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Abstract
Narrow-linewidth high-power fiber amplifiers are demanded in the spectral beam combining technique for power scaling. This article has experimentally studied the seed power spectral density (PSD) effect on the spectral broadening in all-fiber high-power narrow-linewidth fiber amplifier based on fiber Bragg grating (FBG) structure. It was found that the spectral bandwidth of the amplifier output reduces by increasing the PSD of the seed source. During the experiments, a ytterbium-doped fiber (YDF) amplifier was injected by three seed sources with the PSDs of 1.27 W/pm, 1.5 W/pm and 1.8 W/pm to scale up the seed power. After amplification, the resultant 3-dB spectral bandwidth of the amplifier was measured to be 88 pm, 61 pm, and 41 pm, respectively, at kW-level power. Meanwhile, we developed a 990 W fiber amplifier with a 3-dB spectral bandwidth of 41 pm; the narrowest spectrum achieved using a simple FBG-based master oscillator power amplifier configuration at this power level.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power narrow-linewidth fiber lasers with near-diffraction-limited beam quality are highly required in advanced applications areas, such as remote laser communication [1,2], nonlinear frequency conversion [3–5], gravitational wave detection [6,7], and laser beam combining [8]. Spectral beam combining (SBC) [9] and coherent beam combining (CBC) [10,11] are promising approaches to extend the output laser power while maintaining a good beam quality during the last decade. These approaches overcome limitations of further power scaling in single fiber laser sources such as pumping brightness, nonlinear effects, thermal management, and transverse mode instability (TMI) [12]. However, dispersive components which used to combine output laser from multiple sources in the SBC method have a limited reflection or transmission spectral bandwidth. Therefore, the output power of each fiber laser should be increased to boost the combined output power. Moreover, the spectrum of individual fiber lasers should be narrower to raise the number of combined laser wavelengths. So, it is significant to employ fiber laser sources with a high-power spectral density (PSD), which is the ratio of output power to the laser spectral bandwidth [13,14]. In order to control the spectral broadening in a high-power fiber laser, two popular methods have been carried out based on the master oscillator power amplifier (MOPA) configuration. Employing phase-modulated single-frequency seed laser is one of these methods [15,16]. The seed laser enters the main amplifier after broadening the spectral bandwidth by a phase modulator and pre-amplifying. According to experimental data, there is no severe broadening in the output spectrum of the amplifier [17,18]. Since the stimulated Brillouin scattering (SBS) threshold is proportional to the spectral bandwidth of the laser output, the amplifier experiences intense SBS, and to enhance the SBS threshold, a higher speed phase modulator is demanded. However, using complex electronic equipment and multi-stage amplification of low power seed makes this method costly. The other method utilizes traditional fiber Bragg grating (FBG)-based oscillators that are more compact and less complicated than the previous method [19–21]. In this method, increasing the amplifier output power makes it difficult to prevent the spectral bandwidth from over-expanding owing to the nonlinear effects [22]. The SBS threshold significantly depends on the spectral bandwidth of the laser output. A remarkable effort has been made to narrow the spectral bandwidth of high-power FBG-based amplifiers in MOPA configuration. In $2015$, Hao et al. obtained an $823$ W forward-pumped ytterbium-doped fiber (YDF) laser with a $3$-dB spectral bandwidth of $80$ pm and power spectral density of $10.29$ W/pm [23]. A $2$ kW forward-pumped two-stage fiber laser with a $3$-dB spectral bandwidth of $286$ pm and PSD of $7.17$ W/pm was developed by Xu et al. in the same year [19]. In $2016$, Huang et al. demonstrated a forward-pumped fiber laser with a $3$-dB spectral bandwidth of $310$ pm at a power level of $2.9$ kW corresponding to a PSD of $9.35$ W/pm [20]. In $2019$ Wang et al. reported a $2.4$ kW bidirectional-pumped fiber laser with a $3$-dB spectral bandwidth of $240$ pm corresponding to a PSD of $10$ W/pm [24]. In the same year, Huang et al. presented a $2.19$ kW backward-pumped fiber laser with a $3$-dB spectral bandwidth of $86$ pm and a PSD of $25.32$ W/pm [25]. In $2021$ Huang et al. reported a $3$ kW backward-pumped MOPA fiber laser with a $3$-dB spectral bandwidth of $103$ pm corresponding to a PSD of $29.22$ W/pm [26]. The amplifier output PSD is related to several factors, such as gain fiber and pumping power characteristics [23–29]. Table 1 shows the experimental results on the seed PSD and corresponding amplifier output PSD in typical narrow linewidth FBG-based fiber amplifiers in recent years. It seems that the spectral bandwidth of the amplifier output, in addition to the mentioned parameters, is dependent on the PSD of the seed output. It appears that making a high PSD seed may lead to a fewer spectral broadening factor in the amplifier output which is the ratio of increased linewidth to the increased output power in the amplifier output. To ensure this issue, it was decided to keep the amplifier parameters constant, and only the effect of the seed sources with different PSDs on the output spectrum of the amplifier was investigated.
