High power tandem-pumped thulium-doped fiber laser - OSA Publishing

High power tandem-pumped thulium-doped fiber laser Yao Wang, Jianlong Yang, Chongyuan Huang, Yongfeng Luo, Shiwei Wang, Yulong Tang,* and Jianqiu Xu Laboratory for Laser Plasmas (MOE) and Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China * [email protected]

Abstract: We propose a cascaded tandem pumping technique and show its high power and high efficient operation in the 2-μm wavelength region, opening up a new way to scale the output power of the 2-μm fiber laser to new levels (e.g. 10 kW). Using a 1942 nm Tm3+ fiber laser as the pump source with the co- (counter-) propagating configuration, the 2020 nm Tm3+ fiber laser generates 34.68 W (35.15W) of output power with 84.4% (86.3%) optical-to-optical efficiency and 91.7% (92.4%) slope efficiency, with respect to launched pump power. It provides the highest slope efficiency reported for 2-μm Tm3+-doped fiber lasers, and the highest output power for all-fiber tandem-pumped 2-μm fiber oscillators. This system fulfills the complete structure of the proposed cascaded tandem pumping technique in the 2-μm wavelength region (~1900 nm → ~1940 nm → ~2020 nm). Numerical analysis is also carried out to show the power scaling capability and efficiency of the cascaded tandem pumping technique. ©2015 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (140.3510) Lasers, fiber; (140.3480) Lasers, diodepumped; (260.3060) Infrared.

References and links 1.

Y. Tang, L. Xu, Y. Yang, and J. Xu, “High-power gain-switched Tm3+-doped fiber laser,” Opt. Express 18(22), 22964–22972 (2010). 2. Y. Tang, C. Huang, S. Wang, H. Li, and J. Xu, “High-power narrow-bandwidth thulium fiber laser with an allfiber cavity,” Opt. Express 20(16), 17539–17544 (2012). 3. F. Stutzki, F. Jansen, C. Jauregui, J. Limpert, and A. Tünnermann, “2.4 mJ, 33 W Q-switched Tm-doped fiber laser with near diffraction-limited beam quality,” Opt. Lett. 38(2), 97–99 (2013). 4. R. A. Sims, P. Kadwani, A. S. Shah, and M. Richardson, “1 μJ, sub-500 fs chirped pulse amplification in a Tmdoped fiber system,” Opt. Lett. 38(2), 121–123 (2013). 5. A. M. Heidt, Z. Li, J. Sahu, P. C. Shardlow, M. Becker, M. Rothhardt, M. Ibsen, R. Phelan, B. Kelly, S. U. Alam, and D. J. Richardson, “100 kW peak power picosecond thulium-doped fiber amplifier system seeded by a gain-switched diode laser at 2 μm,” Opt. Lett. 38(10), 1615–1617 (2013). 6. F. Stutzki, C. Gaida, M. Gebhardt, F. Jansen, A. Wienke, U. Zeitner, F. Fuchs, C. Jauregui, D. Wandt, D. Kracht, J. Limpert, and A. Tünnermann, “152 W average power Tm-doped fiber CPA system,” Opt. Lett. 39(16), 4671– 4674 (2014). 7. X. Wang, P. Zhou, H. Zhang, X. Wang, H. Xiao, and Z. Liu, “100 W-level Tm-doped fiber laser pumped by 1173 nm Raman fiber lasers,” Opt. Lett. 39(15), 4329–4332 (2014). 8. T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, G. Rines, and P. Moulton, “1-kW, all-glass Tm:fiber laser,” In Fiber Lasers III: Technology, Systems, and Applications, Proc. of SPIE 7580, (SPIE, 2010), paper 7580–112. 9. M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, “415W SingleMode CW Thulium Fiber Laser in all-fiber format,” in CLEO/Europe and IQEC 2007 Conference Digest, (Optical Society of America, 2007), paper CP2_3. 10. Y. Tang, F. Li, and J. Xu, “High peak-power gain-switched Tm-doped fiber laser,” IEEE Photon. Technol. Lett. 23(13), 893–895 (2011). 11. D. Creeden, B. R. Johnson, S. D. Setzler, and E. P. Chicklis, “Resonantly pumped Tm-doped fiber laser with >90% slope efficiency,” Opt. Lett. 39(3), 470–473 (2014). 12. D. Creeden, B. R. Johnson, G. A. Rines, and S. D. Setzler, “High power resonant pumping of Tm-doped fiber amplifiers in core- and cladding-pumped configurations,” Opt. Express 22(23), 29067–29080 (2014).

