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2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Grating for Raman Suppression

2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber... hv photonics Article 2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Grating for Raman Suppression 1 , 2 1 , 2 1 , 2 1 , 2 1 , 2 , 3 1 , 2 , 3 Xin Tian , Chenhui Gao , Chongwei Wang , Xiaofan Zhao , Meng Wang , Xiaoming Xi and 1 , 2 , 3 , Zefeng Wang * College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China; tianxin@nudt.edu.cn (X.T.); gaochenhui@nudt.edu.cn (C.G.); wangchongwei@nudt.edu.cn (C.W.); zhaoxianfan@nudt.edu.cn (X.Z.); wangmeng@nudt.edu.cn (M.W.); xixiaoming@nudt.edu.cn (X.X.) Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China * Correspondence: zefengwang@nudt.edu.cn or zefengwang_nudt@163.com Abstract: We report a high power, narrow linewidth fiber laser based on oscillator one-stage power amplification configuration. A fiber oscillator with a center wavelength of 1080 nm is used as the seed, which is based on a high reflection fiber Bragg grating (FBG) and an output coupling FBG of narrow reflection bandwidth. The amplifier stage adopted counter pumping. By optimizing the seed and amplifier properties, an output laser power of 2276 W was obtained with a slope efficiency of 80.3%, a 3 dB linewidth of 0.54 nm and a signal to Raman ratio of 32 dB, however, the transverse mode instability (TMI) began to occur. For further increasing the laser power, a high-power chirped and tilted FBG (CTFBG) was inserted between the backward combiner and the output passive fiber, experimental results showed that both the threshold of Stimulated Raman scattering (SRS) and TMI increased. The maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, Citation: Tian, X.; Gao, C.; Wang, C.; a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. No TMI was observed and the beam Zhao, X.; Wang, M.; Xi, X.; Wang, Z. quality factor M maintained about 1.6. This work could provide a useful reference for obtaining 2.58 kW Narrow Linewidth Fiber narrow-linewidth high-power fiber lasers with high signal to Raman ratio. Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Keywords: narrow linewidth; fiber lasers; Stimulated Raman scattering; chirped and tilted fiber Grating for Raman Suppression. Bragg gratings Photonics 2021, 8, 532. https:// doi.org/10.3390/photonics8120532 Received: 3 October 2021 1. Introduction Accepted: 22 November 2021 Published: 25 November 2021 In the past years, owing to the great improvement of laser diodes (LDs) brightness and high-quality large mode area (LMA) fiber as well as beam combining technology, Publisher’s Note: MDPI stays neutral the output power of continuous-wave (CW) fiber lasers has been scaled rapidly [1–4]. with regard to jurisdictional claims in However, the further improvement of single fiber output power is limited by various published maps and institutional affil- nonlinear effects, transverse mode instability (TMI), thermal effect, etc. For the present iations. single fiber lasers, further power scaling is even difficult with compromise in bandwidth, beam quality, and so on. Spectral beam combining (SBC) is a promising approach to break through the limitations of the fiber lasers [5,6]. In SBC, the key is that the sub beam needs to be a narrow linewidth fiber laser (NLFL) with high beam quality, which usually realize Copyright: © 2021 by the authors. by a main oscillator power amplifier (MOPA) configuration [7]. At present, there is no Licensee MDPI, Basel, Switzerland. unified definition about the “narrow” of NLFLs. Considering its practical application in This article is an open access article SBC, in this paper, the linewidth of NLFLs is defined as <1 nm. For MOPA structure, there distributed under the terms and are two main types of seeds, namely few longitudinal mode fiber oscillator laser (FOL) conditions of the Creative Commons seed and phase modulated single-frequency laser (PMSFL) seed. The method utilizing a Attribution (CC BY) license (https:// phase modulation seed for power amplification is relatively mature, benefiting from the creativecommons.org/licenses/by/ stable temporal property, high nonlinear effects threshold and spectral purity during the 4.0/). Photonics 2021, 8, 532. https://doi.org/10.3390/photonics8120532 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 532 2 of 10 amplification process [8–10]. Up to now, the power of NLFL based on the PMSFL seed has been scaled to several thousand watts [10–17], and the highest power of NLFL has exceeded 5 kW [16,17]. For MOPA structure based on a FOL seed, because of its simple, compact and economical structure, it has also attracted enormous attention in recent years [18–23]. By this method, the maximum power also reached 3 kW [23]. However, among the factors currently limiting the further power scaling of NLFLs based on FOL seed, Stimulated Raman scattering (SRS) is one of the most important limitations. Since the injecting seed laser is not strictly single mode and is usually broadened during the power scaling process in the amplifier. The onset of SRS would bring about a series of problems, such as signal power declination, beam quality deterioration, fiber component damage, etc. [24,25]. Furthermore, the SRS of NLFLs would affect the beam quality of output and the efficient after spectral beam combining. Thus, high power NLFLs with high signal to Raman ratio are becoming increasingly important for SBC. Many methods have been used to suppress SRS in high-power fiber lasers, including the large-mode-area (LMA) fibers, spectrally selective fibers, long-period fiber gratings, chirped tilted fiber Bragg gratings (CTFBGs), and so on [26–31]. Among these methods, CTFBG is considered a comparatively suitable component for SRS suppression. With a tilt angle introduced chirped FBG, CTFBG can couple the forward core mode, which are originally transmitted only in the core, to backward