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Abstract
Impact of mode instability on dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers has been studied for the first time, which reveals another characterization of mode instability from the aspect of optical spectrum. It shows that, after the onset of mode instability, the measured light spectrums, especially the Raman light spectrums, are different from those without mode instability, which become burr-like. As mode instability evolves into different stages, the intensity of stimulated Raman scattering effects as laser power increasing also behaves differently. During the transition region, the stimulated Raman scattering effect becomes stronger as the lasing power increases until the mode instability evolves into chaotic regions, where the stimulated Raman scattering effect weakens. The effect of stimulated Raman scattering on mode instability has also been studied. Due to that the stimulated Raman scattering effect is weak and that the fraction of Raman light is only a few percent, the stimulated-Raman-scattering-induced mode instability has not been observed in the experiment, and the observed mode instability is induced by ytterbium ion gain of signal laser. It also revealed that the stimulated Raman scattering has negligible influence on the mode instability induced by ytterbium ion gain.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The sudden onset of mode instability currently limits the power scaling of ytterbium doped fiber lasers with near diffraction limited beam quality, which is due to the quantum defect heat from ytterbium ion gain, and has been under extensive investigation during the recent years [1–11]. After the first report of the phenomena, extensive attention has been diverted to it by different research groups to investigate the underlying physical mechanisms [2–5,12–15], and several approaches to mitigating mode instability have been reported including dynamic mode excitation [16], tailoring the Yb-ion distribution [17], shifting the pump or signal wavelength [18–22], increasing the ratio of pump cladding diameter to core diameter [23], increasing the loss of high-order mode [24–26], employing different pump configurations [10,27–29], and so on.
Stimulated Raman scattering (SRS) is another one of the main obstacles in the power scaling of fiber lasers [30–33]. The study of the interaction between SRS and mode instability is important for power scaling of the high power fiber laser sources. It is theoretically predicated that the quantum defect heat during power converting to Stokes light in the Raman gain process of the Raman fiber lasers can also result to mode instability, which is the SRS-induced mode instability [34]. Recently, the SRS-induced mode instability has been observed in high power fiber sources [10,35]. It reports that SRS has negative impact on the SRS-induced mode instability, and the mode instability threshold decreases as the SRS effect intensifies [10,35,36]. However, to the best of our knowledge, the impact of mode instability on the dynamic characteristics of SRS has not been examined yet.
In this manuscript, a high power all-fiberized amplifier has been set up to study the influence of mode instability on the dynamic characteristics of SRS experimentally. It reveals that the onset of mode instability has significant influence on the strength and behavior of the SRS effect. The impact of SRS on mode instability has also been studied, and the results are discussed in light of the current understanding of mode instability.
2. Experimental setup
A co-pumping tandem pumped scheme was used, which is shown in Fig. 1. The seed master oscillator was a linear cavity, which included a pair of fiber Bragg gratings (FBGs). The high-reflector (HR) and output-coupler (OC) FBGs were centered at 1080 nm. The gain medium utilized in the laser oscillator was double cladding ytterbium-doped fiber (YDF) with a core diameter of 10 μm, which can guarantee that the oscillator works in single mode operation. Six fiber-pigtailed 976 nm laser diodes were used to pump the oscillator through a 7 × 1 tapered fiber bundle, which enables a maximal output power of 310W. The remaining central pump port was used to monitor the SRS effect by measuring the backward power and spectrum. The cladding light strippers (CLSs) were employed to dump the residual pump and high-order signal mode propagating in the fiber cladding. Then, the output laser signal of the oscillator was launched into the main power amplifier through the signal port of a (6 + 1) × 1 signal/pump combiner. One should note that the oscillators constructed with core diameter being 20 μm can deliver more output power, but they generally operate in few-mode state [37–40], and the beam quality may vary as the power increases [41], which changes the mode instability threshold [7,10]. This introduces extra interference factors and may leads to confusing conclusion [41], so they are not employed to construct the seed oscillator.
