The 7 kW narrow linewidth fiber amplifier has achieved a significant enhancement in performance through the precise adjustment of the refractive index of the large mode area active fiber

1. Introduction High-power fiber lasers with nearly diffraction-limited (NDL) beam quality have emerged as a focal point of research in laser technology due to their relatively low cost, high output power, and ease of maintenance [see Richardson, Nilsson, and Clarkson1; Zuo and Lin4]. Narrow linewidth fiber amplifiers based on the master oscillator power amplifier (MOPA) and all-fiber format are the most versatile and efficient configurations for power scaling of monolithic fiber sources in applications such as spectral beam combining and coherent beam combining [see Zheng, Yang, Wang, Hu, Liu, Zhao, Chen, Liu, Zhao, He, and Zhou5; Liu, Ma, Su, Tao, Ma, Wang, and Zhou6]. Previous studies have indicated that the output power limitations of NDL fiber amplifiers are governed by pump brightness, non-linear effects, and thermal effects [see Dawson, Messerly, Beach, Shverdin, Stappaerts, Sridharan, Pax, Heebner, Siders, and Barty7; Wang11]. Among these, stimulated Raman scattering (SRS) and transverse mode instability (TMI) have been identified as the primary constraints in high-power NDL fiber amplifiers. For narrow linewidth spectral characteristics, stimulated Brillouin scattering (SBS) also requires careful consideration [see Kobyakov, Sauer, and Chowdhury12]. To date, power levels exceeding 10 kW have been achieved with tandem pump configurations featuring broad spectrum emission [13, see Shiner14]. In particular, direct pumping with semiconductor laser diodes (LDs) has provided up to 6 kW of power in both broad and narrow linewidth emissions by comprehensively balancing SRS, TMI, and even SBS [see Yang, Wang, Zhang, Xi, Shi, Wang, and Xu15; Wang, Song, Chen, Ren, Ma, Liu, Yao, and Zhou16].

2. The thresholds for SBS and SRS are inherently linked to the total net gain of the Raman and Brillouin Stokes lasers [see Smith17; Liu, Ma, Lv, Xu, Zhou, and Jiang19]. In practical narrow linewidth fiber amplifiers, SBS suppression typically focuses on constructing seed lasers for broadening the SBS gain spectrum, where phase modulation of single-frequency lasers is the mainstream approach to alleviate spectral broadening while maintaining a higher SRS threshold during cascaded amplification [see Flores, Robin, Lanari, and Dajani20; Liu, Song, Ma, Xiao, and Zhou23]. For SRS suppression, strategies such as a large effective mode field area (EMFA) for the fundamental mode (FM), high pump absorption coefficient, shorter active fiber length, and backward pumping can be employed [see Dawson, Messerly, Beach, Shverdin, Stappaerts, Sridharan, Pax, Heebner, Siders, and Barty7; Otto, Jauregui, Limpert, and Tünnermann8; Wang11]. The TMI threshold is determined by various factors in system design, such as signal/pump wavelength, core/cladding diameter, core numerical aperture (NA), and pump power distribution. The impact of these factors on the TMI threshold has been extensively studied, and techniques for manipulating thermally induced refractive index gratings and reducing the fiber’s higher-order mode (HOM) losses have been validated [see Tao, Wang, and Zhou24; Jauregui, Stihler, and Limpert25]. Nonetheless, coiling the active fiber to increase the relative loss of HOMs is one of the few effective strategies for raising the TMI threshold to above 5 kW in narrow linewidth systems [see Wang, Song, Chen, Ren, Ma, Liu, Yao, and Zhou16; Ma, Xiao, Liu, Zhang, Wang, Leng, and Zhou26; Liao, Luo, Xiao, Shu, Cheng, Zhang, Xing, Li, Dai, and Li29]. It is noteworthy that the effectiveness of this technique for TMI suppression diminishes with increasing core diameter and NA [see Tao, Su, Ma, Wang, and Zhou30]. Importantly, SRS and TMI are also interrelated in the design of high-power fiber amplifiers, with system design trade-offs existing in terms of thermal load, effective fiber length, and core diameter [see Tao, Wang, and Zhou24; Jauregui, Stihler, and Limpert25; Hejaz, Shayganmanesh, Rezaei-Nasirabad, Roohforouz, Azizi, Abedinajafi, and Vatani31; Distler, Möller, Strecker, Palma-Vega, Walbaum, and Schreiber34]. Moreover, the TMI threshold in narrow linewidth fiber amplifiers is lower than.

