I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. FBG FABRICATION AND C...III. HIGH POWER ALL-FIBER...IV. CONCLUSIONREFERENCESHigh power fiber lasers have been extensively used in many fields due to the advantages of good beam quality, compact structure, high conversion efficiency, flexible power delivery, and so on.1,21. W. Shi, Q. Fang, X. Zhu, R. A. Norwood, and N. Peyghambarian, “Fiber lasers and their applications [Invited],” Appl. Opt. 53, 6554–6568 (2014). https://doi.org/10.1364/ao.53.0065542. M. N. Zervas and C. A. Codemard, “High power fiber lasers: A review,” IEEE J. Sel. Top. Quantum Electron. 20, 0904123 (2014). https://doi.org/10.1109/jstqe.2014.2321279 From the structure of high-power fiber laser, there are mainly two categories: fiber amplifiers3–53. A. Liem, J. Limpert, H. Zellmer, and A. Tünnermann, “100-W single-frequency master-oscillator fiber power amplifier,” Opt. Lett. 28, 1537–1539 (2003). https://doi.org/10.1364/ol.28.0015374. F. Beier, C. Hupel, S. Kuhn, S. Hein, J. Nold, F. Proske, B. Sattler, A. Liem, C. Jauregui, J. Limpert, N. Haarlammert, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier,” Opt. Express 25, 14892–14899 (2017). https://doi.org/10.1364/oe.25.0148925. M. N. Zervas, “Transverse-modal-instability gain in high power fiber amplifiers: Effect of the perturbation relative phase,” APL Photonics 4, 022802 (2019). https://doi.org/10.1063/1.5050523 and fiber oscillators.6–86. Y. Xiao, F. Brunet, M. Kanskar, M. Faucher, A. Wetter, and N. Holehouse, “1-kilowatt CW all-fiber laser oscillator pumped with wavelength-beam-combined diode stacks,” Opt. Express 20, 3296–3301 (2012). https://doi.org/10.1364/oe.20.0032967. M. Ackermann, G. Rehmann, R. Lange, U. Witte, F. Safarzadeh, B. Boden, H. Weber, D. Netz, C. Perne, A. Kösters, and V. Krause, “Extraction of more than 10 kW from a single ytterbium-doped MM-fiber,” Proc. SPIE 10897, 1089717 (2019). https://doi.org/10.1117/12.25093078. Y. Wang, R. Kitahara, W. Kiyoyama, Y. Shirakura, T. Kurihara, Y. Nakanish, T. Yamamoto, M. Nakayama, S. Ikoma, and K. Shima, “8-kW single-stage all-fiber Yb-doped fiber laser with a BPP of 0.50 mm-mrad,” Proc. SPIE 11260, 1126022 (2020). https://doi.org/10.1117/12.2545832 Owing to the advantages of strong anti-reflection ability and easy control logic, fiber oscillators are more popular than fiber amplifiers in industrial applications.99. 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). https://doi.org/10.1117/12.2287624 With the improvement of fiber components and pump sources, the output power of fiber oscillators has been greatly boosted since the last decade.6–106. Y. Xiao, F. Brunet, M. Kanskar, M. Faucher, A. Wetter, and N. Holehouse, “1-kilowatt CW all-fiber laser oscillator pumped with wavelength-beam-combined diode stacks,” Opt. Express 20, 3296–3301 (2012). https://doi.org/10.1364/oe.20.0032967. M. Ackermann, G. Rehmann, R. Lange, U. Witte, F. Safarzadeh, B. Boden, H. Weber, D. Netz, C. Perne, A. Kösters, and V. Krause, “Extraction of more than 10 kW from a single ytterbium-doped MM-fiber,” Proc. SPIE 10897, 1089717 (2019). https://doi.org/10.1117/12.25093078. Y. Wang, R. Kitahara, W. Kiyoyama, Y. Shirakura, T. Kurihara, Y. Nakanish, T. Yamamoto, M. Nakayama, S. Ikoma, and K. Shima, “8-kW single-stage all-fiber Yb-doped fiber laser with a BPP of 0.50 mm-mrad,” Proc. SPIE 11260, 1126022 (2020). https://doi.org/10.1117/12.25458329. 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). https://doi.org/10.1117/12.228762410. B. Yang, H. Zhang, Q. Ye, H. Pi, C. Shi, R. Tao, X. Wang, and X. Xu, “4.05 kW monolithic fiber laser oscillator based on home-made large mode area fiber Bragg gratings,” Chin. Opt. Lett. 16, 031407 (2018). https://doi.org/10.3788/col201816.031407 Up until now, the highest output powers of an all-fiber oscillator and a spatial structure fiber oscillator are 8 kW88. Y. Wang, R. Kitahara, W. Kiyoyama, Y. Shirakura, T. Kurihara, Y. Nakanish, T. Yamamoto, M. Nakayama, S. Ikoma, and K. Shima, “8-kW single-stage all-fiber Yb-doped fiber laser with a BPP of 0.50 mm-mrad,” Proc. SPIE 11260, 1126022 (2020). https://doi.org/10.1117/12.2545832 and 17.5 kW,77. M. Ackermann, G. Rehmann, R. Lange, U. Witte, F. Safarzadeh, B. Boden, H. Weber, D. Netz, C. Perne, A. Kösters, and V. Krause, “Extraction of more than 10 kW from a single ytterbium-doped MM-fiber,” Proc. SPIE 10897, 1089717 (2019). https://doi.org/10.1117/12.2509307 respectively. In