Open Access
Add to BibSonomyAbstract
A ytterbium-doped double-cladding fiber was successfully fabricated by a chelate gas phase deposited technique. The measurement results of dopant concentration distribution and refractive index of preform indicate that radial and longitudinal homogeneity could be controlled perfectly with this technique. The absorption coefficients of fiber are 0.39 dB/m at 915nm and 1.02 dB/m at 976nm respectively. Using this fiber as the laser amplifier stage, 3592W output power at 1080nm with 72.5% slope efficiency was obtained with end-pump technique, which is close to the test results of Nufern commercial fiber. The results demonstrate that the chelate gas phase deposition technique is a relatively promising technique for high quality gain fiber fabrication.
© 2016 Optical Society of America
1. Introduction
Ytterbium (Yb)-doped fiber laser becomes more and more attractive in the applications of industrial processing and national defense [1,2] for the advantages of low maintenance, compactness of fiber lasers, good thermal management of the active fibers, high optical efficiency, excellent beam quality, and power scalability [3–7]. With the emergence of double- cladding fiber and mature of LD pumping technology, the output power of Yb-doped fiber laser has been scaled up multi kW in the last few years [8,9].
Yb-doped fiber, the main element in the fiber laser, remarkably influences the fiber laser properties. To manufacture a high power fiber laser, the high quality fiber is required to have a few typical characteristics including uniform core diameter, accurate control of refractive index profile (RIP), low background loss, high slope efficiency, outstanding thermal management and low photo-darkening. In addition, the quality of fiber relies heavily on the preform fabrication technology. At present, the available commercial Yb-doped fiber is usually prepared by conventional modified chemical vapor deposition (MCVD) in conjunction with the subsequent solution doping technique (SDT). However, the conventional solution doping approach has now been pushed to the limits and new fabrication techniques are required to allow much higher rare earth (RE) doping concentrations with increased uniformity along the length of the preform, increased core size, high precision in both dopant and refractive index profile, and low background loss in fiber with good optical to optical conversion efficiency. All the factors are critical in taking the high power fiber laser technology forward. For the reasons above, some different techniques are arousing researchers’ interests, such as halide-gas-phase-doping technique [10], atomic layer deposition method [11], outside vapordeposition (OVD) [12], powder sinter technology [13], direct nanoparticle deposition [14–16], chelate gas phase deposition technique [17–19] and other technique [20].
In this paper, a Yb-doped preform with good uniformity of refractive index along the length of preform and low OH content was fabricated by chelate gas phase deposition technique. Using the fiber drawn from the preform as the laser amplifier stage, a 3592W output power has been achieved, which is much higher than the output power of 200W reported in other publication with the same deposition technique [19]. So we want to reveal that the chelate gas phase deposition technique is a promising method to fabricate high quality Yb- doped fiber.
2、Experiment
A chelate doping system was employed to deliver the AlCl3, Yb(thd)3, Ce(thd)4 to the substrate tube during the deposition process. The temperature of heating chamber and transmitting line for AlCl3 is between 120°C and 150°C while that for Yb(thd)3 and Ce(thd)4 is between 200°C and 240°C. The purity of all raw materials and process gas is above 5N. The Heraeus F300 silica tube was adopted as substrate tube. The core of preform was deposited through the chemical reaction between raw material SiCl4, AlCl3,Yb(thd)3, Ce(thd)4,SiF4 and process gas O2 at the temperature of 1350°C. After that, the technological processes of sinter and collapse are operated successively at a high temperature (about 2000°C). When this process was finished, a solid rod was prepared and the fabrication of preform is completed. During the collapsing, a small flow of Cl2 was introduced in the process to decrease the content of OH in the core. Compared with solution technique which should take apart and connect the substrate tube one more in the midway of whole manufacturing process in order to soak deposited porous silica layer in RE ions and co-dopants contained solution, the chelate doping technique could conveniently complete the fabrication of preform in one uninterrupted deposition and collapse process. After the fabrication of the preform, suitable jacket process and mechanical treatment were chosen to form octagon preform according to the core/cladding ratio of fiber. In this paper, a 30μm/600μm fiber has been drawn at the temperature of 2000-2100°C using the preform fabricated above. A low index polymer (NA:0.46) was coated as the primary coating to allow pump light to be guided in the cladding and a protect polymer was coated as the second coating to enhance mechanical property.
