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Article

6.7 kW LD-Pumped Nearly-Single-Mode MOPA Fiber Laser Enabled by Low-NA Confined-Doped Fiber

by
Hengyu Tang
1,
Bingyu Rao
1,
Yufei Gan
1,
Baolai Yang
1,2,*,
Fan Wang
3,
Lei Zhang
3,
Meng Wang
3,
Lili Hu
3,
Zilun Chen
1,2,
Hu Xiao
1,2,
Zhixian Li
1,2,
Pengfei Ma
1,2 and
Zefeng Wang
1,2,*
1
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
2
Nanhu Laser Laboratory, National University of Defense Technology, Changsha 410073, China
3
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
*
Authors to whom correspondence should be addressed.
Photonics 2025, 12(10), 971; https://doi.org/10.3390/photonics12100971
Submission received: 27 August 2025 / Revised: 26 September 2025 / Accepted: 29 September 2025 / Published: 30 September 2025
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Abstract

Optimized designs of the ytterbium-doped fiber (YDF) have been effective at mitigating transverse mode instability (TMI) and enabling high-power scaling. In this study, the use of low-NA confined-doped YDFs is explored to achieve high-power nearly-single-mode continuous-wave lasers. Three types of 25/500 µm YDFs are manufactured with ~80% doping ratio and respective NAs of 0.058, 0.053, and 0.048. Experimental results indicate that the corresponding TMI thresholds increase with the descending NA in the YDFs. Based on the YDF with a NA of 0.048, the master oscillation power amplification (MOPA) fiber laser is scaled to 6.79 kW with nearly-single-mode beam quality.

1. Introduction

High-power fiber lasers have been widely employed in the applications of industrial material processing and military defense. The output powers of monolithic fiber lasers have been scaled remarkably in the last decade [1,2,3,4]. The 1018 nm tandem-pumped fiber lasers have exceeded 20 kW level outputs [5]. The laser diode (LD)-pumped fiber lasers enjoy a higher conversion efficiency and a lower cost at the same power level, which is preferred in large-scale productions and industrial applications. However, the LD-pumped fiber lasers endure severe heat-load-induced mode couplings and distortions, which limits the power scaling of fiber lasers with high beam quality. In 2024, a 20 kW level LD-pumped fiber laser was reported, employing a 50/800 µm ytterbium-doped fiber and 915 nm and 976 nm dual wavelength LDs [6]. A multimode laser with M2 ~8.17 was achieved at the operation of 14.2 kW. The poor beam quality restricted its possible application fields. There were also several reports on the nearly-single-mode high power fiber lasers. In 2020, the Fujikura Inc. reported an 8 kW monolithic fiber laser oscillator, which employed a 600 µm2 mode area home-made ytterbium-doped fiber (YDF) with a mode-selection technique [7]. The beam parameter product (BPP) is 0.5 mm·mrad. In 2024, our group reported a 7 kW level narrow linewidth fiber amplifier with a M2 factor of ~1.3, which employed an optimized 20/400 µm YDF in tight coiling [8]. In 2025, IPG Photonics reported a 7 kW single-mode ytterbium fiber laser amplifier with a beam quality parameter of M2 < 1.1 [9]. The ytterbium-doped fiber with a mode field diameter (MFD) of 17 µm and 960 nm LDs were employed. The aforementioned high-beam-quality fiber lasers successfully mitigated the transverse mode instability (TMI) and stimulated the Raman scattering (SRS) effect. The optimized design and manufacture of the YDF is the key factor for mitigating the TMI and enabling high-power scaling.
There have been many YDF optimizations for TMI mitigation reported in recent years, including reducing the core numerical aperture (NA) [10,11,12,13,14,15,16,17], optimizing the Yb-ion distribution [18,19,20,21,22,23,24], increasing high-order modal loss by adding trenches or coupled cores in the cladding [25,26,27,28,29], or simply tightly coiling the fiber [30,31], etc. Reducing the core NA and optimizing the Yb-ion distribution are relatively convenient to manipulate in the fiber preform manufacture. As for the low-NA technique, it has been proven effective, but ultra-low-NA fiber requires a wide bending diameter, which is inconvenient to use [12,13,17]. There were few reports nearly-single-mode fiber lasers of over 5 kW employing ultra-low-NA fiber. As for the confined-doped fibers or gain-tailored fibers featured in several reports, many examples of high-power laser scaling have been reported [23,24,32,33]. In 2023, Zhang et al. reported a nearly-single-mode 3.3 kW fiber laser amplifier with a 33/400 µm YDF with a ~57% doping ratio [23]. In 2024, Cui et al. reported a 5.1 kW TMI-free single-mode oscillating–amplifying integrated fiber laser, achieved by tightly bending the 25/400 µm confined ytterbium-doped fiber [24]. A promising approach appears to be to combine the low-NA technique and the confined-doping technique in the design and manufacture of the YDF, in order to further scale the high-power fiber laser with nearly-single-mode beam quality.
In the manuscript, the use of a low-NA confined-doped YDF is explored to achieve a high-power nearly-single-mode laser. Three types of 25/500 µm YDFs are manufactured with an ~80% doping ratio and respective NAs of 0.058, 0.053, and 0.048. A master oscillation power amplification (MOPA) configuration LD-pumped fiber laser is constructed, and these YDFs are utilized in the power amplifier stage to evaluate the TMI threshold and power-scaling ability. Experimental results indicate that the TMI threshold increases with the decrease in NA in the YDFs. Based on the ~80% doped YDF with an NA of 0.048, the MOPA fiber laser is scaled to 6.79 kW at ~1080 nm. The M2 factors are 1.53 and 1.41 in both directions. The ratio of the signal laser to Raman stokes light is ~42 dB. It is indicated that effective mitigation of the TMI can be achieved by employing the low-NA and confined-doping techniques in a ~25 µm core diameter YDF, promise is shown for further power scaling with appropriate optimizations.