order to achieve high power fiber oscillators, it is urgent to fabricate fiber components in large-core fibers, such as fiber Bragg gratings (FBGs). Some issues should be paid attention to when FBGs are written in large-core fibers as cavity mirrors for high power fiber oscillators. On the one hand, the thermal slope of the FBG should be improved by minimizing the absorption loss, scattering loss, and cladding coupling loss.11,1211. D. Grobnic, C. W. Smelser, S. J. Mihailov, R. B. Walker, and P. Lu, “Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask,” IEEE Photonics Technol. Lett. 16, 1864–1866 (2004). https://doi.org/10.1109/lpt.2004.83123912. I. C. M. Littler, T. Grujic, and B. J. Eggleton, “Photothermal effects in fiber Bragg gratings,” Appl. Opt. 45, 4679–4685 (2006). https://doi.org/10.1364/ao.45.004679 On the other hand, the refractive index modulation region should cover the cross-section of the fiber core completely and homogeneously. The reflectivity of FBG growths increases with the increased of overlap between the refractive index modulation region and the cross-section of the fiber core, which is crucial for inscribing high-reflectivity FBGs (HR-FBGs) in large-core fibers.13–1613. S. Klein, O. Fitzau, M. Giesberts, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Investigation of fiber Bragg gratings for high-power multi-mode XLMA-based fiber lasers,” Proc. SPIE 10897, 1089713 (2019). https://doi.org/10.1117/12.250838514. M. Raguse, S. Klein, P. Baer, M. Giesberts, M. Traub, and H.-D. Hoffmann, “Investigations on high-reflective Fiber-Bragg-Gratings in multimode fibers,” Opt. Continuum 1, 965–973 (2022). https://doi.org/10.1364/optcon.45015015. S. Klein, M. Giesberts, P. Baer, M. Raguse, O. Fitzau, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Fiber Bragg gratings in active multimode XLMA fibers for high-power kW-class fiber lasers,” Proc. SPIE 11260, 1126025 (2020). https://doi.org/10.1117/12.254573816. P. Baer, S. Klein, M. Raguse, M. Giesberts, M. Reiter, and D. Hoffmann, “Monolithic highly multi-mode XLMA-fiber resonator for high power operation,” Opt. Express 30, 33842 (2022). https://doi.org/10.1364/oe.464861 Moreover, because the large-core fibers are usually multimode fibers, the inhomogeneity of cross-sectional refractive index modulation could cause cladding mode coupling and high-order mode excitation.17,1817. H.-G. Yu, Y. Wang, C. Yang, Q.-Y. Xu, X.-L. Yang, and C.-Q. Xu, “Effects of the asymmetric refractive index change profile on the reflection spectra of multimode fiber Bragg gratings,” Proc. SPIE 5970, 597008 (2005). https://doi.org/10.1117/12.62878318. H. Song, Y. Liu, B. Shen, X. Feng, S. Huang, M. Li, J. Wang, L. Li, and R. Tao, “Asymmetry in UV side-inscribed large mode area chirped fiber Bragg gratings,” Optik 228, 166217 (2021). https://doi.org/10.1016/j.ijleo.2020.166217 Normally, the FBGs for high power fiber lasers are fabricated via the ultraviolet (UV) laser exposure method,1919. A. Dragomir, D. N. Nikogosyan, K. A. Zagorulko, P. G. Kryukov, and E. M. Dianov, “Inscription of fiber Bragg gratings by ultraviolet femtosecond radiation,” Opt. Lett. 28, 2171–2173 (2003). https://doi.org/10.1364/ol.28.002171 but hydrogen-loading2020. T. Taunay, P. Bernage, M. Douay, W. X. Xié, G. Martinelli, P. Niay, J. F. Bayon, E. Delevaque, and H. Poignant, “Ultraviolet-enhanced photosensitivity in cerium-doped aluminosilicate fibers and glasses through high-pressure hydrogen loading,” J. Opt. Soc. Am. B 14, 912–925 (1997). https://doi.org/10.1364/josab.14.000912 and thermal annealing2121. P. Holmberg, F. Laurell, and M. Fokine, “Influence of pre-annealing on the thermal regeneration of fiber Bragg gratings in standard optical fibers,” Opt. Express 23, 27520–27535 (2015). https://doi.org/10.1364/oe.23.027520 treatments are needed before and after the inscription. In addition, the larger the fiber core, the longer the time of hydrogen-loading and thermal annealing, which significantly increases the costs of time and economy for large-core FBG fabrication. Moreover, if there is residual hydrogen in fiber after thermal annealing, the thermal slope of FBG would increase.2222. P. Rutthongjan, P. Sudwilai, and O. A. Tangmettajittakul, “The study of effects of hydrogen loading time to the photosensitivity in optical fiber in term of writing time,” in 20th Microoptics Conference (MOC) (IEEE, 2015), pp. 25–28. It should also be noted that