The dopant concentration was determined by electron probe microanalyzer (EPMA, JXA-8230, Japan) on a thin polished preform slice. Meanwhile, the refractive index of preform was measured by the Photon Kinetics 2600 preform analyzer every 20mm interval. In order to measure the fluorescence emission spectra and life decay curve, the 976nm LD was used as the pump light to pump polished preform slice and the fluorescence signal was collected by an InGaAs detector (Zolix instruments Co. Ltd. China). After the preform drawn to fiber, the refractive index profile of fiber was obtained by Photon Kinetics fiber analyzer S14. Based on the cut-back method, the absorption coefficient of homemade fiber at 915nm and 976nm were measured. The performance of fiber as amplifier was tested in a two stage master oscillator power amplifier (MOPA) system (in Fig. 1). About 20m length fiber with 300mm-500mm coiling diameter was used as amplifier stage to test the laser performance. The power of seed light is about 1000W. Six 600W LD matching a (6 + 1) × 1 combiner was used to pump the test fiber. The power was recorded by an Ophir 5kW-ROHS power meter. In order to avoid the damage of the double-cladding fiber end surface by high power laser, a fused silica end-cap with 8 degree cleaved angle was welded with the fiber outputting end surface.
3、Results and discussion
The preform core cross section and dopant concentration of Al2O3, Yb2O3 and CeO2 are shown in Fig. 2(a) and 2(b). It shows that the core has a homogenous radial distribution. As an active fiber, in order to obtain single mode output, the numerical aperture should be small with the value about 0.06 (corresponding refractive index difference of 0.0012). But the dopant of Al2O3, Yb2O3 and CeO2 all will increase the refractive index of the core. So the dopant of fluorine used to lower refractive index was incorporated in core deposition process to adjust refractive index. From the refractive index profile (RIP) of preform shown in Fig. 3, it’s seen that the refractive index is in the range of 1.1~1.3 × 10−3 along the length of preform with the corresponding numerical aperture (NA): 0.057-0.062, which indicates the refractive index has been controlled exactly and the longitudinal value is homogenous. The tip which usually occurs in refractive index profile center in fluorine incorporated preform is not existed in this preform. Also we can see the diameter of the preform core is uniform along the length of preform as shown in the refractive index profile. So it’s demonstrated that the RIP of preform could be adjusted perfectly by the chelate gas phase deposition technique.
Fig. 2 Preform core cross section (a) and concentration profiles of preform core (b) along the dash dot line in (a).
The fluorescence emission spectrum and life decay curve of Yb3+ are shown in Fig. 4. The peak of fluorescence emission occurred at 976nm and 1018nm correspond to the Yb3+ transition from 2F5/2 multiplet manifold to 2F7/2 ground state. The fluorescence lifetime is 856 μs which is consistent with the lifetime value of Yb3+ in silica host glass [21].
Fig. 4 Fluorescence emission spectrum of preform slice excitated at 976nm. The inset is the decay curve monitored at 1080nm.
The facet image and RIP of fiber are shown in Fig. 5(a) and 5(b), an octagon configuration fiber profile could be seen obviously. The cladding diameter (flat to flat) of the fiber is 602.5μm and the core is 30.9μm. The refractive index of fiber is about 1.2 × 10−3 (NA:0.06) which is consistent with that of the preform. The measurement results of absorption coefficient are shown in Fig. 6 and the value at 915nm and 976nm are 0.39 dB/m and 1.02 dB/m respectively, which are close to that of Nufern commercial fiber. By calculating, when 99% 976nm pump light was absorbed, a length of more than 19.6m fiber should be employed as fiber laser gain medium. Also we could see a characteristic peak of OH which occurred at 1390nm, which is lower than Nufern fiber. It indicates that the chelate gas phase deposition technique has good effects in decreasing the OH content. The core attenuation at 1080 nm was tested with the value of 11.2 dB/km, which reveals that loss of the fiber at laser output wavelength is low.
