Open Access
Add to BibSonomy
Abstract
The transverse mode instability (TMI) has been one of the main limitations for the power scaling of single mode fiber lasers. In this work, we report a 6 kW single mode monolithic fiber laser enabled by effective mitigation of the TMI. The fiber laser employs a custom-made wavelength-stabilized 981 nm pump source, which remarkably enhanced the TMI threshold compared with the wavelength of 976 nm. With appropriately distributing bidirectional pump power, the monolithic fiber laser is scaled to 6 kW with single mode beam quality (M2<1.3). The stability is verified in a continuous operation for over 2 hours with power fluctuation below 1%.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power fiber lasers have been widely adopted as laser sources in many applications, owing to the advantages of high conversion efficiency, good beam quality, compact structure and reliable stability. In the application fields such as beam combinations and precise laser manufacture, nearly diffraction limited beam quality are required as well as the high average laser power. However, the occurrence of transverse mode instability (TMI) has been one of the main limitations to achieve single mode high power fiber lasers, which leads to a beam quality deterioration or a rollover output laser power [1–4]. Since the first report of the observation, the TMI in fiber lasers has attracted attentions of the researchers around the world, and many detailed studies have been reported on the physical origin, numerical simulation, experimental characterization, mitigation techniques, and etc [5–17]. Various kinds of mitigation techniques, active or passive category, have been proposed and validated [14–22], which enables the power scaling of single mode fiber lasers.
Although various kinds of TMI mitigation techniques have been reported, the output powers of the directly LD-pumped single mode fiber lasers are still limited at power levels of ∼5 kW, to the best of our knowledge. For the single-stage fiber laser oscillators, the researchers in Fujikura Inc. reported a record output power of 5.05 kW with a beam quality M2 factor of 1.3 in 2018 [23]. Minor information on the design of gain fiber parameters or pump optimizations was revealed. Möller et al. reported a single mode fiber laser oscillator with an output power of 4.8 kW, based on 20/400 μm double cladding ytterbium-doped fiber and femtosecond laser written fiber Bragg gratings [24–26]. The M2 factor of the output laser at 4.8 kW was ∼1.3, and further power scaling was primarily limited by the nonlinear effect of stimulated Raman scattering (SRS). For the fiber lasers with master oscillation power amplification (MOPA) structures, the reports of single mode fiber lasers with around 5 kW level are also rare, although many few-modes high power fiber lasers with power ranging from 5 kW to 10 kW have been reported [27–32]. In 2018, Beier et al. reported a counter-pumped single mode fiber laser amplifier with an output power of 4.4 kW in a spatial configuration, which employed a low numerical aperture (NA) ytterbium-doped fiber at 40 cm bending diameter [33]. In 2021, a narrow linewidth single mode 5 kW fiber laser amplifier with a beam quality M2 factor of ∼1.3 was reported [34]. Bidirectional 915 nm pump and gain fiber with mode area of ∼320 μm2 were employed.
In the power scaling of high power fiber lasers, there exist contradictions between the mitigations of TMI and SRS, in the aspects of fiber mode areas, gain fiber length, pump absorptions [15–36]. For successful power scaling, the designs of TMI mitigation techniques have to take the mitigation of SRS into consideration as well. Improving gain fiber designs is a promising way to mitigate both limitations. Several novel ultra-large mode area fiber designs with mode discrimination techniques were proposed and studied [23,37]. Up to date, detailed reports on 5 kW-level single mode lasers based on such novel gain fiber designs are rare. It is a tremendous challenge to achieve a well-balanced effective mitigation of both the limitations of TMI and SRS.
In this paper, we report a 6 kW single mode monolithic fiber laser operating at ∼1080 nm. Comprehensive TMI mitigation techniques are designed in the fiber laser as well as the SRS mitigation. Ytterbium-doped double cladding fiber with core/inner cladding diameter of 25/400 μm are tightly coiled to suppress the TMI and achieve single mode laser output. Custom-made 981 nm LDs are employed and the enhancement of the TMI threshold is validated compared with the 976 nm LDs. With appropriately distributing bidirectional 981 nm pump power, the monolithic fiber laser achieves a maximum output power of 6 kW with single mode beam quality. Continuous operation for over 2 hours is verified with power fluctuation below 1%, indicating a reliable stability of the monolithic fiber laser.
