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Abstract
A novel method for mitigating photo-darkening and the effective photo-bleaching phenomenon by 532 nm cladding pump in Yb-doped fiber were herein reported. Compared with the pristine fiber, beyond 30% of photo-darkening induced excess loss was suppressed by 532 nm pretreatment. Moreover, the excess loss in the photo-darkened fiber was completely bleached with 532 nm pump. Additionally, the bleached fiber exhibited better photo-darkening resistance. Therefore, for high power application, a 20/400 gamma irradiated fiber was bleached in situ by 532 nm pump and the laser properties were explored. The output power restored to 421W accounting for 82% of the pristine fiber, with the mode instability threshold rising to over 2.6 times and the efficiency increasing from 37% to 63%. The results indicate 532 nm pump has bright prospects for the stable operation of high power fiber lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past few decades, Yb-doped fiber lasers has undergone rapid development with power scaling from watts to tens of kilowatts [1,2]. Possessing the advantages of excellent beam quality, compact structure and high efficiency, Yb-doped fiber lasers are widely used in industry, national security, communication and medical treatment [3,4].
However, due to photo-darkening (PD) effect, the output power of Yb-doped fiber laser dwindles, impeding further power scaling and considerably affecting high power application [5]. PD increases the background loss from ultraviolet to near infrared band and causes the absorption of pump light [6], which will increase the thermal load on optical fibers and cause the beam phase distortion, giving rise to the occurrence of mode instability (MI) that relies on the relative phase between interference pattern and thermal induced gratings [7,8]. Moreover, PD induces phase distortion in coherent combination, which will also diminish the combination efficiency for high power application [8]. The mainstream view on the origin of PD are color centers, however, the formation of the color center is still controversial [9,10].
In spite of the unclear mechanism, appreciable efforts have been devoted to mitigate PD. The suppression scheme starts from fiber fabrication, including changing the glass composition (Ce/P/Al/Na), gas loading (H2/O2) and fiber structure designing [10–15]. Ce is a common co-dopant for suppressing PD in commercial fibers, however, the PD inhibition process that Ce3+/Ce4+ capture electrons and holes will cause additional heat sources [16]. Co-doping with P ions can enhance the solubility of Yb ions in silica, reducing the probability of cluster formation and inhibiting PD [17]. Recently, it has been reported that the addition of Na ions could increase the amount of non-bridge oxygen and improve the PD resistance in Yb-doped fibers [13]. In the pretreatment methods, PD can be significantly suppressed by H2-loading; besides, it also exhibited excellent thermal performance in the H2-loaded fiber [15,18].
In addition to inhibiting PD by optimizing the composition and structure of optical fibers, bleaching methods are also proposed to eliminate the additional loss after the occurrence of PD, including photo-bleaching, thermal bleaching, etc [19–21]. Photo-bleaching can partially or completely bleach the excess loss induced by PD, depending on the light source. I. Manek et al. used 355 nm ultraviolet irradiation to bleach the photo-darkened fibers, which could eliminate the PD loss completely [6]. Similarly, photo-bleaching phenomenon was observed by 405 nm with power of 2 mW, nevertheless, the existence of ground state absorption would cause PD to a certain degree, leading to the incomplete recovery [20]. Guzman Chávez et al. found about 50% of additional loss was bleached under 543 nm irradiation in Yb-doped fibers [21]. They hypothesized that the photo-darkening and photo-bleaching effects might originate from the light-induced conversion between Yb3+ and Yb2+. With the decrease of photon energy and the extension of bleaching wavelength (633 nm, 793 nm, etc), the bleaching effect was limited [22,23]. In the previous reports, the output power of bleaching wavelengths were mostly in the mW magnitude, which could play a certain role in laboratory-level bleaching fibers lasers. However, for the industrial kilowatt-class fiber lasers, it is difficult to access practical application, which lacks research on the effect of photo-bleaching in high power fiber lasers.