core modes and the cladding modes. Due to its simplicity of application and good spectrum stability, CTFBGs utilized as broadband spectral filters have been extensively studied. In 2017, we firstly proposed and demonstrated using CTFBG to suppress SRS in high-power fiber amplifier [26]. Then, with the improvement of CTFBG fabrication technology, the power handling capability of CTFBGs kept refreshing, which has increased from hundred watts to the kilowatts level [27–31]. These studies are all based on the conventional high-power fiber laser, whose linewidth is in several nanometers level and not belongs to narrow linewidth. So far, there is no report that CTFBG used to suppress SRS in NLFLs, especially in the output of fiber laser handling multi-kW laser power to directly filter SRS. Here, we report a counter pumped MOPA configuration NLFL based on a FOL seed and CTFBG suppressing SRS. A CTFBG with carrying high power capability is applied to directly filter forward SRS at the output of multi-kW level fiber laser. With the CTFBG included, both the threshold of SRS and TMI increased, and the output power reached to 2.58 kW with a power improvement of 300 W compared that without CTFBG, and the laser slope efficiency was about 77.1%. At the output spectrum of the maximum power, the signal to Raman ratio was 42 dB, and the 3 dB linewidth was about 0.87 nm. The beam quality factor M maintained about 1.6 during the power scaling. This work is helpful for obtaining high-power narrow-linewidth fiber lasers with high signal to Raman ratio. 2. Experimental Setup The all-fiber FBG-based MOPA configuration fiber laser was established, as shown in Figure 1. The FOL seed consisted of a wavelength stable laser diode (WS LD) worked at 976 nm, a pair of fiber Bragg gratings (FBGs) with a center wavelength of ~1080 nm, a 3 m long 10/130 m Yb-doped fiber (YDF) and a cladding power stripper (CPS). The absorption coefficient of the YDF was 5.2 dB/m at 976 nm. The high reflective (HR) FBG and output coupler (OC) FBG provided a full-width-at-half-maximum (FWHM) of 2.6 nm and 0.04 nm, respectively. The seed laser was injected into the amplifier stage through a mode field adaptor (MFA), of which the input fiber and output fiber had a size of 10/130 m, 20/400 m, respectively. The amplifier stage based on counter pumping had two configurations with a dif- ference that whether CTFBG and band-pass filter (BPF) were used or not. For amplifier configuration 1, the gain fiber was a 11.5 m long double-cladding YDF with 20 m/0.06 NA core and 400 m/0.46 NA inner cladding. The absorption coefficient of the gain fiber was 1.42 dB/m at 976 nm. The YDF was coiled with minimum diameter of 90 mm. Four 976 nm WS LDs were employed as the counter pumping sources. The pumping power was coupled Photonics 2021, 8, 532 3 of 10 into the active fiber via a (6 + 1)  1 fiber combiner. The input signal fiber and output signal fiber of the backward combiner both had a size of 20/400 m. The CPS was used to eliminate residual pump light and the laser was output by a quartz block head (QBH) at the end. Including the backward combiner and QBH, the germanium-doped fiber (GDF) with a core/cladding diameter of 20/400 m has a total length of 3 m for delivering the output laser. Figure 1. Experimental setup of the all-fiber oscillator one-stage amplification high-power narrow linewidth fiber laser (WS LD: wavelength stable laser diode; YDF: Yb-doped fiber; GDF: germanium- doped fiber; CPS: cladding power stripper; QBH: quartz block head). After the narrow-linewidth oscillator for one-stage amplification experiment was completed, the amplifier configuration 2 was established based on configuration 1 by inserting a BPF and a CTFBG. The BPF is commercially available and CTFBG is specially customized. The BPF was inserted after the seed laser to filter out part of the background spectral noise and backward Raman light of the amplifier, preventing affecting seed laser injected. A specially designed and fabricated CTFBG on 20/400 m fiber was inserted between the backward combiner and the CPS for SRS suppression, resulting in 0.7 m increasing of GDF. The CTFBG had an average rejection depth of 20 dB with a rejection bandwidth of more than 20 nm, which covered the whole Raman spectral range of 1080 nm laser. Its Bragg reflection range was longer than 1150 nm [29]. The measured insertion loss was 2.1%. Then the experiment on the filtering effect of the CTFBG on the SRS in amplifier was carried out. In the all-fiber laser system, all the components in the experiment, including YDF, LDs, combiners, FBGs and CPSs, were placed on a water-cooled heat sink to ensure the stability in high power operation. The output power was divided into two parts via an HR mirror, the high power was detected by a power meter. The low power was used to measure the beam quality by a Beam Squared M analyzer manufactured by Ophir. Meanwhile, the spectrum and time domain were also recorded by a Spectrum Analyzer and a photo detector. Photonics 2021, 8, 532 4 of 10 3. Results and Discussion 3.1. Laser Performance with Amplifier Configuration 1 The seed power injected into the main amplifier is set to 17.5 W. Figure 2 illustrates the spectrum of the seed laser, which measured by a Yokogawa AQ6370D Optical Spectrum Analyzer with a spectral resolution of 0.02 nm. The 3 dB and 20 dB bandwidths are about 0.28 nm and1.40 nm. In the fiber oscillator, although only 3 m of YDF was applied, the laser efficiency still reached 76%. Figure 2. The output spectrum of the seed laser. To begin with, the MOPA system was first operated with amplifier configuration 1. During the power scaling process, the output power and spectrum were monitored and recorded. Figure 3a shows the output spectrum at different output power with the GDF length of 6 m. With a pumping power of 1982 W, the output power was 1570 W, but strong nonlinear effects were observed, such as SRS and four wave mixing. The signal to Raman