrication.
To measure the PD induced excess loss, J. J. Koponen et al. proposed to observe the transmission changes at visible wavelengths and therefore predict the PD at signal wavelengths. Furthermore, they found the loss at 633 nm has a near linear correlation with that at signal wavelength, which is about 71 times [27]. Besides, the equilibrium state of PD loss depends on the pump power level, as reported by S. Jetschke [28].
In this work, the impact of Na+ ions doping into the Yb/Al co-doped fiber on PD effect is carefully studied. We report the measurement results of PD effect in Yb/Al co-doped fiber and Yb/Al/Na co-doped fiber. The role of Na+ ions for PD effect suppression is discussed. We also measured the fiber laser properties when doping with Na+ ions and compared the results with Yb/Al co-doped fiber.
2. Fiber parameters and experiments
An Yb/Al/Na co-doped fiber and an Yb/Al doped fiber were fabricated by the conventional modified chemical vapor deposition (MCVD) combined with solution doping technique. The refractive index profiles (RIPs) of the two fibers were also measured by PK104 (PHOTO KINETICS), as shown in Fig. 1. It is clear that the addition of Na+ ions into the fiber core has no significant effect on the RIPs. The two fiber preforms core radius were both 1.35 mm and the numerical apertures (NA) were measured to be 0.089. Besides, no obvious depressions and peaks were observed. Therefore, the addition of Na+ ions does not influence the fiber preform profiles and parameters.
Then, the two fiber preforms were drawn to the same single mode double-clad fibers in a fiber drawing tower. The core and inner-cladding diameters are 10 μm and 130 μm, respectively. The NAs of the two fibers are both around 0.09. The doping concentration of Yb3+ and Al3+ are about 0.745 wt.% and 0.517 wt.% for the two fibers. The Na+ concentration in the Yb/Al/Na fiber is about 2201 ppm. The parameters are listed in Table 1.
Table 1. Preform and fiber samples.
The experimental setup for PD induced excess loss measurement shown in Fig. 2 is the same used in our previous publications [15, 29]. The signal light is a halogen lamp ranging from 600 ~1650 nm and was free spaced into a single mode fiber. The pump source is a 915 nm laser diode. They were both pumped into the Yb-doped fiber through a combiner. The tested Yb-doped fibers were kept about 10 cm long to maintain uniform inversion level through the fiber. A mode stripper was inserted between the Yb-doped fiber and single mode fiber to remove the cladding light. To avoid heat accumulation, an air-cooling fans near the Yb-doped fiber and a water-cooling system with 18 °C for the mode-stripper were applied. The optical spectra of the Yb-doped fiber were obtained after each test. The pump power was kept 5.5 W to provide 45% population inversion during the whole fiber test.
3. Results and discussion
3.1 Photo-darkening excess loss measurement and analysis
The absorption spectrum of the two fiber samples were recorded every 20 minutes when the 915 nm diode was pumping. 633, 702, 810, and 1041 nm were selected as the probe wavelengths, where the PD induced excess loss can be easily observed. Figure 3 shows the time dependent excess loss at 633, 702, 810, and 1041 nm. The data were also fitted by the classical stretched exponential function described in [30].
Fig. 3 PD excess loss and fitting curve at 633 nm, 702 nm, 810 nm, and 1041 nm of (a) Yb/Al co-doped fiber; (b) Yb/Al/Na co-doped fiber.
The results in Fig. 3 show that the PD induced excess loss was decreased by doping Na+ ions into the Yb/Al co-doped fiber. For the fiber without Na+ ions doping, the PD induced excess loss increment within the first 20 minutes was as large as 29 dB/m at 702 nm. It was still far from the equilibrium state when the pumping time was up to 320 minutes. When fitting the curves with the classical stretched exponential function, the equilibrium excess loss at 633, 702, 810, and 1041 nm was calculated to be 349.96, 121.30, 39.30, and 3.52 dB/m, respectively. While for the Yb/Al/Na co-doped fiber, the equilibrium loss was reduced to 115.54, 86.87, 25.51, and 2.92 dB/m at the same wavelengths. About 30% PD loss was reduced with Na+ ions doping. Thus, the addition of Na+ ions into the Yb-doped fiber strongly improved the PD resistance. This proposed method, which can be applied during the fiber fabrication process, offers a convenient way to reduce the impact of PD effect on fiber laser performance.
3.2 Laser properties of Yb/Al/Na co-doped fiber
To investigate the influence of Na+ ions on the fiber laser properties, the laser efficiency of Yb/Al/Na co-doped fiber was measured in contrast with the fiber without Na+ ions doping. The setup is plotted in Fig. 4. A 915 nm LD was used to pump the two fibers. A fiber grating with high peak reflectivity R = 99.9% @ 1080 nm and the 4% Fresnel reflection of the fiber output end were used to provide the laser cavity. The output laser power dependence on the pump power is shown in Fig. 5. The slope efficiency of Yb/Al co-doped fiber is 75.2%, and it is 76.1% for the Yb/Al/Na co-doped fiber. The 1% difference is within the margin of error for this measurement setup. Besides, the background loss of the two fibers was also measured, which is 40.56 and 51dB/km, respectively. Therefore, the addition of Na+ ions has no negative influence on the laser properties, indicating that our method of PD loss mitigation can be applied in high power fiber lasers.
Fig. 5 Laser output power dependence on absorbed pump power of (a) Yb/Al co-doped fiber; (b) Yb/Al/Na co-doped fiber.
3.3 Hypothesis of photo-darkening resistance mechanism with Na+ ions doping
S. Yoo et al proposed that Yb-associated oxygen deficiency centers is the precursor to the PD effect [9]. In Yb-doped fibers, Yb3+ ions preferably occupy oxygen-surrounding voids provided by Al-O tetrahedral or octahedral coordination in silicate glasses [31]. When doping Yb3+ ions heavily, the lack of voids can induce ill-valenced bonds such as Yb-Al or Yb-Yb, which is called oxygen deficiency center. Doping Na+ ions into silica glasses has been proved to introduce much non-bridging oxygen (NBO). The Na2O in silica glass can provide much oxygen atoms and increase its percentage. Therefore it’s impossible for every oxygen atom to be connected with two silicon atoms. Those oxygen atoms bonding with only one silicon atom is called non-bridging oxygen. The NBO might be captured by Yb3+ ions and prevent the formation of ill-valenced bonds. Therefore the PD induced excess loss was partly reduced.
The PD mechanism is complicated and needs further investigation to study the impact of Na+ ions. However, the detailed investigation is our ongoing work. The next step includes different doping concentrations of Na+ ions, the UV-VIS spectroscopy of the two fibers, the analysis of valence bonds and states within the fiber materials. We will continue further investigation to reveal the PD mechanism.
4. Conclusions
In conclusion, we proposed a novel approach of co-doping Na+ ions to suppress the PD induced excess loss and without fiber parameters and laser efficiency deterioration. No change of NA value for the Na+ ions doping fiber preform was observed. About 30% PD induced excess loss was decreased. The laser efficiency of Yb/Al/Na co-doped fiber was measured to be 76.1% and the background loss was 51 dB/km, which were almost the same as Yb/Al co-doped fiber. The results demonstrate that the addition of Na+ ions is a convenient method to improve the PD resistance in Yb-doped fiber and keep fiber lasers operating with good performance.
Funding
The National Key Research and Development Program of China (No. 2016YFB0402200).
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