In this work, a novel method to alleviate PD and a remarkable recovery of laser properties in high power fiber amplifiers was demonstrated with 532 nm irradiation. More than 30% of PD loss was reduced by 532 nm pre-irradiation in the pristine fiber. While almost complete photo-bleaching in the photo-darkened fiber was observed under 532 nm pump, the PD performance was substantially improved after the bleaching process. Thus, in order to investigate the effect of 532 nm photo-bleaching in high power fiber amplifiers, the laser properties of a gamma irradiated fiber were measured before and after 532 nm photo-bleaching. The optical efficiency increased from 37% to 63%, and the MI threshold raised to at least 2.6 times. Based on the experiment results, the mechanism of PD mitigation and photo-bleaching by 532 nm was discussed.
2. Experimental setup
In this experiment, two types of double-clad fiber were fabricated by modified chemical vapor deposition (MCVD) process combined with solution doping method. The doping concentration and fiber parameters are illustrated in Table 1.
Table 1. Fiber samples parameters
Measurement scheme of PD induced excess loss was similar to our previous work, as shown in Fig. 1(a) [23]. A 10 cm fiber sample was pumped by a 915 nm laser diode (LD) with output power of 5.5 W. A 532 nm LD with maximum output power of 5 W was used as the bleaching source. The pigtail of the 532 nm LD was 105/125 um, which was connected with the pump port of the combiner.
Fig. 1. (a) Measurement scheme of PD induced excess loss; (b) structure of the high power fiber amplifier system.
For investigating the effect of 532 nm photo-bleaching in high power fiber amplifiers, a master oscillator power amplifier (MOPA) system was constructed, as shown in Fig. 1(b). In the master oscillator, six 915 nm LDs were coupled into the combiner, and the cavity was composed of a 40 m 20/400 um Yb-doped fiber with a pair of fiber brag gratings (FBGs) centered at 1080 nm. The seed provided output power of 55 W at 1080 nm, and the cladding light was removed by a cladding light stripper (CLS). In the amplification stage, four pump LDs working at 976 nm offered maximum pumping power at 666W. Meanwhile, the 532 nm LD was also connected to the pump port of the combiner. The bending diameter of fiber under test was 14 cm. Another CLS was applied to filtering the cladding light and an endcap was used to abate the backlight. A photodetector received the scattered light from the power meter and connected to the oscilloscope. To accelerate the production of color centers, gamma irradiation was used, and the fiber samples were exposed to Co60 gamma irradiation with total doses of 395 Gy and dose rate of 250 Gy/h.
3. Results and discussion
A series of experiment were conducted to explore the influence of 532 nm pump on the PD properties. The effect of 532 nm pump in the pristine fiber and the photo-darkened fiber were investigated separately, and time-dependent excess loss was measured. For the application prospects of 532 nm bleaching in high power systems, a 20/400 gamma-irradiated fiber with length of 20 m was bleached at 532 nm, and the laser performance and MI threshold were tested.
3.1 The effect of 532 nm pump in the pristine fiber
The 10/130 um Yb/Al fiber (N1) was pretreated by 532 nm cladding pump for 20 hours with output power fixed at 5 W for accelerating the pre-irradiation process. Then the PD characters of the 532 nm pre-irradiated fiber were measured and compared with the pristine fiber. The absorption spectra were recorded every 20 minutes, and based on our previous research, the wavelength of 633 nm, 702 nm, 810 nm and 1041 nm were chosen for comparison [23].
PD induced excess loss for the pristine fiber and the 532 nm pre-irradiated fiber are illustrated in Figs. 2(a) and 2(b). In contrast with the pristine fiber, PD induced excess loss in the 532 nm pre-irradiated fiber declined obviously. After 20 minutes of 915 nm pumping, the excess loss at 702 nm was 12.1 dB/m in the pre-irradiated fiber, falling by 41% in comparison with the pristine fiber. Neither of the two fibers reached equilibrium state at the end of test, and therefore the data were fitted by stretch exponential function. For the pristine fiber, PD loss in the fitted equilibrium at 633 nm, 702 nm, 810 nm and 1041 nm was 397.1 dB/m, 114.4 dB/m, 46.8 dB/m and 2.99 dB/m respectively, while it decreased to 162.2 dB/m,71.1 dB/m,31.8 dB/m and 2.21 dB/m for the 532 nm pre-irradiated fiber. The PD induced excess loss in equilibrium state was mitigated for more than 30% by 532 nm pumping.
