Chinese Optics Letters, Volume. 23, Issue 7, 071408(2025)

6 kW near-single-mode monolithic fiber laser employing a longitudinally asymmetric spindle-shaped ytterbium-doped fiber

Fengchang Li1... Peng Wang1,2,3, Xiangming Meng1, Baolai Yang1,2,3,*, Liangjin Huang1,2,3, Xiaoming Xi1,2,3, Hanwei Zhang1,2,3, Zhiping Yan1,2,3, Zhiyong Pan1,2,3,**, Xiaolin Wang1,2,3, Zefeng Wang1,2,3, and Jinbao Chen1,23,*** |Show fewer author(s)
Author Affiliations
  • 1College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
  • 2Nanhu Laboratory, National University of Defense Technology, Changsha 410073, China
  • 3Hunan Provincial Key Laboratory of High Energy Laser Technology, National University of Defense Technology, Changsha 410073, China
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    In order to balance the suppression of stimulated Raman scattering (SRS) and transverse mode instability (TMI) in high-power fiber lasers, in this Letter, a new type of spindle-shaped ytterbium-doped fiber (YDF) with asymmetric longitude distribution was designed and produced, which had a small-sized input end, large-sized transmission section, and moderate output end, enabling a good fit with a seed laser and mitigating SRS as well as TMI effects. A counter-pumped fiber laser amplifier was established using this YDF, and two kinds of laser diodes (LDs) were adopted for increasing the TMI threshold. Finally, the maximum output power reached 6 kW, and the beam quality (M2 factors) indicated near-single-mode output. The SRS suppression ratio under 6 kW output power was 36 dB, and no dynamic TMI was observed, which revealed that further enhancement of output power was limited only by pump power.

    Keywords

    1. Introduction

    Benefitting from compact structure, high conversion efficiency, good beam quality, convenient thermal management, and many other advantages[13], fiber lasers directly pumped by laser diodes (LDs) have been widely used in plenty of fields including industrial manufacturing, biomedicine, aerospace, and scientific research[4,5]. Early in 2004, Jeong et al. realized a single-mode fiber laser over 1 kW pumped by 975 nm LDs for the first time using a large-mode-area (LMA) ytterbium-doped fiber (YDF) with a core diameter of 40 µm and a core numerical aperture (NA) of 0.05. With the development of cladding pumping technology, high-brightness LDs, and fiber device fabrication processes, in recent years, the output performance of LD-directly-pumped fiber lasers has been rapidly improved. Since 2016, research reports concerning LD-pumped fiber lasers with over 5 kW high-power output appear continuously, including broad-spectral and narrow-linewidth fiber lasers. In continuous pursuit of higher output power, the beam quality of fiber lasers has also been given more attention. However, limited by the pump power, pump brightness, and other factors, LD-pumped high-power fiber lasers with excellent beam quality are hard to realize. At present, the output powers of LD-pumped fiber lasers with M21.5 and M22 are about 6[68] and 8 kW[9], respectively, much lower than those of tandem-pumped fiber lasers[10].

