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Abstract
In this work, a large-mode-area (LMA) step-index constant-cladding tapered-core (CCTC) Yb-doped fiber with a cladding diameter of ∼600 µm is successfully fabricated. The CCTC fiber has a small-core region (diameter of ∼20 µm) at both ends and a large-core region (diameter of ∼36 µm) in the middle. To prove the laser performance of the CCTC fiber, a detailed comparison experiment with conventional uniform fiber with the same effective core diameter is carried out in a multi-kW all-fiber MOPA configuration. The experimental results show that employing the CCTC fiber can effectively mitigate the thermally-induced transverse mode instability (TMI) in both co-pump and counter-pump schemes, and realize high slope efficiency and single-mode beam quality (M2∼1.30). Under the counter-pump scheme, the TMI threshold of the CCTC fiber is observed at ∼2.49 kW with a slope efficiency of 86.2%, while the uniform fiber amplifier exhibits a TMI threshold of ∼2.05 kW. The theoretical analysis based on a semi-analytical model indicates this CCTC fiber can effectively improve the TMI threshold owing to a stronger gain saturation. Our results verify the great potential of such an LMA CCTC fiber to mitigate thermal-induced TMI effect and achieve single-mode operation without sacrifice of laser efficiency in high power monolithic fiber lasers, and the further power scaling is expected by optimizing the fiber design.
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1. Introduction
Fiber lasers have acquired a solid reputation in the laser world, owing to the great advantages such as high conversion efficiency, excellent beam quality, remarkable power scalability and enhanced flexibility [1,2]. Over the past decades, the average power of the single-mode Yb-doped fiber (YDF) lasers has witnessed a dramatic boost in the continuous-wave (CW) regime, benefitting from the development of the double-clad fiber fabrication technology [3–8]. Up to now, the conventional large-mode-area (LMA) fibers with a step-index profile are widely used as a gain medium in high-power fiber lasers due to the simple manufacturing process and packaging. However, with the increasing of output power, the occurrence of stimulated Raman scattering (SRS) [9] and thermally-induced transverse mode instability (TMI) [10–13] have become the main limitations for power scaling in CW broadband fiber lasers. In a conventional step-index fiber (SIF), the SRS effect could be effectively suppressed by increasing the core diameter to enlarge the mode-field area, while it would inevitably support more high-order modes (HOMs) and raise the risk of TMI occurrence [11]. To address the nonlinear effects and TMI effect simultaneously, one of the most promising solutions is to design special LMA active fibers with effective single-mode operation. Various new types of active fibers have been proposed to scale the effective mode-field area and maintain single-mode feature, including photonic crystal fiber [14], leakage-channel fiber [15], large-pitch fiber [16], chirally-coupled-core fiber [17], multi-trench fiber [18], all-solid photonic bandgap fiber [19] and so on. Overall, these special fiber designs, aiming to design the transverse waveguide structure and strengthen the loss of HOMs, seem to be the robust way to raise the TMI threshold in high-power fiber lasers. However, these special fibers have relatively complex structures compared to the SIF, which are restricted by complicated fabrication techniques, as well as free-space configuration.
Besides the fiber with special transversal structure, the tapered double-clad fiber (T-DCF) with a longitudinally varying core diameter has been developed and extensively investigated, thanks to the relatively simple structure and convenient fabrication process [20–26]. The small-core region of the T-DCF can be designed to control the number of supported modes and thus improve the TMI threshold, while the large-core region can increase the effective mode-filed area and help to raise the nonlinear effect threshold [27]. So far, many T-DCFs have been reported in the CW fiber amplifiers and lasers. In 2010, Filippov et al. built a CW tapered fiber laser through free-space mirrors and realized an output power of 750 W with a slope efficiency of 81.9% and beam quality of M2∼1.7 [21]. In 2013, Trikshev et al. demonstrated a 160 W single-frequency laser based on a T-DCF with a core/cladding diameter of 44/700 µm and 7.5/120 µm at the wide and narrow ends, respectively [22]. In 2019, our research group reported an all-fiber oscillator [25] and amplifier [26] based on a long tapered active fiber, respectively, which has a core/cladding diameter of ∼20/400 µm at narrow end and ∼30/600 µm at wide end. An output power of 1.75 kW and 2.17 kW in the oscillator and the amplifier has been achieved respectively, but these two experiments have obtained a poor beam quality (M2 factors were over 2.0) at the maximum output power.