The 32 m gain fiber utilized in the main power amplifier has a core diameter of 30 μm, and the cladding absorption coefficient of the gain fiber is about 3.57 dB/m at 915 nm, which results to that the total pump absorption of the main amplifier at 1018nm is about 19dB. The gain fiber was coiled in a spiral way with a minimal diameter of 12cm [25]. The pump power was provided by twenty one fiber lasers at 1018nm [42–44], which were combined by six power combiners, and then injected into the signal/pump combiner. Totally 5.52 kW pump light was launched into the active fiber, which has the ability to boost the signal power of the laser oscillator to 5 kW level. CLS was performed on a 1.5m long matched passive fiber. An end cap with 2m matched passive fiber was employed to deliver the output signal power and avoid any end-face reflection or damage. All the components were fixed on a water-cooled heat sink with the exception of the end cap and collimator, which were cooled individually. An InGaAs photodetector (150 MHz, 700-1800 nm, Thorlabs) was used to detect the scattering light from the power meter, which has been proved to be an effective method to monitor the onset of mode instability [45]. The spectrum of the output laser was measured by a fiber coupled optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C). The M2 beam quality and beam spot was measured by the Laser Quality Monitor manufactured by PRIMES GmbH while the output was measured by 10K-W power meter manufactured by Ophir photonics. It is shown in Fig. 1 that the signal/pump combiner has been employed to combine the signal laser and pump laser. The combiner may deteriorate the mode content of the laser from the single mode oscillator, which increases the initial fraction of high order mode, and decreases the mode instability threshold [7,46,47]. So it’s necessary to study the influence of combiner on the beam quality of the seed signal laser. The M2 beam quality of the signal laser after the (6 + 1) × 1 signal/pump combiner has been measured, which was less than 1.1 at all power range, and means that the impact of the combiner on beam quality is negligible.
3. Dynamics characteristics of Stimulated Raman scattering and mode instability
3.1 Behavior of Stimulated Raman scattering after the onset of mode instability
Time traces at different output powers are shown in Fig. 2(a) with the corresponding Fourier spectrums. One can see from the Fourier spectrums that the characteristic coupling frequency was at about 1.6 kHz, which is lower than those in 976nm pumped fiber lasers with similar core size, typically 2kHz [48,49]. This is due to that the pump absorption cross-section at 1018nm is significantly lower, and the gain saturation effect is stronger [20,50]. The M2 beam quality and beam spot at different output power was also measured, and the results are shown in Fig. 2(b). The frame frequency is far lower than the coupling frequency of mode instability, which means that the recorded beam spot is an average of hundreds of unstable mode state. It reveals that the beam quality degrades as the lasing power increases. This is due to that the gain fiber of the main amplifier supports several modes, and the high order mode has a larger gain [51]. As the lasing power increases, the fraction of high order mode increases, which results to the beam quality degrades gradually. The value of M2 experienced a sharp increase at 3930W as indicted in Fig. 2(b), which is a direct sign of the onset of mode instability [52]. Before the onset of mode instability, the measured M2 was less than 1.5, which means that the fiber laser system operates in near diffraction limited beam quality in 3560W, and the M2 at 4560W was degraded to 1.96. It can be seen from the inset figure of Fig. 2(b) that there is obvious distortion of the beam spots after the onset of mode instability. During the transition region between 3930W and 4284W, the averaged beam spot still maintained a Gaussian like shape with slight displacement of the peak, which is due to that the power transfer between LP01 mode and LP11 mode [16]. As the mode instability evolved into the chaotic region, the distortion was more obvious, and near-doughnut shape intensity was recorded at 4560W, which is due to that higher order modes set in [16], and the energy density in the fiber is reduced. Using the definition in [49], the threshold of mode instability was about 3900W as shown in Fig. 2(c). The value of σ was calculated from the Fourier spectrogram of the time traces, which is an indicator of the beam fluctuation. Without mode instability, the Fourier spectrogram is flat, and the value of σ is about 0. After the onset of mode instability, there is frequency component in the low frequency of the Fourier spectrogram [16,52,53], which results to the value of σ increases dramatically [49]. The mode instability threshold power is defined as the lasing power at which the value of σ reaches 10%. It also shows that the system was in the stable region for output power was below 3900W, and became chaotically unstable when the output power scaled beyond 4284W.