2. Experimental Setup

The experimental setup of the continuous-wave high-power fiber laser is based on a MOPA configuration, as shown in Figure 1. The seed source is an all-fiber laser oscillator, providing a seeding power of ~100 W in continuous-wave operation. The oscillation cavity is composed of a pair of fiber Bragg gratings (FBG) and YDF with core/inner cladding diameter of 20/400 µm. The YDF has an absorption coefficient of ~1.2 dB/m at 976 nm and a length of ~12 m. The YDFs are coiled in a racetrack-shaped groove on the heat sink. The minimum bending diameter is ~9.5 cm, and the maximum bending diameter is ~13.5 cm. The pair of FBGs operate at the center wavelength of ~1080 nm. The high-reflection fiber Bragg grating (HR-FBG) has a reflectivity of ~99% and a 3 dB reflection bandwidth of ~4 nm. The output coupler fiber Bragg gratings (OC-FBG) have a reflectivity of ~10% and a 3 dB reflection bandwidth of ~2 nm. The broadband FBGs are employed in the seeding laser to achieve an optically stable oscillator laser, which helps to enhance the SRS thresholds of the power amplifier according the previous studies [34]. The pump source of the seed oscillator is a wavelength-stabilized 976 nm LD with a maximum output power of ~250 W. The LD operates in continuous-wave mode. The pump light is coupled into the oscillation cavity via a (2 + 1) × 1 side-pump combiner with a signal fiber of 20/400 µm.
After stripping the cladding light, the seed laser propagates through a mode-field adapter (MFA), which has an input fiber of 20/400 µm and an output fiber of 25/500 µm. The insertion loss of this MFA is ~0.2 dB. The MFA ensures better mode-field matching between the seed laser and the YDF in the amplification stage. The amplifier is counter-pumped by eighteen non-wavelength-stabilized 976 nm LDs. Each LD can provide an output power of ~500 W, resulting in a total pumping power of ~9000 W. The LDs are combined through a (18 + 1) × 1 pump/signal combiner (PSC). The pump ports of the pump/signal combiners are made of multimode fiber, with core/cladding diameters of 135/155 µm and a core NA of 0.22. The signal input and output ports of the pump/signal combiner are made of double-cladding fiber, with core/inner cladding diameters of 25/500 µm and 25/250 µm. The core NA of the signal ports are ~0.06. The nominal insertion loss of the signal propagation through the PSC is 0.15 dB. A cladding light stripper (CLS2) is utilized to filter out the unabsorbed residual pump light in the amplification stage. The YDFs in the amplification stage are specially designed to restrict the doping ratio to ~80% around the center of fiber core, with the aim of mitigating the TMI and help to achieve a nearly single-mode laser. The previous reports showed that the NA also has a remarkable influence on the TMI threshold [23]. In this work, the core/inner cladding diameters of YDFs are 25/500 µm, while the NAs are distributed at 0.058, 0.053, and 0.048. The detailed parameters of the YDFs are shown in Table 1. The amplified signal laser is output through a home-made quartz beam head (QBH) with a pigtailed fiber of 25/250 µm. The total delivery fiber length of the CLS3 and the pigtailed fiber of QBH is ~2.5 m. The performances of the three YDFs, especially the TMI thresholds, are measured and compared.
The YDFs applied in the amplifier stage of the experiment were fabricated using the modified chemical vapor deposition (MCVD) process combined with the solution-doping technique [22,23]. In the experiment, the adopted lengths of all three YDFs are ~28 m, although they differ in their absorption coefficients. In consideration of the wavelength shift in the 976 nm LDs, an abundant total absorption is employed in the experiment. All the total absorptions at the wavelength of 976 nm exceed 20 dB. As the LDs shift from the nominal 976 nm, the abundant pump absorption varies smoothly, avoiding abrupt changes in the laser efficiency. In the counter-pump configuration, the excessive YDF lengths have little influence on the TMI thresholds of the fiber amplifier. The identical YDF lengths enable identical coiling of the YDF, which avoids the influence of the fiber coiling on the TMI thresholds. The coiling of YDF is depicted in Figure 1. The YDFs are coiled in the grooves with a racetrack shape on a heat sink. The heat sink achieves efficient cooling through a circulating water-cooling method. The minimum bending diameter is ~18 cm, and the maximum bending diameter is ~26 cm. The grooves provide effective heat conduction and avoid excessive heat accumulation in the YDFs.
In this work, the TMI thresholds and power-scaling ability of the MOPA fiber laser based on the three YDFs are measured and recorded. The output laser power, optical spectra, and beam-quality factors are recorded in the experiment. The temporal stability of the output laser is measured by a photodetector with a bandwidth of 12 MHz, which helps to identify the modulations as the TMI occurs. The laser beam quality is measured by an Ophir Photonics SG 240S beam-propagation analyzer (BeamSquared, Jerusalem, Israel).

