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Abstract
In this work, we systematically studied the mitigation of photodarkening (PD) in ytterbium-doped silica fiber co-doped with lithium (Li). Adding a proper concentration of Li+ ions to the core glass composition, the PD-induced excess loss can be reduced by 25%. The results showed that the effect on numerical aperture and laser efficiency was very small. Compared with the sodium (Na) co-doped Yb/Al silica fiber at the same concentration, the mitigation of the PD effect through Li+ and Na+ ions co-doping was analyzed and discussed. Furthermore, a hypothesis was proposed for the mechanism of alkali metal Li and Na co-doping to mitigate the PD effect. More effective core composition and doping concentration were proposed to provide guidance of high output power gain fiber preparation.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent decades, possessing the advantages of good beam quality, increasing output power and excellent thermal management, ytterbium-doped fiber lasers (YDFLs) are gradually dominating the mainstream market of high power lasers, and widely used in industrial production, biological medicine, national defense and other important fields [1–3]. The output power of YDFL increased rapidly from milliwatt to 10 kilowatts within just 20 years [4, 5]. However, the constantly refreshed high output power also brings a series of problems that need to be solved, such as: nonlinear effects, mode instability (MI), and photodarkening (PD), etc. Among them, an important factor which affects the long-term stable operation of high-power YDFL is the PD effect.
PD effect refers to the permanent loss of signal light in the fiber core ranging from visible band to near-infrared (NIR) band in the case of pump power launching [6]. The most obvious impact of PD is the irreversible decrease of output power and efficiency of fiber laser. Moreover, it has been reported that PD can cause a significant decrease in the mode instability threshold, which deteriorates beam quality and limits laser power climb [7].
The PD-induced excess losses were believed to be caused by the formation of color centers. The origin of PD effect is still under discussed. Different theories have been proposed, such as: oxygen deficiency centers (ODCs) [8, 9], the presence of divalent ytterbium ions originating from the charge transfer process of trivalent ytterbium ions [10, 11], and Tm3+ impurities [12, 13]. A variety of methods was proposed to mitigate and attenuate PD effect, including ions co-doping (Al, P, Ce, Ca, Na, etc.) [14–19], gas pretreatment, photo-bleaching, and thermal-bleaching. The reported gas includes O2 [20] and H2 [21], which could suppress the generation of ODCs in fiber core. The common wavelengths of photo-bleaching include 355, 405, 543, 633, 793 nm, etc [22–26], and short wavelengths photons could be used to remove the color centers. Thermal-bleaching was the most efficient way to eliminate the excess loss of PD, but it was not applicable in practical engineering due to the fiber performance deterioration under high temperature [27, 28].
Among the methods mentioned above, co-doping with ions is the most feasible and convenient approach. Jetschke et al. reported that [29], as the concentration of aluminum (Al) or phosphorus (P) increases, the PD-induced excess loss gradually decreases. A core composition with equal content of Al and P is most promising to achieve Yb fibers with low PD. Engholm et al. demonstrated that cerium (Ce) ions co-doping in Yb-doped fibers effectively improved the PD resistance [16]. Jetschke et al. showed as Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers [17]. Sugiyama et al. tested the transmittance of Yb fiber and Yb/Ca fiber at 635 nm [18]. The YbCaZSG fiber was insensitive, while the transmittance of YbZSG fiber decreased to 40% of the initial value as time passed. Notice that co-doping with P and Ce ions would increase the fiber numerical aperture (NA), as well as the core background loss [30]. Moreover, the addition of P can also reduce the absorption and emission cross section of Yb ions [31]. Therefore, the output power and slope efficiency of fiber laser are seriously affected.
It has been reported that Na+ ions can improve the PD resistance without NA and laser efficiency deterioration. Doping Na+ ions into silicate glasses could introduce much non-bridging oxygen (NBO), which might be captured by Yb3+ ions and prevented the formation of ill-valenced bonds [19]. As a homologous element of Na, Li has similar chemical properties with Na. But, the atomic radius and atomic mass of Li is smaller than that of Na, which suggesting that lithium is more likely to introduce NBO and maybe more effective to mitigate the PD effect. Thus, we investigated the mitigation of PD in YDF by Li+ ions co-doping. In this paper, we measured the PD-induced excess loss of Yb/Al fiber and Yb/Al/Li fiber with the same doping concentration, and analyzed the suppression of Li+ ions on the PD effect. By comparing with the approximate concentration of Yb/Al/Na fiber, the mitigation of PD effect through Li+ and Na+ ions co-doping was analyzed. Furthermore, the mechanism of alkali metal elements Li and Na co-doping to mitigate the PD effect was discussed.
