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Abstract
In this paper, the amplified spontaneous emission (ASE) suppression in a 1050 nm fiber laser with a pump-sharing oscillator-amplifier (PSOA) structure is studied theoretically and experimentally. A theoretical model of a fiber laser with a PSOA structure is established. The characteristics of the ASE for the PSOA structure and the pump-independent oscillator-amplifier (PIOA) structure are compared and analyzed. The experimental results show that the ASE can be effectively suppressed by utilizing the PSOA structure, which agree with the simulation results. A 1050 nm high-power narrow-linewidth fiber laser with PSOA structure is demonstrated, in which the gain fiber lengths of the oscillator and amplifier are 1.6 m and 9 m, respectively, to ensure the interconnection of pump power between the oscillator and amplifier. Finally, the maximum output power of 3.1 kW has been achieved, the linewidth is 0.22 nm at 3 dB, the beam quality M2 ≈ 1.33, and the optical signal-to-noise ratio (OSNR) is 45.5 dB.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power ytterbium-doped fiber lasers with operating wavelengths less than 1060 nm, especially those with narrow linewidths, are necessary for applications such as spectral beam combining and nonlinear frequency conversion [1–3]. Due to a lower quantum defect heating and a stronger gain saturation effect, fiber lasers operating at short wavelengths (less than 1060 nm) have the advantage of suppressing transverse mode instability (TMI) [4]. There has been much research conducted on high-power narrow-linewidth fiber lasers at short wavelengths in recent years [5–9]. However, because of the amplified spontaneous emission (ASE) effect, it is difficult to achieve high-power output with a high optical signal-to-noise ratio (OSNR) with short-wavelength fiber lasers. In 2016, Naderi et al. suppressed the ASE effect by shortening the amplifier gain fiber, achieving a 1 kW laser output at 1034 nm with a spectral linewidth of 0.011 nm and a spectral OSNR of 40 dB [5]. In 2021, Y Xu et al. demonstrated a 2.4 kW single-mode fiber laser at 1045 nm with ∼33 dB OSNR, which is the highest power in previous studies for a narrow linewidth fiber laser in the wavelength range of 1040 nm−1050 nm band [9]. To further optimize the spectral parameters and acquire a narrower linewidth and high OSNR laser output, different laser structures have become the priority in recent research.
A master oscillator power amplifier (MOPA) with a phase-modulated single frequency (PMSF) seed is an effective spectral control structure [5,8,10–15]. This structure can be used to achieve output powers in the range of several kilowatts with ultranarrow linewidths. In 2020, Chu et al. achieved a 3 kW single-mode output at 1030 nm based on a structure with a 5.5 m amplifier gain fiber. The linewidth of the output laser was 0.18 nm and the OSNR was 37 dB [8]. However, the MOPA structure with PMSF seeds requires high-speed phase modulators to broaden the seed spectrum and improve the stimulated Brillouin scattering (SBS) threshold of the amplifier, a requirement that increases the cost of the laser. Another MOPA structure that uses a fiber Bragg grating (FBG)-based oscillator laser (FOL) as the seed more easily achieves a high-power narrow-linewidth output. However, it is difficult for this structure to restrain excessive spectral broadening once the laser reaches a high power. Previous studies have presented many schemes for compressing the spectral broadening of MOPAs with FBG-based seeds [9,16–18]. In 2016, Huang et al. investigated the association between the number of longitudinal modes of the FBG-based seed and the nonlinear spectrum spreading of the amplifier. Furthermore, seeds with few longitudinal modes are introduced to the amplifier. A narrow-linewidth laser was achieved that exhibited 2.9 kW output power, 0.3 nm linewidth, and 20 dB OSNR [17]. This structure exhibits poor control for spectral OSNR, making it inappropriate for short wavelength high-power narrow-linewidth fiber lasers. In 2019, Huang et al. demonstrated a backward-pumped MOPA structure based on an ultranarrow-linewidth FBG-based seed, achieving a laser output of 0.086 nm linewidth at the 2.19 kW power scale. The amplifier in this structure used a short gain fiber (SGF) and shared the backward pumping power with the oscillator, further suppressing the spectral broadening without sacrificing the optical efficiency [18]. Furthermore, this pump-sharing structure has been applied to inhibit the destabilization of the fiber laser from the reflected laser. Due to this integrated structure, the reflected laser from the fiber core again forms oscillations in the resonant cavity without affecting the laser output [19,20]. However, the effect of the PSOA structure on ASE has not been investigated in these studies.
