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Abstract
In the power scaling of monolithic fiber lasers, the fiber nonlinear effects and transverse mode instability are main limitations. The tapered gain fiber has a longitudinally varying mode area, which has the advantage of mitigating fiber nonlinear effects. However, the transverse mode instability (TMI) was seldom reported in the tapered fiber lasers at high average power levels. In this work, we have constructed a monolithic tapered ytterbium-doped fiber laser oscillator and investigated the laser oscillator performance with respective 976 nm and 915 nm pump, especially on the aspects of the TMI. The double cladding tapered ytterbium-doped fiber has a narrow end of ~20/400 μm and a wide end of ~30/600 μm. Fiber Bragg gratings (FBG) are respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm to match with the narrow and wide end of the tapered ytterbium-doped fiber. When 915 nm pump is employed, the TMI occurs at the output power of ~1350 W. The output power is further scaled to a maximum of 1720 W. The M2 factor of the output laser is ~2.1 and the full width at half maximum (FWHM) of the signal laser is ~3.6 nm. To the best of our knowledge, this is the highest average power for the tapered ytterbium-doped fiber lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power monolithic fiber lasers are widely adopted as laser sources in the industrial applications and beam combinations, for the advantages of high conversion efficiency, good beam quality, compact structure and reliable stability. In the last decade, there has been a remarkable increase in the output power of fiber lasers. The output powers of nearly single mode fiber laser amplifiers and oscillators have reached 20 kW and 5 kW [1–3], respectively. According to the recent studies [4–8], the power scaling of the fiber lasers are mainly limited by the fiber nonlinear effects and the transverse mode instability. Both the limitation effects have to be simultaneously suppressed for the successful power scaling of fiber lasers. Therefore, gain fibers with very large mode areas and specially designed mode discrimination techniques have been proposed and manufactured, such as Chirally-Coupled-Core (3C) fiber [9,10], ultra-low numerical aperture (NA) fiber [11–13], leakage channel fiber (LCF) [14,15], large pitch fiber (LPF) [16], cladding trenched fibers [13,17] and etc. These gain fiber designs require elaborate control of the transverse refractive index in the fiber manufacture.
Different from the longitudinal uniform fibers, tapered ytterbium-doped fiber has a longitudinally varying mode area along the fiber, which can be designed to filter out high-order modes and maintain good laser beam quality. Moreover, the increasing mode area along the tapered fiber has the advantage of mitigating nonlinear effects. In 2013, Trikshev et al. reported a 160 W single-frequency laser based on a tapered double-cladding fiber amplifier [18]. In 2016, Zhou of our research group demonstrated the stimulated Brillouin scattering (SBS) threshold of the single frequency fiber amplifier can be notably enhanced by employing tapered ytterbium-doped fiber [19]. In 2017, Bobkov et al. reported a chirped pulse monolithic fiber amplifier based on a tapered polarization maintaining ytterbium-doped fiber with MW-level peak power and diffraction limited beam quality [20]. In 2018, Fedotov et al. reported amplification of mode-locked fiber laser based on an ytterbium-doped tapered fiber with an ultra-large core diameter of 96 μm [21]. A pulse energy of 28 µJ with 292 kW peak power was reached at an average output power of 28 W for a 1 MHz repetition rate.
Up to date, most studies on the tapered fiber lasers are focused on the single frequency fiber lasers or pulsed fiber lasers at relative low average power levels, while the high average power tapered fiber lasers were rarely reported [22,23]. The record average output power for the tapered fiber laser oscillators were only 750 W, which was achieved in 2010. Since the occurrence of TMI depends on the average power of fiber lasers, there was rare report on the aspects of TMI performance of the tapered ytterbium-doped fiber laser oscillators [24]. The tapered fibers have been employed to mitigate the fiber nonlinear effects, meanwhile, it is meaningful to validate whether the TMI in the fiber lasers can be effectively suppressed by employing the tapered ytterbium-doped fiber design.
In this paper, we report a high power monolithic tapered ytterbium-doped fiber laser oscillator with a record output power of 1.7 kW. The monolithic fiber laser oscillator was constructed based on a double-cladding tapered ytterbium-doped fiber with a narrow end of ~20/400 μm and a wide end of ~30/600 μm. High reflection and output coupler fiber Bragg gratings were respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm, which match with the narrow and wide end of the tapered ytterbium-doped fiber. Detailed TMI characteristics of the high power tapered fiber laser oscillator were studied with 976 nm and 915 nm pump respectively. By employing 915 nm pump, the output laser power was scaled to 1.7 kW with high beam quality.
