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Abstract
Here, we have experimentally demonstrated the selection principle of the seed power in a narrow linewidth fiber amplifier seeded by fiber oscillator based on a pair of fiber Bragg gratings. During the study on the selection of seed power, the spectral instability of the amplifier is found when a low power seed with bad temporal characteristics is amplified. This phenomenon is thoroughly analyzed from seed itself and the influence of the amplifier. Increasing the seed power or isolating the backward light of amplifier could effectively eliminate the spectral instability. Based on this point, we optimize the seed power and utilize a band pass filter circulator to isolate the backward light and filter the Raman noise. Finally, a 4.2 kW narrow linewidth output power is achieved with signal to noise ratio of 35 dB, which has exceeded the value under the highest output power reported in this type of narrow linewidth fiber amplifiers. This work provides a solution for high power and high signal to noise ratio narrow-linewidth fiber amplifiers seeded by FBGs-based fiber oscillator.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Spectral beam combining (SBC) technology has promoted the output power of fiber laser system with maintaining excellent beam quality [1–3]. Narrow-linewidth fiber amplifiers (NLFAs) with near-diffraction-limited beam quality, ensuring the number of fiber lasers that can be spectrally combined and beam quality of combining systems, are the key components for SBC [4,5]. For NLFAs, the limited factors of power amplification are completely different with the conventional broadband fiber lasers due to their narrow linewidth. The stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects are main limited factors for conventional fiber amplifier and usually need to be suppressed simultaneously [6–8]. However, not only SRS and TMI effects, but stimulated Brillouin scattering (SBS) and spectral broadening effects also need to be considered in NLFAs [4,5,9–12]. And the implementation difficulties would be multiplied once increasing the requirement for the laser linewidth. Utilizing master oscillator power amplification (MOPA) structure seeded by narrow linewidth fiber oscillator is a simple method of attaining NLFAs. For this type of NLFA, the characteristics of fiber oscillator laser seed has great influence on spectral broadening and nonlinear effects [13–18]. For example, Liu et al. demonstrated the self-pulsing and temporal fluctuation of fiber oscillator laser is stronger when the laser spectrum is narrower, which would lead to a higher SRS intensity in the amplifier [15]. Wang et al. also found that the self-pulsation of fiber oscillator would decrease the SRS threshold and limit the output power of fiber amplifier [16]. One can see that the seed’s temporal characteristics play a key role in the NLFA. Besides, a discovery that the linewidth of amplifier decreases with the number of longitudinal modes of seed source reducing is also reported [17,18].
Typically, increasing seed power is the simplest method of improving the temporal characteristics of fiber oscillator seed [19,20]. However, the linewidth of the seed will be broadened with the increase of seed power, which is not conductive to controlling the linewidth of amplification. Besides, the higher seed power will introduce more Raman noise into the amplifier so as to decrease the SRS threshold. According to the previous studies, the low seed power has the benefit of suppressing SRS [21,22]. Thus, the seed power cannot be too high. Nevertheless, it does not mean that the lower of seed power, the better. With too low seed power injecting, the time-domain instability or self-pulsing phenomenon maybe occur in narrow linewidth fiber oscillator laser, severely influencing the SRS threshold of amplifier [16,20]. It can be seen the seed’s temporal characteristics, Raman noise, and linewidth have a strong relationship with seed power, thereby influencing the performance of amplified laser. In previous reports about this type of NLFAs, the power range of fiber oscillator seed is usually from 7.3 to 145 W. Wang et al. demonstrated a 2.4 kW fiber laser with a linewidth of 0.24 nm, the fiber oscillator laser seed used a fiber core diameter of 10 µm and seed power was set 7.3 W [23]. Huang et al. adopted narrow linewidth laser seed with fiber core diameter of 20 µm and power of 145 W, achieving 3.01 kW output power with a 3 dB spectrum bandwidth of 103 pm [24]. Du et al. achieved a 3.3 kW narrow linewidth laser output by selecting the seed power of 29.5 W [25]. Since specific seed power directly affects the laser linewidth and SRS effect of amplifiers, how to select appropriate seed power is significant for NLFAs seeded by fiber oscillator. At present, the highest output power of this type of NLFA and corresponding signal to noise ratio (SNR) are 4 kW and 22 dB, respectively [26].