ratio was measured at about 32 dB. The difference between the signal and four wave mixing ratio was about 42 dB. Then, cutting the GDF length to 3 m, the spectrum at different output power is illustrated in Figure 3b. Under the same signal to Raman ratio of 32 dB, the output power could reach to 2278 W with a pumping power of 2840 W, which was 700 W higher than the original one. Furthermore, the nonlinear effect was obviously mitigated, not only the SRS, but also four wave mixing effect was greatly weakened. Figure 3c,d demonstrate the output and optical-optical efficiency versus pump power with a GDF length of 3 and 6 m, respectively. The inserted pictures illustrate the spectral linewidth at their highest powers. By comparing these results, we can see the efficiency of amplifier increasing from 78.8% to 80.3%. At their highest power of 1570 W and 2278 W, the 3 dB are 0.41 nm and 0.54 nm with a linewidth increasing rate of 3.8 pm/100 W and 4.2 pm/100W. Experimental results shows the efficiency and power spectral density of amplifier had improved owing to the weak nonlinear effect caused by the shortening of GDF length. Therefore, the nonlinear effect has a great effect on the amplifier efficiency and output spectrum. Photonics 2021, 8, 532 5 of 10 Figure 3. The output spectrum versus output power (a) with GDF length of 6 m (b) with GDF length of 3 m. The output power and efficiency versus pump power (c) with GDF length of 6 m (d) with GDF length of 3 m. With the GDF length of 3 m, the beam quality at several different output powers is illustrated in Figure 4a. One can see that the beam quality M had been maintained about 1.6 during the process of amplification. With the increase of pumping power, the output power stopped increasing at 2278 W. Figure 4b shows the temporal signals and corresponding FFT spectra at the maximum power a periodic fluctuation of the time domain signal was observed, reasonably indicating TMI occurred [24], which resulted in the power stagflation. However, the beam quality shows no sign of deterioration. The final output was limited by TMI. These results that with GDF length of 3 m are referred to as the case of without CTFBG for the following comparison with the results with CTFBG. Photonics 2021 Photonics , 8, x FO 2021R P , 8, 532 EER REVIEW 6 of 10 6 of 10 0.025 2.2 2.2 2.0 2.0 0.020 -40 1.8 1.8 -60 0.015 1780W 2278W -80 1.6 1.6 -100 2047W 0.010 1361W -120 1.4 1.4 -140 0.005 -160 1.2 1.2 0 2500 5000 7500 10000 12500 Frequency(Hz) 0.000 1.0 1.0 -1.0 -0.5 0.0 0.5 1.0 1250 1500 1750 2000 2250 Time(s) Output power (W) (a) (b) Figure 4. (a) The beam quality at several output power. (b) Temporal signals and corresponding Fourier spectra at the Figure 4. (a) The beam quality at several output power. (b) Temporal signals and corresponding Fourier spectra at the output power of 2278 W. output power of 2278 W. 3.2. Laser Performance with Amplifier Configuration 2 3.2. Laser Performance with Amplifier Configuration 2 In order to further weaken the SRS, experiments were carried out in amplifier con- In order to further weaken the SRS, experiments were carried out in amplifier con- figuration 2. A BPF was inserted between the seed laser and MFA. Not it can filter out figuration 2. A BPF was inserted between the seed laser and MFA. Not it can filter out the the background spectral noise of the seed, but prevent the backward SRS affecting the seed performance. Furthermore, a specially designed CTFBG with high power capability background spectral noise of the seed, but prevent the backward SRS affecting the seed provided an effective method to directly filtering forward SRS at the output of multi-kW performance. Furthermore, a specially designed CTFBG with high power capability pro- level fiber laser. Figure 5a shows the output spectrum at different output power with vided an effective method to directly filtering forward SRS at the output of multi-kW level CTFBG. The maximum output power came to 2576 W at the pump power of 3346 W with fiber laser. Figure 5a shows the output spectrum at different output power with CTFBG. the CTFBG included, 300 W higher than that without CTFBG. The difference between The maximum output power came to 2576 W at the pump power of 3346 W with the the signal and Raman light was 40 dB at the maximum output power. The spectrum at CTFBG included, their highest power 300 W h leveliis gher than compared that with in Figure out CTFBG. The 5b. It can be seen that difference betwe the forward Raman en the sig- light was greatly filtered out, the suppression ratio is about 12.4 dB under the same output nal and Raman light was 40 dB at the maximum output power. The spectrum at their power level. More importantly, it brought an increase in output power with a lower Raman highest power level is compared in Figure 5b. It can be seen that the forward Raman light intensity. Figure 5c shows the Signal to Raman ratio evolution. If the Signal to Raman ratio was greatly filtered out, the suppression ratio is about 12.4 dB under the same output of 50 dB is set as the SRS threshold, the green curve could demonstrate the SRS threshold is power level. More importantly, it brought an increase in output power with a lower Ra- improved by 800 W with CTFBG operated. When the signal power increased to 2.58 kW, man intensity. Figure 5c shows the Signal to Raman ratio evolution. If the Signal to Raman the Signal to Raman ratio came to 40 dB. This value is 850 W higher than no CTFBG ap- plied, as illustrated by the blue curve in Figure 5c. During the process of amplification, the ratio of 50 dB is set as the SRS threshold, the green curve could demonstrate the SRS backward spectrum of the seed be measured. Figure 5d compared the backward spectrum threshold is improved by 800 W with CTFBG operated. When the signal power increased at the output power of 1600 W with and without BPF. The backward SRS was filtered out to 2.58 kW, the Signal to Raman ratio came to 40 dB. This value