In this paper, the amplified spontaneous emission (ASE) suppression in a 1050 nm fiber laser with the PSOA structure is studied. The ASE power of a 1050 nm narrow linewidth fiber laser based on the PSOA structure is simulated under different backward powers. The output characteristics of the PSOA and PIOA structures are also compared experimentally. Based on the theoretical results, a 1050 nm narrow linewidth fiber laser experiment is conducted. The implementation of this structure effectively suppresses the ASE, resulting in a high OSNR narrow-linewidth output laser. The effect of thermal control on the system is also discussed.
2. Theory
Figure 1 shows the theoretical model of a fiber laser based on a PSOA configuration. The design of this laser adopts a bi-directional pump method. The first ytterbium-doped fiber (YDF1) absorbs the photons radiated by the pump light and oscillates through the high and low reflection gratings to form the seed laser, which is then scaled by the amplifier. The seed has a longitudinal mode interval of Δf = c/(2nL), where c, n, and L are the speed of light in vacuum, refractive index of silica fiber, and length of the laser cavity, respectively. As the cavity length L is shortened, the longitudinal mode spacing in the resonant cavity increases, resulting in a decrease in the number of longitudinal modes. Therefore, the short gain fiber (SGF) seed source can be used to effectively suppress spectral broadening due to four-wave mixing in the amplifier [17]. Compared to the MOPA system with an independent pump [9], there is no pump isolation device in PSOA. The seed and the amplifier directly share the bidirectional pump. The power distribution of the PSOA structure laser can be obtained using the rate equation model with a two-level energy structure for Yb3+. For SGF seed sources, ignoring nonlinear effects, the rate equation can be expressed as [21]
Fig. 1. Theoretical model of a fiber laser based on a pump-sharing oscillator-amplifier configuration.
3. Simulation
The steady-state power distribution of the fiber laser based on a PSOA configuration is studied in the simulation. The pump wavelength is 976 nm and the signal wavelength is 1050 nm. σep is 1.77×10−24 m−2, σap is 1.71×10−24 m−2, σes is 4.81×10−25 m−2, σas is 1.42×10−26 m−2 [24], τ is 0.901 ms, α is 3×10−3 m−1, N0 is 7.03 ×1025 m−3, $N_0^\ast$ is 6.43×1025 m−3 [25], Γp is 2.5 × 10−3, $\Gamma_p^\ast$ is 3.9 × 10−3, Γs and $\Gamma_s^\ast$ are all set to 1. The core/cladding diameters of the gain fibers in the oscillator and the amplifier are set to 20/400 µm and 25/400 µm, with numerical apertures (NAs) of 0.065/0.46 (core/cladding) and lengths of 1.6 m and 9 m, respectively. The reflectances R1 and R2 of the HR and the OC are 99% and 14%, respectively, for the oscillator. The forward pump power is constant at 500 W, and the maximum backward pump power is 4 kW.
The red dotted line and the blue dotted line in Fig. 2(a) show the variations in the optical-to-optical power conversion efficiency (PCE) and the output power of the PSOA structure with the backward pump power, respectively. At the maximum pump power, the PCE is 86.8% and the output power is 3.9 kW. The red solid line and the bule solid line in the figure show the variations in the PCE for the pump-independent oscillator-amplifier (PIOA) structure, which includes a pump isolation device between the seed and the amplifier. When only a forward pump is applied, the PCE of the laser is only 37%. At a backward pump power of 4 kW, the PCE is 78% with an output power of 3.51 kW. With the same backward pump power, the PCE of the PSOA structure is significantly higher than that of the PIOA structure due to the pump-sharing feature of the PSOA. The blue solid line with star points in Fig. 2(b) shows the variation in the forward ASE power with the backward pump power for the PIOA structure, while the red solid line with diamond points shows its seed power variation. Due to the existence of the pump isolation device, the seed power of the PIOA structure remains constant with that of the backward pump. The forward ASE power increases to 699 µW at a backward pump power of 4 kW. The variation in the seed power with the backward pump power of the PSOA structure is shown by the curve with red circles in Fig. 2(b). Because the residual backward pump is applied in the seed, the seed signal power increases with the backward pump power. The dynamic variation in the seed power serves to maintain the power proportion in the amplifier gain fiber, reserving the utilization of the seed light for the upper state population of the amplifier. Thus, at the maximum backward pump power, the forward ASE power is approximately 2/3 of that in PIOA, as shown by the blue curve with triangular points in Fig. 2(b). Figure 2(c) represent the variations of the ASE power of the seed and the residual backward pump power with the backward pump power of the PSOA structure, respectively. The forward ASE power of the seed changes slowly when the residual backward pump power continues to increase with backward pump power. At the maximum backward pump power, the forward ASE power of the seed with short-gain-fiber is only up to 21 µW, which is much smaller than the seed power given in Fig. 2(b). Based on the simulation results, it is evident that the PSOA structure can effectively suppress the ASE effect in the 1050 nm fiber laser.