The main amplifier is powered by 12 976 nm wavelength-stable LDs arranged in a bi-directional pumping configuration, and these LDs are coupled into the large mode area ytterbium-doped active fiber (LMA-YDF) through two fiberized (6 + 1) × 1 signal pump combiners. Each LD can provide a maximum pump power of 0.87 kW. The core/cladding diameters of the transmission fibers are 20/250 and 20/400 μm, with a numerical aperture (NA) of approximately 0.063 for the forward signal pump combiner, and 25/400 and 25/250 μm for the transmission fibers with an NA of approximately 0.065 for the backward fiber. Two cladding power strippers (CPS) based on etching technology are connected in series to remove residual pump and cladding signals. A quartz block holder (QBH) is used to interface the fiber laser output to free space. In the experiment, the total length of the transmission fibers after the YDF is approximately 1.8 m.

The YDF used in the main amplifier is a 14 m-long domestic LMA-YDF with a uniform core/cladding diameter of 20/400 μm, manufactured using an improved chemical vapor deposition (MCVD) process. The measured core NA and absorption coefficient at 976 nm are 0.059 and 1.2 dB/m, respectively. For the design of the YDF bending and cooling plate structure, a symmetric dual-track active water-cooling plate is employed to incorporate bending losses to suppress transverse mode instability (TMI). The inner radius is set to 40 mm, with a spacing of 1 mm between the two adjacent circles, as shown in Figure 2. The seed laser is injected into the inner circle of the main amplifier to increase the relative loss of higher-order modes in the large thermal gradient region, thus achieving TMI suppression [see Tao, Su, Ma, Wang, and Zhou30].

3. The optimization of various fiber parameters can simultaneously affect the thresholds for transverse mode instability (TMI) and stimulated Raman scattering (SRS) in high-power fiber amplifiers, such as core/cladding diameter, core numerical aperture (NA), doping area ratio, cladding pump absorption coefficient, or effective fiber length. For most of these factors, the thresholds for SRS and TMI are in conflict with each other in the design of high-power fiber amplifiers [see Tao, Wang, and Zhou24; Jauregui, Stihler, and Limpert25; Hejaz, Shayganmanesh, Rezaei-Nasirabad, Roohforouz, Azizi, Abedinajafi, and Vatani31; Distler, Möller, Strecker, Palma-Vega, Walbaum, and Schreiber34]. It is worth noting that a decrease in NA can increase the effective mode field area (EMFA) of the fundamental mode (FM) and enhance the SRS threshold for near-diffraction-limited (NDL) operation [see Smith17]. Additionally, a reduction in NA can be achieved through a specific winding method to increase the relative loss of higher-order modes (HOMs), which also has the advantage of suppressing TMI [see Tao, Su, Ma, Wang, and Zhou30; Tao, Ma, Wang, Zhou, and Liu38].

Generally, the EMFA and loss characteristics of FM have been widely estimated by using the core diameter, NA, and winding radius of the active fiber [see Schermer and Cole39]. However, the core NA is often an average approximation of the refractive index profile (RIP) cross-section, which is difficult to estimate accurately. Therefore, in this work, the optimization of the active fiber was conducted under the control of the cross-sectional RIP. Figure 3 illustrates the measured RIP of the domestic YDF (Fiber_A), and also provides the RIP of the YDF (Fiber_B) used in the previously reported 6.12 kW narrow linewidth fiber amplifier for comparison [see Wang, Song, Chen, Ren, Ma, Liu, Yao, and Zhou16]. The peak-valley (PV) value is approximately 4 × 10–4 and has been tentatively generated in a inverted triangle with a base of about 5.5 μm. The addition of the settlement region will sacrifice the FM’s EMFA to some extent, but the bending loss for TMI suppression is significantly increased. Furthermore, the decrease in the FM’s EMFA can be compensated by reducing the average core NA. Compared to the RIP of Fiber_B, the average core NA of Fiber_A is reduced from 0.061 to 0.059. Overall, the FM’s EMFA and the bending loss of HOMs can be increased simultaneously (as shown below), which is beneficial for achieving higher fiber amplifier output power by better balancing SRS and TMI.

