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Abstract
We demonstrate an all-fiber high-power narrow-linewidth fiber laser based on a homemade tapered Yb-doped fiber (T-YDF). The laser performance is investigated and systematically compared with different seed powers and pump manners. The experimental results reveal that the injected seed power requires a trade-off designed to take into account the impact of spectral broadening, nonlinear effects, and transverse mode instability (TMI). Compared with the co-pump manner, the counter-pump manner performs well in inhibiting nonlinearities, spectral broadening, and improving the TMI threshold. Under the counter-pump manner, this narrow-linewidth T-YDF amplifier realized a 2.09 kW output power with a 3 dB spectral linewidth of ∼0.34 nm, a beam quality of M2∼1.28 and a high Raman suppression ratio over 53.5 dB, the highest reported power for such a T-YDF-based narrow-linewidth single-mode laser, to the best of our knowledge. This work provides a promising pathway towards implementing monolithic high-power narrow-linewidth single-mode fiber lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power fiber laser sources feature the prominent advantages of high conversion efficiency, convenient thermal management, flexible operation and compact structure, served as many applications in industrial processing, medical treatment, scientific research and so on [1,2]. Thanks to breakthroughs in techniques involving high-brightness laser diodes (LDs) and high-quality large-mode-area (LMA) fibers, the output power of single-mode fiber lasers has been dramatically developed to tens of kW level with a broad linewidth [3–5] and several kW level with a narrow linewidth [6–8]. To facilitate coherent or spectral beam combination of multiple fiber lasers, fiber laser operating with narrow spectral linewidth is the key element [9]. However, the power scaling of narrow-linewidth fiber lasers has been hampered by several physical issues including various nonlinear effects (stimulated Brillouin and Raman scattering (SBS, SRS)) [1] and thermally induced transverse mode instability (TMI) [10]. As for SBS and SRS effects, enlarging the mode area with larger core sizes is a practical and common route for suppressing these nonlinearities. Nevertheless, a larger core size results in a more transverse mode supported in step-index LMA fibers, which degrades the beam quality and decreases the TMI threshold [6,10]. Consequently, there is an intrinsic contradiction for implementing the comprehensive inhibition of the nonlinear effects and TMI in the traditional step-index fiber laser.
Extensive studies have been carried out to tackle the barrier of nonlinear effects and TMI effect simultaneously, involving optimizing of the seed laser properties [11,12], employing a counter/bidirectional pumping manner [13,14], exploiting special designed active fibers [15–19], etc. With regard to these solutions above, developing specialty LMA fibers with robust single-mode operation is considered as a promising approach to realize comprehensive suppression on nonlinearity and TMI effect, some examples of which are chirally-coupled-core fiber [20], all-solid photonic bandgap fiber [21], large-pitch fiber [22], tapered fiber [23,24] and so on. Among them, tapered Yb-doped fibers (T-YDFs) with variable longitudinal geometry, is an economical and effective option to solve these problem due to their advantages of relatively simple structure, ease of manufacturing and superior inherent characteristics [25–27]. Benefiting from the gradually increasing mode area in the longitudinal direction and a diameter-dependent Brillouin frequency shift, the T-YDF offers a significant merit in SRS and SBS suppression [27,28]. Additionally, the characteristics of light propagation from the thin end to the thick end of T-YDF are conducive to maintaining good beam quality, which is in favor of improving the TMI threshold [25,29].
These outstanding features make T-YDFs an excellent candidate for achieving high-power continuous-wave (CW) narrow-linewidth/single-frequency fiber lasers in the 1 µm spectral region. In 2013, Trikshev et al. reported a 160 W tapered Yb-doped single-frequency fiber laser with a core/cladding diameters of 7.5/120 µm and 44/700 µm at the narrow and wide ports, respectively [30]. In 2018, Huang et al. demonstrated a T-YDF-based all-fiber high-power narrow-linewidth fiber laser, and an output power of 260 W with a 3 dB linewidth of ∼2 GHz and beam quality of M2∼2.26 was obtained [31]. In 2020, Lai et al. realized a 550 W single-frequency fiber amplifiers at 1030 nm by utilizing a LMA T-YDF, and the M2 factor was measured to ∼1.47 [32]. In 2021, Li et al. exhibited a domestic long tapered active fiber varied from 10/155 µm to 26/400 µm, and achieved a maximum output power of 1328 W with narrow 3 dB bandwidth of 0.26 nm, the Raman suppression ratio is 34.7 dB [33]. Furthermore, a full output power of 4180 W with a linewidth of ∼0.59 nm was obtained by employing a novel double-tapered Yb-doped fiber and phase modulated single-frequency laser (PMSFL) seed in 2022 [34]. These presented results show the considerable potential of LMA T-YDFs for power handling of high-average-power CW fiber lasers. However, the exploration and investigation of the T-YDF enabled all-fiber narrow-linewidth single-mode laser is still insufficient so far, the systematical investigation has not been conducted in T-YDF-based narrow linewidth fiber lasers.