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Received 27 Oct 2014; revised 19 Dec 2014; accepted 27 Jan 2015; published 2 Feb 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002991 | OPTICS EXPRESS 2991

13. C. Huang, Y. Tang, H. Li, Y. Wang, J. Xu, and C. Du, “A versatile model for temperature-dependent effects in Tm-doped silica fiber lasers,” J. Lightwave Technol. 32(3), 421–428 (2014). 14. J. Yang, Laboratory for Laser Plasmas (MOE) and Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China, and Y. Wang, G. Zhang, Y. Tang, C. Huang, and J. Xu. are preparing a manuscript to be called “Influences of pump transitions on thermal effects of multi-kilowatt thulium-doped fiber lasers.” 15. D. P. Hand and P. S. J. Russell, “Solitary thermal shock waves and optical damage in optical fibers: the fiber fuse,” Opt. Lett. 13(9), 767–769 (1988). 16. Y. Jeong, S. Baek, P. Dupriez, J.-N. Maran, J. K. Sahu, J. Nilsson, and B. Lee, “Thermal characteristics of an end-pumped high-power ytterbium-sensitized erbium-doped fiber laser under natural convection,” Opt. Express 16(24), 19865–19871 (2008).

1. Introduction Tm-doped fiber lasers have attracted increasing attention in recent years [1–7]. Owing to their ‘eye-safe’ characteristics, they can be applied in extensive areas, such as medical surgery, scientific experiments, industrial machining, remote detection, etc. It is common practice to use laser diodes (LDs) as the pump source to scale the output power of a laser system. Combined with the double-cladding pumping technique, 793 nm LD pumped Tm-doped fiber laser has achieved more than 1 kW of output power at ~2-μm [8]. However, this kind of 793 nm LD pumped 2-μm Tm fiber laser has an upper-limited efficiency of ~60% due to large quantum defect. The amount of heat generated can be equivalent to or exceed the amount of output optical power. Under high power operation, the great amount of waste heat will damage the fiber and put an unsolvable issue for further power scaling of this kind of 2-μm Tm fiber laser. In order to find a more efficient energy transfer method for power scaling of the 2-μm Tm fiber laser, resonant pumping with the 1500 nm laser has been proposed [9]. However, this pumping technique still limits the conversion efficiency to lower than 70%. Further power scaling of the 2-μm Tm fiber laser thus calls for more efficient pumping techniques. Such pumping techniques must have less quantum defect and render less thermal loading. For this purpose, tandem pumping technique (pumping at wavelengths close to the emission wavelength) for 2-μm Tm fiber lasers has been proposed [10–12]. As shown in Fig. 1 [13], tandem pumping can greatly decrease the quantum defect of the fiber laser due to the pump wavelength being very close to the laser wavelength. Rather than one-step power scaling of the 2-μm fiber laser, it is more appropriate to scale the output power step by step. Therefore, we propose the cascaded tandem pumping technique for 2-μm Tm fiber lasers, as shown in Fig. 1. The spectral conversion from the ~1900 nm range (from 1890 nm to 1910 nm) to the ~1940 nm range (from 1930 nm to 1950 nm) comprises the first-stage tandem pumping, while the spectral conversion from the ~1940 nm range to the ~2020 nm range (from 2000 nm to 2020 nm) consists of the second-stage tandem pumping. In each stage tandem pumping, the quantum defect (thus heat load) is very small. The total heat loading is distributed among two stages, spreading waste heat over a longer distance, greatly diminishing the thermal problem. To make the tandem pumping technique high efficient, the quantum defect should be as small as possible. Therefore, the pump wavelength should be as closer to the laser emission wavelength as possible. Based on the absorption spectrum of the Tm fiber (see the red curve of Fig. 1), the pump wavelength can be as long as ~2000 nm. However, total laser efficiency will be greatly decreased when the pump wavelength is >1940 nm due to decreased pump absorption and heavy selfabsorption of the ~2-μm signal light in the Tm fiber. After many experiments with the currently available Tm fiber, we found that 1940 nm is approximately the longest pump wavelength for this 2-μm tandem pumping technique to achieve high efficient operation. When the pump wavelength is >1960 nm, laser threshold and laser slope efficiency will significantly increases and decreases, respectively. To push the pump wavelength to even longer range (~2000 nm) with this tandem pumping technique, further advancement of Tm fiber fabrication and engineering is required.

