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Abstract
In this work, we proposed a nanoporous glass phase-separation method to fabricate low photo-darkening Yb-doped fiber. Compared to a modified conventional vapor deposition method, equilibrium photo-darkening induced excess loss at 702, 810, and 1041 nm of the nanoporous glass fiber was reduced to 35.07, 12.49 and 1.69 dB/m, respectively, despite higher Yb3+ concentration. The excess loss mitigation percentage was above 20%, and about 1 dB/m self-bleaching was observed. Besides, slope efficiency of this fiber was measured to be as high as 72.8%. The mechanism of photo-darkening mitigation by the porous glass phase-separation technology was also discussed. The results strongly confirmed that the method is applicable to further develop high-power fiber lasers with low photo-darkening effect.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last decade, Yb-doped fiber lasers, taking advantages of compactness, ruggedness, high beam quality, and flexibility, has been widely applied in industrial processing, military, medical treatment, etc [1–4]. The output power is showing a remarkable increase under the great demand of industry. Over kW continuous wave and MW pulsed wave fiber lasers have been reported [5] and they have penetrated into various applications formerly dominated by other lasers. However, output stability of Yb-doped fiber laser is badly influenced by photo-darkening (PD) effect when operating at high power condition for long time [6]. PD effect is pump-induced broadband (from UV to NIR range) absorption, depending on the pump power [7] and rare-earth dopant concentrations [8]. The PD induced loss covers the pump and laser wavelengths of Yb-doped fibers. It seriously deteriorates the laser performance and hinders the further development of Yb-doped fiber lasers.
Great efforts have been made to study the PD mechanism and mitigation methods. It is generally convinced that PD effect is caused by color centers related to Yb ions or fiber matrix. Charge transfer absorption bands [9], oxygen deficiency center (ODC) [10] and Tm impurities [11] were proposed to explain the formation of color centers and reveal the underlying microscopic process. Based on the mechanisms above, PD mitigation or suppression methods including co-doping with ions (Ce [12], P [13], Al [14], Mg [15], Na [16] etc), loading with H2 [17] or O2 [10] gas, photo-bleaching [18–20] (PB) and thermal bleaching [21](TB) have been reported. Co-doping with ions is easy to realize, while it induces laser properties (including numerical aperture, background loss, absorption and emission cross section, etc) degradation as well [22,23]. As a post-treatment process, H2 or O2 loading needs to operate under high pressure condition [10,17], which increases unsafety risk. Photo-bleaching method introduces extra UV-VIS radiation source in mW magnitude [18–20], so it is not suitable for high power laser. Thermal bleaching only takes place over 500 °C, resulting in unsafety and permanent damage to the fiber [21]. Besides, both PB and TB make the laser setup more complex. Therefore, it is desirable to find a more effective and convenient approach to solve PD problem.
In this work, we reported the effective PD suppression in Yb-doped fibers, which is prepared by nanoporous glass phase-separation (NPGPS) method. Compared to modified conventional vapor method (MCVD) with limited rare-earth doping concentration, the Yb-doped fibers fabricated by NPGPS exhibit low PD induced excess loss under higher Yb3+ ion doping concentration. 72.8% laser efficiency and about 1 dB/m self-bleaching phenomenon were observed. We also discussed the mechanism of PD suppression in the NPGPS fiber. The results indicate the NPGPS method is the excellent candidate of PD suppression method and is promising to fabricate large mode area Yb-doped fibers with high doping concentration.
2. Experimental setup
The experimental setup is shown in Fig. 1. A PK 2500 optical fiber analysis system (PHOTON KINETICS, USA) with high precision provides the signal light, lens coupling system, and data calculation functions, as the dashed box shown in Fig. 1. A halogen lamp emitting from 600 to 1650 nm was used as the signal light and was free spaced coupled into a single mode fiber. To ensure high repeatability, the PK2500 can automatically switch the lens to proper position and provide optimal coupling efficiency. Besides, the single mode fiber was always the same one and was not removed during the whole test, providing a repeatable and consistent coupling efficiency. The pump source was a 915 nm pigtailed laser diode (LD). Both the single mode fiber and laser diode were spliced to the input ends of a (1 + 1) × 1 combiner. The splicing loss can be reduced to nearly 0 dB. The Yb-doped fibers under test were 10 cm long, which could provide uniform inversion level along the whole fiber length. One end of the Yb-doped fiber was spliced with the combiner output pigtail, where the splicing loss could be controlled under 0.05 dB and almost pumping power is injected into the Yb-doped fiber. The other end was connected to a 2 m single mode fiber. A mode stripper was applied in the stage to remove the cladding pump power. After propagating in the single mode fiber, the spectra can be detected by a low noise receiver.
To directly compare the PD level of the fibers under test and ensure the same pumping power, we record the combiner output end pumping power and maintain it to 5.5 W during each measurement.