In this work, we have introduced a high-power narrow-linewidth fiber amplifier based on a homemade LMA T-YDF, of which has a core/cladding diameter of ∼20/400 µm and ∼30/600 µm in the narrow and wide end, respectively. A fiber oscillator based on narrow-bandwidth fiber Bragg gratings (FBGs) is act as seed laser to suppress the SBS effect [35–37]. The laser performance of this T-YDF-based narrow-linewidth MOPA laser for different pump manners and injected seed powers is comprehensively studied, especially on the aspects of SRS and TMI. The results indicate that the counter-pumping manner exhibits a better performance with higher TMI threshold, higher SRS suppression and narrower spectral linewidth than that of the co-pumping manner. Specifically, under the counter-pump scheme, an output power over 2 kW with a 3 dB linewidth of ∼0.34 nm is achieved, the M2 factor is about 1.28 and SRS suppression ratio is higher than 53.5 dB.
2. Experimental system implementation
2.1 Fiber fabrication and characterization
The Yb-doped fiber preform is fabricated through the modified chemical vapor deposition (MCVD) combined with chelate gas deposition technique. The tapered optical fiber is obtained by controlling the drawing speed with a certain constant acceleration during the drawing process of preform. The relationship between the drawing speed of the fiber and the cladding diameter can be expressed as follow:
where K is the proportion coefficient related to the drawing tower, v(z,t) is the drawing speed of the fiber at different time t and different position z, D(z,t) represents the cladding diameter of the tapered fiber at different positions. To suppress the nonlinear effects and maintain good beam quality, in our work, the tapered active fiber is designed to the core/cladding diameter of ∼20/400 µm and ∼30/600 µm in the narrow and wide ends, respectively. The core size distribution of T-YDF along the length is shown in Fig. 1(a). It can be seen that the T-YDF has a total length of ∼8.6 m, and can be divided into three regions along the fiber axis, where the tapered region is as long as 3 m located in the middle region with approximately linear longitudinal profile. The narrow end this T-YDF has a smooth region with length of ∼3.6 m, which is designed to inhibit the generation of high-order modes (HOMs) and improve the beam quality. The wide end owns a smooth region of ∼2 m to mitigate the SRS effect as much as possible. The cross-section photograph of the narrow and wide end of this fiber is shown in the inset of Fig. 1(a), indicating the narrow and wide ends have a core/cladding diameter of 20.02/402.13 µm and 29.86/605.02 µm, respectively. By means of the cut-off method, the cladding pump absorption spectrum of this T-YDF was measured by cutting the narrow-end invariant region, and the result was illustrated in Fig. 1(b). It shows the absorption peak coefficient is about 0.39 dB/m while pumped by 915 nm and 1.32 dB/m at 976 nm, exhibiting a typical absorption spectrum of Yb-doped fiber.
Fig. 1. (a) The core size distribution of the fabricated T-YDF (insets: the cross-sections of narrow and wide ends), (b) the cladding pump absorption spectrum of the T-YDF.
2.2 Experimental setup
The experimental setup of the high-power narrow-linewidth fiber laser based on T-YDF is depicted in Fig. 2. The laser system contains two parts: a fiber oscillator seed and a power amplifier. The fiber oscillator is comprised of a pair of 20/400 µm FBGs with a center wavelength of ∼1080 nm. To fulfil a narrow spectral width and mitigate the SBS effect, the 3 dB bandwidth of the output coupling (OC) FBG in the oscillator stage is ∼0.1 nm, while that of the high reflection (HR) FBG is ∼2 nm. The reflectivity of OC-FBG and HR-FBG is 11% and 99%, respectively. A length of ∼8 m commercial Yb-doped fiber (YDF) with a core/cladding diameter of 20/400 µm is adopted as the gain medium in the oscillator. The nominal cladding absorption of the YDF is ∼1.2 dB/m at 976 nm. To produce a single-mode output from the narrow-linewidth oscillator, the YDF was coiled with a minimum bending diameter of 80 mm. Two 250 W wavelength-locked fiber-pigtailed 976 nm laser diodes (LDs) are utilized as forward pump sources. A cladding power stripper (CPS) is employed to filter out the residual pump light in the oscillator, and the seed power was injected to the amplifier stage.