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Fig. 1. Absorption and emission cross section of the Tm-doped fiber and schematic diagram of the cascaded tandem pumping technique. The 1st stage tandem pumping: ~1900 nm → ~1940 nm; the 2nd stage tandem pumping: ~1940 nm → ~2020 nm. Inset shows absorption cross section of the Tm-doped fiber from 1900 nm to 1940 nm.

Previously, we have demonstrated the first-stage tandem pumping operation of the Tm fiber laser (pumping at ~1900 nm) with high efficiency [10]. In this paper, we verify that the second-stage tandem pumping (pumping at ~1940 nm) is also high efficient and power scalable. Pumped at 1942 nm, more than 35 W laser power at 2020 nm has been achieved with a slope efficiency of ~92%. Both the power and efficiency show record performance for Tm fiber oscillator in an all-fiber configuration. The less than 4% quantum defect from 1942 nm to 2020 nm makes this kind of system almost thermal-free. This system, together with our previous configuration [10], accomplishes the complete chain of our cascaded tandem pumping technique in the 2-μm wavelength regime. Numerical simulation of the first-stage and the second-stage tandem pumping Tm fiber lasers is also implemented and shows great potential of this cascaded tandem pumping technique. Combining this cascaded tandem pumping technique to the conventional cladding pumping technique can potentially scale the output power of 2-μm Tm fiber lasers to an unprecedented level. 2. Experiment and results The experimental setup for the 2nd stage tandem pumping Tm fiber laser is shown in Fig. 2. The pump source is a 1942 nm double-clad Tm-doped fiber laser. The double-clad Tm-doped silica fiber has a ~25 μm diameter, 0.09 NA (numerical aperture) core doped with Tm3+ of ~4wt.% concentration. The pure-silica inner cladding, coated with a low-index polymer, has a 400 μm diameter and a NA of 0.46. A ~4.4 m length of the double-clad Tm-doped silica fiber is used here. The pump sources of the 1942 nm pump laser are six 793 nm high power LDs with 220 μm (0.22 NA) pigtail fibers, which match to the pump fiber of the combiner. A (6 + 1) × 1 high power fiber combiner was used to couple the pump light into the gain fiber, with a coupling efficiency of ~80%. A pair of FBGs was designed with the center wavelength of 1942 nm. One is high reflective (R = 99.5%) with a spectral FWHM (full width at half maximum) of 2 nm, and the other is partially reflective (R = 5.4%) with spectral FWHM of 1 nm. The free fiber ends were angle cleaved by ~8° to eliminate end reflections. The Tmdoped fibers were wrapped on a water-cooled copper drum with a diameter of 25 cm for

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cooling, and all of the fusion splices and the output fiber end were clapped in copper heat sinks. And the output characteristics of the 1942 nm pump laser are shown in Fig. 3 with the maximum output of 49W. The spectral width of the output is about ~2 nm (see the inset of Fig. 3), larger than the FWHM bandwidth of the partially reflective FBG (1 nm) but close to the FWHM bandwidth of the high reflective FBG (2 nm). Considering that the given FWHM bandwidth of FBGs is usually measured at low power levels and the side mode suppression ratio of the FBG is comparatively low (10.24 dB), spectral broadening is reasonably expected due to improved gain under high pump levels.

Fig. 2. Experimental setup for the second-stage tandem pumping Tm fiber laser with the copropagation (upper) and counter-propagation (down) configuration. HR: highly reflective fiber Bragg grating; TDF: Tm-doped fiber; PR: partially reflective fiber Bragg grating; ISO: optical isolator; DM: dichroic mirror.

Fig. 3. Output performance of the 1942 nm pump laser. Symbols: measured data; lines: linear fitting. Inset shows the laser spectrum of the output laser beam.

The output from the 1942 nm pump laser is coupled into the fiber core after crossing an optical isolator with a launch efficiency of ~85%. The double-clad Tm-doped silica fibers are the same as those used in the 1942 nm pump laser, and the fiber length used is 3.5 m. Another pair of FBGs was designed with the center wavelength of 2020 nm. One is high reflective (R = 97%) with a spectral FWHM of 1.4 nm, and the other is partially reflective (R = 10%) with spectral FWHM of 0.5 nm. The free fiber ends of the gratings were angle cleaved by ~8° to eliminate end reflections. The Tm-doped fibers were wrapped on a water-cooled copper drum with a diameter of 25 cm for cooling, and all of the fusion splices and the output fiber end #225730 - $15.00 USD (C) 2015 OSA