UV laser side-inscribing results in an inhomogeneity of large-core FBG in the transverse direction, and the refractive index modulation on the side of the fiber core close to the phase mask is larger due to the absorption of UV laser by the hydrogen-loaded fiber.1818. H. Song, Y. Liu, B. Shen, X. Feng, S. Huang, M. Li, J. Wang, L. Li, and R. Tao, “Asymmetry in UV side-inscribed large mode area chirped fiber Bragg gratings,” Optik 228, 166217 (2021). https://doi.org/10.1016/j.ijleo.2020.166217 Therefore, it is a challenge to fabricate high-quality FBGs in large-core fibers by using the UV laser exposure method.The fs-laser is considered as a promising solution to replace the UV laser for FBG inscription.2323. J. Thomas, C. Voigtländer, R. G. Becker, D. Richter, A. Tünnermann, and S. Nolte, “Femtosecond pulse written fiber gratings: A new avenue to integrated fiber technology,” Laser Photonics Rev. 6, 709–723 (2012). https://doi.org/10.1002/lpor.201100033 The fiber photosensitization is not essential for FBGs written with fs-laser, so the hydrogen-load and thermal annealing treatments are not necessary, which greatly reduces the fabrication process and cost. Nowadays, the fs-laser direct inscription technology24–2624. A. Martinez, M. Dubov, I. Khrushchev, and I. Bennion, “Direct writing of fibre Bragg gratings by femtosecond laser,” Electron. Lett. 40, 1170–1172 (2004). https://doi.org/10.1049/el:2004605025. A. Theodosiou, J. Aubrecht, P. Peterka, I. Kasik, F. Todorov, O. Moravec, P. Honzatko, and K. Kalli, “Er/Yb double-clad fiber laser with fs-laser inscribed plane-by-plane chirped FBG laser mirrors,” IEEE Photonics Technol. Lett. 31, 409–412 (2019). https://doi.org/10.1109/lpt.2019.289689626. X.-P. Pan, Q. Guo, Y.-D. Wu, S.-R. Liu, B. Wang, Y.-S. Yu, and H.-B. Sun, “Femtosecond laser inscribed chirped fiber Bragg gratings,” Opt. Lett. 46, 2059–2062 (2021). https://doi.org/10.1364/ol.422576 and fs-laser phase mask technology27–2927. S. J. Mihailov, C. W. Smelser, P. Lu, R. B. Walker, D. Grobnic, H. Ding, G. Henderson, and J. Unruh, “Fiber Bragg gratings made with a phase mask and 800-nm femtosecond radiation,” Opt. Lett. 28, 995–997 (2003). https://doi.org/10.1364/ol.28.00099528. C. W. Smelser, S. J. Mihailov, D. Grobnic, P. Lu, R. B. Walker, H. Ding, and X. Dai, “Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask,” Opt. Lett. 29, 1458–1460 (2004). https://doi.org/10.1364/ol.29.00145829. J. Thomas, E. Wikszak, T. Clausnitzer, U. Fuchs, U. Zeitner, S. Nolte, and A. Tünnermann, “Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique,” Appl. Phys. A 86, 153–157 (2007). https://doi.org/10.1007/s00339-006-3754-2 are the most commonly used methods for fs-FBGs inscription. The former is often utilized in sensing fields due to its relatively high insertion loss,30,3130. S. J. Mihailov, D. Grobnic, C. Hnatovsky, R. B. Walker, P. Lu, D. Coulas, and H. Ding, “Extreme environment sensing using femtosecond laser-inscribed fiber Bragg gratings,” Sensors 17, 2909 (2017). https://doi.org/10.3390/s1712290931. J. He, B. Xu, X. Xu, C. Liao, and Y. Wang, “Review of femtosecond-laser-inscribed fiber Bragg gratings: Fabrication technologies and sensing applications,” Photonic Sens. 11, 203–226 (2021). https://doi.org/10.1007/s13320-021-0629-2 while the latter is more suitable for FBGs in high power lasers because the low insertion loss can be achieved in large-core FBGs via scanning technology.12,2912. I. C. M. Littler, T. Grujic, and B. J. Eggleton, “Photothermal effects in fiber Bragg gratings,” Appl. Opt. 45, 4679–4685 (2006). https://doi.org/10.1364/ao.45.004679 29. J. Thomas, E. Wikszak, T. Clausnitzer, U. Fuchs, U. Zeitner, S. Nolte, and A. Tünnermann, “Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique,” Appl. Phys. A 86, 153–157 (2007). https://doi.org/10.1007/s00339-006-3754-2 Based on fs-laser phase mask scanning technology, the fs-FBGs were written in large-mode-area ytterbium-doped fibers (YDFs)32,3332. R. G. Kramer, A. Liem, C. Voigtlander, J. U. Thomas, and S. Nolte, “514 W monolithic fiber laser with a femtosecond inscribed fiber Bragg grating,” in Conference on Lasers and Electro-Optics (IEEE, 2013), CJ_1_3.33. R. G. Krämer, C. Matzdorf, A. Liem, V. Bock, W. Middents, T. A. Goebel, M. Heck, D. Richter, T. Schreiber, A. Tünnermann, and S. Nolte, “Femtosecond written fiber Bragg gratings in ytterbium-doped fibers for fiber lasers in the kilowatt regime,” Opt. Lett. 44, 723–726 (2019). https://doi.org/10.1364/ol.44.000723 and passive fibers34–3734. C. Voigtländer, R. G. Krämer, A. Liem, T. Schreiber, A. Tünnermann, and S. Nolte, “1 kW fiber laser oscillator with fs-written fiber Bragg gratings,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2015), CJ_11_13.35. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.38942736. H. Li, X. Tian, M. Wang, X. Zhao, B. Wu, C. Gao, H. Li, B. Rao, X. Xi, and Z. Wang, “Fabrication of fiber Bragg gratings by visible femtosecond laser for multi-kW fiber oscillator,” IEEE Photonics J. 14, 1510904 (2022). https://doi.org/10.1109/jphot.2022.314350137. H. Li, X. Tian, H. Li, B. Wu, X. Zhao, M. Wang, C. Gao, B. Rao, X. Xi, Z. Chen, Z. Wang, and J. Chen, “Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser,” Chin. Opt. Lett. 21, 021404 (2023). https://doi.org/10.3788/col202321.021404 with the core/inner cladding diameters of 20/400 µm. The highest power of all-fiber oscillators using fs-FBGs written in passive fibers is 5 kW,35,3735. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.38942737. H. Li, X. Tian, H. Li, B. Wu, X. Zhao, M. Wang, C. Gao, B. Rao, X. Xi, Z. Chen, Z. Wang, and J. Chen, “Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser,” Chin. Opt. Lett. 21, 021404 (2023). https://doi.org/10.3788/col202321.021404 while the fs-FBGs were written in YDFs to build spatially structured monolithic fiber oscillators with the highest power of 1.9 kW.3333. R. G. Krämer, C. Matzdorf, A. Liem, V. Bock, W. Middents, T. A. Goebel, M. Heck, D. Richter, T. Schreiber, A. Tünnermann, and S. Nolte, “Femtosecond written fiber Bragg gratings in ytterbium-doped fibers for fiber lasers in the kilowatt regime,” Opt. Lett. 44, 723–726 (2019). https://doi.org/10.1364/ol.44.000723 In addition, the investigation of writing FBGs in extra-large mode area (XLMA) fibers was also carried out by using fs-laser phase mask scanning technology.13–1613. S. Klein, O. Fitzau, M. Giesberts, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Investigation of fiber Bragg gratings for high-power multi-mode XLMA-based fiber lasers,” Proc. SPIE 10897, 1089713 (2019). https://doi.org/10.1117/12.250838514. M. Raguse, S. Klein, P. Baer, M. Giesberts, M. Traub, and H.-D. Hoffmann, “Investigations on high-reflective Fiber-Bragg-Gratings in multimode fibers,” Opt. Continuum 1, 965–973 (2022). https://doi.org/10.1364/optcon.45015015. S. Klein, M. Giesberts, P. Baer, M. Raguse, O. Fitzau, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Fiber Bragg gratings in active multimode XLMA fibers for high-power kW-class fiber lasers,” Proc. SPIE 11260, 1126025 (2020). https://doi.org/10.1117/12.254573816. P. Baer, S. Klein, M. Raguse, M. Giesberts, M. Reiter, and D. Hoffmann, “Monolithic highly multi-mode XLMA-fiber resonator for high power operation,” Opt. Express 30, 33842 (2022). https://doi.org/10.1364/oe.464861 A low-reflectivity FBG (LR-FBG) was written in a XLAM active fiber with the core diameter greater than 50 µm, and an 8 kW spatial structure monolithic fiber oscillator was achieved by using a dichroic mirror as a high reflectivity cavity mirror.1515. S. Klein, M. Giesberts, P. Baer, M. Raguse, O. Fitzau, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Fiber Bragg gratings in active multimode XLMA fibers for high-power kW-class fiber lasers,” Proc. SPIE 11260, 1126025 (2020). https://doi.org/10.1117/12.2545738 Excitingly, these results demonstrate the potential of fs-written FBG in high power fiber oscillators, but the above research still has some limitations. For example, the FBGs have to be packaged with water cooling due to the large temperature rise during the power scaling, i.e., the high thermal slope. Therefore, the temperature of FBGs cannot be directly measured, which makes the analysis of the temperature characteristics of the device inevitably inaccurate.15,3515. S. Klein, M. Giesberts, P. Baer, M. Raguse, O. Fitzau, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Fiber Bragg gratings in active multimode XLMA fibers for high-power kW-class fiber lasers,” Proc. SPIE 11260, 1126025 (2020). https://doi.org/10.1117/12.254573835. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.389427 Moreover, the stability and compactness of the 8 kW spatial structure fiber oscillator is poor, the HR-FBG is not written, and the refractive index modulation of the LR-FBG cross-section is inhomogeneous due to the difficulty of fabricating FBGs in XLMA fibers.1515. S. Klein, M. Giesberts, P. Baer, M. Raguse, O. Fitzau, M. Traub, H.