In this work, we have experimentally demonstrated the spectral characteristics of NLFA with different seed powers. The spectral instability of amplifier is observed with low seed power injecting. The phenomenon can be attributed to the seed self with bad temporal stability and the effect of backward Stokes light from amplifier. Based on this point, an optimization of seed power selection is explored by adopting a band pass filter circulator to prevent the effect of backward light and to filter Raman noise. The appropriate seed power can be amplified with high SRS threshold and relatively narrow linewidth simultaneously. Finally, a 4.2 kW narrow linewidth laser is achieved with a slope efficiency of 79.8%, the 3 dB and 20 dB linewidth are 0.62 nm and 2.39 nm. The beam quality M2 factor is ∼1.3 and the SNR reaches 35 dB. The output power and SNR have exceeded simultaneously the reported newest results in this type of NLFAs.
2. Performance of narrow-linewidth fiber amplifier with different seed powers
The laser performance of narrow linewidth fiber amplifier with different seed powers is investigated firstly. The schematic of the narrow-linewidth fiber amplifier with forward pumping structure is shown in Fig. 1. The amplifier uses a narrow linewidth fiber oscillator as seed. The pump source of seed adopts a wavelength stabilized laser diode (WS LD) module with the center wavelength of 976 nm. The laser cavity consists of a pair of FBGs with the center wavelength of ∼1080 nm and a length of Yb-doped fiber (YDF) with the core/inner cladding diameter of 20/400 µm. The high reflector (HR) FBG has a reflectivity of 99.5% and a full width at half maximum (FWHM) of 3 nm. The output coupler (OC) FBG provides a reflectivity of 10% and FWHM of 0.05 nm. To narrow the laser linewidth, except the narrow bandwidth OC-FBG is utilized, a backward pump structure and short resonant cavity with ∼4 m long YDF are adopted. This means the oscillator cavity sacrifices laser efficiency to some extent as the total gain absorption is less than 7 dB with the YDF gain absorption coefficient of 1.78 dB/m. The backward combiner adopts a side pump technology whose multimode pump ports have core/cladding of 220/242 µm and entire signal fiber has core/cladding of 20/400 µm. The forward residual clad light is removed by a clad light stripper (CLS). For the amplifier stage, two 976 nm WS LDs are employed as the pumping sources, each providing a maximum pump power of ∼860 W. The pump power is coupled in the gain fiber via a (2 + 1) × 1 side pump combiner. The gain fiber adopts 12 m long commercial YDF with core/cladding diameter of 20/400 µm. The gain absorption coefficient is 1.14 dB/m at 976 nm. The output fiber includes a CLS and a quartz block head (QBH) with fiber size of 25/400 µm. The laser power and spectrum are measured simultaneously in the experiment. The resolution of the optical spectrum analyzer is 0.02 nm.
Figure 2 shows the laser spectrum with the seed power of 11 W, 24 W and 37 W. The 3 dB linewidth of injected seed laser are 0.036, 0.04 and 0.044 nm respectively. When the seed power is set 11 W for power scaling, the output spectrum of the amplifier is blur and extremely instable at one hundred watts, as shown in Fig. 2(a). A series of random low amplitude pulse components are observed in the spectrum during the repeated measurements. And the SRS threshold is so low that the SNR is 45.6 dB at only 362 W. Figure 2(b) shows the laser spectrum with the seed power of 24 W. With seed power increasing, the spectral blur and pulses are eliminated but there is still spectral instability of amplifier, whose performance is that the SNR is varied with unchangeable pump power. For example, the minimum spectral SNR is 46 dB and the maximum SNR is 40 dB at the output of ∼813 W. In the above two cases, the measured output power only has small fluctuation within the normal error range of the power meter. But the spectral performance shows the actual output laser exists small pulses and fluctuations, which is not conducive to obtain stable high power laser. When the seed power is increased to 37 W, the spectral pulses disappear and the output spectrum is stable and invariable during repeated measurements, as shown in Fig. 4(c). And the SRS threshold is improved obviously from about 100 W to 1000 W with the seed power increasing from 11 W to 37 W.
Fig. 2. The laser spectrum at different output powers with the seed power of (a) 11 W, (b) 24W, (c) 37 W
According to above experiments, the seed power of 37 W is better than 11 W or 24 W, because the spectral instability disappears with improved SRS threshold. But when the injected seed power exceeds a specific value, the SRS threshold will decrease due to more Raman noise introduced the amplifier. Although higher seed power possesses a more stable temporal characteristics, the difference is not big at relatively high seed power. Instead, the Raman noise introduced by high seed power is the major factor affecting the SRS threshold. And the seed laser linewidth of seed is broadened with the increase of seed power, which influences the laser linewidth after power scaling accordingly. This point is not demonstrated in these experiments but has been proved in many previous studies. Thus, for selection principle of seed power in NLFAs, appropriate power must consider that the narrow signal linewidth, stability of laser spectrum after amplification, and the high SRS threshold as possible comprehensively.