    With the increase of pump power, the output characteristics of high-power, high-brightness fiber lasers are limited by many factors, mainly including fiber nonlinear effects such as stimulated Raman scattering (SRS)[11,12], stimulated Brillouin scattering (SBS)[13,14], transverse mode instability (TMI)[15], photon darkening[16,17], and thermal lens effects[18]. Among all the above factors, SRS and TMI have become the most important limitations for high-power continuous-wave (CW) fiber lasers. For traditional fiber lasers using uniform LMA YDF, sharp contradictions exist between methods of suppressing SRS and TMI, resulting in the difficulty of balancing the influence of SRS and TMI for fiber lasers. To simultaneously suppress SRS and TMI as far as possible and improve the performance of CW fiber lasers, a series of specially designed YDFs have been proposed, such as chiral-coupled fibers[19,20], photonic crystal fibers[21,22], core-diameter-variable fibers[23,24], and multi-core fibers[25]. Among these, the core-diameter-variable fiber has some unique advantages including its low requirements for the manufacturing technique, its easy ability to match with commercial passive fiber devices, and its excellent beam quality maintaining characteristics, as well as the extensive amount of research on this type of fiber. Early in 2008, researchers from the Tampere University of Technology and the Russian Academy of Sciences developed a tapered YDF with a taper ratio of 4.8 and a large-end fiber diameter of 27/834 µm, realizing an 84 W 1080 nm CW fiber laser with a high slope efficiency of 92% for the first time[26]. Our group also carried out research on high-power CW fiber lasers using a tapered YDF and obtained some achievements including a 4 kW near-single-mode fiber laser using tapered YDFs with the core/cladding diameters varying from 20/400 to 30/600 µm[27], a 3.42 kW fiber laser oscillator with M2 of 1.88 using a spindle-sharped YDF with a constant cladding diameter and varied core diameters from 24 to 31 to 24 µm[28], and a 6 kW oscillator-amplifier-integrated fiber laser using a spindle-shaped YDF with varied core diameters from 25 to 37.5 to 25 µm[29]. All the above results reveal the great potential of core-diameter-variable YDFs in achieving high-power high-brightness fiber lasers.

    In this paper, a new type of core-diameter-variable YDF, which was designed as an asymmetric spindle-shaped YDF (ASSYDF), was proposed. The ASSYDF has a small-sized injection end for maintaining the beam quality of the input signal, a large-sized middle section for suppressing SRS, and a moderate-sized output end for increasing the TMI threshold. Finally, a near-single-mode fiber laser amplifier over 6 kW was realized using this ASSYDF, which was, to the best of our knowledge, the demonstration of an LD-pumped near-single-mode (M21.5) fiber laser with the highest output power using a spindle-shaped YDF. The SRS suppression ratio was 36 dB, and no TMI was observed, which indicated higher output by increasing pump power.

    2. Fiber Description and Experimental Setup

    When designing the spindle-shaped YDF, based on the rate equations of fiber lasers[29], the SRS suppression characteristics of the fiber laser employing spindle-shaped YDFs (SSYDFs) with different parameters were simulated, and the results are presented in Fig. 1. The optical configuration utilized in the simulation was the oscillating-amplifying integrated fiber laser (OAIFL) described in Ref. [29]. The parameters used in the simulation are listed in Table 1. Three kinds of SSYDFs are designed and applied in the simulation with the core diameters varying from 20 to 30 to 20 µm (named YDF1), 20 to 30 to 25 µm (named YDF2), and 25 to 30 to 25 µm (named YDF3), respectively. The fiber lengths at the input end, middle section, and output end were set as 4, 8, and 7 m, respectively, and the lengths of two transition sections were set as 5 m.

    • Table 1. Main Parameters and Values in Theoretical Simulation

      Table 1. Main Parameters and Values in Theoretical Simulation

      ParameterValue
      Fiber core NA0.065
      Signal wavelength1070 nm
      Pump absorption coefficient0.4 dB/m at 915 nm
      Pump configurationCounter-pump
      Fiber length in oscillation/amplification stage13/29 m
      Pump wavelength976 nm
      Core/cladding diameter in oscillation stage20/400 µm

    Simulation results of output spectra in OAIFL employing different kinds of SSYDFs.

    Figure 1.Simulation results of output spectra in OAIFL employing different kinds of SSYDFs.

    The simulation results are presented in Fig. 1. Evidently from the figure, when compared with YDF1, YDF2 exhibits a remarkable enhancement in SRS suppression. Specifically, its SRS suppression ratio increases from 34.6 to 44.9 dB. This indicates that appropriately increasing the core diameter of the output end of SSYDF can effectively suppress SRS. Nevertheless, if the core diameter of the output end continues to increase, the TMI threshold will be reduced. This is because the number of modes supported by the fiber increases rapidly, which is prone to causing mode coupling between the fundamental mode and the higher-order mode. When further increasing the core diameter of the input end like YDF3 in Fig. 1, the SRS suppression ratio will continue to rise, yet the increase is marginal, merely about 2 dB. Moreover, it will affect the fiber matching of the seed source and amplification stage, and will lead to the deterioration of beam quality.