Fig. 2. (a) Variations in the laser output power and efficiency with laser pump power for different structures. (b) Variations in the oscillator output power and forward ASE power with backward pump power for different structures. (c) Variations in residual backward pump power and seed ASE power with backward pump power for the PSOA structure.
4. Experiment setup
The experimental setup for the fiber laser based on a PSOA configuration is shown in Fig. 3. The laser includes two parts: the oscillator and the amplifier. The linear cavity in the oscillator is composed of a pair of 20/400 µm fiber Bragg gratings (FBGs) that each have a reflectivity spectrum centering at 1050 nm. The reflectance of the high reflection (HR) grating and the output coupling (OC) grating at 1050 nm is 99% and 14%, respectively. The 3 dB bandwidth of HR-FBG is 0.27 nm and that of OC-FBG is 0.07 nm. A 1.6 m ytterbium-doped fiber (YDF) with a core diameter of 20 µm and an inner-cladding diameter of 400 µm is used as the gain medium; its nominal cladding absorption is 1.2 dB/m@975 nm. The bending radius of the 20/400µm YDF is about 40 mm. Two 350 W wavelength-locked fiber-coupled laser diodes (Everbright Photonics, China) are applied as forward pump sources for the oscillator, which are injected into the cavity through a (2 + 1) × 1 multimode pump coupler (MPC1) (Lightcomm, China). The output fiber tail of the OC is directly connected to the amplifier gain fiber. The amplifier gain fiber is a YDF with a core/internal cladding diameter of 25/400 µm and a length of 9 m. The core/inner cladding of the gain fiber has a NA of 0.065/0.46 and a nominal cladding absorption coefficient of 1.8 dB/m@975 nm. The bending radius of the 25/400 µm YDF is about 65 mm. A backward cascaded pump structure is used for the amplifier. Thirty 976 nm non-wavelength-locked LDs (nLGHT, IOFQE-e18) are used as pump sources, where each LD is capable of delivering approximately 120 W pump light. Every three LDs forms a group. Each group of the LDs operates at the rated power. The pump power of the amplifier is determined by the number of LDs groups in operation. In this case, the wavelength of the pump light is all around 976 ± 1 nm. The pump light is coupled into a (6 + 1) × 1 MPC2 (Juhere Photonics, China) through a 7×1 MPC3 (Lightcomm, China) and then injected into the amplifier gain fiber. An end cap is connected to the output fiber of the MPC2 to reduce the laser reflections while outputting a high-power laser. A power metre (Spiricon, 10 kW) is used to measure the output laser power directly. The spectra of the scattered light of the power metre are measured by fiber patch cords with core/inner-cladding diameters of 9/125 µm and 400/440 µm via YOKOGAWA's AQ6370C.The beam quality is measured by the BeamSquared (Ophir, State of Israel). The temporal characteristics of the output laser are measured with an oscilloscope (DPO3032, Tektronix) having a 125 MHz bandwidth photodetector (TIA-525I-FC, Terahertz Technologies), by coupling the scattered light of the power meter into the patch cord.
Fig. 3. Schematic diagram of the fiber laser based on the pump-sharing oscillator-amplifier configuration.