Table 1. Typical experimental results on FBG-based narrow-linewidth fiber amplifiers with different values of PSD for the seed and amplifier.
In this paper, we have experimentally studied the effect of seed output PSD, on the amplifier output PSD in narrow-linewidth FBG-based MOPA fiber lasers. First, three narrow-linewidth seed lasers with different PSDs were constructed using narrow reflection bandwidth FBGs and linear resonant cavities. Then, the seeds were amplified by a forward-pumped amplifier. In the amplifier stage, wavelength stabilized (WS) laser diodes (LDs), and a short gain fiber with a large core were employed to suppress over-expanding of the amplifier spectrum. As a result, it was found that the higher PSD of the seed laser, the higher PSD of the amplifier. Also, the output power of $990$ W with a $3$-dB linewidth of 41 pm was realized in the amplifier output.
2. Experimental setup
The experimental setup of the all-fiber FBG-based MOPA configuration fiber laser, which consists of three seed sources and one forward-pumped fiber amplifier, is demonstrated in Fig. 1(a), 1(b), 1(c), and 1(d) respectively. A $20/400$ $\mu$m YDF with the absorption coefficient of $1.6$ dB/m @ $976$ nm was selected as the gain medium. The pump source used in the seeds is an $80$ W WS $976$ nm LD. The seeds were linear cavities based on a pair of FBGs. The homemade FBGs were fabricated on a hydrogen-loaded $20/400$ $\mu$m photosensitive fiber using a phase-mask and excimer laser. The center wavelength and $3$-dB reflection bandwidth of the high reflection (HR) FBG in all seeds were $1079.98$ nm and $3$ nm. The center wavelength of the low reflection (LR) FBG was measured to be $1079.85$ in seeds $1$ and $2$ and $1080.05$ nm in seed $3$. The $3$-dB reflection bandwidth of the LR was measured to be $0.15$ nm in seeds $1$ and $2$ and $0.1$ nm in seed $3$. The HR FBG and LR FBGs had a reflectivity of $99$% (HR) and $10$% (LR). The pigtail fiber of the pump source had a size of $220/230$ $\mu$m. The pumping power was injected into the gain fiber via a (6+1) $\times$1 combiner. The combiner’s input (output) signal fiber, and pump fiber had a size of $20/400$ $\mu$m and $220/230$ $\mu$m, respectively. The length of YDF is $8$ m in seed $1$ and $7$ m in seeds $2$ and $3$. The gain fiber was coiled in a cylindrical spiral shape with a fixed bending diameter (BD) of $9$ cm for mode selection. In all the seeds, a cladding light stripper (CLS) was employed after the laser cavity to leak out unabsorbed pump light and leaked signal power into the cladding. Moreover, the input signal fiber of the combiner in all seeds was cleaved at $8$ degrees to prevent the occurrence of parasitic lasing. In the amplifier stage, the gain fiber was pumped by six $190$ W WS LDs connected by a (6+1) $\times$1 combiner (similar to the combiner used in the seeds) in the forward direction. The pump sources had a central wavelength of $976\pm 1$ nm and $3$-dB spectral bandwidth of $0.7$ nm. The pigtail fiber of each LD had a size of $200/230$ $\mu$m. A $25/400$ $\mu$m YDF with the absorption coefficient of $1.8$ dB/m @ $976$ nm was chosen as the gain medium, which was coiled on an aluminum cylinder with a BD of $10.5$ cm for mode selection.The reason of coiling the gain fiber in a cylindrical spiral shape instead of circular spiral shape is related to bend loss of higher order modes (HOMs). By coiling the gain fiber in a circular spiral shape, the bend loss of the HOMs, decreases dramatically as the coil diameter increases. If the gain fiber is coiled on a cylinder, the coil diameter is identical for the entire fiber length, which can avoid the disadvantage of the circular spiral coiling design. Furthermore, using a cylinder with a smaller diameter leads to more loss of HOMs and is suitable for mode selection. However, coiling the gain fiber at the diameter of 10 cm and lower is a weakness because of reducing the effective mode area and decreasing the threshold of nonlinear effects especially for the $25/400$ $\mu$m fiber [30]. The length of the gain fiber in the amplifier was shortened to $8.6$ m based on the cut-back method to suppress the SBS effect and spectrum broadening. At the end of the amplifier, a homemade CLS capable of handling a maximum power of $200$ W was used [31,32]. One end cap was employed at the output end of the laser to avoid any end-face reflection.