    Therefore, SRS can be effectively suppressed by controlling the core diameters of the input and output ends to 20 and 25 µm, respectively, while maintaining the beam quality.

    According to the simulation results, the longitudinal structure of the ASSYDF used in our experiment is shown in Fig. 2. The total length is 29 m, and the absorption coefficient is about 1.5 dB/m at 976 nm. It can be divided into five sections, named as S1 to S5, with lengths of 4, 5, 8, 5, and 7 m, respectively, and all sections have a constant core-to-cladding ratio, which is 1/20. S1 is the input end and has an invariable core/cladding diameter of 20/400 µm, which is beneficial for matching with the fiber seed laser and maintaining a good beam quality of the seed laser. S3 is the longest section and has the largest core/cladding diameter of 30/600 µm, which is good for the transmission of high-intensity lasers and SRS suppression. S5 is the output end, which has a moderate core/cladding diameter of 25/500 µm, which is larger than S1 and ensures that, for SRS suppression, it is smaller than S3 and is beneficial for improving the TMI threshold and obtaining good beam quality. The S2 and S4 sections have a gradually varied core/cladding diameter, which is good for the stability of the mode component and ratio in the transmission and amplification of the fiber laser and thus maintaining beam quality.

    Schematic diagram of the longitudinal structure of the ASSYDF.

    Figure 2.Schematic diagram of the longitudinal structure of the ASSYDF.

    As shown in Fig. 3, the 6 kW fiber laser using ASSYDF consists of two parts, which are the seed laser and amplification stage. Different from traditional fiber lasers, no cladding laser stripper (CLS) was inserted between the seed and amplifier stages, and the residual pump laser of the amplification stage can be continuously absorbed by the seed laser, which is able to enhance the pump power utilization rate. The seed laser was a 1070 nm CW fiber laser oscillator counter-pumped by two wavelength-stabilized 976 nm LDs, each with a maximum pump power of 130 W and composed of a high-reflector fiber Bragg grating (HR FBG), an output-coupler fiber Bragg grating (OC FBG), and a 13 m length of YDF, of which the core/cladding diameter is 20/400 µm. The 3 dB reflection bandwidth and reflectivity of the HR FBG and OC FBG are 4 nm, 99% and 2 nm, 10% respectively. According to previous reports, the broadband feature of the oscillator seed laser is beneficial for increasing the SRS threshold of the amplification stage[30]. The core NA and pump absorption coefficient of the 20/400 µm YDF are 0.063 and 1.2 dB/m at 976 nm. It was coiled at the bending diameter of 9.5 cm to achieve single-mode-laser output. A side pump combiner (SPC) was fused with the OC FBG to inject the pump laser into the seed laser. The free port of the HR FBG was fused with CLS1, and the other end of CLS1 was cut at an angle of 8° to eliminate the end-face reflection to suppress the SRS effect.

    Schematic diagram of the 6 kW fiber laser using ASSYDF. CLS, cladding light striper; HR/OC FBG, high reflector/output coupler fiber Bragg grating; YDF, ytterbium-doped fiber; SPC, side pump combiner; CTFBG, chirped and tilted fiber Bragg grating; ASSYDF, asymmetric spindle-shaped YDF; BPSC, backward pump/signal combiner; LD, laser diode; QBH, quartz beam head.

    Figure 3.Schematic diagram of the 6 kW fiber laser using ASSYDF. CLS, cladding light striper; HR/OC FBG, high reflector/output coupler fiber Bragg grating; YDF, ytterbium-doped fiber; SPC, side pump combiner; CTFBG, chirped and tilted fiber Bragg grating; ASSYDF, asymmetric spindle-shaped YDF; BPSC, backward pump/signal combiner; LD, laser diode; QBH, quartz beam head.

