With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.
© 2016 Optical Society of America
Fiber laser has attracted more and more attention thanks to its small volume, high electrical-to-optical transfer efficiency, long lifetime, good and stable beam quality, and high reliability in recent decades [1–4]. Fiber laser has been widely adopted in a wide range of scientific and commercial applications: material processing, remote sensing, free space communication, and military defense [1–17]. To date, IPG photonics (USA) is a world leader in high-power Yb-doped fiber (YDF) laser due to its advanced fiber design and fabrication technologies . Transversal mode instabilities (TMI), nonlinear effects, and thermal issues are the three bottlenecks for multi-kW fiber laser development [1–22]. Two key points to develop high-power YDF laser system working at 10 kW or above are large-mode-area (LMA) YDF materials to provide enough gain and pump/signal combiner components to incorporate enough pump power.
Pump and signal combiners for fiber laser use can be broadly classified into two main categories: end-pumping combiner and side-pumping combiner. YDF lasers usually employ an end-pump scheme and use combiners to inject pump from laser diodes (LDs) at suitable wavelength [6,9,11,15–17]. As the most common end-pumping fiber combiner, fused tapered fiber bundle (TFB) has demonstrated multi-kW pump power injection and laser output even faced with heavy thermal issues [9,11]. Therefore, to further scale laser power toward 10 kW or above with end-pump method is extremely difficult and risky, because the pump light is injected simultaneously at the end-face of the active fiber, obviously leading to uneven gain distribution and accumulating heat to burn out fiber coating polymer [12–14,18–21]. In addition, TFB is unsuitable for application in bi-directional pump configuration due to the signal insertion loss up to ~10% [23–27]. Compared with end-pumping scheme, side-pumping scheme shows dramatic advantages in terms of system design, ease of pump injection, signal extraction and power scalability, and is regarded as a promising pump coupling scheme for future ultra-high power fiber laser over 10 kW [12–14,19–27].
GTWave fiber, a representative of pump and signal combiner based on side-pumping scheme, is a multi-functional component with one laser fiber and at least one pump fiber to simultaneously integrate high pump power and present high power laser output [12,22,26–28]. GTWave fiber was firstly invented by SPI Company (UK) , and is now widely used in kW-level fiber laser systems [5,24–27,30]. KW-level GTWave fiber was fabricated and reported by H. Zimer et.al  but with a low optical-to-optical efficiency of 65%. Measured in an all-fiber oscillator laser cavity, GTWave fiber presented 1kW-class laser output with a slope efficiency of 71% . Cascaded GTWave fiber laser generated 1.009 kW of output power at 1064 nm with an optical-to-optical efficiency of 76.8% . 2kW GTWave fiber amplifier was recently fabricated and reported by our group  with an optical-to-optical efficiency of 67.8% at 1064nm. Aiming for further power-scaling, to develop new-types of GTWave fiber amplifiers with high slope efficiency toward 80% and multi-kW pump power aggregating capability is highly desirable.
In this report, we demonstrated a home-made GTWave fiber suitable for bi-directional pump from multi-ends. Numerical analysis using semi-vector beam propagation method, the propagation and absorption of pump light and thermal characteristics in GTWave fiber were fully characterized. This GTWave fiber allowed for 5.19kW aggregated pump power from commercial 976nm-LDs in a master oscillator power amplifier (MOPA) laser setup and stably presented 5.07kW laser output at 1066.5nm with a slope efficiency of 74.5%.