3. Experimental results and discussion
3.1 Laser performance with seed 1
The power injected into the main amplifier is set to $47$ W in seed $1$. The $3$-dB and $10$-dB spectral bandwidth of the seed output was $37$ pm and $83$ pm, respectively, measured by an optical spectrum analyzer under the resolution of $7$ pm. Therefore, a $3$-dB PSD of $1.27$ W/pm was achieved by the oscillator. With this power, the fiber laser finally reached the output power of $995$ W under the total pumping power of $1220$ W, corresponding to an optical-to-optical efficiency of $81.5$%. The output and the backward power versus the pump power are depicted in Fig. 2. As shown, the backward power increases linearly so, the SBS threshold has not been reached even at the highest output power in this fiber amplifier. The broadened $3$-dB and $10$-dB spectral bandwidth was measured to be $88$ pm and $245$ pm at the maximum output power, respectively, corresponding to a $3$-dB PSD of $11.3$ W/pm and a 3-dB spectral broadening factor of $53.8$ pm/kW. The output spectra of the seed and amplifier are demonstrated in Fig. 3. Spectral broadening of the amplifier output at the $3$ dB and $10$ dB in different power levels is illustrated in Fig. 4. As seen, the spectral broadening in 10 dB is faster than in 3 dB due to the nonlinear effects such as four-wave mixing (FWM) and self-phase modulation (SPM) [21].
Fig. 1. The schematic diagram of (a) seed 1, (b) seed 2, (c) seed 3, and (d) a forward-pumped YDF amplifier for studying the effect of the seed PSD on spectral broadening in the amplifier.
Fig. 2. The output and the backward power versus the pump power when seed 1 was injected into the amplifier.
Fig. 3. The output spectra at the power of $47$ W (seed 1) and $995$ W (corresponding amplified signal). The inset shows the measured laser spectrum in broad range.
Fig. 4. The spectral broadening of the amplifier output at $3$ dB and $10$ dB in different power levels when was injected by seed 1.
3.2 Laser performance with seed 2
The power injected into the main amplifier is set to $45$ W in seed $2$. The $3$-dB and $10$-dB spectral bandwidth of the seed output was $30$ pm and $63$ pm. Therefore, a $3$-dB PSD of $1.5$ W/pm was achieved by the oscillator. With this power, the fiber laser finally reached the output power of $988$W under the total pumping power of $1220$ W, corresponding to an optical-to-optical efficiency of $81$%. The output and the backward power versus the pump power are depicted in Fig. 5. As shown, the backward power increases linearly so, the SBS threshold has not been reached even at the highest output power in this fiber amplifier. The broadened $3$-dB and $10$-dB spectral bandwidth was measured to be $61$ pm and $145$ pm at the maximum output power, respectively, corresponding to a $3$-dB PSD of $16.2$ W/pm and a $3$-dB spectral broadening factor of $32.9$ pm/kW. The output spectra of the seed and amplifier are demonstrated in Fig. 6. Spectral broadening of the amplifier output at the $3$ dB and $10$ dB in different power levels is illustrated in Fig. 7.
Fig. 5. The output and the backward power versus the pump power when seed 2 was injected into the amplifier.
Fig. 6. The output spectra at the power of $45$ W (seed 2) and $988$ W (corresponding amplified signal). The inset shows the measured laser spectrum in broad range.
Fig. 7. The spectral broadening of the amplifier output at $3$ dB and $10$ dB in different power levels when was injected by seed 2.
3.3 Laser performance with seed 3
The output power of seed 3 is $45$ W, and its $3$-dB and $10$-dB spectral bandwidth was measured to be $25$ and $59$pm, respectively, corresponding to a $3$-dB PSD of $1.8$ W/pm. With a total pumping power of $1220$ W, the amplifier reached the maximum output power of $990$ W. Therefore, the optical-to-optical efficiency at the maximum output power was $81.1$%. The output and the backward power versus the pump power are depicted in Fig. 8. There is no sudden increase in the backward power which means that SBS does not initiate in the amplifier. The $3$-dB and $10$-dB spectral bandwidth was measured to be $41$ and $87$ pm at the maximum output power, respectively, corresponding to a $3$-dB PSD of $24.1$ W/pm and a 3-dB spectral broadening factor of 16.9 pm/kW, which is the narrowest spectrum at this power level for a fiber amplifier seeded by an FBG-based oscillator to the best of our knowledge. The spectrum of seed $3$ and amplifier is demonstrated in Fig. 9. As depicted, there is no SBS peak at the amplifier output spectrum. Spectral broadening of the amplifier output at the 3 dB and 10 dB in different power levels is illustrated in Fig. 10.