The EMFA of FM for the two YDFs was calculated using finite element methods, based on the measured RIP shown in Figure 2, as depicted in Figure 4(a). As shown in Figure 4(a), when the winding radii are different, the EMFA of a specific YDF changes very little. In various scenarios, the EMFA of Fiber_A is 12.4% larger than that of Fiber_B. Based on the EMFA of FM for the two YDFs, we can simulate the enhancement ratio of the SRS threshold for the fiber amplifier using Fiber_A compared to the fiber amplifier using Fiber_B, as described in the traditional SRS model [see Liu, Ma, Lv, Xu, Zhou, and Jiang19]. Figure 4(b) illustrates the corresponding SRS enhancement factor when the power ratio of the backward pump is different. As shown in Figure 4(b), the SRS enhancement decreases with an increase in the backward pump ratio, and in different cases, the SRS

The loss coefficients for LP01 and LP11 modes of the two YDFs are displayed in Figures 5(a) and 5(b), respectively. As depicted in these figures, the total loss coefficients for LP01 and LP11 modes of a specific YDF decrease as a function of the winding radius. Figure 5(a) shows that, at the same winding radius, the LP01 mode loss coefficient of Fiber_A is consistently higher than that of Fiber_B. For instance, when the winding radius is set at 4 cm, the LP01 loss coefficients for Fiber_A and Fiber_B are 0.059 dB/m and 0.026 dB/m, respectively. Figure 5(b) reveals that when the winding radius ranges from 4.5 to 6.0 cm, the LP11 mode loss coefficient of Fiber_A is comparable to that of Fiber_B. However, it is noteworthy that when the winding radius is less than 4.25 cm, the LP11 mode loss coefficient of Fiber_A is greater than that of Fiber_B. When the coil radius is set at 4 cm, the LP11 mode loss coefficient of Fiber_A is 227.7 dB/m, which is 1.5 times higher than that of Fiber_B (151.6 dB/m).

4. First, the spectra of the modulated master oscillator (MO) and the spectrum after the PAM were characterized, as shown in Figure 6. The results from Figure 6 indicate that, during the preamplification process, the spectral distribution and linewidth are well-preserved. After the PAM, the measured 3 dB linewidth is 0.83 nm.

Next, the characteristics of the narrow linewidth fiber amplifier with backward pumping were investigated, and the output power conversion with pump power was displayed in Figure 7(a). The slope efficiency is 83.3%, enabling the achievement of 4.20 kW of output power at a pump power of 5.16 kW, with an optical-to-optical conversion efficiency of 81.5%. The output spectra at several typical output powers are shown in Figure 7(b). At the maximum output power, the measured 3 dB spectral linewidth is 0.41 nm. In the experiment, when the backward pump power was increased from 4.90 kW (signal power 4.00 kW) to 5.16 kW (signal power 4.20 kW), a noise-like envelope was observed in the Fourier transform of the time trace within 5 kHz, as shown in Figure 7©. Therefore, for the narrow linewidth fiber amplifier with backward pumping, TMI occurs at a pump power of 5.16 kW.

Similarly, the output power, spectrum, and Fourier transform of the time trace of the narrow linewidth fiber amplifier with forward pumping were examined, with typical results presented in Figures 8(a)–8©. As shown in Figure 8(a), the output power linearly increases to 2.69 kW at a pump power of 4.23 kW, with a slope efficiency of 63.9%. When the pump power is further increased to 4.38 kW, the power saturates at 2.70 kW. At a pump power of 4.23 kW, the optical-to-optical conversion efficiency is 63.7%, and the measured 3 dB spectral linewidth is 0.75 nm (as shown in Figure 8(b)). Figure 8© illustrates the Fourier envelope induced by TMI within 5 kHz. By comparing the power conversion before and after, it can be inferred that there is a significant difference in conversion efficiency in the experiment. This phenomenon may be attributed to the power distribution difference between forward and backward pumping, as the overall loss caused by the bent active fiber is a comprehensive effect of power distribution and loss coefficients.

Figure 9(a) depicts the output power of the fiber amplifier in a bidirectional pumping configuration. In this scenario, we first increased the forward pump power, expanding the output power to 1.01 kW, then increased the backward pump power to further boost the output power to 5.35 kW, and finally increased the forward pump power to 7.03 kW. As shown in Figure 9(a), the output power of the signal laser almost linearly increases with the pump power. At a pump power of 9.4 kW, the maximum output power is 7.03 kW, with an overall conversion efficiency of 74.2%. The corresponding forward and backward pump powers are 4.23 kW and 5.16 kW, respectively. An interesting observation is that the limit of output power in bidirectional pumping is slightly higher than the total power from separate backward and forward pumping, which were 4.23 kW and 5.16 kW, respectively. This result may be attributed to the fact that the temperature increase at the fiber’s end expands the core numerical aperture (NA) and reduces the transmission loss caused by the bending of the forward signal due to bidirectional pumping [see Schermer and Cole39; Brown and Hoffman40].