2. Experiments and results
The fiber samples were fabricated by the conventional modified chemical vapor deposition (MCVD) combined with solution doping technique. Two samples maintained the same concentration of Yb3+ (0.045 mol/L) and Al3+ (0.2 mol/L), and 0.1 mol/L Li+ ions were added in one sample. They were prepared into single-mode double cladding fiber with a core diameter of 10 μm and inner-cladding of 130 μm. The measured doping concentration and NAs of the two fibers are listed in Table 1. The NA of two fiber samples was 0.088 and 0.087, respectively. The difference was very small that could be considered as the same. Their Yb3+ and Al3+ concentrations were also similar. The Li+ content in Yb/Al/Li fiber was 0.069 wt%.
Table 1. Fiber Parameters
In our work, the impact of PD was determined with a classic method of measuring the transmission changes over time at visible wavelengths as an indicative measure of PD at the signal wavelengths [32]. The experimental setup for PD-induced excess loss measurement was the same as our previous publication [26]. The fiber samples in the system were replaced by Yb/Al and Yb/Al/Li fibers with a length of about 10 cm. The fiber samples were pumped with a 915 nm LD of 5.5 W to provide 45% population inversion. As reported, the equilibrium state of PD loss depends on the pump power level [33], so the pump power remained unchanged during the whole test of all samples. In the case of pumping, the optical spectrum of fiber sample was measured every 20 minutes. Then the complete additional loss curve of the fiber sample was obtained. The time dependent excess loss of Yb/Al and Yb/Al/Li fibers at 633, 702, 810, and 1041 nm are shown in Fig. 1. Because the lower limit wavelength of the measuring equipment was 600 nm, the data near 600 nm ~650 nm were affected by background noise. Therefore, the measured data points have a large jump, as the black squares shown in Fig. 1(a), (b). In order to obtain more accurate fitting results, 702 nm and 810 nm with more similar wavelength and more regular data points were selected as the reference wavelengths. Besides, the PD-induced excess loss of NIR band was very small where our measuring equipment cannot response such a small fluctuations. Therefore, 1041 nm with more accurate results was chosen to reflect the variation of PD-induced excess loss in signal waveband.
Fig. 1 PD excess loss and fitting curve at 633, 702, 810, and 1041 nm of (a) Yb/Al [19]; (b)Yb/Al/Li fiber.
The data point in Fig. 1 is the measurement result and the solid line is the fitting curve. The data were also fitted by the classical stretched exponential function described in [34]. According to the variation curves of 702, 810, and 1041 nm in Fig. 1 (a) and Fig. 1(b), each data point of Yb/Al/Li fiber is lower than Yb/Al fiber. The equilibrium excess losses of Yb/Al fiber at 633, 702, 810, and 1041 nm were calculated to be 349.96, 121.30, 39.30, and 3.52 dB/m, respectively. While for the Yb/Al/Li fiber with the same process and doping concentration, the equilibrium excess losses in the corresponding wavelengths were 189.0, 99.06, 29.22, and 3.218 dB/m. It can be seen that after doping a moderate amount of Li+ ions into the fiber, the PD-induced excess loss was significantly reduced, and the maximum reduction was up to 25%.
In order to verify whether Li+ ions co-doping would affects the fiber laser properties, the slope efficiency of the Yb/Al and Yb/Al/Li fiber (pristine fiber) were measured, using the Yb-doped fiber laser efficiency test system described in [19]. A 915 nm LD was used to pump the fiber samples with the lengths of 16 m. 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 relationship between the laser output power and the absorbed pump power is shown in Fig. 2. According to the linear fitting results, the slope efficiency was 75.20% for Yb/Al fiber and 77.88% for Yb/Al/Li fiber. Within the range of measurement error, it was considered to be approximately the same. In addition, the background loss of Yb/Al and Yb/Al/Li fiber samples were also measured, which was 40.56 and 42.99 dB/km @ 1200 nm, respectively. Thus, the co-doping of Li+ ion does not affect the optical performance of fiber laser.