3. Analysis of spectral instability
3.1 Influence of temporal characteristics of seed
In order to study the relationship between spectral instability and characteristics of fiber oscillator seed itself. The temporal characteristics of seed are measured independently. A tap coupler with the signal fiber of 20/125 µm is added after the CLS of seed. A small amount (0.1%) of the signal light is collected to a high-speed photodetector (5 GHz bandwidth, Thorlabs Inc) via a variable optical attenuator (VOA) then analyzed by an oscilloscope (8 GHz bandwidth, 5 GSa/s, Tektronix Inc). The other signal light is monitored by a power meter at the same time. Figure 3 shows the temporal characteristics of fiber oscillator at different output powers. The sampling time interval is 0.2 ns. It can be observed that the laser is no longer a continuous-wave laser. At the power of 11 W, it exhibits a series of spike pulses obviously, indicating the self-pulsations of oscillator. Specifically, there are two types of pulses in the fiber oscillator according to the measured time domain information, sustained self-pulsing (SSP) and self-mode-locking (SML) pulses [27], which are illustrated in Fig. 3(a) and (b) respectively. The amplitude of SSP is large and the pulse width and pulse spacing time are several microseconds, which is related to the relaxation oscillation of fiber oscillator. And the envelope contains a series of self-locking sub pulses once we focus to a smaller time scale, as shown in Fig. 3(b). Different from the random SSP, SML exhibits periodic pulses. The pulse interval is corresponding to the longitudinal mode interval of the oscillator and in nanosecond time scale. In this fiber oscillator, the SML pulse cycle is 74 ns. The phenomenon can usually be observed when the pump power is low, which can be explained by the multimode interference within the cavity [14]. With the pump power increasing, the mode competition decreases which is characterized by the decrease of pulses amplitude and enhancement of temporal stability.
Fig. 3. The temporal properties of the narrow linewidth fiber oscillator measured in different time scale. (a) 40 µs scale (b) 1 µs scale.
The normalized standard deviation (NSTD) defined as $NSTD = STD/{I_{mean}}$ is utilized to quantitively characterize the temporal stability of the fiber oscillator, in which ${I_{mean}}$ is the average value of the temporal intensity. According to the measured time domain results, the calculated NSTD are 1.01, 0.83 and 0.68 at power of 11 W, 24 W and 37 W, corresponding to the seed laser with different temporal stability. With the seed power increasing to 37 W, the decrease of NSTD value indicates the enhancement of laser temporal stability [28]. Notley, the spectral instability after amplification correspondingly disappears and SRS threshold of amplifier is improved with the seed power increasing to 37 W, as shown in Fig. 2. Therefore, we can infer that too low seed power with bad temporal characteristics contributes to the spectral instability and low SRS threshold of the amplifier. Specifically, the spectral instability can be explained that the dynamic changing of seed laser in time domain. A series of random pulses components in seed laser with different intensity are amplified and happened in the microsecond scale according to the seed temporal characteristics. While the measurement of the optical spectrum analyzer is slow, the measured spectrum is a delayed record of the aforementioned fast process, accordingly leads to different signal spectrum for each measurement. And the presence of high peak pulses results in the low SRS threshold.
3.2 Influence of backward light of amplifier
For conventional fiber amplifier whose seed power is usually ∼100 W level with relatively broad laser linewidth, SBS does not need to be considered due to its high threshold. Whereas when utilizing narrow linewidth fiber oscillator seed to power amplification, the influence of SBS effect cannot be ignored. The signal light with ultra-narrow linewidth makes SBS threshold low when the power is amplified. In this case, the backward Stokes light induced by SBS would enter seed and affect the seed characteristics. Correspondingly, the output performance during power amplification would also be influenced.