2. Experimental setup
The experimental setup of the 6 kW single mode monolithic fiber laser is depicted in Fig. 1, which employs a MOPA structure. The seed laser is a continuous-wave broadband all-fiber laser oscillator. Ytterbium-doped fiber (YDF) with core /inner cladding diameter of 20/400 μm and core NA of 0.06 is adopted and coiled at the bending diameter of ∼10 cm to achieve single mode laser output. The high reflector fiber Bragg gratings has a reflection of 99% and a 3 dB reflection bandwidth of ∼4 nm. The output coupler fiber Bragg gratings has a reflection of ∼10% and a 3 dB reflection bandwidth of ∼2 nm. According the previous studies, the broadband feature of the oscillator seed laser helps to enhance the SRS threshold of the fiber main amplifier [38]. Although the oscillator seed have a capability of outputting over 1 kW, the operating output power is controlled at ∼100 W, which is also aimed to ensure a relative higher SRS threshold of the main amplifier [38–40].
Fig. 1. Experimental setup of the monolithic fiber laser. LD: laser diode; PSC: pump/signal combiner, CLS: cladding light stripper, CO: collimator; HR: high reflection mirror; PM: power meter; BS: beam splitter; PD: photodetector.
The main amplifier employs two pump/signal combiners to constitute a bidirectional-pump configuration. Each pump/signal combiner has six multimode pump ports with core/cladding diameters of 220/242 μm and a core NA of 0.22. The signal ports of the pump/signal combiners are 25/400 μm and 25/250 μm double cladding fiber with a core NA of 0.06. The signal insertion loss of the pump/signal combiner is ∼0.15 dB. The gain fiber employed in the main amplifier is ytterbium-doped fiber with a core/inner cladding diameter of 25/400 μm and a core NA of 0.063. The pump absorption coefficients of the YDF at the wavelengths of 915 nm and 981 nm are ∼0.5 dB/m and ∼0.8 dB/m, respectively. The total YDF length of main amplifier is ∼19.5 m. Bending loss of the YDF is exploited in the main amplifier to suppress the TMI and achieve single mode laser output. The YDF is tightly coiled in the grooves with each interval of 1 mm on a water-cooled metal plate, which provide available fiber bending and remove heat accumulation. The minimum bending diameter of the grooves is designed as 8 cm, which have been proved quite effective for suppressing the TMI [41]. After the counter-pump combiner, the total delivery fiber length is ∼2 m, including the cladding light stripper (CLS) and the pigtail fiber of the quartz beam head (QBH).
There are two kinds of high power wavelength-stabilized laser diodes (LDs) available in the experiment, which respectively center at 976 nm and 981 nm. The high power wavelength-stabilized 981 nm LDs are custom-made, which are not usually found in commercial product catalogs. Each LD is able to output a maximum power of ∼900 W in a multimode fiber with core/cladding diameters of 220/242 μm and core NA of 0.22. In the previous studies, there were reports on increasing the TMI thresholds by utilizing pump wavelengths of 915 nm and 973 nm instead of the 976 nm pump [18,42,43]. With a similar physical mechanism, the adoption of the 981 nm is also intended to shift away from the absorption peak of the YDF at ∼976 nm, thus decreasing the pump absorption and heat load in the gain fiber [15]. Furthermore, the longer pump wavelength at 981 nm corresponds to a lower the quantum defect, although the shift of the pump wavelength is minor. To verify the TMI mitigation benefitting from the pump shift, the TMI thresholds of the main amplifier pumped by the 976 nm and 981 nm LDs are respectively measured and compared in the experiment.