2. Numerical simulations
To study light-wave propagation phenomena in optical waveguides [24,29,31–33], semi-vector beam propagation method (BPM) was used to simulate pump absorption and propagation characteristics of GTWave fiber. To simplify the numerical simulation work, we regarded the propagating modes as a whole in our model and assumed the input pump light with intensity set as a constant with random phase. By the same token, the pump absorption coefficient α of the home-made Yb-doped aluminophosphosilicate (Al2O3-P2O5-SiO2, Yb-APS) laser fiber core was simply measured by the cut-back method and set as a constant during the whole calculation procedure. Based on the scalar Helmholtz equation on the assumption of low population inversion, i.e. Nexcited<<Nground, the pump cladding absorption coefficient α ± (z) along z-direction in a GTWave fiber can be obtained in the following equation:
3. Experimental procedure
3.1 GTWave Fiber fabrication
By traditional modified chemical vapor deposition (MCVD) system in combination with newly-developed chelate precursor doping technique (CPDT) reported in our previous work [27,34,35], Yb-APS LMA fiber preform as signal preform and passive fiber preform as pump preform were successfully fabricated at home. Before drawn into fiber, the Yb-APS preform was milled to form an octagonal shape so as to improve pump coupling efficiency. Benefiting from appropriate incorporation of Al and P, Yb-APS signal fiber was found to effectively suppress widely-existing photodarkening phenomenon at multi-kW level laser output . Yb-APS fiber preform and pump fiber preform were firstly drawn into bare fibers separately, assembled together, and finally coated with low-refractive index resin to form a whole body . Drawing speed, UV-curving power and cladding polymer properties play major roles in determining the final quality of the made GTWave fiber, such as mechanical strength, deficiency like bubbles and strippers in polymer cladding, thermal-induced damage threshold and LD pump power aggregating ability at each pump port, and were improved dramatically comparable to that of 2 kW-level (2 + 1) GTWave fiber .
3.2 Fiber and laser performance measurements
The refractive index profile (RIP) of the signal fiber was measured by refracted near field method . Elemental distribution was measured by an electron probe micro-analyzer (EPMA) . In order to characterize GTWave fiber, a MOPA laser system was constructed and depicted in Fig. 1 [27,35]. The well-known 10kW fiber laser reported by IPG photonics (USA) in 2009 , used tandem (in-band) pump method pumped with many low-power 1018nm fiber laser sources to get one high-power 1070 ± 10nm fiber laser output, i.e. from fiber laser to fiber laser. However, the home-made (3 + 1) GTWave fiber amplifier in this study was directly pumped by commercial 976nm-LDs, i.e. from commercial LDs to high quality fiber laser, which is the most distinguished point for our new design in GTWave fiber development. Pump radiation is bi-directionally coupled across to the signal fiber in a distributed manner. The reverse feeding pump power is ~2700W from three groups of 900W 976nm-LDs via three pump ends independently. The forward feeding pump power is ~2490W via three ends, but two pump ends are 900W 976nm-LDs and one pump end is 690W 976nm-LDs integrated through a home-made 7 × 1 combiner. The output laser was collimated by a quartz block holder (QBH) and detected by a power meter (PM) at the end.
4. Results and discussion
4.1 GTWave fiber structure
The cross section of the home-made (3 + 1) GTWave fiber is schematically shown in Fig. 2(a). This (3 + 1) GTWave fiber is with one Yb-APS signal fiber as gain medium and three pump fibers as pump injection with 976nm-LDs, both of which were paralleled longitudinally up to 15~18m in length but physically isolated well from each other. The Yb-APS signal fiber is with ~29.79μm core and ~249.6μm (flat to flat) clad, i.e. 30/250 fiber as usual, and surrounded by three multi-mode pump fibers which are distributed with an angle of 120° in-between indicated in Fig. 2(a). With a refractive index difference Δn of 0.0015 between fiber core and clad, the corresponding N.A. is ~0.066, allowing only a few modes existing in the fiber core to keep good laser beam quality at multi-kW laser output. The dopant concentration of Al and P ions in Yb-APS fiber core was estimated to be 11070ppm and 11274ppm in molar percent, which is effective to prevent ~419ppm Yb ions from clustering, suppress photondarkening effect and further decrease refractive index of fiber core by forming [AlPO4] units . Pump fiber is in agreement with that reported in  in term of size, N.A., configuration, etc. Figure 2(b) principally demonstrates how the bidirectional pump (3 + 1) GTWave fiber amplifier works: Total power of 5.19kW from six 976nm-LD ports simultaneously was utilized to bi-directionally pump the home-made (3 + 1) GTWave fiber. By this way, (3 + 1) GTWave fiber amplifier is able to integrate enough pump power, enlarge the laser signal in the Yb-APS fiber, suppress nonlinear effect by using bidirectional pump method to shorten the used fiber length, and effectively distribute thermal load by long interaction zone up to 15-18m in length to present multi-kW laser output safely. Due to this special design and characteristic, GTWave fiber is also named as ‘pump-gain integrated functional fiber’, a new kind of fiber device to replace the traditional signal laser fiber without any pump function.