Fig. 2. PD induced excess loss and fitting curve at 633nm, 702 nm, 810 nm and 1041nm for (a) the pristine fiber; (b) the 532 nm pre-irradiated fiber.
In previous reports, pre-irradiation treatment of optical fibers can cause significant PD effect, including ultraviolet, 405 nm and 488 nm [10,20,24]. In order to exclude the possibility of additional loss caused by 532 nm irradiation, which may influence the evaluation of PD properties, the transmission spectra before and after 532 nm pumping were measured. The spectral changes of the fiber after 532 nm pumping for 20 hours and 915 nm pumping for 1 hour are shown in Fig. 3. The spectra were basically unchanged after 20 hours of 532 nm light injection. In contrast, the excess loss was remarkably increased after 915 nm pumping for 1 hour. This result confirmed that there was no sign of PD phenomenon by 532 nm pretreatment. Since 532 nm pretreatment will not exert adverse impact on the background loss, it improves the PD resistance for more than 30%, which indicates that 532 nm pre-irradiation was an effective method for PD mitigation. Moreover, increasing the pumping power of 532 nm LD might be a promising way to further enhance the PD suppression effect in the future. The enlightenment of PD suppression by 532 nm pump contributes to not only blazing a new trail of mitigating PD, but also understanding the PD process and the formation mechanism of color center. The formation process of color center can be deduced through the inhibition mechanism, enabling the further investigation on radical elimination of PD.
3.2 The effect of 532 nm pump in the photo-darkened fiber
Firstly, a 10/130 um Yb/Al fiber (N1) was pumped by 915 nm LD for 100 minutes, reaching a PD loss of 36 dB/m at 702 nm. Then, the 915 nm LD was turned off and the 532 nm LD was turned on. A series of pumping power ranging from 25 mW to 5 W were selected and used to bleach the photo-darkened fiber at the same PD loss level. Different pieces of N1 fiber were used for each power. The evolution of PD loss with time and the relation between bleached ratio and pump power of 532 nm LD is shown in Figs. 4(a) and 4(b). With the increase of 532 nm pump power, the bleaching process accelerated and the bleached ratio also raised after 300 minutes bleaching. However, the saturate tendency appeared when the pump power of 532 nm LD uprated to a certain extent, and the effect of increasing pump power on the final bleached ratio reduced. When the pump power increased to 5 W with irradiation intensity at 0.058 MW/cm2, more than 97% of PD loss was bleached, demonstrating the ability of completely photo-bleaching by 532 nm pump.
Fig. 4. (a) PD and photo-bleaching evolution under different pump power of 532 nm LD at 702nm; (b) The relation between bleached ratio and pump power of 532 nm LD.
For verifying the repeatability of 532 nm bleaching, several photo-darkening and photo-bleaching cycles were conducted. The 915 nm LD and the 532 nm LD were pumped alternately, with each pumping time for 300 minutes. To accelerate the bleaching process, the pumping power of 532 nm LD was set to 5 W. The evolution of PD loss in the alternating pumping process is displayed in Fig. 5. First of all, PD loss almost disappeared at the end of each cycle, strongly demonstrating the repeatability of 532 nm photo-bleaching. Then, comparing with the first cycle, the excess loss had declined by 37% and 40% at 702 nm in the PD process of the second and third cycle. It was obvious that PD performance of the bleached fiber was better than that of the pristine fiber, which further proved the ability of mitigating PD effect by 532 nm pump. The 532 nm pump in the photo-darkened fiber exhibited two effects that one was the bleaching effect and the other was the improvement of PD resistance. In contrast with the research of 355 nm photo-bleaching, no difference in PD performance was observed between the pristine fiber and the bleached fiber [6]. The photon energies of 355 nm and 532 nm were 3.5 eV and 2.3 eV respectively. However, the PD inhibition phenomenon did not occur under 355 nm irradiation with higher photon energy, indicating that photon energy was not the decisive factor for causing PD inhibition. Such chemical reactions may only occur under the appropriate wavelength. Considering the coexistence of bleaching effect and PD inhibition effect, 532 nm photo-bleaching will have a great application prospect.