    The seed laser was injected into the amplification stage after passing through a chirped and tilted fiber Bragg grating (CTFBG), which was used to suppress the SRS effect. The main amplification stage employed one backward pump/signal combiner (BPSC) to constitute a counter-pump configuration. The BPSC has 18 multimode pump ports with a core/cladding diameter of 135/155 µm, which were fused with 18 pieces of LDs. The core/cladding diameters of the input and output signal ports of the BPSC are 25/500 and 25/250 µm, respectively. The signal insertion loss of the BPSC is about 0.15 dB. The gain fiber employed in the amplification stage has been described in Fig. 1. The ASSYDF was tightly coiled in a figure-eight pattern in the grooves, with each interval of 1 mm on a water-cooled metal plate, which provided available fiber bending and removed heat accumulation. The minimum bending diameter of both the input end and output end was designed as 8 cm, which has been proven effective for increasing the TMI threshold[31]. After the BPSC was a CLS2 and a quartz beam head (QBH), of which the total delivery fiber length was about 2.5 m. In the experiment, the output laser power, temporal signals, and laser beam quality were measured and used to verify the TMI threshold.

    3. Experimental Results and Discussion

    There were two kinds of LDs used in the experiment for achieving high-power output lasers: the non-wavelength-stabilized 976 nm LDs, each with a maximum pump power of 300W, and the dual-wavelength (DW) LDs located at 969 and 982 nm, each with a maximum pump power of 650W. First, the output characteristics of the amplification stage counter-pumped by 18 pieces of non-wavelength-stabilized 976 nm LDs were recorded in detail. The measured power of the seed laser outputted from the QBH was 104 W, and its beam quality (M2 factor) was 1.41 and 1.23 in the X and Y directions, respectively, indicating single-mode output. Due to the pump wavelength shift under different operating currents, the output power and corresponding optical-to-optical conversion efficiency under different pump powers were measured twice and are shown in Figs. 4(a) and 4(b). Figure 4(c) shows the collected time domain signals under maximum output power and the calculated Fourier spectra in the frequency domain. When the pump wavelength was 976.1 nm, the maximum output power reached 3480 W with a conversion efficiency of 88.7%. However, TMI was observed as shown in Fig. 4(c) and limited the further increase of output power. However, when the pump wavelength decreased to 974.1 nm, the maximum output power greatly increased to 4130 W with a conversion efficiency of 84.1%, lower than before due to a lower absorption coefficient. No TMI appeared as shown in Fig. 4(c), and further increase of output power was just limited by the pump power. The comparison results also reveal that optimization of the pump wavelength has an obvious effect in suppressing the TMI effect.

    Output characteristics of the amplification stage pumped by non-wavelength-stabilized 976 nm LDs. Measured (a) output powers and (b) conversion efficiencies under different pump powers and pump wavelengths; (c) measured time-domain signals and calculated Fourier spectra in the frequency domain under different output powers.

    Figure 4.Output characteristics of the amplification stage pumped by non-wavelength-stabilized 976 nm LDs. Measured (a) output powers and (b) conversion efficiencies under different pump powers and pump wavelengths; (c) measured time-domain signals and calculated Fourier spectra in the frequency domain under different output powers.