2. Experimental setup
The experimental setup of the monolithic fiber laser oscillator is depicted in Fig. 1. The pump sources are high-power 976 nm and 915 nm laser diodes (LDs), which are respectively employed. All the LDs have pigtailed fiber with core/cladding diameters of 220/242 μm. A 7 × 1 tapered fused bundle (TFB) combiner is utilized to combine the pump light into the inner cladding of the output fiber, which has a diameter of 400 μm and numerical aperture (NA) of 0.46. Six pump ports of the TFB combiner are employed, while the unoccupied central pump port of the TFB combiner is angle cleaved to avoid facet reflection. The co-pump light is launched into the laser oscillation cavity through the high reflection fiber Bragg gratings (HR FBG). The gain fiber is double-cladding linearly tapered ytterbium-doped fiber (TYDF), which is provided by the No. 23 institute of China electronics technology group corporation. The narrow end has a core/inner cladding diameter of ~20/400 μm, and the wide end has a core/inner cladding diameter of ~30/600 μm. The core numerical aperture (NA) of the TYDF is ~0.065. The length of TYDF is ~33 m and the total absorption coefficient at 976 nm is ~25 dB. The gain fiber is coiled in circles with diameters of 14 cm~18 cm. The HR FBG is inscribed on double cladding fiber with core/inner cladding diameter of 20/400 μm, which matches with the narrow end of the TYDF. The HR FBG provides a reflectivity of ~99% with a 3 dB bandwidth of ~2 nm at the center wavelength of ~1080 nm. Since there is no available 30/600 μm FBG to match with the wide end of the TYDF, we employ a 30/400 μm output coupler fiber Bragg gratings (OC FBG) instead. The OC FBG provides a reflectivity of ~10% with a 3 dB bandwidth of ~1 nm at the center wavelength of ~1080 nm. To avoid excess heat accumulation at the cladding-mismatched splice point between the 30/600 μm fiber and the 30/400 μm fiber, the cladding light propagating in the inner cladding of 30/600 μm fiber has to be fully stripped out. Therefore, a cladding light stripper (CLS) is utilized before the mismatched splicing point. A length of ~1 m delivery fiber with core/inner cladding diameter of 30/600 μm is spliced between the wide end of tapered gain fiber and the OC FBG. On the 30/600 μm double-cladding delivery fiber, a CLS is realized by removing the polymer cladding of delivery fiber and coating the inner cladding with high refractive index ointment. Thus the cladding light leaks into the air and the temporal characteristic of the stripped cladding light can also be recorded. After the OC FBG, a length of ~5 m 30/400 μm delivery fiber is spliced and an endcap is employed to output the signal laser without facet reflection. Another CLS is also performed on the 30/400 μm delivery fiber to strip the leaked signal laser in the inner cladding. Both CLS are placed on a heat sink to carry away heat accumulation. The core numerical apertures are ~0.065 for both the 30/600 μm and 30/400 μm delivery fibers. In the experiment, the output powers, optical spectra, laser beam quality and temporal characteristics of output laser are recorded. Moreover, the temporal characteristics of cladding light at the CLS are also measured, as a supporting evidence of the occurrence of TMI.
Fig. 1 Experimental setup of the monolithic tapered fiber laser oscillator (TYDF: tapered ytterbium-doped fiber, CLS: cladding light stripper, CO: collimator, HR: high reflector, PD: photodetector, LQM: laser quality monitor).
3. Results and discussion
In the experiment, we investigate the performances of the monolithic tapered fiber laser oscillator, especially on the aspect of TMI threshold, which is crucial to estimate the power scaling ability of the oscillator. The tapered fiber laser oscillator is pumped by the 976 nm and 915 nm LDs, respectively. Compared with the 976 nm pump, the 915 nm pump can enhance the TMI threshold of the fiber laser oscillators, thus enables further power scaling of the tapered fiber laser oscillators.