In order to study whether spectral instability is related to the influence backward light of amplifier, a circulator is utilized between the seed and amplifier stage to prevent the backward light entering the seed. Here, the port 3 of circulator is defined and used to monitored the power or spectrum of backward light. The isolation that preventing return light is 33 dB. The seed power is set 12 W with 3 dB linewidth of 0.036 nm. Figure 4 depicts the backward spectrum from port 3 of circulator during the amplification process, two spectral peaks can be clearly observed at output power of∼300 W, in which one is Rayleigh scattering light and another is Stokes light due to SBS, which can be verified from the frequency shift between signal light and backward light of ∼0.065 nm. The intensity difference between Stokes light and Rayleigh scattering light is ∼6 dB, which indicates the Stokes light is the main component of backward light. Figure 5 compares the output spectrum of amplifier with or without circulator inserting between seed and amplifier. Once preventing the backward light with the circulator, the output spectrum is pure and stable during repeated measurements. And low amplitude pulse components near signal light are eliminated according to the enlarge view of Fig. 5. These low intensity pulses observed in forward spectrum also prove to be the result of SBS. Due to the wavelength of Stokes light induced by SBS is close to signal light, the Stokes light would also oscillate in the resonant cavity owing to the FBGs, which causes the Stokes light to propagate in the same direction as the signal simultaneously. Therefore, except the self-stability of seed laser, the influence of backward SBS light of amplifier is another factor resulting in the spectral instability phenomenon.
Fig. 5. The comparison of laser spectrum with or without circulator inserting between seed and amplifier
4. Optimization of seed power selection
From the results in Fig. 2, the seed power of 37 W seems a proper selection to further power scaling. But the use of circulator provides a new insight for selecting seed power because the spectral instability disappears at seed power of only 12 W. Furthermore, obvious research has demonstrated band pass filter could effectively filter spectral noise to suppress SRS. Thus, a band pass filter circulator (BPFC) combined the function of filtering with isolation could further optimize the seed power. For this reason, the similar amplification experiments with different seed powers are carried utilizing the setup in Fig. 1, with the difference of BPFC inserting between seed and amplifier stage. The pass bandwidth is 2 nm with center wavelength of 1080 nm, other spectral noise which is outside the pass bandwidth can be filtered.
Figure 6 shows the laser spectrum at different output power with BPFC inserting. The output spectrum of the amplifier is stable and the spectral pulses no longer occurs. When the seed power is 12 W, the SNR is 59.2 dB at 1130 W with 3 dB and 20 dB linewidth are 0.30 nm and 0.79 nm. When the seed power is increased to 24 W, the SNR is improved with the value of 63 dB at 1300 W. But when the seed power is increased to 37 W, the SNR decreases with the value of 56.5 dB at 1137 W. Based on horizontal comparison, it is clearly observed that there is an inflection point of seed power whose SRS effect is weakest. When the seed power is increased from 12 W to 24 W corresponding to a better temporal stability, the threshold of SRS and spectral instability are improved simultaneously. In this case, the temporal characteristics is major factor that affect SRS threshold. Combined with the results without circulator inserting in Fig. 2, one can see that the different temporal characteristics of seed really have an influence on the spectral stability and SRS threshold whether the circulator is used or not. While when the seed power is high, the Raman noise is major factor that affect SRS threshold in the process of power amplification. Based on the vertical comparison, the inflection point of seed power is reduced owing to the function of BPFC. Compared with the amplification results utilizing the three seed powers in Fig. 2, the SRS threshold has been improved to varying degrees. For example, with the seed power of 24 W, SRS threshold is greatly improved from ∼700 W to ∼1300 W. This proves our point that both temporal characteristics of seed itself and backward Stokes light make a difference to threshold of spectral instability and SRS gain.
Fig. 6. The laser spectrum at different output power with the BPFC inserting at the seed power of (a) 12 W, (b) 24 W, (c) 37 W
Through longitudinal and transverse comparison and analysis, the seed power of 24 W combining with the function of BPFC is best selection in our experiments to reduce the SRS effect of the amplifier. From the linewidth results after power amplification, the output linewidth is related to the seed laser linewidth. Thus, the seed power of 24 W also balances the SRS, SBS and output laser linewidth well. Overall, we can get the optimization of seed power selection principle for narrow linewidth fiber laser. Utilizing the band pass filter circulator or isolator is very necessary to prevent the backward Stokes light affecting the seed performance and to eliminate the spectral instability phenomenon. Combined with filtering spectral noise, the SRS threshold could be improved further and there is an appropriate seed power for the weakest Raman effect.