Fig. 5 The fiber facet image(a) and the refractive index profile of fiber (b) along the dash dot line in (a).
The preformance of fiber as amplifier was carried on in a two-state MOPA system in the Fig. 1. A maximum output power of 3592W was obtained and the output lasted for 10 munites above 3580W. During the period, mode instability and non-linear effect haven’t been observed. The slope efficiency curve is shown in Fig. 7 (a) and the efficiency light to light is 72.5%. The laser peak was centered at the wavelength of 1080.5 nm and the full width at half maximum of the laser spectrum is 2.6nm. During the process of laser output, the temperature of all splices and fiber are monitored by several infrared thermal imagers. The temperature of input splice is about 50°C and that of fiber is below 40°C. It demonstrates that the fiber has potentiality in increasing the output power. For the limit of the pump power, the fiber hasn’t reached the highest power which it could load. It’s predictable that improved pumped scheme will increase the output power of this fiber further. In order to evaluate the actual state of homemade fiber, a similar test was carried on employing Nufern LMA-YDF-30/600E fiber. It’s shown that the efficiency light to light is 74.3% (in Fig. 7 (b)) which is a little higher than homemade fiber. This result indicates that the homemade fiber has a close performance of slope efficiency but still should be optimized afterward.
4、Conclusion
Yb-doped double-cladding fiber was fabricated successfully by chelate gas phase deposition technique. The RIP of preform indicates that the longitudinal homogeneity of refractive index and core diameter are uniform using this deposition technique. The fiber was drawn and the refractive index is consistent with that of the preform. The absorption coefficient is measured by cut-back method and the value is 0.39 dB/m at 915nm and 1.02dB/m at 976nm. Using this fiber as laser amplifier stage, the performance test is carried out in a MOPA system and an output power of 3592W was obtained with the efficiency light to light of 72.5%. The results are close to Nufern commercial fiber whose output power is 3621 and the efficiency light to light is 74.3%.
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 61205039, No. 61138007).
References and links
1. M. N. Zervas and C. A. Codemard, “High Power Fiber Lasers: A Review,” IEEE J. Sel. Top. Quant. 20(5), 0904123 (2014). [CrossRef]
2. J. Ballato and P. Dragic, “Materials Development for Next Generation Optical Fiber,” Materials (Basel) 7(6), 4411–4430 (2014). [CrossRef]
3. A. Langner, M. Sucha, G. Schötza, F. Justb, M. Leichb, A. Schwuchowb, S. Grimmb, H. Zimerc, M. Kozakc, B. Wedelc, G. Rehmannd, C. Bachertd, and V. Krause, “Multi-kW single fiber laser based on an extra large mode area fiber design,” Proc. SPIE 8237, 82370F (2012). [CrossRef]
4. Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12(25), 6088–6092 (2004). [CrossRef] [PubMed]
5. J. Limpert, A. Liem, H. Zellmer, and A. Tünnermann, “500 W continuous-wave fibre laser with excellent beam quality,” Electron. Lett. 39(8), 645–647 (2003). [CrossRef]
6. D. A. V. Kliner, K. Chong, J. Franke, T. Gordon, J. Gregg, W. Gries, H. Hu, H. Ishiguro, V. Issier, B. Kharlamov, A. Kliner, M. Kobayashi, K.-H. Liao, J. Lugo, J. Luu, D. Meng, J. J. Morehead, M. H. Muendel, L. E. Myers, K. Nguyen, H. Sako, K. Schneider, J. Segall, K. Shigeoka, R. Srinivasan, D. Tucker, D. Woll, D. L. Woods, H. Yu, and C. Zhang, “4-kW fiber laser for metal cutting and welding,” Proc. SPIE 7914, 791418 (2011). [CrossRef]