In our experiment, we measured the backward power collected by the optical circulator (OC) to monitor the SBS effect. The backward power ratio (Rb), defined as the ratio of the backward power to the output power, varies with the output power as shown in Figure 9(b). Generally, the slope of the Rb increase with forward pumping is greater than that with backward pumping. Specifically, when the output power is increased from 1.01 kW to 5.35 kW with backward pumping, Rb changes from 0.017‰ to 0.031‰. When the forward pump power is injected, Rb increases nearly linearly with power scaling. At the maximum output power, Rb is calculated to be within 0.05‰. Throughout the amplification process, we did not observe an exponential nonlinear growth trend, indicating that the fiber amplifier operates below the SBS threshold.

Figure 10(a) illustrates the output spectra of the signal laser at various output powers. As depicted in Figure 10(a), the spectral wings broaden with the enhancement of SRS. This phenomenon can be understood through the spectral wing broadening induced by four-wave mixing [see Liu, Song, Ma, Xiao, and Zhou23]. At the maximum output power, the 3 dB and 20 dB spectral linewidths of the signal laser are 0.76 nm and 2.92 nm, respectively. The spectral component near 1135 nm starts to increase at an output power of 5.35 kW. At the maximum output power, the peak signal-to-noise ratio (SNR) is 31.4 dB, which is higher than the Raman Stokes light. The corresponding power ratio of the Raman Stokes light (defined by the Raman ratio) is calculated by dividing the integrated spectrum from 1110 to 1150 nm by the integrated spectrum from 1050 to 1150 nm, as shown in Figure 10(b). The results indicate that the Raman ratio remains around –49.0 dB when the output power is below 5.35 kW, slightly increases to –43.7 dB at 6.10 kW, and rapidly reaches –24.5 dB (0.36%) at the maximum output power.

Figure 10© and 10(d) demonstrate the normalized time traces and corresponding Fourier transforms of the signal laser at different output powers. The increasing trend in intensity fluctuations and frequency envelopes suggests that the narrow linewidth fiber amplifier is operating near its TMI threshold. At the maximum output power, the beam quality (M2 factor) is measured using M2-200s, with Mx2 = 1.26 and My2 = 1.25 for the x and y directions, respectively. The inset in Figure 10(d) is the beam profile at the focal length of M2-200s, which also indicates that NDL beam quality has been achieved.

Figure 11 illustrates the 3 dB spectral linewidth of the signal laser measured under various output powers and pumping processes. As depicted in Figure 11, the overall 3 dB spectral linewidth is influenced by the output power, the direction of pump power, and the ratio of forward to backward pump power, and it becomes unstable as the power scales up. The phenomenon observed in Figure 11 may be attributed to the intensity noise fluctuations caused by the phase modulation process, which can generate frequency chirp through the effect of self-phase modulation [see Liu, Ma, Lai, Song, Zhang, Li, Xiao, and Zhou41; Liu, Feng, Wang, Wang, Li, Liu, Shi, Wei, Yan, Peng, Sun, Shang, Ma, Gao, and Tang43]. This can either enhance or attenuate the original phase modulation at different output powers, ultimately leading to changes in the 3 dB spectral linewidth.

5. In this work, we have demonstrated a record-breaking 7 kW all-fiber narrow linewidth fiber amplifier with near-diffraction-limited (NDL) beam quality. The use of a single-frequency seed laser with white noise signal (WNS) modulation effectively suppressed the impact of stimulated Brillouin scattering (SBS). By optimizing the refractive index of the large mode area (LMA) active fiber, we achieved a balanced suppression of stimulated Raman scattering (SRS) and transverse mode instability (TMI). A quantitative analysis was performed to highlight the benefits of the optimized active LMA fiber in simultaneously increasing the effective mode field area (EMFA) of the fundamental mode (FM) and the loss coefficient of higher-order modes (HOMs). The power scaling capabilities, spectral, and temporal characteristics of both forward and backward pumping processes were separately investigated. Additionally, the signal-to-noise ratio (SNR) is 31.4 dB, and the beam quality factor Mx2 = 1.26, My2 = 1.25 of the 7.03 kW narrow linewidth fiber laser was achieved through bidirectional pumping. We believe that this work can provide new insights and design strategies for high-power narrow linewidth fiber lasers with NDL beam quality.

Acknowledgment: This work was supported by the National Key R&D Program of China (No. 2022YFB3606400) and the National Natural Science Foundation of China (No. U22A6003).