3.1 Performance of tapered fiber laser oscillator with 976 nm-pumping
The performance of the tapered fiber laser oscillator with 976 nm pumping is firstly measured. In the power scaling process, the output power and corresponding temporal signals of the output laser are recorded. The standard deviations of the temporal signals are calculated to quantify the stability of the temporal signals [25]. The output power increases with the pump power at a slope efficiency of ~60%, as shown in Fig. 2(a). A maximum output power of 950 W is achieved at the pump power of ~1.6 kW. As shown in Fig. 2(a), the standard deviations of the temporal signals increase remarkably as the output power exceeds ~860 W. In the experiment, the temporal characteristics of the dumped cladding light are also recorded, as a supplementary proof of the evidence of TMI. When the TMI occurs in the fiber laser oscillator, there is dynamical coupling between the fundamental mode and the high-order modes. Due to the fiber coiling, a certain proportion of the high-order modes experience higher bending loss and leak into the inner cladding, which can be detected by the photodetector. Figures 2(b)-2(d) show the temporal signals and corresponding Fourier spectra of the dumped cladding light at the operation of 805 W, ~860 W and ~950 W. At the operation of 805 W, the time domain signal is quite stable, and there is no notable frequency component in the corresponding frequency domain. When the output laser power reaches ~860 W, periodic fluctuation of the time domain signal is observed, and a frequency component at ~3.7 kHz appears in the corresponding frequency domain. When the output laser power is further scaled to ~950 W, the temporal fluctuation grows more intense in the time domain and the frequency components broaden remarkably in the frequency domain. Since the recorded temporal signals reflect the dynamic characteristics of mode coupling in the tapered fiber, it is obvious that the Fourier spectra reveal the mode coupling frequencies of the TMI. The evolution of the temporal signals shown in Figs. 2(b)-2(d) are consistent with the typical stable region, transitory region and chaotic region in the route to TMI [25,26]. With the measured temporal characteristics, the occurrence of the TMI in the tapered ytterbium-doped fiber laser oscillator can be validated at the output power of ~860 W, when the 976 nm LDs are employed as the pump.
Fig. 2 (a) output laser power and standard deviation of the temporal signals of the output laser at different pump powers, (b)-(d) temporal signals and corresponding Fourier spectra at the operation of 805 W, ~860 W and ~950 W.
The beam quality of the output laser is measured with a laser quality monitor, and the M2 factors of the output laser at different power levels are shown in Fig. 3(a). The M2 factors of output laser are around ~2.0 at both axes in the power scaling. Even at low power levels far below the TMI threshold, there are high-order modes contained in the output laser beam, judging from the measured M2 factors. This is mainly due to the adoption of multimode delivery fiber with a V-parameter of ~5.7. The imperfect splicing between the delivery fibers and the modes deformation in the delivery fibers, cause the coupling of high-order modes and the beam quality deterioration of the output laser. Although there is dynamic mode coupling between the fundamental mode and the high-order modes as the TMI occurs, the measured M2 factors of the output laser show little further deterioration. The optical spectra of the output laser at different power levels are depicted in Fig. 3(b). It is obvious that the bandwidth of the signal laser broadens with the increase of output power. At the operation of ~950 W, the laser operates at the center wavelength of ~1080 nm with a full width at half maximum (FWHM) of ~3 nm. No pump light or Raman Stokes light is observed in the optical spectra.
Fig. 3 (a) beam quality of the output laser at different power levels, (b) optical spectra of the output laser at different power levels.
3.2 Performance of tapered fiber laser oscillator with 915 nm-pumping
To enhance the TMI threshold of the tapered fiber laser oscillator, the 915 nm LDs are employed as pump instead of the 976 nm LDs [27,28]. For the identical ytterbium-doped fiber, the absorption of the 915 nm pump is much weaker than the 976 nm pump and the total absorption coefficient is less than 10 dB, which induces a lower slope efficiency and a much stronger intensity of the stripped cladding light at the CLS. In the power scaling process, the output power dependence on the pump power is shown in Fig. 4(a). The output power increases with the pump power at a slope efficiency of 56%. A maximum output power of 1720 W is achieved at the pump power of ~3.1 kW. The decreased optical efficiency is mainly due to the inadequate pump absorption. In the experiment, the temporal signals of the output laser at different power levels are recorded and the standard deviations of the temporal signal are calculated to quantify the temporal stability, which are also shown in Fig. 4(a). The standard deviations of the temporal signals increase remarkably as the output laser power exceeds ~1350 W. The temporal signals of the stripped cladding light are also recorded, and the corresponding Fourier spectra are obtained by performing the Fourier transform on the temporal signals, which are shown in Figs. 4(b)-4(d). At the operation of 1270 W, the time domain signal of the stripped cladding light is quite stable, and there is no notable frequency component in the corresponding Fourier spectrum. When the output laser power reaches ~1350 W, remarkable fluctuation of the stripped cladding light is observed. In the corresponding Fourier spectrum, featured modulation frequency components are at ~kHz levels. When the output laser power increases to ~1480 W, the temporal fluctuation of the stripped cladding light grows stronger, and the corresponding modulation frequency components broadens in the Fourier spectrum. It is an interesting phenomenon that the modulation frequencies of TMI are different when respective 976 nm and 915 nm pump is employed. Since the laser oscillation cavity maintains unchanged and only the pump LDs are switched, the differences in modulation frequencies are mainly due to the heat load distribution changes in the tapered gain fiber, which is correlated with the formation of thermally-induced refractive index gratings. Judging from the temporal characteristics of the output laser and the stripped cladding light, it is verified that the TMI occurs at the output power of ~1350 W when the 915 nm LDs are employed as the pump.