7. H. Zimer, M. Kozak, A. Liem, F. Flohrer, F. Doerfel, P. Riedel, S. Linke, R. Horley, F. Ghiringhelli, S. Demoulins, M. Zervas, J. Kirchhof, S. Unger, S. Jetschke, T. Peschel, and T. Schreiber, “Fibers and fiber-optic components for high power fiber lasers,” Proc. SPIE 7914, 791414 (2011). [CrossRef]
8. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]
9. Y. Jeong, A. J. Boyland, J. K. Sahu, S. Chung, J. Nilsson, and D. N. Payne, “Multi-kilowatt single-mode ytterbium-doped large-core fiber laser,” J. Opt. Soc. Korea 13(4), 416–422 (2009). [CrossRef]
10. K. Peng, Y. Y. Wang, L. Ni, Z. Wang, C. Gao, H. Zhan, J. J. Wang, F. Jing, and A. X. Lin, “Yb-doped large-mode-area laser fiber fabricated by halide- gas- phase- doping technique,” Laser Phys. 25(6), 065801 (2015). [CrossRef]
11. J. J. Montiel i Ponsoda, L. Norin, C. Ye, M. Bosund, M. J. Söderlund, A. Tervonen, and S. Honkanen, “Ytterbium-doped fibers fabricated with atomic layer deposition method,” Opt. Express 20(22), 25085–25095 (2012). [CrossRef] [PubMed]
12. J. Wang, S. Gray, D. T. Walton, M. J. Li, X. Chen, A. P. Liu, and L. A. Zenteno, “Advanced Vapor-doping, All-glass Double-clad Fibers,” Proc. SPIE 6890, 6890061 (2008). [CrossRef]
13. A. Langner, G. Schötz, M. Such, T. Kayser, V. Reichel, S. Grimm, J. Kirchhof, V. Krause, and G. Rehmann, “Comparison of silica-based materials and fibers in side- and end-pumped fiber lasers,” Proc. SPIE 7195, 71950Q (2009). [CrossRef]
14. S. Tammela, P. Kiiveri, S. Sarkilahti, M. Hotoleanu, H. Valkonen, M. Rajala, J. Kurki, and K. Janka, “Direct Nanoparticle Deposition process for manufacturing very short high gain Er-doped silica glass fiber,” in 28TH European Conference on Optical Communication, Vol. 4 of 2002.
15. M. C. Paul, A. V. Kir’yanov, Yu. O. Barmenkov, S. Das, M. Pal, S. K. Bhadra, S. Yoo, A. J. Boyland, J. K. Sahu, A. Martínez-Gamez, and J. L. Lucio-Martínez, “Yb2O3 Doped Yttrium-Alumino-Silicate Nano-Particles Based LMA Optical Fibers for High-Power Fiber Lasers,” J. Lightwave Technol. 30(13), 2062–2068 (2012). [CrossRef]
16. S. Tammela, M. Söderlund, J. Koponen, V. Philippov, and P. Stenius, “The potential of direct nanoparticle deposition for the next generation of optical fibers,” Proc. SPIE 6116, 61160G (2006). [CrossRef]
17. E. H. Sekiya, P. Barua, K. Saito, and A. J. Ikushima, “Fabrication of Yb-doped silica glass through the modification of MCVD process,” J. Non-Cryst. Solids 354(42-44), 4737–4742 (2008). [CrossRef]
18. M. Saha, A. Pal, M. Pal, and R. Sen, “Large Core Yb-doped Optical Fiber through Vapor Phase Doping Technique,” Proc. SPIE 8775, 87750A (2013). [CrossRef]
19. A. J. Boyland, A. S. Webb, S. Yoo, F. H. Mountfort, M. P. Kalita, R. J. Standish, J. K. Sahu, D. J. Richardson, and D. N. Payne, “Optical Fiber Fabrication Using Novel Gas Phase Deposition Technique,” J. Lightwave Technol. 29(6), 912–915 (2011). [CrossRef]
20. A. Langner, G. Schötz, M. Such, T. Kayser, V. Reichel, S. Grimm, J. Kirchhof, V. Krause, and G. Rehmann, “A new material for high power laser fibers,” Proc. SPIE 6873, 687311 (2008). [CrossRef]
21. S. Unger, F. Lindner, C. Aichele, M. Leich, A. Schwuchow, J. Kobelke, J. Dellith, K. Schuster, and H. Bartelt, “A highly efficient Yb-doped silica laser fiber prepared by gas phase doping technology,” Laser Phys. 24(3), 035103 (2014). [CrossRef]