is 850 W higher than no clean and after adding BPF, there was no difference between the forward spectrum and the CTFBG applied, as illustrated by the blue curve in Figure 5c. During the process of ampli- original one. fication, the backward spectrum of the seed be measured. Figure 5d compared the back- ward spectrum at the output power of 1600 W with and without BPF. The backward SRS was filtered out clean and after adding BPF, there was no difference between the forward spectrum and the original one. 2 2 M M Vlotage(V) Amplitude(dB) Photonics 2021, 8, 532 7 of 10 Photonics 2021, 8, x FOR PEER REVIEW 7 of 10 -10 -10 With CTFBG 1003W Without CTFBG 2278W 32dB -20 1474W -20 With CTFBG 2231W 44.4dB 1838W With CTFBG 2576W 42dB -30 2231W -30 2576W 40dB -40 -40 -50 -50 -60 -60 -70 -70 -80 -80 1050 1100 1150 1200 1050 1100 1150 1200 Wavelength(nm) Wavelength (nm) (a) (b) -20 without CTFBG without BPF Backward with CTFBG with BPF -30 -40 40 -50 -60 -70 -80 1000 1500 2000 2500 1050 1100 1150 1200 Output power(W) Wavelength (nm) (c) (d) Figure 5. (a) The output spectrum versus output power. (b) The forward spectrum comparison with and without CTFBG Figure 5. (a) The output spectrum versus output power. (b) The forward spectrum comparison with and without CTFBG at at their highest power. (c) The signal to Raman ratio versus output power. (d) The backward spectrum comparison with their highest power. (c) The signal to Raman ratio versus output power. (d) The backward spectrum comparison with and and without BPF. without BPF. Figure 6a shows the output power and efficiency versus pump power. The slope ef- Figure 6a shows the output power and efficiency versus pump power. The slope ficiency of the system is 77.1%, which is slightly lower than before due to the insertion efficiency of the system is 77.1%, which is slightly lower than before due to the insertion loss of CTFBG. The inserted picture shows the 3 dB and 20 dB signal bandwidth of 0.87 loss of CTFBG. The inserted picture shows the 3 dB and 20 dB signal bandwidth of 0.87 nm nm and 4.62 nm. Figure 6b compared the 3 dB and 20 dB bandwidth of the signal laser and 4.62 nm. Figure 6b compared the 3 dB and 20 dB bandwidth of the signal laser versus versus output laser power between with and without CTFBG. From the results of band- output laser power between with and without CTFBG. From the results of bandwidth width comparison, with the increase of 0.7 m GDF introduced by CTFBG, the linewidth comparison, with the increase of 0.7 m GDF introduced by CTFBG, the linewidth was was wider than that without CTFBG, especially for 20 dB linewidth, resulting in a decrease wider than that without CTFBG, especially for 20 dB linewidth, resulting in a decrease of linewidth increasing rate obviously. The beam quality with different output powers is of linewidth increasing rate obviously. The beam quality with different output powers is illustrated in Figure 6b. When the output power reached to 2576 W, the beam quality was illustrated in Figure 6b. When the output power reached to 2576 W, the beam quality was maintained about 1.6 and didn’t deteriorate. Figure 6d shows the temporal signals and maintained about 1.6 and didn’t deteriorate. Figure 6d shows the temporal signals and corresponding FFT spectra at the maximum power. When the maximum output power corresponding FFT spectra at the maximum power. When the maximum output power was reached, TMI didn’t appear. This means the threshold of TMI was improved with the was reached, TMI didn’t appear. This means the threshold of TMI was improved with the suppression on SRS. By analyzing, the reason may be that the CTFBG coupled the forward suppression on SRS. By analyzing, the reason may be that the CTFBG coupled the forward Raman light into backward-propagating cladding, which decreased the heat deposition Raman light into backward-propagating cladding, which decreased the heat deposition in i the n the fi fiber ber, so the TMI threshol , so the TMI threshold d woul would d i incr ncrea ease se w with ith the the suppr suppres ession sion on on SRS. SRS. Th The e further further power power improvement wa improvement was s limited limited by pump power. by pump power. Intensity (dBm) Signal Raman ratio(dB) Intensity (dBm) Intensity(dBm) Photonics 2021, 8, 532 8 of 10 Photonics 2021, 8, x FOR PEER REVIEW 8 of 10 With CTFBG Without CTFBG 3dB 2500 5 slope efficiency: 77.1% With CTFBG 3dB Without CTFBG 20dB 2000 4 With CTFBG 20dB 1500 3 -1 0 3 dB: 0.87nm 60 2576W 20dB: 4.62nm -2 0 1000 2 -3 0 -4 0 10 76 1 07 8 10 80 10 82 Wavelen gth(nm) 0 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 3000 3500 Output power(W) Pump power (W) (a) (b) 2.2 2.2 0.025 2.0 2.0 0.020 -40 1.8 1.8 -60 0.015 2000W -80 990W 2576W 1.6 1.6 -100 2300W 1530W 0.010 -120 1.4 1.4 -140 0.005 -160 1.2 1.2 0 2500 5000 7500 10000 12500 Frequency(Hz) 0.000 1.0 1.0 -1.0 -0.5 0.0 0.5 1.0 750 1000 1250 1500 1750 2000 2250 2500 2750 Output power (W) Time(s) (c) (d) Figure 6. (a) The output power and optical-optical efficiency versus pump power with CTFBG. (b) The 3-dB and 20-dB Figure 6. (a) The output power and optical-optical efficiency versus pump power with CTFBG. (b) The 3-dB and 20-dB bandwidths of the signal laser versus output laser power. (c) The beam quality at several output power. (d) Temporal bandwidths of the signal laser versus output laser power. (c) The beam quality at several output power. (d) Temporal signals and corresponding Fourier spectra at the output power of 2576 W. signals and corresponding Fourier spectra at the output power of 2576 W. Next, when using CTFBG to suppress SRS in NLFL, the influence of the system fiber Next, when using CTFBG to suppress SRS in NLFL, the influence of the system length on the output linewidth needs to be considered. Other methods to suppress spec- fiber length on the output linewidth needs to be considered. Other methods to suppress trum broadening and TMI also need to be taken. Furthermore, the