In this work, the pore size distribution was measured by an ASAP 2020M (Micromeritics, USA) automated surface area and pore size analyzer. The pore structure of the silica rod was measured by field emission scanning electron microscope (FESEM) Sirion 200 (FEI, NL). The fiber preform index difference was measured by the PK104 (PHOTON KINETICS, USA). The concentration distribution of Yb3+ and Al3+ in the fiber core was measured by electron probe micro analyzer (EPMA) JXA-8230 (JEOL, Japan). The fiber background loss was measured by PK2500 (PHOTON KINETICS, USA) based on cutback method.
3. Fiber fabrication
An Yb/Al co-doped fiber was fabricated by NPGPS method as following steps, which has been reported in our previous publication [24]. Firstly, high purity reagents of SiO2, H3BO3 and Al(OH)3 were mixed with special ratio and were fabricated into alkali-borosilicate glass under 1600°C high-temperature melting condition in a platinum crucible. Then the alkali-borosilicate glass was cut and polished into a cylinder with a diameter of 4 mm and a length of 300 mm. Thirdly, when heating the rod with high temperature between 550 and 650 °C, the alkali-borosilicate glass was separated into boron-rich phase and pure silica phase. The boron-rich phase can be easily dissolved in hot acid. The pure silica phase cannot be dissolved in most acid solutions, except hydrofluoric acid. Afterwards, the rod was soaked in hot diluted chlorhydric acid (90-100°C) to leach away the boron-rich phase, leaving the pure silica rod with uniformly distributed interconnected nanopores and channels. At this stage, the nanoporous silica rod was obtained.
Figure 2 shows the pore size distribution of the nanoporous silica rod, which was measured by nitrogen adsorption method. It can be seen that the pore size is mainly located at 5.2 nm. The full width at half maximum (FWHM) of the pore size distribution is about 1.06 nm, which is narrow and indicates the uniformly distribution of the nanopores. This relatively smaller pores take advantage in tailoring the clusters and suppressing the concentration quenching [25]. The pore size distribution could be further modified through adjusting the above process parameters. The inset in Fig. 2 is the FESEM image of the glass rod, clearly showing the pore size and pore structures in the porous glass rod.
Fig. 2 Nitrogen adsorption cumulative pore size distribution and the FESEM image of the porous glass rod (inset).
After the above steps, Yb3+ and Al3+ ions were doped into the nanoporous glass by immersing the rod into solution containing corresponding nitrate compounds, namely solution doping technology. Then, the glass rod was heated over 1800°C to remove the OH- and collapse nanopores. Finally, an active silica rod with a diameter of 3 mm and a length of 270 mm was obtained. It was drawn to Yb/Al co-doped fiber using rod-in tube and fiber drawing procedures.
The core and inner-cladding diameters are 10 and 130 μm, respectively. The numerical aperture (NA) is 0.085. The concentration of Yb3+ and Al3+ are about 1000 ppm and 1200 ppm, measured by EPMA. For comparison, an Yb/Al co-doped fiber was also prepared by MCVD combined with solution doping technique and drawn to the same structure single mode double-cladding fiber. The parameters of the two fibers are listed in Table 1.
Table 1. Fiber Samples Parameters
Figure 3 depicts the refractive index profile of the MCVD fiber preform and NPGPS fiber preform provided by PK104. Some slightly dips and peaks are observed in the MCVD fiber preform, although this degree of fluctuation does not affect the fiber properties. For the NPGPS fiber preform, the index difference profile looks more smoothly.
The concentration distribution of Yb3+ and Al3+ across the two fibers’ active core are characterized by EPMA and shown in Fig. 4. For MCVD fiber, the doping concentrations of Yb3+ and Al3+ are about 650 and 3000 ppm. While, they are 1000 and 1200 ppm for NPGPS fiber. Higher doping concentration of Yb3+ ions is obtained by nanoporous glass fabrication method. Based on cutback method, the fiber background loss at 1200 nm for MCVD fiber and NPGPS fiber is measured to be 0.023 and 0.04 dB/m, respectively. The fiber loss can be effectively reduced by decreasing the impurities and OH- ions. The insets in Fig. 3 present the octagonal inner cladding shape of the fiber cross section. The core and cladding diameters’ offset is within 1 μm, indicating that the fabricated fibers possess high longitudinal uniformity.
Fig. 4 Electron microprobe analysis of Yb3+ and Al3+ in the active core and the cross section (inset) of (a) MCVD fiber; (b) NPGPS fiber.
4. Results and discussion
As reported by Stefano [8], Yb3+ ionic concentration has a square law dependence on the photo-darkening induced excess loss at equilibrium state. This indicates that high doping concentration results in more serious PD effect. In this work, we measured the PD loss of heavier doping NPGPS fiber and less doping MCVD fiber for comparison.