3.3 Application of 532 nm pump in high power fiber amplifiers
Previous reports on photo-bleaching were conducted in low power systems in the mW magnitude, which are subject to various restrictions in practical applications. To investigate the potential application of the 532 nm photo-bleaching in the high power fiber laser/amplifier systems, a series of experiment were conducted. 20/400 double clad Yb-doped fiber (N2) samples were adopted in the experiment. The absorption coefficient of the N2 fiber was 1.02 dB/m, and the optimized fiber length was 20 m according to Dawson [25]. In the series of comparative experiments, the length of fiber samples was identical.
Since part of color centers induced by PD and gamma irradiation were identical, gamma irradiation was applied to accelerating the process of producing color centers in the experiment [26]. In order to exclude the possibility of self-bleaching effect after gamma irradiation and thermal bleaching effect during 976 nm pump, thermal bleaching with 976 nm pump was conducted one year after gamma irradiation. Firstly, the gamma irradiated fibers was pumped by 976 nm LD at 257 W for 8 hours to avoid thermal bleaching effect during pumping. Laser performance was tested at the end of pumping, when the thermal bleaching effect basically reached equilibrium. Then, the fiber was bleached with 532 nm light for 20 hours. Due to coupling loss, the maximum input power of 532 nm light was 4 W, with irradiation intensity at 0.003MW/cm2. Laser performance was tested again at the end of photo-bleaching. Moreover, the temperature of the fiber was monitored during 532 nm photo-bleaching which remained under 25 degrees Centigrade in the whole process.
The output power and standard deviation (STD) as a function of pump power for the pristine fiber, the gamma irradiated fiber before and after 532 nm photo-bleaching are illustrated in Figs. 6(a) and 6(b). For the pristine fiber, the maximum output power was 513 W with 77% of optical-optical efficiency, and there was no sign of MI according to the STD. The efficiency declined sharply after gamma irradiation. Even though the fiber was bleached due to the thermal load in the Yb-doped fiber, the optical-optical efficiency could only recover to 37% with maximum output power of 160 W. After 532 nm photo-bleaching, the optical efficiency increased to 63% and the output power recovered to 421W, accounting for 82% of the pristine fiber. However, the laser performance did not restore to the original state after 532 nm photo-bleaching. The reason might be that the irradiation intensity was only 5% of the completely bleaching. Due to the maximum output power limit of the 532 nm LD, the irradiation intensity could not be further increased. Under these circumstances, the effect of photo-bleaching was still obvious, which could be also seen from MI threshold. Power rollover phenomenon appeared when the output power was 160 W in the gamma irradiated fiber before photo-bleaching. Meanwhile, the STD raised dramatically, indicating the occurrence of MI, in which the energy transferred between fundamental mode and higher order modes. After 532 nm bleaching for 20 hours, no power rollover phenomenon was observed, and MI did not appear when the output power reached 421 W. The MI threshold was restored to at least 2.6 times after photo-bleaching.
Fig. 6. (a) Output power and (b) standard deviation as a function of pump power for the pristine fiber, the gamma irradiated fiber before and after 532 nm photo-bleaching.
The above experimental results manifested that the color centers induced by gamma irradiation or pump light had a strong impact on the efficiency and the MI threshold of fiber amplifiers, which could be significantly restored by 532 nm photo-bleaching. In the gamma-irradiated fiber, the background loss intensified drastically from ultraviolet to near infrared, leading to the intense absorption of the pump energy. The pumping energy absorbed by Yb3+ ions declined, which was the main reason for deteriorating the optical fiber efficiency. In the meantime, a large amount of heat was generated due to pump light absorption in the color centers, which may result in the occurrence of MI according to the research [7]. They pointed out that slightly additional loss induced by PD would introduce the double thermal load. The color centers in the fiber would consume energy from the NIR pump light, converting into heat and accentuating the thermal refractive index grating, thus tremendously reducing the MI threshold. The elimination of color centers by 532 nm photo-bleaching eliminated most of the additional loss and thermal load in optical fibers, therefore, the optical efficiency and the MI threshold increased enormously. The results suggested that the mitigation of PD would be beneficial for promoting MI threshold.