-D. Hoffmann, V. Krause, and G. Rehmann, “Fiber Bragg gratings in active multimode XLMA fibers for high-power kW-class fiber lasers,” Proc. SPIE 11260, 1126025 (2020). https://doi.org/10.1117/12.2545738 Therefore, the characteristics of fs-written FBGs applied in high-power fiber oscillators should be further investigated, and the fabrication method of large-core FBGs also needs to be optimized by improving the scanning strategy of fs-laser phase mask scanning technology.In the common scanning strategies, the cylindrical lens38–4438. D. Grobnic, S. J. Mihailov, C. W. Smelser, and R. T. Ramos, “Ultrafast IR laser writing of strong Bragg gratings through the coating of high Ge-doped optical fibers,” IEEE Photonics Technol. Lett. 20, 973–975 (2008). https://doi.org/10.1109/lpt.2008.92296739. M. Bernier, R. Vallée, B. Morasse, C. Desrosiers, A. Saliminia, and Y. Sheng, “Ytterbium fiber laser based on first-order fiber Bragg gratings written with 400 nm femtosecond pulses and a phase-mask,” Opt. Express 17, 18887–18893 (2009). https://doi.org/10.1364/oe.17.01888740. M. Bernier, F. Trépanier, J. Carrier, and R. Vallée, “High mechanical strength fiber Bragg gratings made with infrared femtosecond pulses and a phase mask,” Opt. Lett. 39, 3646–3649 (2014). https://doi.org/10.1364/ol.39.00364641. C. Hnatovsky, D. Grobnic, and S. J. Mihailov, “Nonlinear photoluminescence imaging applied to femtosecond laser manufacturing of fiber Bragg gratings,” Opt. Express 25, 14247–14259 (2017). https://doi.org/10.1364/oe.25.01424742. N. Abdukerim, D. Grobnic, C. Hnatovsky, and S. J. Mihailov, “Through-the-coating writing of tilted fiber Bragg gratings with the phase mask technique,” Opt. Express 27, 38259–38269 (2019). https://doi.org/10.1364/oe.27.03825943. A. Halstuch, A. Shamir, and A. A. Ishaaya, “Femtosecond inscription of fiber Bragg gratings through the coating with a Low-NA lens,” Opt. Express 27, 16935–16944 (2019). https://doi.org/10.1364/oe.27.01693544. L. Talbot, P. Paradis, and M. Bernier, “All-fiber laser pump reflector based on a femtosecond-written inner cladding Bragg grating,” Opt. Lett. 44, 5033–5036 (2019). https://doi.org/10.1364/ol.44.005033 or fiber32–35,45,4632. R. G. Kramer, A. Liem, C. Voigtlander, J. U. Thomas, and S. Nolte, “514 W monolithic fiber laser with a femtosecond inscribed fiber Bragg grating,” in Conference on Lasers and Electro-Optics (IEEE, 2013), CJ_1_3.33. R. G. Krämer, C. Matzdorf, A. Liem, V. Bock, W. Middents, T. A. Goebel, M. Heck, D. Richter, T. Schreiber, A. Tünnermann, and S. Nolte, “Femtosecond written fiber Bragg gratings in ytterbium-doped fibers for fiber lasers in the kilowatt regime,” Opt. Lett. 44, 723–726 (2019). https://doi.org/10.1364/ol.44.00072334. C. Voigtländer, R. G. Krämer, A. Liem, T. Schreiber, A. Tünnermann, and S. Nolte, “1 kW fiber laser oscillator with fs-written fiber Bragg gratings,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2015), CJ_11_13.35. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.38942745. S. J. Mihailov, D. Grobnic, R. B. Walker, C. W. Smelser, G. Cuglietta, T. Graver, and A. Mendez, “Bragg grating writing through the polyimide coating of high NA optical fibres with femtosecond IR radiation,” Opt. Commun. 281, 5344–5348 (2008). https://doi.org/10.1016/j.optcom.2008.07.05646. R. Wang, J. Si, T. Chen, L. Yan, H. Cao, X. Pham, and X. Hou, “Fabrication of high-temperature tilted fiber Bragg gratings using a femtosecond laser,” Opt. Express 25, 23684–23689 (2017). https://doi.org/10.1364/oe.25.023684 are placed on a translation stage, and the refractive index modulation region of the cross-section is extended by making the translation stage move perpendicular to the fiber axis. However, these two scanning strategies require that the movement of the translation stage be strictly coaxial and not offset in other directions; otherwise, the fiber alignment would be affected, resulting in poor grating quality. Though a high-accuracy translation stage can be used, it is still doubtful that the long-term stability of the inscription system can meet the needs of industrial mass production. In addition, the high-accuracy translation stages are expensive, causing a high economic cost of the inscription system, and writing FBGs in large-core fibers could be limited by the travel range of the translation stage.