To improve the beam quality and further scale the output power, a modified tapered fiber with a spindle-shaped core have been proposed and demonstrated recently [28,29], which is also known as the double-tapered YDF. In 2020, Zeng et al. reported an all-fiber oscillator based on a longitudinally spindle-shaped YDF and achieved a single-mode output power of 3 kW under the bidirectional pumping scheme [28]. Later, a spindle-shaped YDF with a larger core diameter has been applied in the monolithic fiber amplifier, and realized 5 kW output power with a beam quality of ∼1.9 and conversion efficiency of 66.6% [29]. It should be noted that the core and cladding diameters of the above tapered fibers vary simultaneously in the longitudinal direction, so the core-to-cladding diameter ratios (CCDRs) of these fibers remain constant along the fiber length, and the heat profile is exponential-like as well as its pump power profile [30]. Recently, another kind of active tapered fiber, with a constant cladding and tapered core profile was proposed and fabricated [31–33]. This fiber has a gradually varying CCDR in the longitudinal direction and leads to a smoother thermal load along the fiber, which may help to mitigate the TMI in fiber lasers. However, the presented experimental results, such as slope efficiency and beam quality are not ideal, and the detailed TMI characteristics of this fiber are rarely reported to date.
In this contribution, we demonstrate an LMA step-index tapered-core YDF with a constant cladding diameter along the fiber, called constant-cladding tapered-core (CCTC) fiber. The fabricated CCTC fiber has a small-core region at both ends with a core diameter of ∼20.8 and ∼20.3 µm respectively, and a large-core region with a core diameter of ∼36.0 µm in the middle. The inner-cladding diameter is kept constant at ∼600 µm. Based on an all-fiberized master oscillator power amplifier (MOPA) setup operating at ∼1080 nm, the laser performances of this CCTC fiber have been systematically investigated. By comparing with the conventional uniform fiber (core/cladding diameter of ∼28/600 µm) sharing the same effective core diameter, the experimental results manifest that the CCTC fiber can effectively improve the TMI threshold, and obtain a high slope efficiency and single-mode beam quality. In addition, a semi-analytical model is adopted to theoretically analyze the TMI threshold of both fibers. This work provides an effective solution of active tapered fiber design to mitigate TMI effect and realize single-mode operation as well as high conversion efficiency in high power monolithic fiber amplifiers.
2. Fiber design and characterization
The CCTC fiber can be successfully fabricated by conventional modified chemical vapor deposition (MCVD) process combined with solution doping technique, as shown in Ref. [33]. The fabrication process of CCTC fiber mainly includes the following steps. Firstly, the oxyhydrogen flame is used as a heating source to heat a certain point on the ready preform, both ends of the preform are simultaneously pulled to form a cone in opposite direction at the same speed, repeat the fabrication of multiple cones on the preform. Secondly, the stretched preform is milled into a round shape and jacketed with suitable silica tube to form a resulted preform that can fully meet the CCDR design requirement. Thirdly, the fiber preform is milled into an octagonal shape to enhance cladding absorption, and then drawn into the fiber at invariant drawing speed and coated with a low-index acrylic coating to form a double clad fiber. Finally, a CCTC fiber is picked from the drawn fibers based on the position of the cone in the preform.
In this work, we fabricated the CCTC fiber with a large cladding diameter of ∼600 µm, and the core diameter distribution along the fiber length are shown in Fig. 1(a), which is followed a parabolic model [34]. The CCTC fiber can be divided into three regions in the longitudinal direction, including two gradually-varying tapered regions (region I and III) at both ends, and a large-core region (region II) in the middle. The core diameter of region I gradually changes from 20.8 µm to 36.0 µm, and the region II has an invariant core diameter of ∼36 µm, while the core diameter of region III varies from 36.0 µm to 20.3 µm. The lengths of these three regions are about 11.1 m, 6 m and 11.4 m. The cross-sections of the small-core end and the large core in the middle are shown in the insets of Fig. 1(a), respectively. The measured absorption cross section of this CCTC fiber is shown in Fig. 1(b), and the average absorption coefficient is measured to be ∼0.78 dB/m at 976 nm. The effective core diameter of the CCTC fiber can be defined as follows [21]:
where D(z) is the core diameter of the CCTC fiber along the fiber length, L is the length of the fiber. Here the calculated Deff of this CCTC fiber is about 28 µm.