    To achieve higher output power, eight pieces of non-wavelength-stabilized 976 nm LDs were replaced with 650 W DW LDs, and the amplification stage was simultaneously hybrid pumped by two kinds of LDs. The pump wavelength far from 976 nm of DW LDs is beneficial for increasing the TMI threshold. The output characteristics of the amplification stage were recorded in detail and drawn in Figs. 5(a)5(d), when the pump wavelength of non-wavelength-stabilized 976 nm LDs was controlled at 975.5 nm. When the total pump power was 6.6 kW, in which the pump powers of 976 nm LDs and DW LDs were 2.7 and 3.9 kW, respectively, the output power of the amplification stage reached 5 kW. The corresponding optical-to-optical conversion efficiency was only 74%, much lower than that shown in Fig. 4(b). The main reason for this resulting phenomenon is that the absorption coefficient of ASSYDF in 969/982 nm is much lower than that in 976 nm, which also caused the conversion efficiency to gradually decrease after the DW LDs were put into action as shown in Fig. 5(a). The measured output spectra of the seed laser and 5 kW high-power laser are depicted in Fig. 5(b). The figure reveals that, compared with the seed laser, obvious spectral broadening emerged in the 5 kW high-power laser, and the full width at half-maximum (FWHM) was about 3 nm. Owing to the CTFBG and specially designed ASSYDF, no SRS was observed, and the SRS suppression ratio was over 40 dB. However, the time-domain signals and calculated Fourier spectra in the frequency domain shown in Fig. 5(c) reveal that TMI has already appeared, which limited further enhancement of output power. The M2 factors were measured and shown in Fig. 5(d), which were 1.65 and 1.34 in the X and Y directions. Compared with the seed laser, the beam quality of the 5 kW high-power laser shows a slight degradation, and the main reason was analyzed to be the generation and amplification of the higher-order mode in the amplification stage. The inset in Fig. 5(d) is the beam profile at the focal point measured under 5 kW output, which indicates a near-single-mode output.

    Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the first time. Measured (a) output powers and conversion efficiencies under different pump powers, (b) spectra of the seed laser and 5 kW high-power laser, (c) time-domain signals and Fourier spectra in the frequency domain, and (d) beam quality (M2 factors) of the 5 kW high-power laser.

    Figure 5.Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the first time. Measured (a) output powers and conversion efficiencies under different pump powers, (b) spectra of the seed laser and 5 kW high-power laser, (c) time-domain signals and Fourier spectra in the frequency domain, and (d) beam quality (M2 factors) of the 5 kW high-power laser.

    To further increase the output power, two methods were conducted, one being enlarging the power of the seed laser to 470 W and the other being optimizing the pump wavelength of non-wavelength-stabilized 976 nm LDs to 974 nm by controlling the operating current. Finally, the output power of the main amplification stage reached 6 kW, and the output performance is shown in Figs. 6(a)6(e). As shown in Fig. 6(a), when the total pump power was 8.1 kW, in which the pump powers of 976 nm LDs and DW LDs were 2.5 and 5.6 kW, the maximum output power was 6020 W, and the corresponding optical-to-optical conversion efficiency was about 69.2%, which was much lower than that shown in Figs. 5(a) and 4(b). The laser spectra under different output powers were recorded and drawn in Fig. 6(b). As can be seen, the spectral bandwidth gradually increased with the enhancement of output power. When the output power reached 6 kW, the FWHM increased to about 4 nm, and obvious SRS appeared with the SRS suppression ratio of 36 dB. Figure 6(c) plots the measured time-domain signals and calculated Fourier spectra in the frequency domain under 6 kW output. As can be seen, the time-domain signals were quite stable, and no characteristic frequency appeared, which indicates that the TMI threshold was higher than 6 kW for the amplification stage. Having an acceptable SRS suppression ratio and no TMI reveals that further enhancement of output power of the amplification stage is just limited by the pump power. As shown in Fig. 6(d), the M2 factors were measured to be 1.61 and 1.34, respectively, in the X and Y directions, indicating a near-single-mode output. The beam profile in the focal point shown in the inset in Fig. 6(d) also reveals that the transverse intensity distribution of the 6 kW output laser fit well with Gaussian distribution. The beam quality and standard deviation (STD) of the normalized photodetector (PD) signals under different output powers were recorded in detail and depicted in Fig. 6(e). The figure reveals that, along with the increase of output power, the M2 factors in both X and Y directions gradually degraded from 1.19/1.41 to 1.34/1.61, indicating the ratio enhancement of the higher-order mode laser. However, no abrupt change appeared in the STD of normalized PD signals, which reveals again that no TMI was detected in the whole output power-increasing process.

    Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the second time. (a) Output powers and conversion efficiencies under different pump powers, (b) laser spectra under different output powers, (c) time-domain signals and Fourier spectra in the frequency domain, (d) beam quality (M2 factors) of the 6 kW high-power laser, and (e) M2 factors and STDs of normalized PD signals under different output powers.