In the experiment, the output laser power, temporal signals and laser beam quality are measured and used to verify the TMI threshold. The collimated output laser beam is split by a high reflection mirror with a reflectivity of >99%. The reflected laser power is measured with a power meter, and the transmitted laser beam is further split to measure the temporal signals and the laser beam quality. The temporal signals are recorded by a photodetector with a bandwidth of 12 MHz. The laser beam quality is measured with a beam propagation analyzer (BeamSquared). The optical spectra of the output laser are also measured with an optical spectrum analyzer.
3. Results and discussion
3.1 Validation of TMI mitigation in the MOPA fiber laser with a 981 nm pump
The TMI mitigation in the main amplifier is experimentally validated, with the 976 nm pump source replaced by the 981 nm pump source. According the previous theoretical studies [44], the increase of the TMI threshold corresponding to the 981 nm pump is prospective, since both the pump absorption coefficient and the quantum defect decrease with the pump wavelength shift. It is still necessary to verify how much extent the TMI threshold can be enhanced. Moreover, there is a demand of increasing the proportion of counter-pump power as much as possible, in order to mitigate the SRS effect [45]. The TMI thresholds of the main amplifier with respective 976 nm and 981 nm LDs in the counter-pump structure are recorded and compared.
The proof of the TMI threshold is explained briefly. The output laser powers and corresponding temporal signals at different pump power levels are recorded. The TMI thresholds of the main amplifier are confirmed by observation of the typical TMI features, which are rollover output powers together with a simultaneous temporal fluctuation with kHz-level modulation frequencies. The physical mechanism of the simultaneity lies in the tight coiling of the YDF in the main amplifier. Owing to the tight coiling of YDF, the high order modes experience high modal losses in the propagation, and only the fundamental mode outputs with minor loss. When the TMI occurs in the main amplifier, there exists dynamic mode coupling between the fundamental mode and the high order modes. The coupled high order modes leak into the inner cladding and corresponding laser powers get stripped in the region of CLS. Therefore, with the presence of tight fiber coiling, the dynamic mode couplings result in a rollover output power and temporal intensity fluctuation with similar mode coupling frequencies. The method has been proved effective [8,46], and the measurement of the output power and temporal intensity fluctuation can be quite convenient.
The performances of the main amplifier counter-pumped by 976 nm LDs are shown in Fig. 2. The output laser power dependence on the pump power is depicted in Fig. 2(a). The output laser power increase almost linearly, until a maximum of 2494 W is reached. A decline of the output power is observed as the pump power further increase. As the pump power increase from 2820 W to 2975 W, the output laser power decrease from 2494 W to 2380 W. Correspondingly, the optical efficiency decreases from 85% to 76%, and the standard deviations of the recorded temporal signals increase dramatically. For simplicity, the operation powers are denoted as P1 and P2, as shown in the Fig. 2(a). The temporal signals at the operations of P1 and P2 are depicted in Fig. 2(c), and the corresponding Fourier spectra are shown in Fig. 2(d). At the operation of P1, the temporal signal of the output laser is quite stable. When the pump power further increases and operates at P2, the temporal intensity fluctuation can be clearly observed, and the modulation frequencies spread at the range of 0-4 kHz. The phenomenon of simultaneous rollover output power and temporal intensity fluctuation has a threshold feature related with the average power, which was repeatedly verified. The optical spectrum at the operation of P1 is depicted in Fig. 2(b). There is no obvious evidence of Raman Stokes or amplified spontaneous emission (ASE). The beam quality of the output laser at P1 is measured, and M2 factors in the x and y directions are 1.3 and 1.2. The beam quality at the operation of P2 is missed deliberately. The occurrence of TMI leads to a distinct increase of the cladding light power, which induces extra heat accumulation inside the pump/signal combiner. To avoid detriment caused by the extra heat accumulation, the operation time at P2 is controlled and the beam quality factor measurement is given up. Although no direct mode coupling in beam profiles is present, we believe the occurrence of the TMI can be confirmed with the recorded phenomenon. The TMI threshold of the main amplifier counter-pumped by the 976 nm LDs is ∼2.49 kW.