Fig. 1. (a) The core diameter distribution of the fabricated CCTC fiber (the insets are the cross-sections of small and large core), (b) the measured absorption cross section of the CCTC fiber.
For a reasonable comparison with this CCTC fiber, a conventional uniform double-clad fiber with a core/cladding diameter of 28/600 µm is employed in the experiment. The uniform fiber is manufactured from the same preform with the CCTC fiber, which can rule out the influence of the fiber material on the experimental results. The fabrication process of the uniform fiber is straightforward, where the ready fiber is jacketed with silica tube to ensure that the CCDR meets the design requirement, then milled into an octagonal shape and drawn into the fiber. Hence, such a fiber design can ensure that the fabricated fibers have almost the same average absorption coefficient, thereby eliminating the effect caused by the different absorption lengths [35]. The main parameters of the CCTC fiber and the uniform fiber are listed in Table 1. Both fibers have the identical numerical aperture (NA) of ∼0.065. The measured average absorption coefficient of the uniform fiber is about 0.8 dB/m, which is close to the fabricated CCTC fiber. To ensure the same total pump absorption in the MOPA setup, the adopted fiber lengths of both YDFs in the experiment are about 28.5 m and 27.8 m, respectively.
Table 1. Fiber parameters
3. Experimental setup
To directly characterize the laser performance of these two fibers, an all-fiber MOPA platform is constructed as depicted in Fig. 2. The seed laser is a linear cavity broadband laser oscillator operating at a central wavelength of ∼1080 nm. The active fiber adopted in the seed laser is commercial double-clad fiber with a core/cladding diameter of 20/400 µm. The seed laser is controlled to generate output power of ∼50 W into the power amplifier stage. To remove the residual pump light in the cladding, a home-made cladding light stripper (CLS) is performed on the passive fiber before injecting into the amplifier stage.
Fig. 2. Experimental setup of the all-fiber MOPA laser with bidirectional pump configuration. (a) Experimental setup, (b) schematic diagram of the fiber groove. (The measuring system in the inset, including CO: collimator, HR: high-reflectivity mirror, PM: power meter, OSA: optical spectrum analyzer (Yokogawa, 600-1700nm), PD: photodetector (150 MHz, 700-1800nm)), DM: dichroic mirror, BQA: beam quality analyzer (Beam Squared, Ophir)).
The power amplifier is built as a bidirectional pumping configuration via two (6 + 1)×1 pump/signal combiners, which serve as the forward combiner and backward combiner, respectively. Both combiners have six multimode pump ports with core/cladding diameters of 220/242 µm and a core NA of 0.22. Through the pump ports, four LD modules used for forward pump power and six LD modules used for backward pump power, where each LD module emits at a non-stabilized wavelength of 976 nm and can launch a maximum power of ∼700 W. The signal ports of the forward combiner are 20/400 µm and 20/250 µm double-clad fiber with a core NA of 0.06. To match with two different core sizes of active fiber, the signal input and output fibers of the backward combiner are 30/400 µm and 30/250 µm, respectively. The active fiber in the amplifier stage is the CCTC fiber and conventional uniform fiber, respectively, where the schematics of both YDFs are shown in the inset of Fig. 2(a). For each measurement, the active fiber is coiled on the water-cooled plate with an “8”-shaped fiber groove as shown in Fig. 2(b), which has a total length of 30 m. The fiber groove has a minimum coiling diameter of 12 cm in both input and output end, which could ensure that the small-core regions at both ends of the CCTC fiber are coiled in a small bending diameter and implement an effective mode control. In the experiment, the bending diameter of the CCTC fiber ranges from 12 cm to 18.3 cm to 12.5 cm, and that of the uniform fiber ranges from 12 cm to 18.3 cm to 12.9 cm. We kept the same bending conditions as much as possible to reduce the influence of the fiber coiling, and make sure that the advantages and disadvantages of the both fibers can be most truly reflected [35]. An about 4-m-long passive fiber with core/cladding diameter of 30/400 µm is spliced to deliver the signal power. A home-made CLS is utilized on the passive fiber to strip the unwanted cladding light and a commercial quartz block holder (QBH) is used to avoid harmful feedback. All components are fixed on the water-cooled plates to achieve an efficient thermal management.