3. Comparison with Na+ co-doping
According to the previous publication [19], Na+ ions co-doping in Yb/Al fiber can also mitigate the PD-induced excess loss. In this paper, Yb/Al, Yb/Al/Li, and Yb/Al/Na fiber were compared at the same doping concentrations. The solution doping concentration of Li+ and Na+ were both 0.1 mol/L. The PD-induced excess loss at the four reference wavelengths of the three fiber samples is shown in Fig. 3. At each reference wavelength, Yb/Al fiber had the highest amount of PD-induced excess loss (black squares), indicating that both Li+ and Na+ ions could effectively suppress the excess loss of PD. However, the PD-induced excess loss of Yb/Al/Na fiber (blue circles) was lower than that of Yb/Al/Li fiber (red triangles), implying the suppression of Na+ ions at the same concentration was stronger than that of Li+ ions. Similarly, we compared the slope efficiency of the three fiber samples (pristine fiber), as shown in Fig. 4. The slope efficiency curves of the three were basically coincident, which means the co-doping of Li+ and Na+ ions will not affect the fiber laser properties.
Fig. 3 The PD-induced excess loss of Yb/Al, Yb/Al/Li, and Yb/Al/Na [19] fiber at (a) 633nm, (b) 702nm, (c) 810nm, (d) 1041nm.
As the alkali metal elements, both Li and Na belong to the network modifier, which could be free in the glass structure. Because of their electro-positivity, more NBO could be introduced into silica glasses. The NBO might be captured by Yb3+ ions and prevented the formation of ill-valenced bonds. Therefore PD-induced excess loss was partly reduced [19]. Compared with Na+ ion, Li+ ion had smaller atomic mass and radius, but it was too active and less stable to mitigate the color centers effectively. Besides, under the same mass fraction, the damage of Li+ ions to the glass structure was stronger than that of Na+ ions. Therefore, compared with the same doping concentration of Yb/Al/Li and Yb/Al/Na fibers, the suppression of Na+ ions co-doping was better.
In order to verify this inference, much more fiber samples co-doping with different concentrations of Li+ was tested. This series of fibers had the same concentration of Yb3+ and Al3+ ions of previous fibers. The Li+ ions content of Yb/Al fibers was changed to 0.075, 0.05 and 0.2 mol/L. Similarly, the sample was prepared into a single-mode double cladding fiber with a core of 10 μm and inner-cladding of 130 μm. The measured doping concentration of this fiber is 0.052, 0.035, and 0.139 wt%, respectively. The differences between core NA could be negligible. The parameters of Li co-doping fibers are listed in Table 2. The same test system was used to measure the variation of the PD-induced excess loss in 300 min. The comparison with the previous three fiber samples is shown in Fig. 5.
Table 2. Parameters of Li Co-doping Fibers
Figure 5 shows the variation of the PD-induced excess loss over time of Yb/Al, Yb/Al/Na, and the series of Yb/Al/Li fibers at 702 nm. The measurement results of Yb/Al/Li-1 fiber (purple) were located below the data points of the Yb/Al/Na fiber (blue) in the first few minutes. After the cross point, the measurement results were slightly higher than that of Yb/Al/Na fiber. In addition, the results of the lower co-doped Yb/Al/Li fiber were always below Yb/Al/Li fiber (red). By comparing the fitting results of these fiber samples, although the data points were fluctuating up and down, the PD-induced excess loss in steady state were almost the same. The experimental results showed that the mitigation of PD-induced excess loss could be improved by appropriately reducing the concentration of Li+ ions, which could almost achieve the same suppression as Na+ ions.
On the contrary, Yb/Al/Li-2 fiber with the lowest Li co-doping and Yb/Al/Li-3 fiber with the highest Li co-doping had no suppression on PD. Even worse, the PD losses were higher than Li-free fiber. We hypothesize that for extreme low co-doped Yb/Al/Li-2 fiber, Li+ ions played the role as impurities. The impurities caused more kinds of losses in fiber, including PD loss. Besides, for Yb/Al/Li-3 fiber, the concentration of Li was too high to damage the glass structure of the fiber. The slope efficiency of Yb/Al/Li-3 fiber was significantly lower than that of other fibers (only 33.64%). Therefore, the fiber with high Li concentration not only increased PD loss, but also deteriorated dramatically the slope efficiency. Because the Li+ ions were more free and active than Na+ ions under the same mass fraction, Na+ ions were more advantageous in the mitigation of PD. By decreasing the doping concentration of Li+ ions in a moderate amount, the damage to the glass structure could be reduced and may realize more effective enhancement of PD suppression.