    Figure 6.Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the second time. (a) Output powers and conversion efficiencies under different pump powers, (b) laser spectra under different output powers, (c) time-domain signals and Fourier spectra in the frequency domain, (d) beam quality (M2 factors) of the 6 kW high-power laser, and (e) M2 factors and STDs of normalized PD signals under different output powers.

    The aforementioned experimental results demonstrate that we have achieved a 6 kW near-single-mode fiber laser output using an LD-directly-pumped ASSYDF laser, without being limited by SRS and TMI effects, indicating significant potential for further performance enhancement. Although there have been reports of LD-directly-pumped uniform YDFs achieving 6 kW single-mode laser output, the experimental results presented in this Letter do not represent a significant breakthrough in output power. However, it provides valuable insights for future endeavors to achieve higher-power near-single-mode fiber lasers. Unlike traditional ytterbium-doped fiber lasers (YDFLs), which struggle to simultaneously suppress SRS and TMI effects to enhance output characteristics, the greatest technical advantage of the SSYDF lies in its ability to concurrently mitigate both SRS and TMI effects. Of course, this does not imply that the SSYDF can entirely resolve both SRS and TMI issues simultaneously. In practical applications, it is still necessary to employ techniques such as pump wavelength optimization and pump method optimization to comprehensively suppress SRS and TMI. Naturally, spindle-shaped fibers also have certain limitations, primarily in their challenging manufacturing process and difficulty matching with conventional passive fiber components. Moreover, the technical advantages of the SSYDF make the realization of LD-directly-pumped 10-kW-class single-mode fiber lasers a possibility. Specific technical solutions have been detailed in Refs. [32,33]. In addition to utilizing specific core-diameter-variable YDFs (including SSYDFs), it is also necessary to optimize the design of various parameters such as the fiber laser structures, pump sources, and passive components to achieve high-power, high-brightness laser output.

    4. Conclusion

    In conclusion, a novel type of tapered fiber named ASSYDF was proposed and designed in this Letter, which had technical advantages in balancing the suppression of SRS and TMI in high-power fiber lasers. A counter-pumped fiber laser amplifier was established using the ASSYDF. By optimizing the pump wavelength and adopting appropriate pump LDs, 6 kW near-single-mode fiber laser amplifiers were successfully realized with M2 factors of 1.61 and 1.34. The SRS suppression ratio was 36 dB, and no TMI appeared under 6 kW output, indicating that the output laser power was able to be further promoted by increasing the pump power. To the best of our knowledge, this is the highest outpower demonstration of LD-directly-pumped near-single-mode fiber lasers using the SSYDF. The results prove the advantages of the core-diameter-variable fiber in achieving high-power high-beam-quality fiber lasers and provide a reference for the design and optimization of tapered YDFs. The following research will be focused on increasing the pump power and optimizing the design parameters of the ASSYDF and pump wavelength for further promoting the output performance of the fiber laser.

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    Fengchang Li, Peng Wang, Xiangming Meng, Baolai Yang, Liangjin Huang, Xiaoming Xi, Hanwei Zhang, Zhiping Yan, Zhiyong Pan, Xiaolin Wang, Zefeng Wang, Jinbao Chen, "6 kW near-single-mode monolithic fiber laser employing a longitudinally asymmetric spindle-shaped ytterbium-doped fiber," Chin. Opt. Lett. 23, 071408 (2025)

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    Paper Information

    Category: Lasers, Optical Amplifiers, and Laser Optics

    Received: Nov. 13, 2024

    Accepted: Mar. 18, 2025

    Published Online: Jun. 20, 2025

    The Author Email: Baolai Yang (yangbaolai1989@163.com), Zhiyong Pan (panzy168@163.com), Jinbao Chen (kdchenjinbao@aliyun.com)

    DOI:10.3788/COL202523.071408

    CSTR:32184.14.COL202523.071408

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