4. Results and discussion
4.1. Experimental results
The output power characteristics of the amplifiers based on these two fibers have been studied under both co-pump and counter-pump schemes, respectively. As shown in Fig. 3(a), the output power has a good linear relationship with respect to the launched pump power in the co-pump scheme. The slope efficiency of both CCTC fiber and uniform fiber amplifier is ∼80.9% and ∼71.6%, respectively. It can be seen that the CCTC fiber amplifier presents a higher slope efficiency than that of the uniform fiber amplifier. The higher laser efficiency is mainly attributed to that the CCTC fiber has a gradually-varying tapered region at both fiber ends, which can effectively suppress the HOMs generation, so that few HOMs power leaks into the cladding. Furthermore, the relatively poor beam quality of uniform fiber in the following measurement indicates that there are more HOMs filtered out by CLS, which can reduce the slope efficiency of the uniform fiber amplifier. In the counter-pumping case, the output power versus the pump power is depicted in Fig. 3(b). The CCTC fiber amplifier achieves a maximum output power of about 2.5 kW with a high slope efficiency of ∼86.2%, while the uniform fiber amplifier has a slope efficiency of ∼83.4%. However, it is found that both fiber amplifiers suffer a power roll-over in the co-pump and counter-pump schemes, and the corresponding optical-to-optical efficiency encounters a significant decrease in the meantime, which implies the occurrence of TMI effect [5].
Fig. 3. The output power characteristics and corresponding optical-to-optical efficiency of both fiber amplifiers, (a) in co-pump scheme, (b) in counter-pump scheme.
Then, to validate the onset of TMI, the temporal signals of the output laser of both fiber amplifiers were measured respectively through a photodetector (PD) connected to an oscilloscope. The standard deviation (STD) of the temporal signals can be employed to quantify the temporal stability and verify the TMI threshold, as explained in [35–37]. With the increase in output power, it can be seen that the STD raised abruptly when the output power reached certain value in both CCTC fiber and uniform fiber amplifier, as shown in Fig. 4(a), (b). It is found that the rapid increase in the STD matched the power roll-over shown in Fig. 3, confirming the occurrence of TMI. As introduced in [37], the TMI threshold can be defined as the output power of more than twofold increase of the average standard deviation. Applying this definition, in the CCTC fiber amplifier, the TMI thresholds of co-pump and counter-pump schemes are respective around 1324 W and 2494 W, while TMI thresholds of the uniform fiber amplifier are around 1135 W and 2056 W respectively. The latter two graphs of Fig. 4 depict 100-ms of the normalized time traces at power levels around the TMI threshold in counter-pumped CCTC fiber amplifier and its corresponding Fourier spectra, which are shown as examples to prove that the observed effect is indeed TMI. When the output power reaches 2494 W (red curve), the temporal signal fluctuation can be clearly observed, and the fluctuation frequency components distribute in the range of 0∼3 kHz, which are the typical TMI characteristics [36]. Moreover, with the output power further increasing (green curve), the temporal fluctuation grows more intense and the frequency components broaden remarkably in the frequency domain. From the comparison of the TMI threshold, one can conclude that the CCTC fiber presents a higher TMI threshold than that of the uniform fiber in the condition of the same effective core diameter. Furthermore, the TMI threshold of the CCTC fiber amplifier under the counter-pump scheme is significantly higher than that of the co-pump scheme, which benefits from the small-size tapered region in the output end of the CCTC fiber. This causes higher gain saturation and enhancement of the TMI threshold [12].
Fig. 4. (a) The STD of normalized PD-signal of the CCTC fiber amplifier under co-pump and counter-pump schemes. (b) The STD of normalized PD-signal of the uniform fiber amplifier under co-pump and counter-pump schemes, (c) the normalized temporal signal at power levels around the TMI threshold in counter-pumped CCTC fiber amplifier, (d) its corresponding Fourier spectra.