The amplifier stage has two configurations, including co-pumped scheme and counter-pumped scheme. For the co-pumped scheme, the output fiber from the narrow-linewidth oscillator is directly connected to the signal input port of the forward (6 + 1) × 1 pump/signal combiner (F-PSC), where the signal input fiber has size of 20/250 µm. The signal output fiber with a size of 20/400 µm of the F-PSC is connected to the narrow end of the T-YDF. Five groups of 976 nm wavelength-locked LDs enter the gain fiber via the pump ports of the combiner, where each LD group is capable of launching ∼850 W pump power. The amplifier gain fiber is the double-clad T-YDF with a core numerical aperture is ∼0.065. To achieve effective single-mode operation, the T-YDF from narrow end to wide end was coiled in a racetrack groove with a diameter ranged from 82 mm to 130 mm, and the narrow end experiences a smaller fiber coiling to increase the loss of HOMs in the amplifier. To deliver the output laser power, a 2-m-long passive fiber with a core/cladding diameter of 30/400 µm was high-precisely spliced with the wide end of T-YDF. A CPS is used to eliminate unwanted cladding light and the signal laser is output by a quartz block head (QBH) at the end.
After completing the co-pumped experiment, the amplifier configuration was changed to the counter-pumped scheme. We removed the F-PSC and put the output fiber of the seed oscillator is connected to the narrow end of T-YDF, then a backward (6 + 1) × 1 pump/signal combiner (B-PSC) was inserted between the wide end of T-YDF and the passive output fiber. The B-PSC has a signal input fiber size and output fiber size of 30/400 µm and 30/250 µm, respectively. Although the pump cladding diameter of the combiner does not match the wide end of T-YDF, the fusion loss is reduced by ensuring core alignment during the splice process to avoid the excitation of HOMs. In the process of constructing the fiber amplifier, all fusion points are carefully processed to prevent the heat generation, and the temperature of all fusion points are monitored in real-time during the experiment. The delivery fiber and QBH at the end of the counter-pumped scheme is the same as that of the co-pump. The laser performance involving output power, backward power, optical spectrum, temporal signals, and beam quality is measured and recorded in the whole experiment.
3. Results and discussion
3.1 Laser performance with co-pumping scheme
Firstly, to investigate the influence of the injected seed power of the narrow linewidth FBG-based MOPA laser, the seed power from the fiber oscillator is set to ∼33W and ∼78W by controlling the driving current, respectively. Figure 3 shows the output spectra of the different seed powers, which were measured by an optical spectrum analyzer. The 3 dB and 20 dB bandwidths of the 33W seed power are respective 0.25 nm and 0.49 nm, while that of the 78 W seed power are about 0.27 nm and 0.68 nm, respectively. Meanwhile, the beam quality (M2 factor) of different seed powers was measured by a commercial beam quality analyzer. The M2 factor in the x and y directions is 1.25 and 1.20 for 33 W seed power, while the 78 W seed power has a M2 factor of 1.24 and 1.20, as shared in the inset of Fig. 3, meaning that the injected seed is operated at near-diffraction-limited beam quality.
Fig. 3. The output spectrum of the (a) 33 W seed power and (b) 78 W seed power (inset: the corresponding beam profile and beam quality M2).
In the co-pumping amplification experiment, the laser performance of this narrow linewidth fiber amplifier with different seed power injection was measured and recorded. The output power varies with the launched pump power as shown in Fig. 4(a) and (b), respectively. It is shown that with the 33 W seed power injection, the slope efficiency is up to 84.6%, while that of the 78 W seed power is about 85.4%. The slightly higher slope efficiency of the 78 W injected seed power is mainly attributed to that higher seed power stimulates the enhanced absorption of the pump power by gain ions. At the highest output power of 1947 W, the measured M2 factor is 1.28 and 1.24 in the x and y directions, as shown in the inset of Fig. 4(a), indicating this T-YDF-based narrow linewidth amplifier performs well in maintaining single-mode beam quality. However, a power roll-over and a decreasing optical-to-optical conversion efficiency was observed near the maximum output powers, which relates to the appearance of TMI effect [10].
Fig. 4. The output laser power and corresponding optical-to-optical efficiency versus the pump power for (a) 33 W seed power and (b) 78 W seed power.