5. High power amplification experiments with optimized seed
With the optimized seed, we setup a high-power narrow linewidth fiber amplifier with bidirectional pumping configuration, as shown in Fig. 7. Based on the setup in Fig. 1, the BPFC is inserted between the seed and amplifier stage. And a backward (6 + 1) × 1 signal/pump combiner is utilized to inject more pump power. For the design of system, we comprehensively consider the nonlinear effects and TMI effect. In order to improve the TMI threshold of the laser, the gain fiber of the amplifier is coiled to a minimum diameter of 8 cm. To mitigate the SRS effect, the output fiber of combiner and the delivery fiber have a core diameter of 25 µm. As for SBS effect, whose threshold will increase along with the signal linewidth broadening, is mainly ups to the injected linewidth of seed laser. In the fiber laser system, all fibers and fiber components are placed on a water-cooled heat sink to realize efficient heat dissipation and ensure the stability in high power operation. During the process of amplification, laser characteristics such as the output power, backward power, optical spectrum, and beam quality are measured and recorded in the experiment.
The seed power injected into the amplifier is set to 24 W according to the above experiments. The forward pump is firstly injected until the output power reaches 1300 W then the backward pump is injected. Figure 8(a) shows the output power and backward power versus pump power. With a total pumping power of 5315 W, the laser reaches a maximum output power of 4193 W with a slope efficiency of 79.8%. Correspondingly, the backward SBS power increases to 1.9 W at the maximum output power. Figure 8(b) shows the output spectrum at different output powers. The SNR is 35 dB at the power of 4193 W.
Fig. 8. (a) The output power and backward power versus pump power (b) the output spectrum at different output powers.
As shown in Fig. 9(a), the measured beam quality (M2 factor) maintains near single-mode and dose not degrade during the power amplification. The x direction of M2 is 1.44 and y direction is 1.24 at the maximum power, as shown in inserted Fig. 9(a). The reason for the difference between the M2x and M2y is the asymmetry of spot caused by the presence of high order mode [29], which is excited inevitably through imperfect splice point or twist of fiber [30]. Figure 9(b) illustrates the evolution of signal linewidth versus output power. The 3-dB linewidth of the signal laser grows gradually from 0.29 nm to about 0.62 nm at the maximum output power. For the 20 dB spectral width, it grows quickly under only forward pump then grows steadily after adding the backward pump. The 20 dB linewidth increases from 0.5 nm to 2.39 nm at the maximum output power.
Figures 10(a) and (b) show the temporal signals and corresponding Fourier spectral intensity of the scattering light at the operation of 4193 W. The temporal evolution of the signal laser remains stable in millisecond time scale and the corresponding standard deviation is only about 0.5%, as shown in Fig. 10(a). The corresponding Fourier spectral intensity remains relatively flat in the frequency range from 0 to 30 kHz, and there is no sign of typical frequency component of TMI effect, indicating that this fiber amplifier works well below the TMI threshold at the maximum output power. The further power scaling is limited by pump power.
Fig. 10. (a) The temporal signals and (b) corresponding Fourier spectral density of the dumped cladding light at the operation of 4193 W.
6. Conclusions
In summary, we have experimentally found the spectral instability phenomenon in NLFA seeded by a fiber oscillator when seed power is too low. The phenomenon can be attributed to the seed with temporal instability self and the effect of backward Stokes light from amplifier. By increasing the seed power or isolating the backward light of amplifier, the spectral instability could be effectively eliminated. According to the experimental results, the optimized seed power is explored by adopting BPFC preventing the effect of backward light and filtering Raman noise. The appropriate power balances the SRS, SBS and narrow linewidth of amplifier well. Furthermore, a NLFA is setup with bidirectional pump structure based on the optimized seed. A 4.2 kW output power is achieved with a slope efficiency of 79.8%. The 3 dB linewidth and 20 dB linewidth of 0.62 nm and 2.39 nm and the beam quality M2 factor is ∼1.3. The SNR is 35 dB at 4.2 kW, which have exceeded simultaneously the reported newest results in this type of NLFAs. Since this NLFA is free of TMI, the higher output power could be expected to achieve with more backward pump injecting. Overall, this work provides a valuable reference for high power NLFAs with high SNR seeded by FBGs-based fiber oscillator.
Funding
State Key Laboratory of Pulsed Power Laser (SKL-2020-ZR05, SKL-2021-ZR01); Science and Technology Innovation Program of Hunan Province (2021RC4027); National Natural Science Foundation of China (11974427, 12004431).
Disclosures
The authors declare no conflicts of interest.
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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