In the experiment, the beam quality (M2 factor) of both fiber amplifiers at different power levels were measured with a beam quality analyzer, and illustrated in Fig. 5(a). Each M2 factor is averaged in the x and y directions. In the uniform fiber amplifier, it can be found that the value of M2 factor experiences a sharp increase as the output power raise from 1009 W to 1135 W in the co-pump scheme and from 2025 W to 2105 W in counter-pump scheme, respectively. Furthermore, when the output power is increased continuously, the beam quality is more obviously degraded, and the beam profile became irregular. The rapid degeneration in the beam quality of the uniform fiber amplifier is consistent with the sudden increase of STD [as shown in Fig. 4(b)], which further confirms the occurrence of TMI. However, the evolution of M2 value of the CCTC fiber amplifier is different from that of uniform fiber amplifier. The measured M2 factors in both co-pump and counter-pump schemes almost maintain about 1.3 without any deterioration in beam quality, indicating a single-mode laser output in the CCTC fiber amplifier. Compared to the uniform fiber amplifier, the CCTC fiber amplifier has a better beam quality in spite of the same experimental parameters, and preserve single-mode operation during the power scaling. This is mainly due to that the CCTC fiber has a small-sized tapered region at both ends, which can effectively control the guided-modes, and maintain single-mode beam quality through appropriate fiber coiling. It is worth noting that the beam quality of the CCTC fiber amplifier still maintains M2∼1.30 at the output power of 2529 W and the beam profile is not obvious distorted, as shown in Fig. 5(b), although the TMI effect is observed in the time traces. It is because that at this power point, the TMI-induced HOMs contents are minor and the coupled HOMs get dumped by the CLS, which results in a better beam quality in the output laser.
Fig. 5. (a) The beam quality evolution during the power scaling of both fiber amplifiers, (b) the M2 factor and beam profile at an output power of 2.5 kW.
In addition, to characterize the strength of nonlinear effects of both fiber amplifiers, the spectral linewidth evolution during the power scaling is calculated. As shown in Fig. 6(a), the 3 dB spectral linewidths of the both fiber amplifiers under the co-pump and counter-pump scheme broadens with the growth of output power. Due to the nearly same fiber length and equivalent core area, the spectral broadening of the two fiber lasers is almost the same at this power level. The output spectra of the CCTC fiber amplifier during the power scaling are shown in Fig. 6(b). It is shown that the spectra are clean without any sign of residual pump light, owing to sufficient pump absorption and efficient cladding light filtering. At the maximum output power of 2529 W, the signal laser centers at the wavelength of 1080.28 nm with a full width at half maximum (FWHM) of 2.78 nm, and no Raman Stokes light is observed at this power level, which means a promising prospect of the CCTC fiber in SRS suppression.
Fig. 6. (a) The spectral linewidth evolution of both fiber amplifiers during the power scaling, (b) the output spectra of the CCTC fiber amplifier under counter-pump scheme.
4.2. Discussion
From the above experimental results, it can be found that the CCTC fiber has a great performance in high power fiber lasers due to the unique longitudinal geometrical profile. It has been shown that employing the CCTC fiber can effectively improve the TMI threshold in both co-pump and counter-pump schemes compared to the conventional uniform fiber, and maintain excellent beam quality without sacrifice of laser efficiency. With a comparable experimental parameter as the uniform fiber amplifier sharing the almost same bending condition, the TMI threshold of CCTC fiber amplifier can be enhanced by a factor of 16.7% and 21.3% in the co-pump and counter-pump schemes, respectively. To further discuss the TMI effect of the CCTC fiber and uniform fiber amplifier, a semi-analytical model reported in [38] is adopted to theoretically illustrate and analyze the TMI threshold.
In the model, we define the core diameter of the small-core region at both ends and large-core region in the middle of the CCTC fiber as D1 and D2, respectively. Assume that the lengths of the three regions of CCTC fiber are L1, L2, L3, respectively, then the core diameter distribution of the CCTC fiber is given as follows,
Based on the semi-analytical model of TMI in fiber amplifiers, the growth of the HOM power ratio can be written as the following expression,
Table 2. Parameters used for the simulation
In order to meet the experimental fiber parameters as much as possible, the D1 and D2 of the CCTC fiber in the simulation is 20 µm and 36 µm, respectively. The length of the CCTC fiber is 28 m, in which the lengths of the three regions are 11 m, 6 m, 11 m, respectively. The initial signal power is 50 W. For these two fiber amplifiers, the nonlinear coupling coefficients under the co-pump and counter-pump schemes are calculated and illustrated in Fig. 7(a). It is clear that the coupling coefficient of both fiber amplifiers under the co-pump scheme is much higher than that of the counter-pump scheme. The coupling coefficient in the co-pump scheme is remarkably high in the front of YDF, while the coupling coefficient in the counter-pump scheme is almost smoothing along the fiber length owing to a strong degree of population inversion saturation, which is consistent with the previous report [39]. Accordingly, compared to the co-pump scheme, the counter-pump scheme of both fiber amplifier has a higher TMI threshold due to the stronger gain saturation [12,39], which agrees well with the experimental result. Moreover, the HOM power ratio evolution versus the output power under the co-pump scheme of both fiber amplifiers is plotted in Fig. 7(b). The TMI threshold is regarded as the signal power at which the HOM power ratio reaches 5% [38], which means that ξ(z) = 0.05. It can be seen that the TMI threshold of both fiber amplifiers is ∼1319 W and ∼1167 W, respectively, indicating that the theoretical TMI threshold of CCTC fiber amplifier is higher than that of the uniform fiber amplifier.