To confirm the reliability of the experimental setup, we measured the spectra of an Yb-doped fiber for three times and plotted the results within Fig. 5 for comparison. It is obvious that the three lines almost overlapped and proved the high repeatability of the measurement results. To eliminate the influence of fiber properties (e.g. dopant distribution, background loss) on measurement results, we’d like to explain them from three aspects. Firstly, the NPGPS fiber and MCVD fiber both have relatively homogeneous index profile, as shown in Fig. 3, meaning the dopant distribution is uniform. Therefore dopant distribution has no influence on the PD mitigation. Secondly, the fiber length used for measurement is only 10 cm. The background loss in such short fiber length can be neglected. Moreover, the fiber absorption spectrum was measured first as a basement. Then the PD loss was calculated by subtracting the basement. Therefore influence of the negligible background loss can be removed. Thirdly, the MCVD fiber and NPGPS fiber inner claddings are processed to octagonal shape to avoid helical light. The NA of inner cladding is 0.46, which can ensure sufficient absorption of the pumping light. Therefore, the pumping power to induce PD loss is in the same amount.
The 915 nm LD was used to cladding pump the two fiber samples for 300 minutes. The fiber spectra were recorded every 20 minutes and the PD induced absorption loss were plotted in Fig. 6. The results showed clearly reduction of absorption loss induced by PD effect in the NPGPS fiber, despite the heavier doping concentration of Yb3+ ions in comparison with MCVD method. It seems that the PD induced loss at short wavelengths is very close for the two fibers. However, the detection wavelength range in our experiment is 600 nm −1650 nm and the short wavelengths are near the edge position, where the signal-to-noise ratio is very bad. The measured data are unstable and unreliable. While the 702 and 810 nm are far from the detection edge and PD induced excess loss is still obvious to record. Therefore, the PD losses at the two wavelengths are selected for comparison.
Fig. 6 Absorption loss spectra of Yb-doped fibers during 915 nm LD pumping (a) MCVD fiber; (b) NPGPS fiber.
In Fig. 6(a), the MCVD fiber’s absorption loss around 702 nm raised 16.14 dB/m within the first 20 minutes and then increased to nearly 30.72 dB/cm in the following 80 minutes. When continuously pumping the Yb-doped fiber, the loss still showed an upward trend but the increment was reduced gradually. The loss at 702 nm finally reached to 43.73 dB/m. While, for porous glass fiber in Fig. 6(b), the loss increment induced by the first 20 minutes’ pumping was no more than 10 dB/m, which showed above 50% reduction. At the point of 300 minutes pumping, the increment was observed to be 36.77 dB/m.
To further study the PD equilibrium state of the fibers, we selected 702, 810, and 1041 nm as the probe wavelengths, and plotted the time dependent loss revolution within 300 minutes in Fig. 7. The excess loss of the MCVD fiber at the four wavelengths was 43.73, 21.72, and 2.98 dB/m, respectively. While for the NPGPS fiber, it declined to 35.07, 12.49, and 1.69 dB/m at the same wavelengths.
Fig. 7 PD excess loss and fitting curve at 702 nm, 810 nm, and 1041 nm (a) MCVD fiber; (b) NPGPS fiber.
The data were also fitted by the classical stretched exponential function [26] described as below:
where α(t) is the induced loss on period of t under pump; is the loss at the final equilibrium state; τ is the time constant; β is the stretching parameter ranging between 0 and 1.The fitting results in Table 2 and Table 3 visually demonstrated the PD loss suppression in the NPGPS fiber. For the MCVD fiber, the equilibrium excess loss at 702, 810, and 1041 nm was calculated to be 70.73, 36.50, and 4.06 dB/m, respectively. For the nanoporous glass fiber, the fitting results were decreased to 55.59, 23.66 and 2.37 dB/m. It is suggested that a PD loss mitigation of over 20% can be reached even with heavier doping concentration in the NPGPS fiber.
Table 2. Fitting parameters of fiber samples fabricated by MCVD.
Table 3. Fitting parameters of fiber samples fabricated by porous glass phase-separation method.
We also recorded the absorption spectra of the two fibers when the 915 nm LD was turned off after 16 hours. Figure 8 depicted the subtraction value of stopping pumping after 16 hours and 300 minutes pumping photo-darkened fiber. A slight increase of loss in MCVD fiber, namely the black dotted line, was observed. The NPGPS fiber showed decrease of excess loss after the pump source was switched off, and the loss recovery value was about 1 dB/m, which was called self-bleaching [27].
Fig. 8 The subtraction value of loss spectrum between the photo-darkened fiber and the same fiber after 16 hours (Black scatters and line: MCVD fiber; Red scatters and line: NPGPS fiber).