Here we report a galvanometer-based scanning strategy used in fs-laser phase mask technology, which makes the fs-laser beam scan across the fiber core via the vibration of a galvanometer. Using this new scanning technology, the FBGs are fabricated in large-core fibers with a core diameter of 30 µm, and the homogeneous cross-sectional refractive index modification can be ensured. The reflectivity of HR-FBG is more than 99% with the 3 dB bandwidth of 3.6 nm, and the reflectivity of LR-FBG is about 10% with the 3 dB bandwidth of 2 nm. Furthermore, an all-fiber oscillator is built based on large-core FBGs, and the output power of 7920 kW at 1080 nm is realized with an optical–optical conversion efficiency of 74%. To the best of our knowledge, this is the highest power of the all-fiber oscillator based on fs-written FBGs. The FBGs are neither recoated nor packaged but simply fixed to the water-cooled plate without using thermal interface materials, and the thermal slopes of the HR-FBG and LR-FBG are about 9.2 and 5 °C/kW, respectively. This work demonstrates that the fs-written large-core FBG has excellent performance, which is of great significance for the application of fs-written FBGs in high power fiber oscillators. Moreover, the new scanning strategy with its advantages of flexibility, stability, and economy is conducive to promoting industrial mass production of fs-written FBGs.

II. FBG FABRICATION AND CHARACTERIZATION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. FBG FABRICATION AND C... <<III. HIGH POWER ALL-FIBER...IV. CONCLUSIONREFERENCESThe FBGs are inscribed by femtosecond laser phase mask scanning technology, and the schematic of the inscription system is shown in Fig. 1(a). The collimated fs-laser beam (190 fs pulse duration, 1 kHz repetition rate, 270 µJ pulse energy, 515 nm wavelength) passes through the reflecting mirror, galvanometer, cylindrical lens (25 mm focal length), linear chirped phase mask (1.488 µm center period, 2 nm/cm chirp rate), and optical fiber (core/inner cladding diameter of 30/600 or 30/250 µm) along the X, Y, and Z axes successively. The phase mask is placed between the cylindrical lens and fiber to generate diffraction. By using the walk-off effect,4747. C. W. Smelser, D. Grobnic, and S. J. Mihailov, “Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask,” Opt. Lett. 29, 1730–1732 (2004). https://doi.org/10.1364/ol.29.001730 pure interference of the +1 and −1 orders of diffraction can be realized with a fiber-mask distance greater than 0.86 mm, resulting in the periodic grating pattern in the fiber. The Gaussian fs-laser beam with a diameter of 3 mm is focused by the cylindrical lens to generate enough peak intensity for a refractive index change in the fiber. The focused beam waist width of about 5.4 µm can be calculated according to ω ≈ 4λf/πωo, where λ is the wavelength of the fs-laser, f is the focal length of the cylindrical lens, and ωo is the beam diameter of the incident fs-laser. In addition, the depth of focused beam is L = 8λf2/πωo2 ≈ 91.1 µm. The depth of the focused beam is greater than the fiber core diameter, but the waist width of the focused beam is much smaller than the core diameter. Therefore, the scanning technology should be utilized to expand the overlap between the focused beam and fiber core cross-section for sufficient grating reflectivity.Figure 1(b) presents the schematic illustration of a galvanometer-based scanning strategy, and the elliptic pattern is the region in which the focused laser beam has enough peak intensity to change the refractive index. When the galvanometer vibrates at an angle Δθ, the elliptic pattern scans perpendicular to the fiber axis, so that the refractive index modulation covers the whole fiber core. Obviously, the galvanometer-based scanning strategy has significant advantages in inscribing FBG in large-core fiber because it is flexible enough to achieve a large area of refractive index modulation by a small angle vibration of the galvanometer. Furthermore, the galvanometer-based scanning strategy does not change the relative positions of the cylindrical lens, phase mask, and fiber, and the cost of the galvanometer is also very low, which greatly improves the stability and reduces the cost of the inscription system. In addition to extending the homogeneous refractive index modulation over the whole cross-section, it is also necessary to increase the grating length for larger bandwidths of chirped FBG. Therefore, the reflecting mirror, galvanometer, and cylindrical lens are all placed on a one-dimensional translation stage which is able to move along the X axis [see Fig. 1(a)]. Noting that the fs-laser beam is always incident on a reflecting mirror parallel to the X axis and the incident point remains unchanged when the one-dimensional translation stage