experimental design spectrum broadening and TMI also need to be taken. Furthermore, the experimental design strategies for higher output power should mainly focus on the character of injected seed strategies for higher output power should mainly focus on the character of injected seed laser, active fiber, and the co/counter pumping power ratios to achieve a comprehensive laser, active fiber, and the co/counter pumping power ratios to achieve a comprehensive suppression for both SRS and TMI. suppression for both SRS and TMI. 4. Con 4. Conclusions clusions In summary, we have presented an all-fiber narrow linewidth fiber amplifier seeded In summary, we have presented an all-fiber narrow linewidth fiber amplifier seeded by a narrow reflection FBG-based oscillator, and a CTFBG was used to suppress the SRS by a narrow reflection FBG-based oscillator, and a CTFBG was used to suppress the SRS ffor or llaser aser power i power incr ncreasi easing. ng. Wi Without thout the the CT CTFBG, FBG, the the ma maximum ximum output laser power wa output laser power was s 2276 W, which was mainly limited by TMI. With the CTFBG being inserted between the 2276 W, which was mainly limited by TMI. With the CTFBG being inserted between the backward combiner and the output passive fiber, the increasing of both the threshold of backward combiner and the output passive fiber, the increasing of both the threshold of SRS and TMI were observed, and the maximum laser power was improved to 2576 W SRS and TMI were observed, and the maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. At the maximum power, no TMI was observed and the beam quality factor M 2 0.87 nm. At the maximum power, no TMI was observed and the beam quality factor M maintained about 1.6. By further optimizing the system parameters, such as the power and maintained about 1.6. By further optimizing the system parameters, such as the power the linewidth of the seed, the active fiber length and its coiled way in the amplifier, the and the linewidth of the seed, the active fiber length and its coiled way in the amplifier, position of the CTFBG, and so on, this system could be expected to reach 5 kilowatts level the position of the CTFBG, and so on, this system could be expected to reach 5 kilowatts in the future. This work could provide good reference for obtaining compact high-power level in the future. This work could provide good reference for obtaining compact high- narrow-linewidth fiber lasers with high signal to Raman ratio. power narrow-linewidth fiber lasers with high signal to Raman ratio. 2 2 M M Output power (W) Intensity(dB) Efficiency (%) Vlotage(V) Bandwidth(nm) Amplitude(dB) Photonics 2021, 8, 532 9 of 10 Author Contributions: Conceptualization, Z.W.; methodology, X.X. and M.W.; validation, X.T., X.Z., X.X. and M.W.; formal analysis, X.T. and X.X.; investigation, X.T., C.G. and C.W.; resources, X.X. and X.T.; data curation, X.Z., C.W. and C.G.; writing—original draft preparation, X.T.; writing—review and editing, Z.W.; visualization, X.T. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Outstanding Youth Science Fund Project of Hunan Province Natural Science Foundation (2019JJ20023), National Natural Science Foundation of China (NSFC) (11974427, 12004431), and State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR05, SKL2021ZR01). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zervas, M.N.; Codemard, C.A. High power fiber lasers: A review. IEEE J. Sel. Top. Quantum Electron. 2014, 20, 219–241. [CrossRef] 2. Stiles, E. New developments in IPG fiber laser technology. In Proceedings of the 5th International Workshop on Fiber Lasers, Dresden, Germany, 30 September 2009. 3. Shiner, B. The Impact of Fiber Laser Technology on the World Wide Material Processing Market; CLEO: Applications and Technology: San Jose, CA, USA, 2013; AF2J.1. 4. Shcherbakov, E.A.; Fomin, V.V.; Abramov, A.A.; Ferin, A.A.; Mochalov, D.V.; Gapontsev, V.P. Industrial grade 100 kW power CW fiber laser. In Advanced Solid-State Lasers Congress; Huber, G., Moulton, P., Eds.; OSA Technical Digest (online); Optical Society of America: Paris, France, 2013; ATh4A. 2. 5. Wirth, C.; Schmidt, O.; Tsybin, I.; Schreiber, T.; Eberhardt, R.; Limpert, J.; Tünnermann, A.; Ludewigt, K.; Gowin, M.; Ten Have, E.; et al. High average power spectral beam combining of four fiber amplifiers to 8.2 kW. Opt. Lett. 2011, 36, 3118–3120. [CrossRef] [PubMed] 6. Zheng, Y.; Yang, Y.; Wang, J.; Hu, M.; Liu, G.; Zhao, X.; Chen, X.; Liu, K.; Zhao, C.; He, B.; et al. 10.8 kW spectral beam combination of eight all-fiber super-fluorescent sources and their dispersion compensation. Opt. Express 2016, 24, 12063–12071. [CrossRef] [PubMed] 7. Yu, C.; Shatrovoy, O.; Fan, T.; Taunay, T. Diode-pumped narrow linewidth multi-kilowatt metalized Yb fiber amplifier. Opt. Lett. 2016, 41, 5202–5205. [CrossRef] 8. Ma, P.; Tao, R.; Su, R.; Wang, X.; Zhou, P.; Liu, Z. 1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality. Opt. Express 2016, 24, 4187–4195. [CrossRef] 9. Su, R.; Liu, Y.; Yang, B.; Ma, P.; Wang, X.; Zhou, P.; Xu, X. Active polarization control of a 1.43 kW narrow linewidth fiber amplifier based on spgd algorithm. J. Opt. 2017, 19, 045802. [CrossRef] 10. Li, T.; Zha, C.; Sun, Y.; Ma, Y.; Ke, W.; Peng, W. 3.5 kW bidirectionally pumped narrow-linewidth fiber amplifier seeded by white-noise-source phase-modulated laser. Laser Phys. 2018, 28, 105101. [CrossRef] 11. Huang, Z.; Liang, X.; Li, C.; Lin, H.; Li, Q.; Wang, J.; Jing, F. Spectral broadening in high-power yb-doped fiber lasers employing narrow-linewidth multilongitudinal-mode oscillators. Appl.Opt. 2016, 55, 297–302. [CrossRef] 12. Qi, Y.; Yang, Y.; Shen, H.; He, B.; Zhou, J. 2.7 kW CW Narrow Linewidth Yb-Doped All-Fiber Amplifiers for Beam Combining Application. Advanced Solid-State Lasers: ATu3A.1; Optical Society of America: Paris, France, 2017. [CrossRef] 13. Su, R.; Tao, R.; Wang, X.; Zhang, H.; Ma, P.; Zhou, P.; Xu, X. 2.43 kW narrow linewidth