Therefore, it was proved that the PD resistance was greatly improved in the NPGPS fiber. With the advantages of large core preparation and high doping concentration, NPGPS fibers are considered as the excellent candidate of high power fiber lasers with low PD effect.
Based on the PD loss suppression result, we also investigated the laser efficiency of the NPGPS fiber, which is of great concern for fiber lasers. Figure 9 shows the experimental setup for fiber laser efficiency measurement. The same NPGPS fiber preform was drawn to 30/400 μm double cladding fiber for laser property measurement. The inner cladding was manufactured to octagonal shape to ensure sufficient pump absorption. The pump source was a 976 nm LD and free-space coupled into the Yb-doped fiber through a collimator lens and focusing lens. The laser cavity consists of a dichroic mirror (HT@976 nm, HR@1080 nm) and the fiber end with 4% Fresnel reflectivity. After another collimator lens, the laser power and residual pump power were separated by the dichroic mirror. Figure 10 depicted the laser output power dependence on the absorbed pump power. The slope efficiency was measured to be 72.8% with center wavelength at 1071 nm. The fiber length was optimized to 4.5 m, corresponding to 2.8 dB/m absorption at 976 nm. Using cutback method, we got 0.04 dB/m of fiber background loss at 1200 nm. The slope efficiency can be further improved by decreasing the impurities and OH- groups in the doped core, and the fiber attenuation would be accordingly reduced.
Fig. 9 Experimental setup for laser efficiency measurement. HR: high reflectivity, HT: high transmission, DM: dichroic mirror.
The mechanism of PD mitigation in NPGPS fibers was discussed below. Yb-associated oxygen deficiency centers is widely considered as the precursor to induce PD effect. According to Ref [9], oxygen-surrounding voids would be occupied by Yb3+ ions, and ill-valenced bonds such as Yb-Al or Yb-Yb are formed in the case of heavily Yb3+ ions doping, namely oxygen deficiency center.
In silica glass, the flexibility of Si-O-Si bond angle causes the disorder of silica-oxygen tetrahedron, resulting in the existence of broken bonds and non-bridging oxygen (NBO) [28]. These bonds and NBO can be coordinated with rare earth ions [29]. During the process of MCVD fabrication, only few NBOs generated in the high purity glass, which cannot provide enough oxygen coordination to prevent ODCs formation. The contradiction between low NBOs and high coordination numbers of rare earth ions limits the doping concentration. Yb3+ ions will cluster to share the limited broken bonds and NBOs.
In NPGPS fibers, abundant broken bonds within the nanoporous channel surface after acid treatment introduced enough NBOs [30]. In addition to the large specific surface area, NBOs in NPGPS fiber are much more than silica fiber fabricated by MCVD. Besides, some metal ions were introduced as network modifiers during porous glass preparation. These modifiers including alkali metal ions, alkaline-earth metal ions, and aluminum ions provided extra NBOs and could improve rare-earth ions doping concentration [31,32]. Low PD level was observed by Sakaguchi [33] et al in Yb/Ca co-doped fibers. In our previous publication, we also demonstrated the PD excess loss reduction by co-doping with Na+ ions into the Yb-doped fiber [16]. Besides, the active core fabricated by the NPGPS method does not rely on a silica substrate tube, therefore it is suitable to fabricate large core size fibers.
In our future work, we’d like to improve our fiber fabrication method from the following three aspects. Firstly, the codopant in our Yb-doped fiber at present is only Al3+ ions. By adding P5+ or Ce3+ ions into the fiber core and adjusting the doping ratios, we expect much lower PD effect or even without PD loss. The critical step is to doping these ions uniformly in the glass nanopores. Secondly, the doping level of Yb3+ ions can be further increased through higher concentration of the solution and optimizing the pore size distribution in glass. This work can shorten the fiber length for laser output and will reduce the nonlinear effects. At last, we will adjust the heating rate, gas proportion and gas flow rate to further remove OH- content, therefore reduce the fiber background loss and improve the laser efficiency. The achievable loss lower than 0.02 dB/m is expected at least.
5. Conclusion
In conclusion, we proposed a novel approach to suppress the PD induced excess loss through NPGPS method. Compared with the Yb-doped fiber by MCVD method, the NPGPS fiber performed over 20% PD loss suppression with heavier Yb3+ ions doping concentration. The laser slope efficiency was up to72.8%. A self-bleaching phenomenon about 1dB/m was also observed in the NPGPS fiber. More NBOs are introduced during NPGPS fiber fabrication, which can be coordinated with Yb3+ to avoid the formation of ill-valenced bonds such as Yb-Al or Yb-Yb. The results demonstrated that NPGPS fibers with advantages of large core size and high rare earth ions doping concentration, is greatly promising to promote the further development of high power Yb-doped fiber laser with low PD effect.
Funding
National Natural Science Foundation of China (Grant No. 61575066).
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