4.2 Numerical analysis
In order to investigate bidirectional pump method for the case of our home-made (3 + 1) GTWave fiber with Yb-APS signal fiber and three passive pump fibers, numerical calculation was carried out thoroughly with the well-known BPM method mentioned above. Figure 3(a) presents the numerical simulation results of absorption coefficient α for the case of forward, reverse, and bi-directional pump, respectively. The forward and reverse pump absorption gradually decreases from ~1.59 to ~0.47dB/m while the bi-directional pump absorption presents no obvious change along the whole fiber length, justifying even gain distribution along beam propagation. This fact enables us to use bidirectional pump and therefore introduce more pump power at no risk of burning fibers. Thus, (3 + 1) GTWave fiber can allow for multi-kW pump power injection without ultra-high temperature hot-spots. To evaluate the thermal load in a bidirectional pump (3 + 1) GTWave fiber amplifier , the temperatures of the core and polymer cladding were shown in Fig. 3(b), both of which are found to be in the range of 35~83°C, much lower than the usual thermal damage threshold of 5kW-level fiber amplifier.
4.3 Laser performance of bi-directional pump amplifier
Figure 4(a) shows the computation results of pump light and signal light distributions in the (3 + 1) GTWave fiber amplifier studied here. It can be found that the injected pump power was absorbed strongly at the beginning length of 30~50cm after the combining point between the Yb-APS signal fiber and the pump fiber, rescaled to its 1/e at the longitudinal propagation position of 2~3m, and finally tended to be stable afterwards regardless of pump injection direction. With pump light continuously coupled into the centered Yb-APS signal fiber, signal power increased dramatically at both ends of the GTWave fiber but steadily climb at the middle stage, much differently from the pump-gain behavior of the traditional fiber lasers using end-pump method like TFB. Laser output power were measured experimentally and linearly fitted as a function of the input pump power in Fig. 4(b): The overall simultaneously-injected pump power is ~5.19kW limited by the available 976nm-LDs, and 5.07kW laser output was obtained at 1066.5nm from our home-made end-cap, which is with ~8° angle to reduce feed-back laser signal as shown in . The seed light from the first oscillator stage was 1.21kW, and the newly generated 3.86kW laser was extracted from the second MOPA stage with a corresponding optical-to-optical efficiency of 74.5%, higher than that of the already-reported GTWave fiber amplifiers till now [12,27]. No saturation was found with the increment of pump power up to 5.19kW, indicating that much higher laser output could be achieved if with more pump power added in the future.
Aiming to verify the feasibility of our experimental results, the slope efficiency, i.e. the output power as a function of the pump power, was numerically calculated. The slope efficiency is linearly fitted and estimated to be 75.9% in Fig. 4(c), slightly larger than the experimental result 74.5% in Fig. 4(b). Furthermore, the 18m-long 30/250 Yb-APS fiber was independently measured using traditional end-pumping method [34,35] in Fig. 4(d). The optical-to-optical efficiency is linearly fitted to be 77.9%, and the slope efficiency was measured to be 76.4% at 65W. Based on these facts, it is easy to conclude the slope efficiency 74.5% of our home-made (3 + 1) GTWave fiber is reasonable below 80% and mainly originated from our home-made signal fiber with relative low Yb3+ ions concentration, high N.A. and relative long fiber length. Furthermore, at 5kW-level and for 30μm-core laser fiber, TMI effect was not observed for the time being but may be a limitation factor for 10kW laser in the future.
The output laser spectrum was centered at 1066.5nm with an optical signal-to-noise ratio (OSNR) of >20dB, and the full width at half maximum (FWHM) is ~0.51nm (see Fig. 5). The accuracy of the laser output power and the calculated efficiency is guaranteed and calibrated by using a home-made clad light stripper (CLS) to eliminate the residual pump power at 976nm and high-order mode light existing in the signal light. It is important to note that there is a SRS peak found in the laser output spectrum due to high seed power as high as 1.2kW . Thanks to fast heat dissipation function for GTWave fiber, rapidly-cooling with 12°C water, expanded core diameter and large mode area of 30/250 Yb-APS fiber, the SRS peak was suppressed well and the intensity ratio of the signal light to SRS is around 15.6 dB. Cooled with 12°C water, the surface temperature distribution of the polymer cladding is almost in accordance with that in Fig. 3(b) and lies in the range of 40~60°C at 5kW-level, validating our simulation result and justifying the feasibility and safety of bi-directional high-power pump scheme for (3 + 1) GTWave fiber directly pumped by 976nm-LDs.