linearly polarized all-fiber amplifier based on mode instability suppression. Laser Phys. Lett. 2017, 14, 085102. [CrossRef] 14. Ma, P.; Xiao, H.; Meng, D.; Liu, W.; Tao, R. High power all-fiberized and narrow-bandwidth MOPA system by tandem pumping strategy for thermally induced mode instability suppression. High Power Laser Sci. Eng. 2018, 6, e57. [CrossRef] 15. Lin, H.; Tao, R.; Li, C.; Wang, B.; Guo, C.; Shu, Q.; Zhao, P.; Xu, L.; Wang, J.; Jing, F.; et al. 3.7 kW monolithic narrow linewidth single mode fiber laser through simultaneously suppressing nonlinear effects and mode instability. Opt. Express 2019, 27, 9716–9724. [CrossRef] 16. Ma, P.; Xiao, H.; Liu, W.; Zhang, H.; Wang, X.; Leng, J.; Zhou, P. All-fiberized and narrow-linewidth 5 kW power-level fiber amplifier based on a bidirectional pumping configuration. High Power Laser Sci. Eng. 2021, 9, e45. [CrossRef] 17. Huang, Z.M.; Shu, Q.; Tao, R.M.; Chu, Q.H.; Luo, Y.; Yan, D.L.; Feng, X.; Liu, Y.; Wu, W.J.; Zhang, H.Y.; et al. >5 kW Record high power narrow linewidth laser from traditional step-index monolithic fiber amplifier. IEEE Photonics Technol. Lett. 2021, 33, 1181–1184. [CrossRef] 18. Huang, Y.; Yan, P.; Wang, Z.; Tian, J.; Li, D.; Xiao, Q.; Gong, M. 2.19 kW narrow linewidth fbg-based mopa onfiguration fiber laser. Opt. Express 2019, 27, 3136–3145. [CrossRef] Photonics 2021, 8, 532 10 of 10 19. Wang, Y.; Ma, Y.; Peng, W.; Ke, W.; Chang, Z.; Sun, Y.; Zhu, R.; Tang, C. 2.4 kW, narrow linewidth, near-diffraction-limited all-fiber laser based on a one-stage master oscillator power amplifier. Laser Phys. Lett. 2019, 17, 015102. [CrossRef] 20. Lee, J.; Lee, K.H.; Jeong, H.; Park, M.; Seung, J.H.; Lee, J.H. 2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg grating-stabilized laser diode seed source. Appl. Opt. 2019, 58, 6251–6256. [CrossRef] 21. Zhang, L.; Zhang, K.; Zhao, H.; Li, Y.; Zhu, C.; Zhang, D.; Gao, P.; Liu, Y.; Chen, N.; Zhou, S. 2 kW narrow linewidth all fiber laser. In Proceedings of the SPIE 11717, 24th National Laser Conference & Fifteenth National Conference on Laser Technology and Optoelectronics, Shanghai, China, 2 December 2020. 117173H. 22. Wang, Y.; Ke, W.; Peng, W.; Chang, Z.; Feng, Y.; Sun, Y.; Gao, Q.; Ma, Y.; Zhu, R.; Tang, C. 3 kW, 0.2 nm narrow linewidth linearly polarized all-fiber laser based on a compact mopa structure. Laser Phys. Lett. 2020, 17, 075101. [CrossRef] 23. Huang, Y.; Xiao, Q.; Li, D.; Xin, J.; Wang, Z.; Tian, J.; Wu, Y.; Gong, M.; Zhu, L.; Yan, P. 3 kW narrow linewidth high spectral density continuous wave fiber laser based on fiber bragg grating. Opt. Laser Technol. 2021, 133, 106538. [CrossRef] 24. Tao, R.; Xiao, H.; Zhang, H.; Leng, J.; Wang, X.; Zhou, P.; Xu, X. Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode instabilities. Opt. Express 2018, 26, 25098–25110. [CrossRef] 25. Chu, Q.; Shu, Q.; Chen, Z.; Li, F.; Yan, D.; Guo, C.; Lin, H.; Wang, J.; Jing, F.; Tang, C.; et al. Experimental study of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers. Photonics Res. 2020, 8, 595–600. [CrossRef] 26. Wang, M.; Zhang, Y.; Wang, Z.; Sun, J.; Cao, J.; Leng, J.; Gu, X.; Xu, X. Fabrication of chirped and tilted fiber Bragg gratings and suppression of stimulated Raman scattering in fiber amplifiers. Opt. Express 2017, 25, 1529–1534. [CrossRef] 27. Wang, M.; Wang, Z.; Liu, L.; Hu, Q.; Xiao, H.; Xu, X. Effective suppression of stimulated Raman scattering in half 10 kW tandem pumping fiber lasers using chirped and tilted fiber Bragg gratings. Photonics Res. 2019, 7, 167–171. [CrossRef] 28. Jiao, K.; Shu, J.; Shen, H.; Guan, Z.; Yang, F.; Zhu, R. Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators. High Power Laser Sci. Eng. 2019, 7, 76–82. [CrossRef] 29. Tian, X.; Zhao, X.; Wang, M.; Hu, Q.; Li, H.; Rao, B.; Xiao, H.; Wang, Z. Influence of Bragg reflection of chirped tilted fiber Bragg grating on Raman suppression in high-power tandem pumping fiber amplifiers. Opt. Express 2020, 28, 19508–19517. [CrossRef] 30. Tian, X.; Zhao, X.; Wang, M.; Wang, Z. Effective suppression of stimulated Raman scattering in direct laser diode pumped 5 kilowatt fiber amplifier using chirped and tilted fiber Bragg gratings. Laser Phys. Lett. 2020, 17, 085104. [CrossRef] 31. Song, H.; Yan, D.; Wu, W.; Shen, B.; Feng, X.; Li, L.; Chu, Q.; Li, M.; Wang, J.; Tao, R. SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG. Opt. Express 2021, 29, 20535–20544. [CrossRef] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Grating for Raman Suppression

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hv photonics Article 2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Grating for Raman Suppression 1 , 2 1 , 2 1 , 2 1 , 2 1 , 2 , 3 1 , 2 , 3 Xin Tian , Chenhui Gao , Chongwei Wang , Xiaofan Zhao , Meng Wang , Xiaoming Xi and 1 , 2 , 3 , Zefeng Wang * College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China; tianxin@nudt.edu.cn (X.T.); gaochenhui@nudt.edu.cn (C.G.); wangchongwei@nudt.edu.cn (C.W.); zhaoxianfan@nudt.edu.cn (X.Z.); wangmeng@nudt.edu.cn (M.W.); xixiaoming@nudt.edu.cn (X.X.) Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China * Correspondence: zefengwang@nudt.edu.cn or zefengwang_nudt@163.com Abstract: We report a high power, narrow linewidth fiber laser based on oscillator one-stage power amplification configuration. A fiber oscillator with a center wavelength of 1080 nm is used as the seed, which is based on a high reflection fiber Bragg grating (FBG) and an output coupling FBG of narrow reflection bandwidth. The amplifier stage adopted counter pumping. By optimizing the seed and amplifier properties, an output laser power of 2276 W was obtained with a slope efficiency of 80.3%, a 3 dB linewidth of 0.54 nm and a signal to Raman ratio of 32 dB, however, the transverse mode instability (TMI) began to occur. For further increasing the laser power, a high-power chirped and tilted FBG (CTFBG) was inserted between the backward combiner and the output passive fiber, experimental results showed that both the threshold of Stimulated Raman scattering (SRS) and TMI increased. The maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, Citation: Tian, X.; Gao, C.; Wang, C.; a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. No TMI was observed and the beam Zhao, X.; Wang, M.; Xi, X.; Wang, Z. quality factor M maintained about 1.6. This work could provide a useful reference for obtaining 2.58 kW Narrow Linewidth Fiber narrow-linewidth high-power fiber lasers with high signal to Raman ratio. Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Keywords: narrow linewidth; fiber lasers; Stimulated Raman scattering; chirped and tilted fiber Grating for Raman Suppression. Bragg gratings Photonics 2021, 8, 532. https:// doi.org/10.3390/photonics8120532 Received: 3 October 2021 1. Introduction Accepted: 22 November 2021 Published: 25 November 2021 In the past years, owing to the great improvement of laser diodes (LDs) brightness and high-quality large mode area (LMA) fiber as well as beam combining technology, Publisher’s Note: MDPI stays neutral the output power of continuous-wave (CW) fiber lasers has been scaled rapidly [1–4]. with regard to jurisdictional claims in However, the further improvement of single fiber output power is limited by various published maps and institutional affil- nonlinear effects, transverse mode instability (TMI), thermal effect, etc. For the present iations. single fiber lasers, further power scaling is even difficult with compromise in bandwidth, beam quality, and so on. Spectral beam combining (SBC) is a promising approach to break through the limitations of the fiber lasers [5,6]. In SBC, the key is that the sub beam needs to be a narrow linewidth fiber laser (NLFL) with high beam quality, which usually realize Copyright: © 2021 by the authors. by a main oscillator power amplifier (MOPA) configuration [7]. At present, there is no Licensee MDPI, Basel, Switzerland. unified definition about the “narrow” of NLFLs. Considering its practical application in This article is an open access article SBC, in this paper, the linewidth of NLFLs is defined as <1 nm. For MOPA structure, there distributed under the terms and are two main types of seeds, namely few longitudinal mode fiber oscillator laser (FOL) conditions of the Creative Commons seed and phase modulated single-frequency laser (PMSFL) seed. The method utilizing a Attribution (CC BY) license (https:// phase modulation seed for power amplification is relatively mature, benefiting from the creativecommons.org/licenses/by/ stable temporal property, high nonlinear effects threshold and spectral purity during the 4.0/). Photonics 2021, 8, 532. https://doi.org/10.3390/photonics8120532 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 532 2 of 10 amplification process [8–10]. Up to now, the power of NLFL based on the PMSFL seed has been scaled to several thousand watts [10–17], and the highest power of NLFL has exceeded 5 kW [16,17]. For MOPA structure based on a FOL seed, because of its simple, compact and economical structure, it has also attracted enormous attention in recent years [18–23]. By this method, the maximum power also reached 3 kW [23]. However, among the factors currently limiting the further power scaling of NLFLs based on FOL seed, Stimulated Raman scattering (SRS) is one of the most important limitations. Since the injecting seed laser is not strictly single mode and is usually broadened during the power scaling process in the amplifier. The onset of SRS would bring about a series of problems, such as signal power declination, beam quality deterioration, fiber component damage, etc. [24,25]. Furthermore, the SRS of NLFLs would affect the beam quality of output and the efficient after spectral beam combining. Thus, high power NLFLs with high signal to Raman ratio are becoming increasingly important for SBC. Many methods have been used to suppress SRS in high-power fiber lasers, including the large-mode-area (LMA) fibers, spectrally selective fibers, long-period fiber gratings, chirped tilted fiber Bragg gratings (CTFBGs), and so on [26–31]. Among these methods, CTFBG is considered a comparatively suitable component for SRS suppression. With a tilt angle introduced chirped FBG, CTFBG can couple the forward core mode, which are originally transmitted only in the core, to backward core modes and the cladding modes. Due to its simplicity of application and good spectrum stability, CTFBGs utilized as broadband spectral filters have been extensively studied. In 2017, we firstly proposed and demonstrated using CTFBG to suppress SRS in high-power fiber amplifier [26]. Then, with the improvement of CTFBG fabrication technology, the power handling capability of CTFBGs kept refreshing, which has increased from hundred watts to the kilowatts level [27–31]. These studies are all based on the conventional high-power fiber laser, whose linewidth is in several nanometers level and not belongs to narrow linewidth. So far, there is no report that CTFBG used to suppress SRS in NLFLs, especially in the output of fiber laser handling multi-kW laser power to directly filter SRS. Here, we report a counter pumped MOPA configuration NLFL based on a FOL seed and CTFBG suppressing SRS. A CTFBG with carrying high power capability is applied to directly filter forward SRS at the output of multi-kW level fiber laser. With the CTFBG included, both the threshold of SRS and TMI increased, and the output power reached to 2.58 kW with a power improvement of 300 W compared that without CTFBG, and the laser slope efficiency was about 77.1%. At the output spectrum of the maximum power, the signal to Raman ratio was 42 dB, and the 3 dB linewidth was about 0.87 nm. The beam quality factor M maintained about 1.6 during the power scaling. This work is helpful for obtaining high-power narrow-linewidth fiber lasers with high signal to Raman ratio. 