Fig. 2 (a) Time series at different lasing power, (b) M2 at different lasing power, and (c) the value of σ as a function of lasing power. The inset figure in (a) is the corresponding Fourier frequency spectrums while that in (b) is the corresponding beam spots.
Figure 3 shows the measured output power, backward power and backward spectrum at different power. After the onset of mode instability, the output laser power still increases linearly as the pump power increases until the lasing power reaches 4284W, which shows a slight sign of roll over above 4kW (Fig. 3(a)). The linear behavior before 4284W is due to that the mode instability is in the transition region, and the mode coupling is mainly occurs between LP01 mode and LP11 mode [16,54,55], which is well supported by the fibers with core size being 30 μm, and that the efficiency of the laser is not affected [56]. When the lasing power scales beyond 4284W, mode instability evolves into the chaotic regions, and higher order mode sets in, which suffers significantly larger bend loss, and can be easily stripped by the CLS [19,20]. It is shown in Figs. 3(b) and 3(c) that, due to that the SRS effect strengthens, the backward power increases exponentially as the lasing power increases until the lasing power reaches 4284W, which becomes decrease after the lasing power exceeds 4284W. The decrease of the backward power is due to that the mode instability evolves into chaotic region for lasing power above 4282W, where large mount of power is transferred into higher order mode with large mode field area as shown in the inset figure of Fig. 2(b) [16]. This results to the decrease of the signal laser intensity, which weakens the SRS effect and reduces the backward stokes light.
Fig. 3 The power character of the fiber laser systems. (a) Lasing power as a function of pump power, (b) backward power as a function of lasing power, and (c) backward spectrum of the backward power at 3930W.
Spectrums at different output laser have been measured, which is shown in Fig. 4. It can be seen from Fig. 4(a) that, as the lasing power increases, the spectrums broaden while the SRS increases. However, after the onset of mode instability near 3930W, the measured spectrum of the Raman light becomes different, which seems that a noise has been superimposed on the spectrum as detailed in Figs. 4(b) and 4(c). The SRS effect weakens as the output power increases beyond 4284W, and the Raman light at 4678W is about 4dB lower than that at 4343W, which is indicated in Fig. 4(d). The observed new spectrum behaviors result from the dynamic evolving of mode instability. For the case that the line becomes thicker as shown in Fig. 4(b), it is due to that, after the onset of mode instability, the beam spot on power meter becomes unstable on millisecond scale, which results to that the scattering light from power meter fluctuates, and that the detected power collective by the fiber of the OSA changes [45]. Due to that the power fluctuation process occurs on the millisecond scale and the measurement of the OSA is slow, the measured spectrum is a record of the aforementioned fast process, which leads to burr-like and thicker curves. For the case in Fig. 4(c) that the curve for the Raman light is thicker than elsewhere, this is due to that the Raman light is additionally influenced by the energy coupling during mode instability. At the transition region of mode instability, the energy couples mainly between fundamental mode and LP11 mode. When the energy couples to fundamental mode, the energy density is high and the SRS effect is strong while the SRS effect is weak when the energy couples to LP11 mode, which results to that the SRS effect in the fiber is like on a roller coaster, and is changes dynamically. Due to that the coupling process occurs on the millisecond scale and the measurement of the OSA is slow, the measured spectrum is a record of the aforementioned fast process, which recorded simultaneously the strong and weak SRS effect, and manifests a noisy shape and thicker linewidth in Fig. 4(c). As the lasing power increases beyond 4284W, the mode instability evolves into the chaotic region, which means that more high order modes are involved and that the average power is mainly in high order mode. This results in that the beam profile in the fiber is not a Gaussian shape with high peak power in the center (as shown in the inset figure of Fig. 2(b)) [53], which can reduce the peak density and weaken the SRS effect.