5. Experiment results
5.1 Suppression of the amplified spontaneous emission (ASE)
The blue solid line and the blue dotted line in Fig. 4(a) show the variations in the output power (Po) of the PSOA structure and the PIOA structure with the backward pump power at the same forward pump power of 0.4 kW, respectively. The PIOA system is built by adding a cladding power stripper (CPS) between the oscillator and the amplifier in the PSOA system described above. The PCE of the PSOA structure is 76.1% with an output power of 1.4 kW at the backward pump power of 1.44 kW, while The PCE of the PIOA structure is 10.9% lower than that of the PSOA with the maximum outpower of 1.2 kW. The changes of the partial cladding signal power ($P_{\textrm{cs}}^{\textrm{partial}}$) with the backward pump power of the PSOA and PIOA structures, represented by the red solid line and the red dotted line in Fig. 4(a), respectively. The partial cladding signal power is measured through the tail fiber of the backward (6 + 1) × 1 MPC of the two structures. The partial cladding signal light of the PSOA increases with the backward pump power and by 24.1 W at the maximum pump power, while that of the PIOA increases more slowly ending up with only 7.5 W. Although the PCE can be improved by the PSOA structure in short-fiber lasers, the cladding signal light of the seed is not stripped off and scaled up in the amplifier of the structure. Thus, MPCs and LDs in the PSOA structure endure more cladding signal power than those in the PIOA structure. As shown in the inset of Fig. 4(a), the beam quality of the PSOA structure is M2x = 1.35 and M2y = 1.22 at 1.4 kW, while the beam quality of the PIOA structure is M2x = 1.28 and M2y = 1.20 at 1.2 kW. The M2 results of the PSOA and PIOA structures are relatively consistent, which can be attributed to the power stripper ability of the backward pump-signal combiner. Thus, the pump-sharing structure has little effect on the beam quality of the amplifier.
Fig. 4. (a) Variations in the laser output power and the partial cladding signal power with laser pump power for different structures, (b) Comparisons of the laser spectra with different configurations at the pump power of 1.44 kW.
The blue curve in Fig. 4(b) shows the output spectrum of the PSOA structure at a forward pump power of 0.4 kW and a backward pump power of 1.44 kW. The output spectrum has only one spectral line at 1050 nm with an OSNR greater than 53 dB. The red curve in Fig. 4(a) shows the output spectrum of the PIOA system with the same forward and backward pump power. The output spectrum shows an ASE light with an OSNR of 44 dB compared to the 1050 nm signal light spectrum. From the contrast of the OSNR for the two structures, it is clear that the PSOA structure exhibits a better suppression of the ASE effect than the PIOA structure, which agree with the simulation results.
5.2 Laser output characteristics
The blue curve in Fig. 5 shows the output power of the fiber laser based on the PSOA configuration versus the backward pump power (the forward pump power is constant at 400 W). The laser output power increases linearly with the backward pump power, reaching a maximum of 3.1 kW at a backward pump power of 3.64 kW. The power conversion efficiency (PCE) of the laser is only 62.6% when no backward pump power is injected. This result is mainly caused by the relatively low PCE of the oscillator with only a 1.6 m YDF. The PCE can be increased to 76.7% when the backward pump power increases to 3.64 kW, as shown by the red curve in Fig. 5. With the increase of the pump power, the variation of the PCE is decreased gradually. The trend of fluctuations in the PCE of the laser may due to the tolerance of the central wavelength of the non-wavelength-locked LDs.
Fig. 5. Output power characteristics of the fiber laser based on the pump-sharing oscillator-amplifier configuration.
Figure 6(a) shows the output spectra of the PSOA-based fiber laser in the wavelength range of 1030 nm∼1120 nm as the output power increases from 0.25 kW to 3.1 kW. The spectra are measured by coupling the scattered light from the target surface of the power meter into a multimode patch cord (the core/cladding diameter is 400/440 µm). At the maximum output power of 3.1 kW, the OSNR is 45.5 dB. To the best of our knowledge, this is the highest reported output spectrum OSNR for a 1050 nm laser with a magnitude of 3 kW. The spectral noise between 1060 nm and 1110 nm is caused by both the ASE and stimulated Raman scattering (SRS). The ASE light at 1070 nm is not obvious, indicating that the PSOA structure exhibits a good suppression of the ASE effect at high power. The inset in Fig. 6(a) represents the spectral linewidth of the laser. The 3 dB linewidth of the output spectrum is 0.06 nm for an output power of 250 W (forward pumping only) and 0.22 nm for an output power of 3.1 kW. Based on the few-longitudinal-modes oscillator, a narrowband seed laser with a 3 dB linewidth of 0.06 nm is obtained. By sharing the pump power between the oscillator and amplifier, the nonlinear effect-induced spectral broadening can be suppressed by using the short gain fiber without sacrificing PCE. As a result of the spectrum broadening suppression methods, the 3 dB spectral bandwidth at of the 3.1 kW laser can reach 0.22 nm.