The beam quality factor M2 was measured using the knife-edge method as demonstrated in  and . Figure 6 presents the spatial beam quality of the constructed (3 + 1) GTWave fiber amplifier in this study. Transmitting through the un-pumped amplifier, the beam quality factor M2 of the seed light was measured to be ~1.35, mainly induced by a mode-field adaptor (MFA) with 20/400μm input and 30/250μm output and the excitation of the high-order modes in the 30 μm core with an N.A of 0.066. Considering the accumulating heat at the quartz block holder (QBH), M2 is measured to be ~2.3 at 4.1kW for safety rather than at the highest 5.07 kW at risk as shown in Fig. 6(b). Serious distortion of the laser beam quality is mainly derived from the seed light with M2~1.35 and the strongly amplified high-order mode through 15-18 m GTWave fiber. Considering that there are at least 6 LP modes (e.g. LP01, LP02, LP11, LP12, LP21 and LP31) supported by the 30μm-core with 0.066 NA, TMI, i.e. energy exchange between LP01 fundamental mode and the other five higher-order modes, might occur and lead to beam quality degradation. Limited by our experimental conditions, however, TMI can’t be directly measured at 4.1kW without high-speed camera and photodiode . Further, M2 in our experiment was found to be mainly controlled by that of the used seed laser, and the tested GTWave fiber amplifier can preserve the beam quality of the seed light reasonably well. To optimize M2 to be ≤1.5, a few measures should be taken in the future work: To introduce a better seed light with M2≤1.1, to decrease seed laser power as low as tens of watts but feed much more pump power, to shorten GTWave fiber length to 8~10 m by doubling or tripling Yb3+ doping concentration, to reduce active fiber core size and N.A. supporting fewer modes, i.e. nearly single-mode operation but under large-mode-area conditions.
In summary, we reported on the numerical simulation and fabrication of (3 + 1) GTWave fiber with Yb-doped aluminophosphosilicate fiber preform and specialty fiber assembly and coating techniques. We used (3 + 1) GTWave fiber-based MOPA system to bidirectionally aggregate 5.19kW pump light from commercial 976nm laser diodes and then presented 5.07kW fiber laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the highest publically reported result in this area. No roll-over and thermal issue were presented at the maximum laser output. This study justifies that GTWave fiber can directly collect high-power pump energy from commercial laser diodes and support MOPA configuration to present multi-kW fiber laser with high slope efficiency and acceptable beam quality.
National Natural Science Foundation of China (NSFC) (11474257, 51602295) and China Postdoctoral Science Foundation Grant (2015M582756XB).
References and links
1. 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), 63–92 (2010). [CrossRef]
2. K. Ueda and A. Liu, “Future of high-power fiber lasers,” Laser Phys. 8(3), 774–781 (1998).
3. M. N. Zervas and C. A. Codemard, “High power fiber lasers: A Review,” IEEE J. Sel. Top. Quantum Electron. 20(5), 0904123 (2014). [CrossRef]
4. M. N. Zervas, “High power ytterbium-doped fiber lasers-fundamentals and applications,” Int. J. Mod. Phys. B 28(12), 1442009 (2014). [CrossRef]
5. I. P. G. Photonics, “IPG Photonics successfully tests world’s first 10 kilowatt single-mode production laser,” (June 15, 2009).
6. V. Khitrov, J. D. Minelly, R. Tumminelli, V. Petit, and E. S. Pooler, “3kW single-mode direct diode-pumped fiber laser,” in Fiber Lasers XI,Photonic West Conference (2014), pp. 89610V.