Fig. 2. Output laser performance of the main amplifier counter-pumped by 976 nm LDs, (a) output power and temporal signal standard deviation dependence on the pump power, (b) optical spectrum at 2494 W, (c) time domain signals recorded by the PD and (d) Fourier spectra in the frequency domain at the operations of P1 and P2.
The performance of the main amplifier counter-pumped by the 981 nm pump source is measured, by replacing the 976 nm LDs with custom-made 981 nm LDs. Only the LDs are switched and pump ports of the pump/signal combiner are re-spliced. Both the seed laser and main amplifier maintain unchanged, which avoids introducing other perturbations affecting TMI threshold. The output laser power dependence on the 981 nm pump power is shown in Fig. 3(a). The output laser power increases with the pump power at a slope efficiency of 83.5%. No rollover power or optical efficiency decline is observed in the power scaling. At the pump power of ∼5.43 kW, a maximum output power of 4.48 kW is achieved. Further power scaling is only limited by the available pump power each LD can provide. There is no obvious temporal fluctuation observed in the recorded temporal signals at different power levels. The time domain signal and corresponding Fourier spectra at the operation of 4.48 kW are depicted in Fig. 3(b). There are no obvious periodic modulations related with the TMI features. The optical spectrum at 4.48 kW is depicted in Fig. 3(c). The full width at half maximum is 5.3 nm. The intensity of Raman Stokes light is ∼45 dB lower than the signal laser intensity. The beam quality of the output laser at 4.48 kW is shown in Fig. 3(d). The inset is the beam profile at the beam waist. The measured M2 factors in the x and y directions are 1.3 and 1.2, which also verify the absence of the TMI.
Fig. 3. Output laser performance of the main amplifier counter-pumped by 981 nm LDs, (a) output power and temporal signal standard deviation dependence on the pump power, (b) time domain signals and Fourier spectra in the frequency domain at the operation of 4.48 kW, (c) optical spectrum at 4.48 kW, (d) output laser beam quality at 4.48 kW.
By replacing the 976 nm LDs with the 981 nm LDs, the TMI threshold of the counter-pumped main amplifier is remarkably enhanced, from ∼2.49 kW to over 4.48 kW. The underlying physical mechanism is mainly attributed to the decreased pump absorption coefficient and quantum defect. Considering the change of the quantum defect is little, the decreased pump absorption coefficient is the primary factor, which distinctly decreases the heat load in the gain fiber. Although there are concerns that the identical gain fiber has different total absorptions at the respective 976 nm and 981 nm, the additional absorption at the 976 nm have minor influence on the TMI threshold in the counter-pump structure. Moreover, theoretical study shows that the influence of fiber length on TMI threshold is weak for gain fiber without photodarkening effect [47]. And we have not found obvious signs of photodarkening for the main amplifier during the whole experiment. Indeed, achieving as much laser power as possible with only counter-pump is vital to mitigate SRS effect and enable further power scaling.
3.2 Power scaling of the MOPA fiber laser with a bidirectional 981 nm pump
With the limitation of TMI removed, the use of all pump power in bidirectional pumping configuration can be explored. There are abundant pump powers in the co-pump distribution, but the co-pump power proportion has to be controlled in order to mitigate the SRS. The output laser performances with bidirectional 981 nm pump are measured and recorded. As shown in Fig. 4(a), the output laser power increases with the pump power at a slope efficiency of 82.3%. When the co-pump power is 2060 W and the counter-pump power is 5430 W, the monolithic fiber laser achieves a maximum output power of 6.01 kW. The optical spectrum of the output laser at 6.01 kW is depicted in Fig. 4(b). The signal laser centers at the wavelength of ∼1080 nm, and the full width at half maximum is ∼6.3 nm. There is obvious Raman Stokes light in the spectrum, and the intensity is ∼20 dB below the intensity of signal laser. For the laser safety, further power scaling is not pursued to avoid the exponential increase of the Raman Stokes light.