were clapped in copper heat sinks. At the output fiber end, a double-convex lens was used to collimate the 2020 nm laser output, and a dichroic mirror (R > 96% @2020 nm & T > 90% @1942 nm, 45°coated) was used to filter the un-absorbed 1942 nm pump light. The 2020 nm laser output was calibrated by subtracting the un-filtered pump light and incorporating the filter-mirror-rejected laser light. This was accomplished by first spectral analysis of the deflected light from the dichroic mirror. The integrated areas of the 1942 nm spectrum and the 2020nm spectrum determined the power ratio of the filtered pump light to the rejected laser light. Then by combining the transmission values of the 45°dichroic mirror at these two wavelengths, the 2020 nm output power was obtained. The laser power was measured with a FieldMate power meter (Coherent Co.) and the laser spectrum was tested with an Omni-λ750 spectrometer (Zolix Instruments Co.). In the experiment, two kinds of laser propagating cavity configurations were employed. One is the co-propagating laser oscillation structure (the upper diagram of Fig. 2), in which the laser output direction is the same as the pump beam propagation direction. The other is the counter-propagating laser oscillation structure (the down diagram of Fig. 2), in which the laser output direction is opposite to the pump beam propagation direction. With the co- and counter-propagating laser configuration, the output characteristics of the 2020 nm laser are shown in Fig. 4.

Fig. 4. Output performance of the 2020 nm tandem pumping fiber laser with the co- and counter-propagating laser configuration. Symbols: measured data; lines: linear fitting.

With the co-propagating laser configuration, we generated 34.68 W of 2020 nm output power with 41.11 W 1942 nm pump power, yielding an 84.4% optical efficiency and 91.7% slope efficiency with respect to launched pump power. With the counter -propagating laser configuration, we generated 35.15 W of 2020 nm output power with 40.75 W 1942 nm pump power, yielding an 86.3% optical efficiency and 92.4% slope efficiency with respect to launched pump power. To our knowledge, this is the highest optical efficiency and slope efficiency ever reported for Tm-doped fiber oscillators in an all-fiber configuration at any wavelength region. In the co- and counter-propagating laser configuration, the output power shows a linear increase with the pump power, indicating the possibility of further power scaling simply by enhancing pump power. Further improving the laser efficiency is possible by optimizing the laser cavity (fiber length, output coupling, etc.). Laser spectra recorded with the co-propagating configuration and the counter-propagating configuration were very similar. Here, we only discuss the spectra obtained in the copropagating configuration. Laser spectra measured under different output power levels (0.35 W, 12.19 W, and 28.46 W) are shown in Fig. 5. Under all measurements, the center laser wavelength was confined at around 2019 nm and the laser spectral bandwidth (FWHM) was confined to around 1 nm, showing high wavelength- and spectral bandwidth-stabilizing effect of the designed FBGs of the tandem pumping 2-μm Tm-doped fiber lasers. The spectral width

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of the output is about ~1 nm, larger than the FWHM bandwidth of the partially reflective FBG at 0.5 nm, which can be explained just as for the 1942 nm pump laser.

Fig. 5. Laser spectra of the 2020 nm tandem pumping fiber laser with the co-propagating configuration under different output power levels.