Fig. 4 The spectral character of the high power fiber laser systems. (a) Spectrums at different lasing power, (b) details of the signal light spectrums in (a), (c) details of the Raman light spectrums in (a), and (d) spectrums at different lasing power.
The 3dB linewidth is calculated and shown in Fig. 5, which shows that the linewidth of the fiber systems broadens near linearly to about 6nm as the lasing power increases until the lasing power reaches 4284W, beyond which it rolls down to 4.3nm. This is due to that, in the transition region, the average power is mainly in fundamental mode [16], which results in that the broadening of linewidth is not significantly impacted. As the lasing power increases beyond 4284W, the mode instability evolves into the chaotic region, which means that more modes are involved and the average power is mainly in high order mode [16]. This results in that the averaged beam profile in the fiber is not a Gaussian shape, which reduces the peak density and weakens the nonlinear effects. The suppression of nonlinear effects, such self phase modulation and four wave mixing, weakens the broadening of the linewidth, even narrowing the linewidth as shown in Fig. 5.
3.2 Impact of Stimulated Raman scattering on mode instability
One way to change the intensity of SRS effect is to detune the seed power: as the seed power increases, the power gain factor reduces, which increases the effective length of the fiber, and intensifies the SRS effect [30,31]. The power of the oscillator has been increased, and the measured results are shown in Fig. 6. In Fig. 6(a), the output power is about 3930W for blue curve, 3855W for red curve and 3796W for green curve, and the corresponding Raman power fraction is about 0.14%, 0.18% and 0.28%. The maximal Raman power fraction at ~4050W is 0.59% for seed power of 250W while that for seed power of 310W is estimated to be ~2%, which is calculated by integrating the power ranging from 1120nm to 1145nm. It is shown from Figs. 6(a) and 6(b) that, the Stokes peak and the backward power increases as the seed power increases, which indicates that the intensity of SRS has been increases as the seed power increases. By taking similar way in Fig. 2(c), one can found that the mode instability threshold was increased by about 300W when the seed power was increased to 255W, which agrees with the previous results [57]. For the case that the seed power is 310W, the backward power at 4050W increases constantly as shown in Fig. 6(b), which indicates a strong SRS effect. For the safety consideration, the measuring process has been stopped, and the mode instability threshold was not measured. One can conclude that, although strong SRS effect has occurred, dramatic reduce of mode instability threshold is not observed, and the mode instability threshold still increases as the seed power increases.
Fig. 6 (a) Spectrums at different seed power, and (b) the backward power as a function of lasing power for different seed power.