Fig. 4. Output laser performance of the main amplifier bidirectional-pumped by 981 nm LDs, (a) output power dependence on the pump power, (b) optical spectrum at 6.01 kW, (c) output laser beam quality at 6.01 kW, (d) recorded output laser power during the stability test.
The beam quality of the output laser is measured with a beam propagation analyzer. The beam quality at the operation of 6 kW is shown in Fig. 4(c). The inset is the beam profile at the beam waist. The measured M2 factors in the x and y directions are 1.3 and 1.2, which indicate a single mode laser output. Although gain fiber with 25 μm core diameter and 0.063 NA supports several high order modes, the effective mitigation of TMI and appropriate mode discrimination technique still enable an output laser with high beam quality. In this work, the mode discrimination is mainly attributed to the designed tight gain fiber coiling, which filters out high order modes and suppresses the TMI. The employment of a bidirectional 981 nm pump source also helps to mitigate the TMI and avoid laser beam deterioration.
The power stability of the monolithic fiber laser is also tested with continuous operation at the power of ∼6 kW for 145 minutes. The output laser powers are recorded with an interval of 0.1 second. During the continuous operation, the output laser power reaches a maximum of 6.05 kW and never drops below 5.99 kW, corresponding to an average power of 6.018 kW. The power fluctuation during the period is within 1%, which indicates a reliable stability of the monolithic fiber laser. There are no signs of photodarkening or TMI threshold decrease observed in the continuous operation.
There is possibility to further scale the output laser power, as long as the limitation of SRS can be mitigated. With only the co-pump 981 nm power injected, the MOPA laser could be scaled to 2.64 kW without any sign of TMI. The abundant co-pump power could be further exploited. However, increasing the co-pump power leads to a rapid exponential increase of the SRS. For SRS suppression, the optimizations of the seed laser temporal stability and pump distribution have been exploited. The total fiber length has also been optimized and the delivery fiber is relatively short. We believe there is little room for further power scaling in this MOPA fiber laser due to the limitation of SRS effect. Design and employ of gain fiber with ultra-large mode area might be a possible solution for further power scaling, as long as the TMI can be effectively mitigated.
4. Conclusion
In summary, we have reported a monolithic fiber laser with a maximum of 6 kW output power and single mode beam quality. The fiber laser employs a MOPA structure with a fiber laser oscillator seed operating at ∼1080 nm. High power custom-made wavelength-stabilized 981 nm LDs are employed as pump source. For comparison, the 976 nm LDs are also employed, and the TMI thresholds of the MOPA fiber laser corresponding to the 976 nm and 981 nm pumps are respectively measured. Experimental results indicate that the TMI threshold can be remarkably enhanced by replacing the 976 nm LDs with 981 nm LDs. With appropriately distributing the bidirectional 981 nm pump power, the MOPA fiber laser has been scaled to 6 kW with a slope efficiency of 82.3%. At the operation of 6 kW, the M2 factors of the output laser are 1.3 and 1.2 in the respective x and y directions. The monolithic fiber laser is tested at the operation of 6 kW for over 2 hours, and power fluctuation is within 1%, which verifies a reliable stability.
Funding
National Natural Science Foundation of China (61905282, 62005315); Training Program for Excellent Young Innovators of Changsha (kq2009004).