moves. Therefore, elongated FBGs could be fabricated by moving the focused laser beam with respect to the phase mask and fiber. Compared with the two scanning strategies mentioned earlier, although a translation stage is also used in our inscription setup, any inscription setup of fs-written FBG requires a translation stage to achieve fs-laser scanning along the fiber axis to increase the grating physical length. The flexible, stable, and economic advantages of the galvanometer-based scanning strategy are mainly reflected in the expansion of refractive index modulation of cross-section, especially for inscribing large-core FBG.The HR-FBG and LR-FBG are written in large-core fibers with the core/cladding diameters of 30/600 and 30/250 µm, respectively, and the coating is stripped before the FBG inscription. Figure 2(a) displays the measured reflection spectra of the FBGs. The background intensity difference between HR-FBG and LR-FBG is attributed to later image processing, which is to observe the spectral profiles more clearly. The sidelobes of FBGs are well suppressed, and their center wavelengths are around 1080 nm. The HR-FBG has a 3 dB bandwidth of 3.6 nm at a physical length of 20 mm. For the LR-FBG, a smaller bandwidth of 2 nm is implemented since it is written with a shorter physical length of 12 mm. The reflectivity of the FBG is extracted by the transmission spectrum, and the reflectivity of HR-FBG and LR-FBG is about 99% and 10%, respectively. The HR-FBG has a 3 dB bandwidth of 3.6 nm and a reflectivity greater than 99% at a physical length of 20 mm. For the LR-FBG, a lower reflectivity of about 10% and a smaller bandwidth of 2 nm are implemented since it is written with a shorter physical length of 12 mm, and its exposure time is also only 6% of that of the HR-FBG. Moreover, an optical microscope is used to characterize the FBG, as shown in Figs. 2(b) and 2(c). Figure 2(b) presents the microscope images of the side of FBGs viewed along the fs-laser beam axis. We can see that the grating structure completely covers the fiber core, indicating that the galvanometer-based scanning achieves uniform index modulation with a width of about 30 µm centered on the fiber core. Moreover, the period of refractive index modulation is half the pitch of the phase mask, which is consistent with pure interference of +1 and −1 orders of diffraction by using the walk-off effect.12,4712. I. C. M. Littler, T. Grujic, and B. J. Eggleton, “Photothermal effects in fiber Bragg gratings,” Appl. Opt. 45, 4679–4685 (2006). https://doi.org/10.1364/ao.45.004679 47. C. W. Smelser, D. Grobnic, and S. J. Mihailov, “Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask,” Opt. Lett. 29, 1730–1732 (2004). https://doi.org/10.1364/ol.29.001730 It is noteworthy that the grating structure of HR-FBG is more obvious than that of LR-FBG due to the longer exposure time. Similarly, the refractive index modulation area in the cross-section of HR-FBG is also more easily observed, as shown in Fig. 2(c). The upper left insert is a whole microscope image of the cross-section of HR-FBG, and the image near the fiber core is zoomed in to clearly observe the refractive index change region. We can see that there are two strip shadows near the fiber core in the cladding, indicating the obvious refractive index change. To give a more accurate analysis, the refractive index distribution of a cross-section of HR-FBG is measured by a fiber refractive index measuring instrument, as shown in Fig. 2(d). It is clear that there are two strip regions with larger refractive index modulation on both sides of the fiber core, but the refractive index modulation between the two strip regions is homogeneous. This phenomenon can be explained by the vibration process of the galvanometer. Because the galvanometer is driven by a square wave signal, it stays in the turn back position for a longer time, and the focused beam is just on the upper or lower sides of the fiber core along the Z axis at this time, resulting in a greater refractive index modulation. The maximum refractive index modulation of ∼0.8 × 10−3 in these two strip regions is type II modulation, and these two strip regions are in the cladding region, which does not cause broadband losses of core modes. It is also worth noting that because the angular velocity of the galvanometer is constant during the vibration process, the fs-laser scanning area between the two strip regions has a homogeneous refractive index modulation of ∼0.4 × 10−3. The refractive index distribution in the HR-FBG core region seems a little inhomogeneous, which is attributed to the slight inhomogeneity of the used 30/600 µm fiber caused by fiber fabrication technology. Consequently, it can be demonstrated from Fig. 2(d) that the scanning area covers the entire fiber core completely and homogeneously.