4. Conclusion
In this paper, we adopted the method of co-doping with Li+ ions into YDF to mitigate the PD effect. We measured the PD-induced excess loss of Yb/Al fiber and Yb/Al/Li fiber with the same process and doping concentration, and experimentally verified the mitigation of PD-induced excess loss in Yb/Al/Li fiber, which could be reduced by about 25%. Comparing with the approximate concentration of Yb/Al/Na fiber, it was found that both Yb/Al/Li and Yb/Al/Na fibers were able to mitigate the PD effect, but the mitigation of Na+ ions co-doping was better. Based on the atomic properties of element Li and Na, we hypothesized the suppression mechanism of co-doping Li or Na on PD effect were the same. More NBO were introduced through co-doping with Li or Na to prevent the formation of ill-valenced bonds, and finally realize the reduction of PD-induced excess loss. However, due to the active properties of Li+ ions, the mitigation of the co-doped Li+ ions was less than that of Na+ ions under the same mass fraction. Therefore, by co-doping appropriate concentration of Li+ ions, the mitigation of PD-induced excess loss could be improved, which may almost achieve the same effect as Na+ ions. The optimal concentration of Li, Na, and K elements and the laser performance under high power condition need further research and exploration.
Funding
The National Key Research and Development Program of China (No. 2017YFB1104400); and National Natural Science Foundation of China (No. 61735007)
References and links
1. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. 33(7), 1049–1056 (1997). [CrossRef]
2. J. Nilsson and D. N. Payne, “Physics. High-power fiber lasers,” Science 332(6032), 921–922 (2011). [CrossRef] [PubMed]
3. M. N. Zervas and C. A. Codemard, “High power fiber lasers: a review,” IEEE J. Sel. Top. Quant. 20(5), 0904123 (2014). [CrossRef]
4. B. Shiner, “The impact of fiber laser technology on the world wide material processing market,” CLEO: Applications and Technology, AF2J. 1 (2013).
5. V. Fomin, V. Gapontsev, E. Shcherbakov, A. Abramov, A. Ferin, and D. Mochalov, “100 kW CW fiber laser for industrial applications,” IEEE International Conference on Laser Optics, 1 (2014). [CrossRef]
6. C. G. Ye, L. Petit, J. J. Koponen, I.-N. Hu, and A. Galvanauskas, “Short-term and long-term stability in ytterbium-doped high-power fiber lasers and amplifiers,” IEEE J. Sel. Top. Quant. 20(5), 1–12 (2014).
7. H.-J. Otto, N. Modsching, C. Jauregui, J. Limpert, and A. Tünnermann, “Impact of photodarkening on the mode instability threshold,” Opt. Express 23(12), 15265–15277 (2015). [CrossRef] [PubMed]
8. K. E. Mattsson, “Photo darkening of rare earth doped silica,” Opt. Express 19(21), 19797–19812 (2011). [CrossRef] [PubMed]
9. S. Yoo, C. Basu, A. J. Boyland, C. Sones, J. Nilsson, J. K. Sahu, and D. Payne, “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett. 32(12), 1626–1628 (2007). [CrossRef] [PubMed]
10. S. Rydberg and M. Engholm, “Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers,” Opt. Express 21(6), 6681–6688 (2013). [CrossRef] [PubMed]
11. M. Engholm and L. Norin, “The role of charge transfer processes for the induced optical losses in ytterbium doped fiber lasers,” Proc. SPIE 7195, 71950T (2009). [CrossRef]
12. R. Peretti, A. M. Jurdyc, B. Jacquier, C. Gonnet, A. Pastouret, E. Burov, and O. Cavani, “How do traces of thulium explain photodarkening in Yb doped fibers?” Opt. Express 18(19), 20455–20460 (2010). [CrossRef] [PubMed]
13. S. Jetschke, S. Unger, A. Schwuchow, M. Leich, J. Fiebrandt, M. Jäger, and J. Kirchhof, “Evidence of Tm impact in low-photodarkening Yb-doped fibers,” Opt. Express 21(6), 7590–7598 (2013). [CrossRef] [PubMed]
14. S. Jetschke, S. Unger, M. Leich, and J. Kirchhof, “Photodarkening kinetics as a function of Yb concentration and the role of Al codoping,” Appl. Opt. 51(32), 7758–7764 (2012). [CrossRef] [PubMed]