The temporal signals of the output laser were monitored to validate the onset of TMI through a high-speed photodetector connected to an oscilloscope. The time traces were recorded for 2-second at each output level and repeated several times to eliminate noise-induced random error. With each original piece of data, the temporal signal is divided into 100 equal-sized traces, and the standard deviation (STD) of every temporal signal is calculated to determine the TMI threshold, of which method is detailed described in [38]. As shared in Fig. 5, the STD remains stable and low level before the output power reaches 1958 W and 2130 W in the experiments of 33 W and 78 W seed power, respectively. Then, a sudden increase of STD of the temporal signals exists as the output power further raises, where the STD value increases from 0.06% to 5.10%, indicating the onset of TMI effect. Moreover, near the TMI threshold, the characteristic frequency components ranged from 0-5 kHz appear on their Fourier spectra (as shown in the inset of Fig. 5(a) and (b)), further proving the observation of TMI effect during the power process. Therefore, the TMI thresholds of the co-pumping scheme is respective 1958 W and 2130 W in the case of 33 W and 78 W seed power.
Fig. 5. The STD value of temporal signals versus different output powers in case of (a) 33 W seed power and (b) 78 W seed power.
The evolution of the output spectra as a function of the output power is shown in Fig. 6. It can be seen that the spectral linewidth increases gradually with the increase of the output power no matter when the seed power is 33 W or 78 W. In the case of 33W seed power, the measured 3 dB and 20 dB linewidth are 0.42 nm and 1.69 nm at the output power of 1947 W, as illustrated in Fig. 6(a). When the seed power is 78 W, the 3 dB and 20 dB linewidth are 0.56 nm and 2.68 nm at the output power of 2121 W, where the spectral broadening is remarkably higher than that of 33W seed power. This result is mainly due to the fact that the higher seed power leads to a broader spectral linewidth, which will be further broadened after being amplified. Besides, at their highest output power, there is an obvious Raman component at near 1135 nm and the SRS intensity is 42.6 dB and 36.6 dB in both experiments. It can be found that, the lower seed power can obtain a narrower spectrum and higher SRS suppression compared to the higher seed power injection in this narrow-linewidth FBG-based MOPA system, which is compatible with the experimental and theoretical results shown in [39,40].
3.2 Laser performance with counter-pumping scheme
After implementing the co-pumping scheme, the laser performances of the counter-pumped narrow linewidth fiber amplifier were also studied, and the relevant results were recorded and shown in Fig. 7. The output power and the corresponding optical-to-optical efficiency with respective to the pump power are presented in Fig. 7(a). Under the different seed powers injection of 33 W and 76 W, the output laser power exhibits a high linear growth with a slope efficiency of 84.4% and 84.5% respectively before the power roll-over occurs. A maximum output power of 2092 W and 2260 W is achieved in both seed powers respectively. It is shown that the slope efficiency for counter-pumping schemes is almost the same as the co-pumping ones, which is mainly because the negligible vignetting effect due to smaller backward pump cladding area [23]. The STD in dependence on the output power in these two cases is calculated and depicted in Fig. 7(b), and then, the STD value increases abruptly when the output power raises to 1993 W and 2164 W, respectively, which corresponds to the occurrence of TMI. During the power scaling process, the evolution of the beam quality is shared in the inset of Fig. 7(b). The M2 factor experiences a rapid increase from 1.28 to 1.59 when the output power enhances from 1993 W to 2092 W in the case of 33 W seed power, which is consistent with the rapidly increasing of STD value. It follows that the TMI thresholds of these two cases are 1993 W and 2164 W, respectively.
Fig. 7. Experimental results of the counter-pumping scheme. (a) Output power and the corresponding efficiency dependence on the pump power. (b) The STD evolution versus the output power (inset: the evolution of M2 factor). (c) Output spectra with injected seed power of 33 W. (d) Output spectra with injected seed power of 76 W.
The output spectra versus the output power are illustrated in Fig. 7(c) and (d) with respective to the 33 W seed power and 76 W seed power injection, respectively. When the seed power is 33 W, the spectral linewidth of 3 dB and 20 dB is measured to 0.34 nm and 1.48 nm at the highest output power of 2092 W, and no Raman Stokes light is observed, an SRS suppression ratio is greater than 54.1 dB in this instance. While in the case of 76 W seed power, the output spectrum broadens remarkably, and the 3 dB and 20 dB linewidth increases to 0.63 nm and 2.49 nm at the maximum output power. Compared both spectra at the same output power of 2 kW-level, the average spectral linewidth of 33 W is 41.38% narrower than that of 76 W seed power. In the case of 76 W seed power, when operating at output power of 2260 W, the Raman Stokes light was observed with a weak peak, as shown in Fig. 7(d), and the intensity is ∼50.8 dB below the signal light. It is manifested that the lower seed power injection holds better performance in Raman suppression and spectral narrowing compared to the higher seed power.