III. HIGH POWER ALL-FIBER OSCILLATORS

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. FBG FABRICATION AND C...III. HIGH POWER ALL-FIBER... <<IV. CONCLUSIONREFERENCESIn order to investigate the high power performance of femtosecond-written FBGs, an all-fiber oscillator is built, and the experimental setup is shown in Fig. 3. A 40-m-long YDF with a core/inner cladding diameter of 30/600 μm and a core NA of 0.06 is coiling at a bending diameter of 19 to 30 cm. The pump absorption coefficient of the YDF is about 0.6 dB/m at the pump wavelength. One end of the YDF is spliced with the HR-FBG, and the other is spliced with the (18 + 1) × 1 pump-signal combiner. The input and output signal ports of the combiner are 30/600 and 30/250 μm double cladding fibers, respectively. Pump light is provided by a series of wavelength-stabilized fiber-coupled laser diodes (LDs) at 981 nm with a maximum output power of about 850 W each. The reason for choosing 981 nm as the pump wavelength is to balance the nonlinear effect, increasing the threshold of stimulated Raman scattering (SRS) and transverse mode instability (TMI) in the fiber laser. The cladding light stripper (CLS) is used to remove the residual pump power before laser output. The feedback at the forward and backward output ends is suppressed via splicing with a quartz block head (QBH) and an angle-cleaved fiber end, respectively.Figure 4(a) displays the output power and optical–optical conversion efficiency dependence on the pump power. When the system is pumped with a total of 10.2 kW, the maximum output power of 7920 W is achieved, corresponding to an optical–optical conversion efficiency of 77.3%. The output spectra for different output powers are measured, as shown in Fig. 4(b). Because the Raman threshold of fiber laser can be effectively improved by counter-pumping,4848. Y. Ye, B. Yang, X. Wang, H. Zhang, X. Xi, C. Shi, P. Zhou, and X. Xu, “Experimental study of SRS threshold dependence on the bandwidths of fiber Bragg gratings in co-pumped and counter-pumped fiber laser oscillator,” J. Opt. 21, 025801 (2018). https://doi.org/10.1088/2040-8986/aafa65 no Stokes light is observed in the spectra. Obviously, the output spectrum of signal light is broadened with the increase in output power, and the 3 dB bandwidth of signal light is 4.9 nm at the maximum output power, which can be attributed to the nonlinear effects in optical fiber such as the self-phase modulation (SPM). Furthermore, the center wavelength of the signal light shifts slightly to the longer wavelength with the output power scaling. This is because the temperature of LR-FBG increases with the output power scaling, resulting in the shift of its Bragg resonance wavelength to the longer wavelength.The temperature of the FBGs dependence on the output power is measured by a thermal camera, as shown in Fig. 5(a). In our experiment, the FBG self-annealing effect3535. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.389427 was observed, but we only show the temperature slope of the FBGs that finally achieve an output power of 8 kW in Fig. 5(a), which is attributed to the factor that the fiber oscillator structure had been changed and improved in different runs for increasing the output power. Noting that the FBGs are neither recoated nor packaged and are just simply fixed to the water-cooled plate without using thermal interface materials. The thermal slopes of the HR-FBG and the LR-FBG are about 9.2 and 5 °C/kW via data fitting, respectively. The thermal slope of the HR-FBG is greater than that of the LR-FBG, which can be explained by the absorption of signal light at defects such as color centers in the FBGs.3535. 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, 1447–1450 (2020). https://doi.org/10.1364/ol.389427 There are more defects in the HR-FBG due to the longer exposure time during the fs-laser writing process.3939. M. Bernier, R. Vallée, B. Morasse, C. Desrosiers, A. Saliminia, and Y. Sheng, “Ytterbium fiber laser based on first-order fiber Bragg gratings written with 400 nm femtosecond pulses and a phase-mask,” Opt. Express 17, 18887–18893 (2009). https://doi.org/10.1364/oe.17.018887 The temperatures of the HR-FBG and the LR-FBG at the maximum pump power are about 92 and 55 °C, respectively, and the corresponding thermograms are shown in Fig. 5(b). Because fs-written FBGs have good high temperature resistance,3333. R. G. Krämer, C. Matzdorf, A. Liem, V. Bock, W. Middents, T. A. Goebel, M. Heck, D. Richter, T. Schreiber, A. Tünnermann, and S. Nolte, “Femtosecond written fiber Bragg gratings in ytterbium-doped fibers for fiber lasers in the kilowatt regime,” Opt. Lett. 44, 723–726 (2019). https://doi.org/10.1364/ol.44.000723 both the HR-FBG and the LR-FBG can work stably at this temperature.The beam quality of an output laser is measured at different output power, as shown in Fig. 6(a). When the output power is lower than 5 kW, the beam quality changes little with the output power scaling, and the minimum M2 factor of 2.29 is measured at the output power of 4950 W. The beam quality degrades after output power exceeds 6 kW, and the M2 factor is 2.53 at the maximum output power. It is possible that TMI occurs in the fiber laser. However, it can be seen from the inset shown in Fig. 6(a) that the beam profile of the output laser changes little when the output power increases from 4950 to 7920 W, and the existence of high-order modes cannot be observed, which is different from the phenomenon described in Ref.