Fig. 8. The output and the backward power versus the pump power when seed 3 was injected into the amplifier.
Fig. 9. The output spectra at the power of $45$ W (seed 3) and $990$ W (corresponding amplified signal). The inset shows the measured laser spectrum in broad range.
Fig. 10. The spectral broadening of the amplifier output at $3$ dB and $10$ dB in different power levels when was injected by seed 3.
3.4 Discussion
The summary of laser performance with different seeds is presented in Table 2. It was realized that the spectral characteristics of the amplifier output spectrum at similar output power levels were different when it was injected with seeds $1$, $2$ and $3$. As shown in Table 2, the $3$-dB PSD of seed $3$ ($1.8$ W/pm) was higher than seed $2$ ($1.5$ W/pm), which was higher than seed $1$ ($1.27$ W/pm). Furthermore, it was seen that the $3$-dB PSD of the amplifier output was higher when it was injected by seed $3$ ($24.1$ W/pm) than by seed $2$ ($16.2$ W/pm), which was higher than by seed $1$ ($11.3$ W/pm). In our experiments, the pump power, gain fiber, pumping configuration and fiber bending type of all seeds are identical, so the difference in their spectral bandwidth must be related to the YDF length and reflection bandwidth of the LR FBGs. As is shown in Fig. 1, the seed sources $1$ and $2$ are the same, except that the YDF length of seed $1$ is $8$ m while it is $7$ m in seed $2$. It is clear that the frequency separation between adjacent modes in the laser cavity is given by $\Delta f=c/2nL$, in which $c$ is the speed of light, $n$ is the effective index of the fundamental fiber mode, and L is the cavity length. This equation indicates that the interval between the adjacent longitudinal modes in seed $1$ is less than in seed $2$. Therefore, the longitudinal modes number in seed 1 is larger than in seed 2. Furthermore, according to Fig. 1, seeds $2$ and $3$ differ only in their reflection bandwidth of LR FBG. The reflection bandwidth of LR in seed $2$ is $0.15$ nm, while it is $0.1$ nm in seed $3$. From our point of view, the LR FBG with a broader reflection bandwidth leads to a larger number of longitudinal modes in the seed output spectrum. A larger number of longitudinal modes implies more nonlinear effects, contributing to spectral broadening. So, it is expected to have a higher $3$-dB spectral bandwidth in seed $1$ ($37$pm) than in seed $2$ ($30$ pm), which is higher than in seed $3$ ($25$ pm) as shown in Table 2. Accordingly, the amplifier which is injected by seed with a larger number of longitudinal modes, undergoes stronger nonlinear effects and more spectral broadening. In our experiments, as demonstrated in Table 2, the $3$-dB spectral bandwidth of the amplifier when injected with seeds $1$, $2$, and $3$ was measured to be $88$ pm, $61$ pm, and $41$ pm, respectively. In other words, for a given seed output power, the sidebands of the broader output spectrum dedicated a higher portion of the total power than the narrower output spectrum. Therefore, by amplification, the nonlinearity will cause more side-mode generations and more spectral broadening in the amplifier output spectrum. In our experiments, as shown in Table 2, seeds $2$ and $3$ have the same output power ($45$ W), but the $3$-dB spectral bandwidth of seed $2$ ($30$ pm) is broader than seed $3$ ($25$ pm), and therefore, the $3$-dB spectral bandwidth of the amplifier output when injected by seed $2$ is broader than by seed $3$.
Table 2. Summary of laser performance with different seeds.
4. Conclusion
In this paper, we have studied the effect of the seed source output PSD on the amplifier output PSD in kW-level narrow linewidth FBG-based fiber amplifiers. The seeds $1$, $2$, and $3$ with different PSDs of $1.27$ W/pm, $1.5$ W/pm and $1.8$ W/pm were amplified by a forward-pumped all-fiber amplifier. The corresponded PSDs of the amplifier were measured to be $11.3$ W/pm, $16.2$ W/pm and $24.1$ W/pm, respectively. It is found that the ratio of the seed output power to the seed output spectral bandwidth is determinative in the spectral broadening of the amplifier output. Meanwhile, we obtained a $41$ pm, $990$ W fiber amplifier at $1080$ nm employing a MOPA configuration, which is the narrowest spectrum achieved by an all-fiber FBG-based fiber laser at this power level to the best of our knowledge. The results of this investigation should be helpful in the construction of high-power narrow-linewidth fiber amplifiers.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting this study’s findings are available from the corresponding author on request.
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