15. B. Morasse, S. Chatigny, É. Gagnon, C. Hovington, J.-P. Martin, and J.-P. Sandro, “Low photodarkening single cladding ytterbium fibre amplifier,” Proc. SPIE 6453, 64530H (2007). [CrossRef]
16. M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett. 34(8), 1285–1287 (2009). [CrossRef] [PubMed]
17. S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and M. Jäger, “Role of Ce in Yb/Al laser fibers: prevention of photodarkening and thermal effects,” Opt. Express 24(12), 13009–13022 (2016). [CrossRef] [PubMed]
18. S. Sugiyama, Y. Fujimoto, M. Murakami, H. Nakano, T. Sato, and H. Shiraga, “Suppression of photo-darkening effect by Ca additive in Yb-doped silica glass fibre,” Electron. Lett. 49(2), 148–149 (2013). [CrossRef]
19. N. Zhao, Y. Liu, M. Li, J. Li, J. Peng, L. Yang, N. Dai, H. Li, and J. Li, “Mitigation of photodarkening effect in Yb-doped fiber through Na+ ions doping,” Opt. Express 25(15), 18191–18196 (2017). [CrossRef] [PubMed]
20. S. Yoo, C. Basu, A. J. Boyland, C. Sones, J. Nilsson, J. K. Sahu, and D. Payne, “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett. 32(12), 1626–1628 (2007). [CrossRef] [PubMed]
21. M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al-doped fiber lasers,” Proc. SPIE 6873, 68731E (2008). [CrossRef]
22. I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. Bello Doua, and F. Salin, “Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber,” Opt. Express 15(4), 1606–1611 (2007). [CrossRef] [PubMed]
23. R. Piccoli, T. Robin, D. Méchin, T. Brand, U. Klotzback, and S. Taccheo, “Effective mitigation of photodarkening in Yb-doped lasers based on Al-silicate using UV/visible light,” Proc. SPIE 8961, 896121 (2014). [CrossRef]
24. A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photo-bleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett. 4(10), 734–739 (2007). [CrossRef]
25. H. Gebavi, S. Taccheo, L. Lablonde, B. Cadier, T. Robin, D. Méchin, and D. Tregoat, “Mitigation of photodarkening phenomenon in fiber lasers by 633 nm light exposure,” Opt. Lett. 38(2), 196–198 (2013). [CrossRef] [PubMed]
26. N. Zhao, Y. B. Xing, J. M. Li, L. Liao, Y. B. Wang, J. G. Peng, L. Y. Yang, N. L. Dai, H. Q. Li, and J. Y. Li, “793 nm pump induced photo-bleaching of photo-darkened Yb(3+)-doped fibers,” Opt. Express 23(19), 25272–25278 (2015). [CrossRef] [PubMed]
27. J. Jasapara, M. Andrejco, D. DiGiovanni, and R. Windeler, “Effect of heat and H2 gas on the photo-darkening of Yb3+ fibers,” Conference on Lasers and Electro-Optics, CTuQ5 (2006).
28. M. J. Söderlund, J. J. Montiel i Ponsoda, J. P. Koplow, and S. Honkanen, “Thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers,” Opt. Lett. 34(17), 2637–2639 (2009). [CrossRef] [PubMed]
29. S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, “Efficient Yb laser fibers with low photodarkening by optimization of the core composition,” Opt. Express 16(20), 15540–15545 (2008). [CrossRef] [PubMed]
30. S. Unger, A. Schwuchow, S. Jetschke, V. Reichel, A. Scheffel, and J. Kirchhof, “Optical properties of Yb-doped laser fibers in dependence on codopants and preparation conditions,” Proc. SPIE 6890, 689016 (2008). [CrossRef]
31. M. A. Mel’kumov, I. A. Bufetov, K. S. Kravtsov, A. V. Shubin, and E. M. Dianov, “Lasing parameters of ytterbium-doped fibres doped with P2O5 and Al2O3,” Quantum Electron. 34(9), 843–848 (2004). [CrossRef]
32. J. J. Koponen, M. J. Söderlund, H. J. Hoffman, and S. K. Tammela, “Measuring photodarkening from single-mode ytterbium doped silica fibers,” Opt. Express 14(24), 11539–11544 (2006). [CrossRef] [PubMed]
33. S. Jetschke, S. Unger, U. Röpke, and J. Kirchhof, “Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power,” Opt. Express 15(22), 14838–14843 (2007). [CrossRef] [PubMed]
34. S. Jetschke and U. Röpke, “Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers,” Opt. Lett. 34(1), 109–111 (2009). [CrossRef] [PubMed]