3. Results and Discussion

The performances of MOPA fiber lasers based on three YDFs are measured and compared, respectively. In the experiment, the fiber lasers are scaled to the power levels around the TMI thresholds. The output powers and optical conversion efficiencies are shown in Figure 2a. Performances of three YDFs correspond to different symbols in the figures. As fiber-I is employed in the amplifier stage, the fiber laser is scaled to a maximum of 4.14 kW, with an optical efficiency of ~83.6%. As fiber-II is employed, the fiber laser is scaled to a maximum of 5.24 kW, with an optical efficiency of ~81.8%. Both are limited by the onset of the TMI effect. As the fiber-III is employed in the fiber amplifier stage, an output power of 6.79 kW is achieved, with an optical efficiency of ~76.5%. Further power scaling is primarily limited by the pump/signal combiner operating at a high temperature, which combines a pump power of ~8.83 kW.
The recorded laser M2 factors are shown in Figure 2b. As the output laser power increases, the M2 factors deteriorate slightly. As for the fiber-I, the M2 factor of the output laser is ~1.27 at a low power level and deteriorates to 1.57 around the TMI threshold of 4.14 kW. As for the fiber-II, the M2 factor of the output laser is ~1.30 at the low power level and deteriorates to 1.56 around the TMI threshold of 5.24 kW. As for the fiber-III, the beam-quality factors are superior along the power scaling. The M2 factor is ~1.14 at a low power level. As the power reaches ~6.7 kW, the M2 factor increases to ~1.47, which is mainly caused by heat-induced distortions. In comparison, the YDF with a lower NA shows a better power-scaling ability and beam quality.
When the TMI effect occurs in the fiber laser amplifier, the output laser will show a dynamic energy coupling phenomenon between the fundamental mode and high-order modes. At this time, the frequency spectrum corresponding to the time-domain signal will show significant frequency-characteristic peaks, and the standard deviation of the measured time-domain signal shows a non-linear growth trend. To verify the occurrence of TMI in the fiber lasers, the temporal signals are recorded and the temporal stabilities are evaluated. The standard deviations (STD) of the normalized temporal signal are shown in Figure 3. As for the fiber-I and fiber-II, the STD increased exponentially around the TMI thresholds of 4.14 kW and 5.24 kW. The time-domain signals and corresponding frequency-domain Fourier spectra around the TMI threshold are shown in Figure 4a–d. There are obvious periodical modulations at kHz-level, which correspond to features of TMI. Combined with the output power, optical efficiencies, and laser beam quality, the occurrence of TMI can be confirmed at the operation of 4.14 kW and 5.24 kW as the fiber-I and fiber-II are employed, respectively. As for the fiber-III, the STD of the normalized temporal signals are all below 0.01, along the power scaling to 6.79 kW. The time-domain signals of the MOPA fiber laser operating at 6.5 kW and 6.79 kW are depicted in Figure 4e. The time-domain signals are quite stable. The corresponding Fourier spectra are shown in Figure 4f. There is no evidence of TMI occurrence at 6.5 kW and 6.79 kW. It can be concluded that fiber-III has a higher TMI threshold than fiber-I and fiber-II. The experimental results indicate that TMI threshold can also be remarkably enhanced by reducing the NA of the confined-doped YDF. It is simple and effective, but requires elaborate control of the refractive index of the ytterbium-doped fiber core.
The optical spectra of the MOPA fiber laser based on fiber-III are depicted in Figure 5a. The signal laser is centered at the wavelength of 1080.0 nm, with a full width at half maximum (FWHM) of ~3.2 nm. There is weak Raman Stokes light in the spectrum at the operation of 6.79 kW, and the intensity is ~42 dB below the intensity of signal laser. The SRS is not the primary limitation in the MOPA fiber laser, which benefits from the temporal stable oscillator seed and the counter-pumped fiber amplifier configuration. The laser beam quality at the operation of 6.79 kW is shown in Figure 5b. The measured M2 factors in the x and y directions are 1.53 and 1.41. The inset is the beam profile at the beam waist. The laser beam quality maintains nearly-single-mode, although there is slight mode distortion at high-power operation.
For further power-scaling, the pump/signal combiner has to be optimized, especially the power handling ability. In the experiment, there is a significant temperature rise in the PSC as the laser power increases. For every 1.0 kW increase in laser power, the temperature of PSC rises by ~10 °C. When the output power reaches 6.79 kW, the combiner temperature has already reached 87 °C. For the sake of safety, further power scaling is not pursued to avoid heat accumulation in the combiner, which may induce a breakdown. It is promising to further scale the output laser power based on the fiber-III, as long as the limitation of combiner temperature is mitigated.

4. Conclusions

In summary, the use of a low-NA confined-doped designed YDF has been explored to achieve a high-power nearly-single-mode laser. Three types of 25/500 µm YDFs are manufactured with an ~80% doping ratio and respective NAs of 0.058, 0.053, and 0.048. The YDFs are utilized in an LD-pumped high-power fiber amplifier, and corresponding TMI threshold are studied. Based on the YDF of 0.048 NA and an ~80% confined-doping ratio, an output power of 6.79 kW with nearly single-mode beam quality has been achieved, with no evidence of TMI phenomenon. Experimental results indicate that reducing the fiber core NA is a simple and reliable technique to mitigate the TMI. Future work will focus on further power-scaling with available pump power and pump/signal combiners.