Fig. 7. (a) The coupling coefficients of the CCTC fiber and uniform fiber amplifier under the co-pump and counter-pump schemes, (b) HOM power ratio of both CCTC fiber and uniform fiber amplifier under the co-pump scheme.
In addition, the TMI threshold powers of both fiber amplifiers with various pump power distribution are also calculated and shown in Fig. 8. It can be found that under different pump power distribution, the CCTC fiber amplifier holds a higher TMI threshold than that of the uniform fiber amplifier. Moreover, the TMI thresholds in both fiber amplifiers increase as backward pump power ratio increases until it reaches the maximum point, and then decrease with the backward pump power ratio increasing, which indicates that there is an optimal backward pump power fraction for both fiber amplifiers. The reason for the higher TMI threshold is that this CCTC fiber has a smaller core area at the input and output ends compared to the uniform fiber, which causes the population inversion saturation strengthens and the value of nonlinear coupling coefficient falls, thus improving the TMI threshold.
Fig. 8. The TMI threshold powers as a function of backward pump power ratio in both CCTC fiber and uniform fiber amplifiers.
From all of the above, both theoretical analysis and experimental results show the CCTC fiber exhibits a higher TMI threshold and superior beam quality than that of the conventional uniform fiber sharing the same effective core area. Compared with the traditional tapered fiber whose cladding diameter changes simultaneously [25–29], the CCTC fiber can provide a larger cladding pump area and has a flexible designed CCDR along the longitudinal direction. This results in a decreased pump power density injected into both ends of the fiber and a smoother thermal load along the fiber, which could be beneficial to improve the TMI threshold [30]. Compared to the previous CW laser systems with conventional SIFs [6,8], this fiber can realize a well-balanced trade-off between the mitigation techniques of the TMI and SRS for further power scaling. The relatively low threshold power in this work is mainly due to the poor properties of the fiber material. Although the limitation of TMI has been observed at an output power of 2.5 kW in the counter-pumped CCTC fiber amplifier, the higher output power with excellent beam quality is promising to be achieved by further optimizing the fiber material, fiber designs and laser systems. For instance, reducing the core NA and optimizing the ytterbium-doped profile is helpful to reduce HOMs gain. Besides, the fiber bending and pump wavelength can be further optimized to raise the TMI threshold.
5. Conclusion
In summary, we have successfully fabricated and demonstrated an LMA step-index CCTC YDF with a constant cladding diameter of ∼600 µm. The CCTC fiber has a small-core tapered region at both ends with a core diameter of ∼20.8 µm and ∼20.3 µm, respectively, and a large-core region with a core diameter of ∼36.0 µm in the middle. The laser performance of this fiber is carefully investigated in a high power all-fiber MOPA configuration by comparing with a conventional uniform fiber. The experimental results indicate that the CCTC fiber can exhibit a higher TMI threshold in both co-pump and counter-pump schemes than that of the conventional uniform fiber, while maintaining a single-mode beam quality and high slope efficiency in the meantime. Specifically, the CCTC fiber amplifier can achieve a maximum output power ∼2.5 kW with a high slope efficiency of ∼86.2% and an excellent beam quality of M2 ∼1.30. The numerical analysis shows this CCTC fiber holds a stronger degree of saturation and is beneficial to raise the TMI threshold. These results exhibit the potential of the CCTC YDFs to overcome the conventional tradeoff between beam quality, LMA, and high laser efficiency in order to enable further power scaling in high-power single-mode monolithic fiber lasers.
Funding
Training Program for Excellent Young Innovators of Changsha (kq2106008); National Natural Science Foundation of China (61735007, 62005315).
Acknowledgments
The authors wish to thank Mr. Xiaoyong Xu, Mr. Pengfei Zhong, Mr. Lingfa Zeng, Mr. Yingchao Wan, and Mr. Zhejian Hong for their helpful assistance in the experiment.