7. D. L. Sipes Jr, J. D. Tafoya, D. S. Schulz, B. G. Ward, and C. G. Carlson, “Advanced components for multi-kW fiber amplifiers,” Proc. SPIE 8237, 82370P (2012). [CrossRef]
8. N. Holehouse, J. Magné, M. Auger, and M. Quebec, “High power performance limits of fiber components,” Proc. SPIE 9344, 93441F (2015). [CrossRef]
9. A. Rosales-Garcia, H. Tobioka, K. Abedin, H. Dong, Z. Várallyay, Á. Szabó, T. Taunay, S. Sullivan, and C. Headley, “2.1 kW single mode continuous wave monolithic fiber laser,” Proc. SPIE 9344, 93441G (2015). [CrossRef]
10. Q. Tan, T. Ge, X. Zhang, and Z. Wang, “Cascaded combiners for a high power CW fiber laser,” Laser Phys. 26(2), 025102 (2016). [CrossRef]
11. M. Plötner, T. Eschrich, O. de Vries, J. Kobelke, S. Unger, M. Jäger, T. Schreiber, H. Bartelt, R. Eberhardt, and A. Tünnermann, “Demonstration of >5kW emissions with good beam quality from two different 7:1 all-glass fiber coupler-types,” Proc. SPIE 9346, 93460W (2015). [CrossRef]
12. H. Zimer, M. Kozak, A. Liem, F. Flohrer, F. Doerfel, P. Riedel, S. Linke, R. Horley, F. Ghiringhelli, S. Desmoulins, 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]
13. C. Jauregui-Misas, S. Böhme, G. Wenetiadis, J. Limpert, and A. Tünnermann, “All-fiber side pump combiner for high-power fiber lasers and amplifiers,” Proc. SPIE 7580, 7580 (2010).
14. J. K. Kim, C. Hagemann, T. Schreiber, T. Peschel, S. Böhme, R. Eberhardt, and A. Tünnermann, “Monolithic all-glass pump combiner scheme for high-power fiber laser systems,” Opt. Express 18(12), 13194–13203 (2010). [CrossRef] [PubMed]
15. H. Zhou, Z. Chen, X. Zhou, J. Hou, and J. Chen, “All-fiber 7x1 pump combiner for high power fiber laser,” Opt. Commun. 347, 137–140 (2015). [CrossRef]
16. C. X. Yu, S. J. Augst, S. M. Redmond, K. C. Goldizen, D. V. Murphy, A. Sanchez, and T. Y. Fan, “Coherent combining of a 4 kW, eight-element fiber amplifier array,” Opt. Lett. 36(14), 2686–2688 (2011). [CrossRef] [PubMed]
17. D. Jain, Y. Jung, P. Barua, S. Alam, and J. K. Sahu, “Demonstration of ultra-low NA rare-earth doped step index fiber for applications in high power fiber lasers,” Opt. Express 23(6), 7407–7415 (2015). [CrossRef] [PubMed]
18. P. Peterka, I. Kašík, V. Matějec, V. Kubeček, and P. Dvořáček, “Experimental demonstration of novel end-pumping method for double-clad fiber devices,” Opt. Lett. 31(22), 3240–3242 (2006). [CrossRef] [PubMed]
19. T. Weber, W. Lüthy, and H. P. Weber, “Side-pumped fiber laser,” Appl. Phys. B 63, 131–134 (1996). [CrossRef]
20. D. J. Ripin and L. Goldberg, “High efficiency side-coupling of light into optical fibres using imbedded v-grooves,” Electron. Lett. 31(25), 2204–2205 (1995). [CrossRef]
21. T. Theeg, H. Sayinc, J. Neumann, L. Overmeyer, and D. Kracht, “Pump and signal combiner for bi-directional pumping of all-fiber lasers and amplifiers,” Opt. Express 20(27), 28125–28141 (2012). [CrossRef] [PubMed]
22. V. Fomin, V. Gapontsev, E. Shcherbakov, A. Abramov, A. Ferin, and D. Mochalov, “100 kW CW fiber laser for industrial applications,” in International Conf. of Laser Opt. (2014). [CrossRef]
23. J. Nilsson, J. K. Sahu, J. N. Jang, R. Selvas, D. C. Hanna, and A. B. Grudinin, “Cladding-pumped Raman fiber amplifier,” in Proceedings of Topical Meeting on Optical Amplifiers and Their Applications (2002). paper PDP2.