Fig. 4 (a) output laser power and standard deviation of the temporal signals of the output laser at different pump powers, (b)-(d) temporal signals and corresponding Fourier spectra at the operation of ~1270 W, ~1350 W and ~1480 W.
The optical spectra of the output laser at different power levels are depicted in Fig. 5(a). At the operation of ~1.7 kW, the output laser operates at the center wavelength of ~1080 nm with a FWHM of ~3.6 nm. Although the tapered ytterbium-doped fiber has a total length of ~33 m, there is no Raman Stokes light observed in the optical spectrum at ~1.7 kW. The beam quality of the output laser at ~1.7 kW is measured, and the beam profile at the beam waist is shown in Fig. 5(b). The M2 factor of the output laser is ~2.1, indicating a certain amount of high-order modes are contained in the output laser. Since the wide end of the tapered ytterbium-doped fiber, the OC FBG and the delivery fibers have core diameter of ~30 μm, imperfect splicing between the fibers and mode deformation in the delivery fibers induce the excitation of the high-order modes inevitably. To achieve near single mode laser output, some improvements of the monolithic tapered fiber laser oscillator are required, which include reducing the NA of the tapered ytterbium-doped fiber and employing low NA delivery fiber and OC FBG with matched mode area.
Fig. 5 (a) optical spectra of the output laser at different power levels, (b) the laser beam quality of the output laser at ~1.7 kW.
Although the TMI threshold is enhanced to ~1.35 kW by employing the 915 nm pump, the limitation of TMI has to be further mitigated in the tapered fiber laser oscillator. Optimizing the parameters of the tapered ytterbium-doped fiber are the most promising techniques, which include reducing the core NA of the tapered fiber, further reducing core diameter of the narrow end of the tapered fiber and optimizing the ytterbium-doped region. Moreover, the bidirectional pump configuration can also be employed to further enhance the TMI threshold and enable further power scaling.
4. Conclusion
In summary, we have constructed a monolithic tapered ytterbium-doped fiber laser oscillator and investigated the performance of the output laser with respective 976 nm and 915 nm pump, especially on the aspects of the TMI. The double cladding tapered ytterbium-doped fiber had a narrow end of ~20/400 μm and a wide end of ~30/600 μm. HR FBG and OC FBG were respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm, to match with the narrow and wide end of the tapered ytterbium-doped fiber. When 976 nm pump was employed, the TMI occurred at the output power of ~860 W and detailed temporal characteristics were described. When 915 nm pump was employed, the TMI occurred at the output power of ~1350 W and the output power was further scaled to a maximum of 1720 W. At the operation of ~1.7 kW, the M2 factor of the output laser was ~2.1 and the FWHM of the signal laser was ~3.6 nm. To the best of our knowledge, this is the highest average power for the tapered ytterbium-doped fiber lasers.
Funding
National Natural Science Foundation of China (61735007, 61505260); National Key Research and Development Program of Ministry of Science and Technology of China (2016YFB0402204).
Acknowledgments
The authors wish to thank Mr. Jiawei He, Mr. Zhaokai Lou, Mr. Yun Ye, and Miss Qiong Gao for help in measuring the performance of the laser oscillator in the experiment. Portions of this work were presented at the Advanced Solid State Laser Conference in 2018, AM6A.24.
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