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. 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]
2. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7(11), 861–867 (2013). [CrossRef]
3. H. Yu, X. Wang, R. Tao, P. Zhou, and J. Chen, “1.5 kW, near-diffraction-limited, high-efficiency, single-end-pumped all-fiber-integrated laser oscillator,” Appl. Opt. 53(34), 8055 (2014). [CrossRef]
4. Y. Mashiko, H. K. Nguyen, M. Kashiwagi, T. Kitabayashi, K. Shima, and D. Tanaka, “2 kW single-mode fiber laser with 20-m long delivery fiber and high SRS suppression,” Proc. SPIE 9728, 972805 (2016). [CrossRef]
5. 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 (2016). [CrossRef]
6. 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(13), 14892 (2017). [CrossRef]
7. 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]
8. B. Yang, W. Peng, H. Zhang, X. Xi, C. Shi, X. Wang, and X. Xu, “6 kW single mode monolithic fiber laser enabled by effective mitigation of the transverse mode instability,” Opt. Express 29(17), 26366 (2021). [CrossRef]
9. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240 (2008). [CrossRef]
10. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218 (2011). [CrossRef]
11. M. N. Zervas, “Transverse mode instability, thermal lensing and power scaling in Yb3+-doped high-power fiber amplifiers,” Opt. Express 27(13), 19019 (2019). [CrossRef]
12. A. V. Smith and J. J. Smith, “Increasing mode instability thresholds of fiber amplifiers by gain saturation,” Opt. Express 21(13), 15168 (2013). [CrossRef]
13. R. Tao, P. Ma, X. Wang, P. Zhou, and Z. Liu, “1.3 kW monolithic linearly polarized single-mode master oscillator power amplifier and strategies for mitigating mode instabilities,” Photonics Res. 3(3), 86–93 (2015). [CrossRef]
14. M. M. Johansen, K. R. Hansen, M. Laurila, T. T. Alkeskjold, and J. Laegsgaard, “Estimating modal instability threshold for photonic crystal rod fiber amplifiers,” Opt. Express 21(13), 15409 (2013). [CrossRef]
15. L. Dong, H. A. McKay, L. Fu, M. Ohta, A. Marcinkevicius, S. Suzuki, and M. E. Fermann, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding,” Opt. Express 17(11), 8962 (2009). [CrossRef]
16. F. Stutzki, F. Jansen, T. Eidam, A. Steinmetz, C. Jauregui, J. Limpert, and A. Tünnermann, “High average power large-pitch fiber amplifier with robust single-mode operation,” Opt. Lett. 36(5), 689 (2011). [CrossRef]
17. X. Ma, C. Zhu, I. Hu, A. Kaplan, and A. Galvanauskas, “Single-mode chirally-coupled-core fibers with larger than 50µm diameter cores,” Opt. Express 22(8), 9206 (2014). [CrossRef]
18. D. Jain, C. Baskiotis, and J. K. Sahu, “Bending performance of large mode area multi-trench fibers,” Opt. Express 21(22), 26663 (2013). [CrossRef]
19. M. Kashiwagi, K. Saitoh, K. Takenaga, S. Tanigawa, S. Matsuo, and M. Fujimaki, “Effectively single-mode all-solid photonic bandgap fiber with large effective area and low bending loss for compact high-power all-fiber lasers,” Opt. Express 20(14), 15061 (2012). [CrossRef]
20. V. Filippov, Y. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov, “Double clad tapered fiber for high power applications,” Opt. Express 16(3), 1929 (2008). [CrossRef]
21. V. Filippov, J. Kerttula, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Highly efficient 750 W tapered double-clad ytterbium fiber laser,” Opt. Express 18(12), 12499 (2010). [CrossRef]
22. A. I. Trikshev, A. S. Kurkov, V. B. Tsvetkov, S. A. Filatova, J. Kertulla, V. Filippov, Y. K. Chamorovskiy, and Y. K. Chamorovskiy, “A 160 W single-frequency laser based on an active tapered double-clad fiber amplifier,” Laser Phys. Lett. 10(6), 065101 (2013). [CrossRef]