Fig. 6. (a) Spectral characteristics of the fiber laser based on the pump-sharing oscillator-amplifier configuration. (b) Comparisons of the output spectra at 3.1 kW measured with different patch cords
Figure 6(b) shows the output spectra measured with different patch cords in the range of 1040 nm∼1060 nm at 3.1 kW. The red spectra measured with 400/440 µm patch cord is of higher signal intensity than the blue spectra measured with 9/125 µm patch cord. The 3 dB linewidth of the red spectra is 1.11 nm, which is 5 times the 3 dB linewidth of the blue spectrum, while the 13 dB linewidth of the red spectra is only 1.2 times that of the blue spectra. The difference in measurement results is caused by the core diameter of different patch cords. More scattered light can be coupled into the OSA by the patch cords with a large core, allowing spectral signals such as ASE effects and SRS effects to be detected. The signal intensity and the wavelength resolution of the measurement results for the spectra are limited by the type of patch cords used [26]. Since the wavelength resolution measured with the patch cord with a core diameter of 400 µm is only 1 nm, the 9/125 µm patch cord with the obtained wavelength resolution of 0.02 nm is more appropriate for 3 dB linewidth measurements.
Figure 7(a) shows the beam quality factors M2 of the laser at different powers. The spot patterns taken at the focal position of the BeamSquared (Ophir, State of Israel) for 500 W, 1.72 kW and 3.1 kW are also shown in the figure. The beam quality comprises M2x = 1.34 and M2y = 1.32 at 3.1 kW. The results show that with the scaling of the output power, the fiber laser maintains a near-diffraction-limited beam quality. Figure 7(b) shows the intensity curve of the laser, and no TMI was observed. In the experiment, the higher-order modes in the fiber core of the amplifier are filtered out by coiling, which prevents the formation of high-contrast interference fields in the fiber core. As a result, the threshold power of the TMI is effectively increased [27]. In addition, the backward pump scheme of the amplifier also contributes to the suppression of TMI [28]. One can conclude that further increases in the output power are limited only by the available pump power.
Fig. 7. (a) M2 factors at different output powers and (b) time domain curves at the maximum output power.
6. Discussion
In this paper, the effect of the temperature of the oscillator and the amplifier on the output spectrum is investigated experimentally. The experimental setup is shown in Fig. 8. Two water-cooled heat sinks (WCHSs) with fiber optic grooves engraved on the surface are installed for controlling the temperatures of the resonant cavity of the oscillator and the gain fiber of the amplifier, respectively. The gain fibers are cured in the fiber grooves with silica gel. The temperature of the WCHSs is controlled by adjusting the temperature of the cooling water in the water chiller.
Fig. 8. Schematic diagram of the fiber laser based on the PSOA structure with temperature control devices
Figure 9(a) shows the output spectra of the fiber laser based on the PSOA in forward-pumping operation at different cooling-water temperatures. In this case, the forward pump power remains at 400 W. The output laser is produced from the seed light amplified by the forward residual pump light. The blue curve in Fig. 9 is the laser output spectrum at 25 °C. Multiple spectral peaks exist in the spectrum. As the temperature decreases, the number of sideband signal peaks in the spectral curve decreases. When the temperature drops to 15 °C, the spectrum has only one main signal peak at 1050 nm, as shown by the black curve in Fig. 9(a). The blue curve in Fig. 9(b) represents the 3 dB linewidth of the output spectrum versus the cooling-water temperature. The 3 dB linewidth increases from 0.06 nm to 0.18 nm and then decreases to 0.12 nm as the applied temperature is regulated from 15 °C to 25 °C. The red curve in Fig. 9(b) shows the variation in the 10 dB linewidth of the output spectrum with the applied temperature. Spectral broadening is suppressed with decreasing applied temperature. The 10 dB linewidth is clearly compressed from 0.62 nm to 0.19 nm when the temperature is reduced from 25 °C to 15 °C. Figures 9(a) and 9(b) show that reducing the oscillator gain fiber temperature contributes positively to the suppression of spectral broadening. The temperature control affects the sidebands of the spectrum more strongly than the main spectral peak. The spectral broadening and the generation of the sidebands in Fig. 9(a) are more likely caused by the wavelength drift of FBGs with temperature. The wavelength drift arises mainly from the temperature dependence of the effective refractive index and the grating period of the FBGs.