Acknowledgments
The authors wish to thank Mr. Kun Zhang, Mr. Tao Song and Mr. Chuanchuan Zhang, for the help in the measurement and test of the monolithic fiber laser in the experiment. The authors also wish to thank BWT Beijing Ltd. for providing the custom-made LDs.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nature Photon 7(11), 861–867 (2013). [CrossRef]
2. Y. Wang, G. Chen, and J. Li, “Development and prospect of high-power doped fibers,” High Power Laser Sci 6(3), e40 (2018). [CrossRef]
3. M. N. Zervas and C. A. Codemard, “High power fiber lasers: A review,” IEEE J. Select. Topics Quantum Electron 20(5), 219–241 (2014). [CrossRef]
4. C. Jauregui, C. Stihler, and J. Limpert, “Transverse mode instability,” Adv. Opt. Photon. 12(2), 429–484 (2020). [CrossRef]
5. A. V. Smith and J. J. Smith, “Mode instability in high power fiber amplifiers,” Opt. Express 19(11), 10180–10192 (2011). [CrossRef]
6. A. V. Smith and J. J. Smith, “Overview of a Steady-Periodic model of modal instability in fiber amplifiers,” IEEE J. Sel. Top. Quant. 20(5), 472–483 (2014). [CrossRef]
7. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tunnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218–13224 (2011). [CrossRef]
8. H. 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]
9. F. Beier, M. Plötner, B. Sattler, F. Stutzki, T. Walbaum, A. Liem, N. Haarlammert, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Measuring thermal load in fiber amplifiers in the presence of transversal mode instabilities,” Opt. Lett. 42(21), 4311–4314 (2017). [CrossRef]
10. S. Naderi, I. Dajani, T. Madden, and C. Robin, “Investigations of modal instabilities in fiber amplifiers through detailed numerical simulations,” Opt. Express 21(13), 16111–16129 (2013). [CrossRef]
11. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Laegsgaard, “Theoretical analysis of mode instability in high-power fiber amplifiers,” Opt. Express 21(2), 1944–1971 (2013). [CrossRef]
12. J. Lægsgaard, “Static thermo-optic instability in double-pass fiber amplifiers,” Opt. Express 24(12), 13429–13443 (2016). [CrossRef]
13. V. Scarnera, F. Ghiringhelli, A. Malinowski, C. A. Codemard, M. K. Durkin, and M. N. Zervas, “Modal instabilities in high power fiber laser oscillators,” Opt. Express 27(4), 4386 (2019). [CrossRef]
14. F. Zhang, Y. Wang, X. Lin, Y. Cheng, Z. Zhang, Y. Liu, L. Liao, Y. Xing, L. Yang, N. Dai, H. Li, and J. Li, “Gain-tailored Yb/Ce codoped aluminosilicate fiber for laser stability improvement at high output power,” Opt. Express 27(15), 20824 (2019). [CrossRef]
15. C. Jauregui, H. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann, “Passive mitigation strategies for mode instabilities in high-power fiber laser systems,” Opt. Express 21(16), 19375–19386 (2013). [CrossRef]
16. Z. S. Eznaveh, L. Gisela, E. Antonio-López, and R. Amezcua Correa, “Bi-directional pump configuration for increasing thermal modal instabilities threshold in high power fiber amplifiers,” Proc. SPIE 9344, 93442G (2015). [CrossRef]
17. R. Tao, R. Su, P. Ma, X. Wang, and P. Zhou, “Suppressing mode instabilities by optimizing the fiber coiling methods,” Laser Phys. Lett. 14(2), 025101 (2017). [CrossRef]
18. R. Tao, P. Ma, X. Wang, P. Zhou, and Z. Liu, “Mitigating of modal instabilities in linearly-polarized fiber amplifiers by shifting pump wavelength,” J. Opt. 17(4), 045504 (2015). [CrossRef]
19. A. V. Smith and J. J. Smith, “Increasing mode instability thresholds of fiber amplifiers by gain saturation,” Opt. Express 21(13), 15168–15182 (2013). [CrossRef]
20. C. Jauregui, H. Otto, S. Breitkopf, J. Limpert, and A. Tünnermann, “Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities,” Opt. Express 24(8), 7879–7892 (2016). [CrossRef]
21. B. Yang, H. Zhang, C. Shi, X. Wang, P. Zhou, X. Xu, J. Chen, Z. Liu, and Q. Lu, “Mitigating transverse mode instability in all-fiber laser oscillator and scaling power up to 2.5 kW employing bidirectional-pump scheme,” Opt. Express 24(24), 27828–27835 (2016). [CrossRef]
22. 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]
23. K. Shima, S. Ikoma, K. Uchiyama, Y. Takubo, M. Kashiwagi, and D. Tanaka, “5-KW single stage all-fiber Yb-doped single-mode fiber laser for materials processing,” Proc. SPIE 10512, 105120C (2018). [CrossRef]
24. F. Möller, R. G. Krämer, C. Matzdorf, S. Nolte, M. Strecker, F. Stutzki, M. Plötner, V. Bock, T. Schreiber, and A. Tünnermann, “Comparison Between Bidirectional Pumped Yb-doped All-fiber Single-mode Amplifier and Oscillator Setup up to a Power Level of 5 kW,” in Laser Congress 2018 (ASSL) (Optical Society of America, Boston, Massachusetts, 2018), AM2A.3.