23. K. Bobkov, A. Andrianov, M. Koptev, S. Muravyev, A. Levchenko, V. Velmiskin, S. Aleshkina, S. Semjonov, D. Lipatov, A. Guryanov, A. Kim, and M. Likhachev, “Sub-MW peak power diffraction-limited chirped-pulse monolithic Yb-doped tapered fiber amplifier,” Opt. Express 25(22), 26958 (2017). [CrossRef]
24. A. Fedotov, T. Noronen, R. Gumenyuk, V. Ustimchik, Y. Chamorovskii, K. Golant, M. Odnoblyudov, J. Rissanen, T. Niemi, and V. Filippov, “Ultra-large core birefringent Yb-doped tapered double clad fiber for high power amplifiers,” Opt. Express 26(6), 6581 (2018). [CrossRef]
25. B. Yang, H. Zhang, C. Shi, X. Wang, Z. Pan, Z. Wang, P. Zhou, and X. Xu, “High power monolithic tapered ytterbium-doped fiber laser oscillator,” Opt. Express 27(5), 7585 (2019). [CrossRef]
26. Y. Ye, X. Xi, C. Shi, B. Yang, X. Wang, H. Zhang, P. Zhou, and X. Xu, “Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber,” Laser Phys. Lett. 16(8), 085109 (2019). [CrossRef]
27. V. Filippov, Y. Chamorovskii, J. Kerttula, A. Kholodkov, and O. G. Okhotnikov, “Single-mode 212 W tapered fiber laser pumped by a low-brightness source,” Opt. Lett. 33(13), 1416–1418 (2008). [CrossRef]
28. L. Zeng, X. Xi, Y. Ye, H. Zhang, X. Wang, Z. Pan, Z. Wang, and X. Xu, “Near-single-mode 3 kW monolithic fiber oscillator based on a longitudinally spindle-shaped Yb-doped fiber,” Opt. Lett. 45(20), 5792–5795 (2020). [CrossRef]
29. L. Zeng, 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 homemade spindle-shaped ytterbium-doped fiber,” Opt. Lett. 46(6), 1393–1396 (2021). [CrossRef]
30. Z. Li, Z. Huang, H. Lin, S. Xu, Z. Yang, J. Wang, and F. Jing, “Mitigate transverse mode instability by linear inner-cladding fiber,” Opt. Eng. 56(11), 1 (2017). [CrossRef]
31. X. Lin, Z. Zhang, Y. Chu, Y. Wang, Y. Xing, G. Chen, J. Peng, H. Li, N. Dai, and J. Li, “Fabrication and laser performance of cladding uniform core tapered fiber,” Opt. Fiber Technol. 64, 102561 (2021). [CrossRef]
32. Y. Ye, X. Lin, X. Xi, L. Zeng, B. Yang, H. Zhang, X. Wang, J. Li, and X. Xu, “2 kW monolithic fiber amplifier based on constant-cladding tapered-core Yb-doped fiber,” Proc. SPIE 11717, 130 (2020). [CrossRef]
33. Y. Ye, X. Lin, X. Xi, C. Shi, B. Yang, H. Zhang, X. Wang, J. Li, and X. Xu, “Novel constant-cladding tapered-core ytterbium-doped fiber for high-power fiber laser oscillator,” High Power Laser Sci. Eng. 9(2), e21 (2021). [CrossRef]
34. W. K. Burns, M. Abebe, and C. A. Villarruel, “Parabolic model for shape of fiber taper,” Appl. Opt. 24(17), 2753 (1985). [CrossRef]
35. F. Beier, F. Moller, 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 (2018). [CrossRef]
36. 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 (2012). [CrossRef]
37. M. M. Johansen, M. Laurila, M. D. Maack, D. Noordegraaf, C. Jakobsen, T. T. Alkeskjold, and J. Laegsgaard, “Frequency resolved transverse mode instability in rod fiber amplifiers,” Opt. Express 21(19), 21847 (2013). [CrossRef]
38. R. Tao, P. Ma, X. Wang, P. Zhou, and Z. Liu, “Theoretical study of pump power distribution on modal instabilities in high power fiber amplifiers,” Laser Phys. Lett. 14(2), 025002 (2017). [CrossRef]
39. Z. Li, Z. Huang, X. Xiang, X. Liang, H. Lin, S. Xu, Z. Yang, J. Wang, and F. Jing, “Experimental demonstration of transverse mode instability enhancement by a counter-pumped scheme in a 2 kW all-fiberized laser,” Photonics Res. 5(2), 77–81 (2017). [CrossRef]