24. P. Polynkin, V. Temyanko, M. Mansuripur, and N. Peyghambarian, “Efficient and scalable side pumping scheme for short high-power optical fiber lasers and amplifiers,” IEEE Photonics Technol. Lett. 16(9), 2024–2026 (2004). [CrossRef]
25. Q. Xiao, P. Yan, H. Ren, X. Chen, and M. Gong, “A side-pump coupler with refractive index valley configuration for fiber lasers and amplifiers,” J. Lightwave Technol. 31(16), 2715–2722 (2013). [CrossRef]
26. Z. Huang, J. Cao, Y. An, S. Guo, Z. Pan, J. Leng, J. Chen, and X. Xu, “A kilowatt all-fiber cascaded amplifier,” IEEE Photonics Technol. Lett. 27(16), 1–4 (2015). [CrossRef]
27. H. Zhan, Y. Wang, K. Peng, Z. Wang, L. Ni, X. Wang, J. Wang, F. Jing, and A. Lin, “2kW (2+1) GT-wave fiber amplifier,” Laser Phys. Lett. 13(4), 045103 (2016). [CrossRef]
28. K. H. Yla-Jarkko, C. Codemard, J. Singleton, P. W. Turner, I. Godfrey, S.-U. Alam, J. Nilsson, J. K. Sahu, and A. B. Grudinin, “Low noise, intelligent cladding pumped L-band EDFA,” in Proc. ECOC 2002, (Copenhagen, Denmark, 2002). p. PD1.6.
29. A. B. Grodinin, J. Nilaaon, and P. W. Turocr, “New generation of cladding pumped fibre lasers and amplifiers,” in Conf. Lasers and Electro-Optics Europe (CLEO, 20000), paper CWA3.
30. S. Norman, R. Sieberth, A. Appleyard, A. Hassey, A. Wetzig, and E. Bayer, “Application and performance of kw-class single-mode fibre lasers in the cutting of non-oriented electrical steel,”(2012).
31. Q. Xiao, H. Ren, P. Yan, X. Chen, and M. Gong, “Theoretical study of pumping absorption in a co-linear side-pumping coupler,” Opt. Commun. 300, 220–224 (2013). [CrossRef]
32. Z. Huang, J. Cao, S. Guo, J. Hou, and J. Chen, “The characteristics of pump light in side-coupled cladding-pumped fibers,” Opt. Fiber Technol. 19(4), 293–297 (2013). [CrossRef]
33. Z. Huang, J. Cao, S. Guo, J. Chen, and X. Xu, “Comparison of fiber lasers based on distributed side-coupled cladding-pumped fibers and double-cladding fibers,” Appl. Opt. 53(10), 2187–2195 (2014). [CrossRef] [PubMed]
34. T. Shi, Z. Zhou, L. Ni, X. Xiao, H. Zhan, A. Zhang, and A. Lin, “Ytterbium-doped large-mode-area silica fiber fabricated by using chelate precursor doping technique,” Appl. Opt. 53(15), 3191–3195 (2014). [CrossRef] [PubMed]
35. Z. Wang, H. Zhan, L. Ni, K. Peng, X. Wang, J. Wang, F. Jing, and A. Lin, “Research progress of chelate precursor doping method to fabricate Yb-doped large-mode-area silica fibers for kW-level laser,” Laser Phys. 25(11), 115103 (2015). [CrossRef]
36. 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]
38. F. Beier, C. Hupel, J. Nold, S. Kuhn, S. Hein, J. Ihring, B. Sattler, N. Haarlammert, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Narrow linewidth, single mode 3 kW average power from a directly diode pumped ytterbium-doped low NA fiber amplifier,” Opt. Express 24(6), 6011–6020 (2016). [CrossRef] [PubMed]
39. ISO 11146 “Lasers and laser-related equipment-Test methods for laser beam parameters-Beam width, divergence, angle and beam propagation factor” (2005).
40. H. J. Otto, F. Stutzki, F. Jansen, T. Eidam, C. Jauregui, J. Limpert, and A. Tünnermann, “Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers,” Opt. Express 20(14), 15710–15722 (2012). [CrossRef] [PubMed]