25. F. Möller, R. G. Krämer, C. Matzdorf, M. Strecker, M. Plötner, F. Stutzki, T. Walbaum, T. Schreiber, R. Eberhardt, S. Nolte, and A. Tünnermann, “Multi-kW performance analysis of Yb-doped monolithic single-mode amplifier and oscillator setup,” Proc. SPIE 10897, 108970D (2019). [CrossRef]
26. R. G. Krämer, F. Möller, C. Matzdorf, T. A. Goebel, M. Strecker, M. Heck, D. Richter, M. Plötner, T. Schreiber, A. Tünnermann, and S. Nolte, “Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power,” Opt. Lett. 45(6), 1447–1450 (2020). [CrossRef]
27. H. Zhan, Q. Liu, Y. Wang, W. Ke, L. Ni, X. Wang, K. Peng, C. Gao, Y. Li, H. Lin, J. Wang, F. Jing, and A. Lin, “5KW GTWave fiber amplifier directly pumped by commercial 976 nm laser diodes,” Opt. Express 24(24), 27087 (2016). [CrossRef]
28. Q. Xiao, D. Li, Y. Huang, X. Wang, Z. Wang, J. Tian, P. Yan, and M. Gong, “Directly diode and bi-directional pumping 6 kW continuous-wave all-fibre laser,” Laser Phys. 28(12), 125107 (2018). [CrossRef]
29. Y. Wang, C. Gao, X. Tang, H. Zhan, P. Kun, N. Li, S. Liu, Y. Li, C. Guo, X. Wang, Z. Lihua, J. Yu, L. Jiang, H. Lin, J. Wang, F. Jing, and A. Lin, “30/900 Yb-doped aluminophosphosilicate fiber presenting 6.85 kW laser output pumped with commercial 976 nm laser diodes,” J. Lightwave Technol. 36(16), 3396–3402 (2018). [CrossRef]
30. H. Zhan, K. Peng, S. Liu, Y. Wang, Y. Li, X. Wang, L. Ni, S. Sun, J. Jiang, J. Yu, L. Jiang, J. Wang, F. Jing, and A. Lin, “Pump-gain integrated functional laser fiber towards 10 kW-level high-power applications,” Laser Phys. Lett. 15(9), 095107 (2018). [CrossRef]
31. S. K. Kalyoncu, B. Mete, and A. Yeniay, “Diode-pumped triple-clad fiber MOPA with an output power scaling up to 4.67 kW,” Opt. Lett. 45(7), 1870–1873 (2020). [CrossRef]
32. Z. Lingfa, Z. Pan, X. Xi, H. Yang, Y. Ye, L. Huang, H. Zhang, X. Wang, Z. Wang, P. Zhou, X. Xu, and J. Chen, “5 kW monolithic fiber amplifier employing home-made novel spindle-shaped ytterbium-doped fiber,” Opt. Lett. 46, 1393 (2021). [CrossRef]
33. F. Beier, F. Möller, B. Sattler, J. Nold, A. Liem, C. Hupel, S. Kuhn, S. Hein, N. Haarlammert, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Experimental investigations on the TMI thresholds of low-NA Yb-doped single-mode fibers,” Opt. Lett. 43(6), 1291–1294 (2018). [CrossRef]
34. Z. Huang, Q. Shu, Q. Chu, H. Zhang, D. Yan, Y. Luo, R. Tao, X. Tang, Y. Liu, W. Wu, H. Song, Q. Wang, R. Liao, J. Wen, Y. Li, F. Li, H. Lin, J. Wang, and F. Jing, Chinese Journal of Lasers (in Chinese) 48(6), 616001 (2021).