To explore the power scaling capability of the tandem pumping Tm fiber laser and show the advantages of our cascaded tandem pumping technique, a numerical model was built to analyze the lasing and thermal characteristics under different pump transitions. The detailed modeling process is shown in our recent paper [14]. To check the validity of our model and numerical algorithm, we calculate the slope efficiency in four transition schemes (~793 nm→ ~1940 nm, ~1940 nm → ~2020 nm, ~1900 nm → ~1940 nm, ~1900 nm → ~2020nm), with similar laser configurations and fiber parameters as that employed in the experiments in this paper and several other reports [10, 11]. The calculated slope efficiencies are 50.6% (~793 nm→ ~1940 nm), 95.9% (~1940 nm → ~2020 nm), 85.2% (~1900 nm → ~1940 nm), and 92.6% (~1900 nm → ~2020nm). The discrepancy between the calculated slope efficiencies and those from experiments and reports is about 3%. This verifies the validity of our model and it is thereby reasonable to use it to predict the behaviors of Tm-doped fiber lasers. In the next simulation, we compare another four pump transitions (~793 nm→ ~2020 nm, ~1900 nm → ~1940 nm, ~1940 nm → ~2020 nm, ~1900 nm → ~2020nm). The first one is for cladding pumping and the others are for core pumping. The adopted Tm-doped fiber has a core/cladding diameter of 25/400 μm and a Tm3+ doping concentration of 3.5 × 1026 m−3. One FBG is high reflective (R = 100%) and the other FBG is partially reflective (R = 4%). For the four different pump transitions (~793 nm→ ~2020 nm, ~1900 nm → ~1940 nm, ~1940 nm → ~2020 nm, ~1900 nm → ~2020nm), the lengths of the Tm-doped fibers are optimized to achieve maximum slope efficiencies (3.2 m, 4.8 m, 2.3 m, 1.4m). The simulated lasing performance under different pump transitions is shown in Fig. 6. The slope efficiencies for the four transitions are 50.0%, 95.4%, 94.7%, 94.0%, respectively. The slope efficiency in either stage of our cascaded tandem pumping (~1900 nm → ~1940 nm or ~1940 nm→ ~2020 nm) is higher than the slope efficiency of the 793 nm pumping transition (~793 nm→ ~2020 nm) or the conventional tandem pumping transition (~1900 nm → ~2020nm). Therefore, our cascaded tandem pumping technique brings forth smaller heat load and possesses greater potential in scaling output power of 2-μm fiber lasers than single-stage tandem pumping, particularly when such technique is employed in cladding pumped configurations.

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Fig. 6. Lasing performance of the Tm-doped fiber laser under different pump transitions.

When developing high power Tm-doped fiber lasers, we must pay attention to thermal effects. The temperature rise of the gain fiber is caused by waste heat generated in the fiber. If the temperature is too high, damage to fiber core or burning of fiber coating will occur. Based on numerical simulation, we compare the four transitions in this respect. Here damage to the fiber core and coating is taken into account, which is related to a critical temperature of 700 °C [15] and 200 °C [16]. Under different transitions, the temperature changes of the fiber core and coating with launched pump power are shown in Fig. 7. In the ~793 nm→ ~2020 nm transition, the temperature increase of the fiber core (coating) is not linear, and the average temperature rise of the fiber core is 0.112 (0.076) °C/W. In the ~1900 nm → ~1940 nm, ~1940 nm → ~2020 nm, ~1900 nm → ~2020 nm transitions, the temperature increases of the core (coating) are linear and show an increase rate of 0.020 (0.014), 0.043 (0.029), 0.058 (0.039) ◦C/W, respectively. When the fiber core and coating are not burned, the maximum launched pump power in the ~793 nm→ ~2020 nm transition is 440 W (corresponding to ~2μm output power of 220 W); while the maximum launched pump power in the ~1900 nm→ ~2020 nm transition is 4615 W (corresponding to ~2-μm output power of 4338 W). In our cascaded tandem pumping (~1900 nm→~1940 nm→~2020 nm), the maximum tolerable launched pump power even for the ~1940→~2020 nm transition (has a larger quantum defect than the ~1900 nm→~1940 nm transition) is 6206 W (corresponding to ~2-μm output power of 5877 W). Therefore, our cascaded tandem pumping technique has a power scaling capability 26.7 times (1.4 times) higher than the conventional 793 nm pumping technique (the conventional tandem pumping technique).When used in cladding pumping configurations

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(multiple pump sources are combined with a fiber combiner), the cascaded tandem pumping technique has greater power scalability than the conventional single-stage tandem pumping.

Fig. 7. Core (a) and coating (b) temperature of the Tm-doped fiber laser under different pump transitions.

3. Conclusion In conclusion, cascaded tandem pumping technique at the ~2-μm wavelength region is proposed and high-power efficient operation of the second-stage tandem pumping Tm fiber laser is confirmed. Pumped at 1942 nm, the Tm-doped fiber laser achieves ~35 W output power at 2020 nm with 92.4% slope efficiency and 86.3% optical-to-optical efficiency, showing record performance in tandem pumping all-fiber Tm oscillators. Numerical modeling helps to show that this cascaded tandem pumping technique possesses significant advantages over the conventional 793 nm pumping method and the conventional single-stage tandem pumping technique. Combined with the potential cladding pumping configuration at ~1900 nm, this cascaded tandem pumping technique opens up a practical way to scale the output power of the ~2-μm Tm-doped fiber laser to compete the 1-μm Yb fiber laser, i.e., to approach the 10 kW level. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 61138006, No. 61275136, and No. 11121504) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20120073120085).

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