It is shown that the mode instability threshold increases with the seed power increasing [57], which may surpass the effect of SRS on the mode instability threshold, and confuse the understanding of the effect of SRS on the mode instability. Fortunately, it is theoretically shown that if the total pump absorption is larger than 18dB, the dependence of MI threshold on the fiber length is weak and can be ignored [58–60], which means that only the SRS effect is affected by shortening the gain fiber length. So another way to mitigating SRS, shortening the fiber length [30,31,61], is employed. To mitigate the SRS effect, the length of the gain fiber as well as the passive fiber has been reduced by 1m, respectively. The reduced passive fiber is from the CLS part, and the length of the passive fiber cannot be reduced further for the convenience of fiber fusion splicing operation and laser measuring operation. The total pump absorption is about 18.4dB after the gain fiber is shortened by 1m, which means that the mode instability threshold is nearly unchanged. The output power, spectrums and backward power are shown in Figs. 7(a)-7(c). The data for the pristine systems has also been shown in Fig. 7(a) for comparison. One can conclude from Fig. 7(a) that the efficiency of the systems was nearly intact, so the change of mode instability due to fiber length varying is ignorable [58,60]. One can see from Fig. 7(b) and 7(c) that, compared with the results in Fig. 3(b), the backward power has been reduced after the fiber length was shortened, which means that the SRS effect has been effectively suppressed. It can also be seen from Fig. 7(c) that the optical spectrum is clean without any burrs, which means that the mode instability is higher than 3930W. By taking similar way in Fig. 2(c), the mode instability threshold was measured to be about 3.95kW, and is 50W higher than the pristine one (increased by 1.3%). This means that the mode instability threshold of the system with shortened fiber length is nearly the same as the pristine one when the variation of backward reflection from fiber fusion splice point for each fusion splicing operation [62,63] and/or the uncertainty of the optical power meter [64] are considered. One can conclude that the varying of SRS has little influence on the mode instability threshold. Due to that the SRS effect has been effectively suppressed for the case with the seed power being 310W (as indicated in Fig. 7(d)), the mode instability threshold can be measured, which is about 4.3kW. One can see that the mode instability threshold has been increased by 350W, which is due to the increase of seed power.
Fig. 7 Experimental results of the optimized fiber systems. (a) the output power as a function of pump power, (b) the backward power as a function of lasing power, (c) the spectrums at 3930W for different length of fiber, (d) the backward power as a function of lasing power for seed power of 310W.
Further work has been carried out to see whether the 1.3% increment is due to the suppressing of SRS effect. Due to that the passive fiber cannot be shortened for operation and measuring convenience, the length of the gain fiber has been further shortened by 1m to 30m, and the total pump absorption is about 17.7dB, which means that the mode instability threshold variation due to the shortening of the gain fiber length is still negligible. The measured data is shown in Fig. 8, which was obtained with the seed power being 250W. The data from the pristine systems has also been shown in Fig. 8(a). One can conclude from Fig. 8(a) that the efficiency of the systems is reduced slightly, which means that the gain fiber length cannot be shortened any longer. By comparing the results in Fig. 8(b) with those in Fig. 6(b), one can find that the SRS effect has been effectively suppressed. By taking similar way in Fig. 2(c), the mode instability threshold was measured to be about 4.23kW (the variation is about 0.7%), which is nearly equal to that of the pristine fiber and confirms again that the varying of SRS has little influence on the mode instability threshold in the experiment.
Fig. 8 (a) The output power as a function of pump power, and (b) the backward power as a function of lasing power.
4. Discussions
From the recent understanding of the mode instability physical mechanism, the SRS effect can affect mode instability mainly from two aspects: one is the backward Stokes power, which induces a extra short period refractive index grating and reduces the mode instability threshold [63,65], and the other is the additional heat load from the quantum defect from Raman power conversion, which triggers SRS-induced mode instability [34,35]. Due to that the Stokes light is shifted about 13 THz away from the signal wavelength, the backward Stokes light should has little impact on mode instability [63,65]. As revealed in [34], the quantum defect between the signal light and the Raman light results in heat load during Raman amplifying process, which leads to a thermally-induced refractive index grating and the onset of SRS-induced mode instability. Similar to those induced by ytterbium ion gain, the SRS-induced mode instability requires heat accumulation, which means that the Raman light from Raman gain process should reaches certain threshold. It is predicated that the power of the Raman light should reaches several hundred watts [34]. Due to that there is backward Raman light after the onset of SRS effect, which is not considered in Ref [34], the backward Raman light will reduce the SRS-induced mode instability [64,66], and result to that the SRS-induced mode instability is lower than those predicted in Ref [34]. In addition, Raman light can increase the noise content in the system, which will also decreases the mode instability threshold [35].