Fig. 9. (a) Output spectra for seeds at different applied temperatures, (b) 3 dB linewidth curve (diamonds) and 10 dB linewidth curve (circles) for the output laser at different temperatures.
Figure 10 shows the variations in the laser’s 10 dB linewidth with the oscillator cooling-water temperature at different output powers for the PSOA fiber laser. According to the red and blue curves in Fig. 10, the change in the spectral linewidth with temperature increases gradually from 0.34 nm to 1.47 nm when the laser power increases from 0.5 kW to 3.1 kW. The changes in the output spectral linewidth for the amplifier can be attributed to the fact that the seed linewidth changes with the water temperature and the backward pump power.
Fig. 10. The 10 dB linewidths of the output spectra versus the oscillator gain fiber temperature at different output powers.
Figure 11 presents the output spectral curves of the PSOA fiber laser operating at different oscillator water temperatures in the wavelength range of 1030 nm∼1120 nm. The blue line represents the output spectrum of the oscillator at a cooling-water temperature of 25 °C, where this spectrum has a strong Raman spectral peak and an OSNR of 24 dB. The Raman Stokes peak amplitude decreases substantially when the temperature of the oscillator drops to 15 °C, resulting in a spectral signal-to-noise ratio of 45.5 dB, which is 22 dB lower than that at 25 °C. This phenomenon indicates that the oscillator of the PSOA structure reaches the SRS threshold as the temperature is increased in the oscillator gain fiber. Reducing the temperature of the oscillator gain fiber can effectively suppress the SRS light from the oscillator and improve the output spectrum of the system. The generation of this Raman Stokes light is induced by the partial overlap of the Raman gain spectrum with the Yb3+ emission spectrum in short-wavelength fiber lasers, indicating that the ASE power is positively correlated with the Raman power of the laser [8]. Decreasing the oscillator gain fiber temperature attenuates the accumulation of the upper state population on Yb3+, resulting in a weaker spontaneous radiation in the Yb3+ band. Consequently, the ASE power decreases, allowing the SRS threshold of the oscillator to be increased.
Fig. 11. Output spectra for different amplifier gain fiber temperatures at an output power of 3.1 kW.
Separate from the temperature control of the oscillator gain fiber, the temperature regulation of the amplifier gain fiber can also have an impact on the output spectrum. Figure 12 shows the output spectra of the PSOA laser with different amplifier temperatures. As shown, the spectral peaks at different temperatures almost overlap, and the intensities of the ASE and Raman spectra fade when decreasing the temperature of the amplifier gain fiber. Similar to the temperature regulation of the oscillator, the temperature control of the amplifier gain fiber attenuates spontaneous radiation during amplification, suppressing the ASE and SRS effects [29].
Fig. 12. Output spectra of the laser at 3 kW output power for different amplifier gain fiber temperatures.
7. Conclusions
In conclusion, the suppression of the ASE effect by the PSOA structure is analysed theoretically and experimentally. The theoretical model of the fiber laser with PSOA structure is established. According to the experimental results, it is clear that the PSOA structure exhibits a better suppression of the ASE effect than the PIOA structure, which agree with the simulation results. The PCE can be improved by the PSOA structure in short-fiber, and lasers the pump-sharing structure has little effect on the beam quality of the amplifier. a 1050 nm narrow-linewidth fiber laser based on pump-sharing oscillator-amplifier structure has been built. With an oscillator gain fiber of 1.6 m and an amplifier gain fiber of 9 m, the fiber laser delivers 3.1 kW output power with an OSNR of 45.5 dB and a 0.22 nm spectral linewidth. The beam quality M2 of the laser is approximately 1.33 at the maximum power. No transverse mode instability is observed during the experiments, indicating the potential for further power scaling. The results suggest that the PSOA structure is ideally applicable in high-power narrow-linewidth fiber lasers at short wavelengths.
Funding
National Natural Science Foundation of China (61875087).
Disclosures
The authors declare no conflicts of interest. This work is original and has not been published elsewhere.
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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