3.3 Discussion and analysis
From the above experimental results, it is clear that the lower seed power offers higher nonlinearities threshold and narrower spectral linewidth whether in the co-pump and counter-pump scheme. This is mainly due to the fact that with the injected seed power increasing, the more Raman noise is introduced by the higher seed power, leading to a lower SRS threshold [41]. To further illustrate the spectral evolution characteristic of this narrow linewidth fiber amplifier for different pump manners, the spectral linewidths versus the output power of the 33 W seed power and 76 W seed power injection were presented in Fig. 8. Under the same seed power injection, the counter-pump manner has a narrower spectral linewidth than that of the co-pump manner, especially in the 20-dB linewidth regime. In the case of 33 W seed power, as shown in Fig. 8(a), the co-pump manner exhibits a growing rate of 8.88 pm/100W in 3 dB linewidth and 62.2 pm/100W in 20 dB linewidth, while that of the counter-pump manner is 4.37 pm/100W and 48.1 pm/100W, indicating that the counter-pump manner provides a positive impact on inhibiting spectral broadening during the power scaling.
Fig. 8. The 3 dB and 20 dB spectral linewidths evolution versus the output power in the case of (a) 33 W seed power and (b) 76 W seed power.
Apart from these nonlinear effects, the TMI effect is another limitation affecting the power improvement of this narrow-linewidth fiber amplifier. Figure 9 shows the TMI threshold comparison of various seed powers and different pump manners in the experiment. Although the higher seed power leads to the spectrum broadening, it can obtain a higher threshold of TMI. As the seed power increases from 33 W to 76 W, the TMI threshold of the co-pump and counter-pump scheme is enhanced by a factor of 8.78% and 8.58%, respectively. The increase in TMI threshold with the seed power growing can be explained that the gain saturation effect, but the increasing rate gradually slows down because the role that the gain saturation plays becomes weaker and weaker [12]. Therefore, in the narrow linewidth fiber amplifier, the injected seed power should be trade-off designed to consider the influence of the spectral broadening effect, SRS and TMI effects comprehensively. Besides, it can be seen that the TMI threshold of the counter-pump manner is higher than that of co-pumping one regardless of the injected seed power, while the spectral linewidth of counter-pump manner is narrower at the same time, which means that the counter-pump manner is better for narrow linewidth fiber amplifiers.
Compared to previous demonstrations on T-YDF-based narrow-linewidth/single-frequency fiber lasers [31,33,42], this fiber laser exhibits a high slope efficiency, perfect beam quality and high Raman to signal ratio, even though an FBG-based fiber oscillator is utilized as seed source. This is mainly attributed to the intrinsic advantages of the T-YDF, good splicing quality and reasonable coiling state, thereby suppressing the generation of HOMs. Our experimental results manifest that the T-YDF has huge potential for achieving narrow linewidth single-mode all-fiberized lasers. Next, by optimizing the material properties of the tapered fiber itself and the output characteristics of the seed laser, as well as adopting the bidirectional pumping scheme to comprehensively inhibit the SRS and TMI effects, it is prospective to further promote the output power of the T-YDF-based narrow-linewidth fiber laser with near diffraction-limited beam quality and high SRS suppression ratio.
4. Conclusion
In conclusion, we have experimentally demonstrated an all-fiberized narrow-linewidth T-YDF amplifier seeded by an FBG-based fiber oscillator. The T-YDF has a narrow and wide end of ∼20/400 µm and ∼30/600 µm, respectively. The experimental results indicate that the injected seed power should be trade-off designed to take into account the inhabitation strategies of spectral broadening, SRS and TMI effects. In addition, it shows that the counter-pumping manner offers a narrower spectral width, higher TMI threshold, higher SRS suppression ratio than that of the co-pumping one. As a result, a 2 kW-level output power with a 3 dB spectral linewidth of ∼0.34 nm and excellent beam quality of M2∼1.28 is obtained in this counter-pumped T-YDF-based amplifier with a high Raman suppression ratio over 53.5 dB. To the best of our knowledge, this is the highest power ever reported from such a T-YDF-based narrow-linewidth single-mode laser. This work demonstrates a great prospect of LMA tapered active fibers for realizing high power single-mode narrow-linewidth fiber lasers with high signal to Raman ratio.
Funding
Training Program for Excellent Young Innovators of Changsha (kq2106008); National Natural Science Foundation of China (62005315); Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology (SKL2022ZR02).
Acknowledgments
The authors wish to thank Mr. Lingfa Zeng, Mr. Jinming Wu, Mr. Xiaoyong Xu, and Mrs. Siliu Liu for their help in experimental operations.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
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