In Refs [35]. and [36], the core size of the main amplifier is 20μm, which is relatively small. The signal wavelength of the amplifier in [36] is 1064nm, which results to that the Raman light arises at 1120nm and experiences higher ytterbium ion gain when propagating in the amplifier [66]. The effective length of the main amplifier system can be calculated by Leff = LYDF/ln(G) + LGDF (G is the fiber amplifier power gain factor, LYDF and LGDF are the length of YDF and passive fiber, respectively [30,67]), which is about 15-19m for the main amplifier system in Ref [35], about 9-14m for the main amplifier system in Ref [36], and about 11-14m for the main amplifier system in our paper. Then one can see that, in Ref [35], the effective length is longer and the core size is smaller than those in our experiment while the signal wavelength is equal. Meanwhile, in Ref [36], the signal wavelength is shorter and the core size is smaller than those in our experiment while the effective length is almost equal. Due to the difference in the system parameters, the SRS effect in Refs [35]. and [36] is stronger, and the Raman power is above 100W near the mode instability threshold. So the SRS-induced mode instability has been triggered, and it comes very naturally that the mode instability threshold decreases as the SRS effect strengthens and can be increased by suppressing SRS.
As shown in the previous experiment and in Fig. 9, the power converts into Raman power is less than 25W (the highest Stokes peak at 4050W in Fig. 6B is still 22dB lower than the signal peak, and the Raman power fraction is 0.59%) and is far smaller than the cases in Refs [35] and [36], so the quantum defect heat from Raman power conversion and the influence of SRS on noise content is negligible, which means that the SRS-induced mode instability has not been triggered and that the mode instability in the experiment is the ytterbium-ion-gain-induced mode instability. Naturally, the SRS effect has little influence on ytterbium-ion-gain-induced mode instability. So one can see that the SRS effect has opposite impact on ytterbium-ion-gain-induced mode instability and SRS-induced mode instability, and the results in previous part are not conflict with those in Refs [35]. and [36] but root in different physical processes, which is a supplement of the work of K. Hejaz et. al.
Fig. 9 The linear spectrums of the high power fiber laser systems. The spectrums in (a) are at the same power of Fig. 4(a), and the spectrum in (b) is at ~4050W in Fig. 6(b), which corresponds to the strongest SRS effect except for the case that the systems has been shut down for safety consideration. The inset figure in (b) is the corresponding logarithm spectrum.
5. Conclusions
In this work, based on a high power tandem pumped all-fiberized platform, the influence of mode instability on SRS has been investigated for the first time. It revealed that mode instability can have significant impact on the dynamic characteristics of SRS, and apart from the traditional temporal and spatial characterization, the onset of mode instability can also be monitor from the optical spectrum. After the onset of mode instability, the measured optical spectrums become burr-like, especially the Raman light spectrums, which is due to the dynamic mode evolution of mode instability, and it presents a new characterization of mode instability. During the transition region of the mode instability, the SRS effect becomes stronger as the lasing power increases, and weakens as the stage of mode instability evolves into chaotic regions. By employing a suitable filter, the Raman light can be separated from the signal power [36], and the mode-instability-induced noise of the spectrums can be measured more accurately in the time domain or frequency domain, which will be studied in the future work.
The effect of SRS on mode instability has also been studied. By increasing the seed power and shortening the fiber length, the intensity of SRS has been varied. It revealed that, due to that the Raman power is very small (only a few percent), the SRS-induced mode instability has not been observed in the experiment, and the quantum defect heat during signal power converting to Stokes light can contribute little to the ytterbium-ion-gain-induced mode instability. The experimental results provide a new characterization for mode instability from the spectral domain, and a further understanding of the relationship between mode instability and SRS in the power scaling of fiber lasers.
Funding
National Natural Science Foundation of China (61735007, 61505260, 61322505); National Key R&D Program of China (2016YFB0402204).
Acknowledgements
The authors wish to thank Mr. Jiawei He, Mr. Chuanchuan Zhang and Mr. Kun Zhang for the help in measuring the laser performance in the experiment.
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