35. H. Otto, C. Jauregui, J. Limpert, and A. Tünnermann, “Average power limit of fiber-laser systems with nearly diffraction-limited beam quality,” Proc. SPIE 9728, 97280E (2016). [CrossRef]
36. M. N. Zervas, “Power scaling limits in high power fiber amplifiers due to transverse mode instability, thermal lensing, and fiber mechanical reliability,” Proc. SPIE 10512, 1051205 (2018). [CrossRef]
37. F. Stutzki, F. Jansen, H. Otto, C. Jauregui, J. Limpert, and A. Tünnermann, “Designing advanced very-large-mode-area fibers for power scaling of fiber-laser systems,” Optica 1(4), 233–242 (2014). [CrossRef]
38. W. Liu, P. Ma, H. Lv, J. Xu, P. Zhou, and Z. Jiang, “General analysis of SRS-limited high-power fiber lasers and design strategy,” Opt. Express 24(23), 26715–26721 (2016). [CrossRef]
39. J. Wang, D. Yan, S. Xiong, B. Huang, and C. Li, “High power all-fiber amplifier with different seed power injection,” Opt. Express 24(13), 14463–14469 (2016). [CrossRef]
40. J. Zhang, X. Chen, G. Bai, Y. He, K. Liu, B. He, and J. Zhou, “Optimization of seed power for suppression of stimulated Raman scatting in all-fiber amplifiers,” Laser Phys. Lett. 16(3), 035101 (2019). [CrossRef]
41. H. Lin, R. Tao, C. Li, B. Wang, C. Guo, Q. Shu, P. Zhao, L. Xu, J. Wang, F. Jing, and Q. Chu, “3.7 kW monolithic narrow linewidth single mode fiber laser through simultaneously suppressing nonlinear effects and mode instability,” Opt. Express 27(7), 9716 (2019). [CrossRef]
42. B. Yang, H. Zhang, X. Wang, R. Su, R. Tao, P. Zhou, X. Xu, and Q. Lu, “Mitigating transverse mode instability in a single-end pumped all-fiber laser oscillator with a scaling power of up to 2 kW,” J. Opt. 18(10), 105803 (2016). [CrossRef]
43. K. Hejaz, R. Norouzey, A. Poozesh, A. Heidariazar, R. Roohforouz, N. Rezaei Nasirabad, A. Tabatabaei Jafari, A. Hamedani Golshan, M. Babazadeh, and Lafouti, “Controlling mode instability in a 500 W ytterbium-doped fiber laser,” Laser Phys. 24(2), 025102 (2014). [CrossRef]
44. R. Tao, X. Wang, and P. Zhou, “Comprehensive theoretical study of mode instability in High-Power fiber lasers by employing a universal model and its implications,” IEEE J. Select. Topics Quantum Electron. 24(3), 1–19 (2018). [CrossRef]
45. P. Yan, Y. Huang, J. Sun, D. Li, X. Wang, and Q. Xiao, “3.1 kW monolithic MOPA configuration fibre laser bidirectionally pumped by non-wavelength-stabilized laser diodes,” Laser Phys. Lett. 14(8), 080001 (2017). [CrossRef]
46. B. Yang, H. Zhang, C. Shi, R. Tao, R. Su, P. Ma, X. Wang, P. Zhou, X. Xu, and Q. Lu, “3.05 kW monolithic fiber laser oscillator with simultaneous optimizations of stimulated Raman scattering and transverse mode instability,” J. Opt. 20(2), 025802 (2018). [CrossRef]
47. N. Xia and S. Yoo, “Investigation of thermal loads for transverse mode instability in Ytterbium-Doped large mode area fibers,” J. Lightwave Technol. 38(16), 4478–4489 (2020). [CrossRef]