3.4 Mechanism of 532 nm induced PD inhibition and photo-bleaching effect
In Yb-doped aluminosilicate fibers, Al-Oxygen hole center (AlOHC) and non-bridging oxygen hole center (NBOHC) were considered to be vital defects for causing PD [27,28]. Previous studies showed that the conversion between Yb3+ ions and Yb2+ ions also played an crucial role both in the photo-darkening and photo-bleaching process [9,21]. In the photo-darkening process, charge transfer transition occurred, where the electron belonging to the surrounding ligands transferred to the Yb3+ ion and enabled the transformation of Yb3+ ion into Yb2+ ion, leaving a hole around the ligands [9]. This reaction created electron-related color centers, e.g. Yb2+, as well as hole-related color centers, e.g. AlOHC, NBOHC. While for the photo-bleaching process, it was a reverse process of photo-darkening. 543 nm photo-bleaching could bleach 50% of PD loss, which was explained that Yb2+ ion recombined with the released hole of surrounded ligands with 543 nm photo-bleaching, converting Yb2+ ion into Yb3+ ion and simultaneously eliminating the hole-related color centers such as AlOHC and NBOHC [21].
Based on the above experimental results, it can be inferred that Yb2+ ions might be converted into Yb3+ ions during 532 nm photo-bleaching. When 532 nm pumping the photo-darkened fiber, the photon energy was enough to rearrange the electrons around the color centers [22]. Under 532 nm pumping, holes could be released by color centers such as AlOHC and NBOHC, and they could combine with Yb2+ ions to form Yb3+ ions, thus eliminating color centers. Furthermore, when the Yb/Al fiber was co-doped with Ce, the redox couple Ce3+/Ce4+ could capture both electrons and holes, however, the addition of Ce into the matrix would not affect the basic Yb/Al/Si related mechanism [11,29].
While for the pristine fiber, M. Engholm et al. proved the existence of Yb2+ ion in the pristine fiber, even if the fiber was prepared under oxidation condition [30]. Therefore, it could be speculated that analogous reaction might happen when 532 nm pumping the pristine fiber. Similar reaction might occur in defects, for instance, NBOHC and AlOHC, liberating holes which could recombine with the Yb2+ and producing Yb3+ ion. As a consequence, the amount of defects in the fiber decreased and PD induced excess loss was mitigated. However, there are still many doubts on the PD and photo-bleaching mechanism, and the exploration is still continuing.
The 532 nm LD showed excellent performance in PD inhibition and photo-bleaching, which proved that 532 nm bleaching could be used for high power fiber systems. In addition, the output power of commercial 532 nm LDs can reach hundred watts which are more appropriate for photo-bleaching in high power fibers systems in comparison with other bleaching sources.
4. Conclusion
An extended investigation on 532 nm pump induced PD inhibition and photo-bleaching effect was conducted in Yb-doped fiber. With the approach of 532 nm pre-irradiation, more than 30% of PD effect was mitigated in the pristine fiber. While both effects of PD suppression and complete photo-bleaching were observed in the photo-darkened fiber under 532 nm pump. The bleaching effect might come from the transformation of Yb2+ into Yb3+. Experimental results showed that 532 nm pump had remarkable effect in high power applications, enabling the significant recovery in the gamma irradiated fibers. Comparing the laser properties before and after photo-bleaching, the efficiency increased from 37% to 63% after bleaching, and meanwhile, the MI threshold also increased to over 2.6 times. Considering the excellent performance of 532 nm pump, it is a competitive method for long-term operation of high power lasers.
Funding
National Natural Science Foundation of China (61735007); National Key Research and Development Program of China (2017YFB1104400).
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