Author Contributions

Conceptualization, B.Y.; methodology, F.W., L.Z., M.W. and L.H.; validation, H.T., B.R. and Y.G.; formal analysis, H.T., B.R. and B.Y.; data curation, H.T.; writing—original draft preparation, H.T.; writing—review and editing, B.Y.; visualization, Z.C., H.X., Z.L. and P.M.; supervision, B.Y.; project administration, Z.W. and B.Y.; funding acquisition, Z.W. and B.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Science (XDB0650000), Open Research Fund of State Key Laboratory of Pulsed Power the Laser Technology, Electronic Countermeasure Institute, National University of Defense Technology (SKL2022ZR02), and the Science and Technology Innovation Program of Hunan Province (2021RC4027).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Experimental setup of the MOPA fiber laser.
Figure 1. Experimental setup of the MOPA fiber laser.
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Figure 2. Comparison of output laser test results based on three fibers amplifiers: (a) power and efficiency curves; (b) beam-quality test results.
Figure 2. Comparison of output laser test results based on three fibers amplifiers: (a) power and efficiency curves; (b) beam-quality test results.
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Figure 3. Variation in time-series signal standard deviation with power based on three fiber amplifiers.
Figure 3. Variation in time-series signal standard deviation with power based on three fiber amplifiers.
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Figure 4. Output characteristics of time-domain and frequency-domain signals from three fibers: (a) Fourier spectra at 3.79 kW and 4.14 kW for Fiber-I; (b) time trace at 3.79 kW and 4.14 kW for Fiber-I; (c) Fourier spectra at 4.75 kW and 5.24 kW for Fiber-II; (d) time trace at 4.75 kW and 5.24 kW for Fiber-II; (e) Fourier spectra at 6.50 kW and 6.79 kW for Fiber-III; (f) time trace at 6.50 kW and 6.79 kW for Fiber-III.
Figure 4. Output characteristics of time-domain and frequency-domain signals from three fibers: (a) Fourier spectra at 3.79 kW and 4.14 kW for Fiber-I; (b) time trace at 3.79 kW and 4.14 kW for Fiber-I; (c) Fourier spectra at 4.75 kW and 5.24 kW for Fiber-II; (d) time trace at 4.75 kW and 5.24 kW for Fiber-II; (e) Fourier spectra at 6.50 kW and 6.79 kW for Fiber-III; (f) time trace at 6.50 kW and 6.79 kW for Fiber-III.
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Figure 5. Output laser performance of Fiber III: (a) optical spectrum; (b) output laser beam quality at 6.79 kW.
Figure 5. Output laser performance of Fiber III: (a) optical spectrum; (b) output laser beam quality at 6.79 kW.
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Table 1. The detailed parameters of the fibers.
Table 1. The detailed parameters of the fibers.
Fiber TypeCore Diameter (µm)NAClad Diameter (µm)Yb-Doping
Ratio (%)		Absorption Coefficient at 915 nm (dB/m)Fiber Length (m)
Fiber-I25.10.058499.0~800.34~28
Fiber-II25.20.053500.1~800.31~28
Fiber-III25.40.048500.6~800.26~28
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Tang, H.; Rao, B.; Gan, Y.; Yang, B.; Wang, F.; Zhang, L.; Wang, M.; Hu, L.; Chen, Z.; Xiao, H.; et al. 6.7 kW LD-Pumped Nearly-Single-Mode MOPA Fiber Laser Enabled by Low-NA Confined-Doped Fiber. Photonics 2025, 12, 971. https://doi.org/10.3390/photonics12100971

AMA Style

Tang H, Rao B, Gan Y, Yang B, Wang F, Zhang L, Wang M, Hu L, Chen Z, Xiao H, et al. 6.7 kW LD-Pumped Nearly-Single-Mode MOPA Fiber Laser Enabled by Low-NA Confined-Doped Fiber. Photonics. 2025; 12(10):971. https://doi.org/10.3390/photonics12100971

Chicago/Turabian Style

Tang, Hengyu, Bingyu Rao, Yufei Gan, Baolai Yang, Fan Wang, Lei Zhang, Meng Wang, Lili Hu, Zilun Chen, Hu Xiao, and et al. 2025. "6.7 kW LD-Pumped Nearly-Single-Mode MOPA Fiber Laser Enabled by Low-NA Confined-Doped Fiber" Photonics 12, no. 10: 971. https://doi.org/10.3390/photonics12100971

APA Style

Tang, H., Rao, B., Gan, Y., Yang, B., Wang, F., Zhang, L., Wang, M., Hu, L., Chen, Z., Xiao, H., Li, Z., Ma, P., & Wang, Z. (2025). 6.7 kW LD-Pumped Nearly-Single-Mode MOPA Fiber Laser Enabled by Low-NA Confined-Doped Fiber. Photonics, 12(10), 971. https://doi.org/10.3390/photonics12100971

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