2. Experimental Setup The all-fiber FBG-based MOPA configuration fiber laser was established, as shown in Figure 1. The FOL seed consisted of a wavelength stable laser diode (WS LD) worked at 976 nm, a pair of fiber Bragg gratings (FBGs) with a center wavelength of ~1080 nm, a 3 m long 10/130 m Yb-doped fiber (YDF) and a cladding power stripper (CPS). The absorption coefficient of the YDF was 5.2 dB/m at 976 nm. The high reflective (HR) FBG and output coupler (OC) FBG provided a full-width-at-half-maximum (FWHM) of 2.6 nm and 0.04 nm, respectively. The seed laser was injected into the amplifier stage through a mode field adaptor (MFA), of which the input fiber and output fiber had a size of 10/130 m, 20/400 m, respectively. The amplifier stage based on counter pumping had two configurations with a dif- ference that whether CTFBG and band-pass filter (BPF) were used or not. For amplifier configuration 1, the gain fiber was a 11.5 m long double-cladding YDF with 20 m/0.06 NA core and 400 m/0.46 NA inner cladding. The absorption coefficient of the gain fiber was 1.42 dB/m at 976 nm. The YDF was coiled with minimum diameter of 90 mm. Four 976 nm WS LDs were employed as the counter pumping sources. The pumping power was coupled Photonics 2021, 8, 532 3 of 10 into the active fiber via a (6 + 1)  1 fiber combiner. The input signal fiber and output signal fiber of the backward combiner both had a size of 20/400 m. The CPS was used to eliminate residual pump light and the laser was output by a quartz block head (QBH) at the end. Including the backward combiner and QBH, the germanium-doped fiber (GDF) with a core/cladding diameter of 20/400 m has a total length of 3 m for delivering the output laser. Figure 1. Experimental setup of the all-fiber oscillator one-stage amplification high-power narrow linewidth fiber laser (WS LD: wavelength stable laser diode; YDF: Yb-doped fiber; GDF: germanium- doped fiber; CPS: cladding power stripper; QBH: quartz block head). After the narrow-linewidth oscillator for one-stage amplification experiment was completed, the amplifier configuration 2 was established based on configuration 1 by inserting a BPF and a CTFBG. The BPF is commercially available and CTFBG is specially customized. The BPF was inserted after the seed laser to filter out part of the background spectral noise and backward Raman light of the amplifier, preventing affecting seed laser injected. A specially designed and fabricated CTFBG on 20/400 m fiber was inserted between the backward combiner and the CPS for SRS suppression, resulting in 0.7 m increasing of GDF. The CTFBG had an average rejection depth of 20 dB with a rejection bandwidth of more than 20 nm, which covered the whole Raman spectral range of 1080 nm laser. Its Bragg reflection range was longer than 1150 nm [29]. The measured insertion loss was 2.1%. Then the experiment on the filtering effect of the CTFBG on the SRS in amplifier was carried out. In the all-fiber laser system, all the components in the experiment, including YDF, LDs, combiners, FBGs and CPSs, were placed on a water-cooled heat sink to ensure the stability in high power operation. The output power was divided into two parts via an HR mirror, the high power was detected by a power meter. The low power was used to measure the beam quality by a Beam Squared M analyzer manufactured by Ophir. Meanwhile, the spectrum and time domain were also recorded by a Spectrum Analyzer and a photo detector. Photonics 2021, 8, 532 4 of 10 3. Results and Discussion 3.1. Laser Performance with Amplifier Configuration 1 The seed power injected into the main amplifier is set to 17.5 W. Figure 2 illustrates the spectrum of the seed laser, which measured by a Yokogawa AQ6370D Optical Spectrum Analyzer with a spectral resolution of 0.02 nm. The 3 dB and 20 dB bandwidths are about 0.28 nm and1.40 nm. In the fiber oscillator, although only 3 m of YDF was applied, the laser efficiency still reached 76%. Figure 2. The output spectrum of the seed laser. To begin with, the MOPA system was first operated with amplifier configuration 1. During the power scaling process, the output power and spectrum were monitored and recorded. Figure 3a shows the output spectrum at different output power with the GDF length of 6 m. With a pumping power of 1982 W, the output power was 1570 W, but strong nonlinear effects were observed, such as SRS and four wave mixing. The signal to Raman ratio was measured at about 32 dB. The difference between the signal and four wave mixing ratio was about 42 dB. Then, cutting the GDF length to 3 m, the spectrum at different output power is illustrated in Figure 3b. Under the same signal to Raman ratio of 32 dB, the output power could reach to 2278 W with a pumping power of 2840 W, which was 700 W higher than the original one. Furthermore, the nonlinear effect was obviously mitigated, not only the SRS, but also four wave mixing effect was greatly weakened. Figure 3c,d demonstrate the output and optical-optical efficiency versus pump power with a GDF length of 3 and 6 m, respectively. The inserted pictures illustrate the spectral linewidth at their highest powers. By comparing these results, we can see the efficiency of amplifier increasing from 78.8% to 80.3%. At their highest power of 1570 W and 2278 W, the 3 dB are 0.41 nm and 0.54 nm with a linewidth increasing rate of 3.8 pm/100 W and 4.2 pm/100W. Experimental results shows the efficiency and power spectral density of amplifier had improved owing to the weak nonlinear effect caused by the shortening of GDF length. Therefore, the nonlinear effect has a great effect on the amplifier efficiency and output spectrum. Photonics 2021, 8, 532 5 of 10 Figure 3. The output spectrum versus output power (a) with GDF length of 6 m (b) with GDF length of 3 m. The output power and efficiency versus pump power (c) with GDF length of 6 m (d) with GDF length of 3 m. With the GDF length of 3 m, the beam quality at several different output powers is illustrated in Figure 4a. One can see that the beam quality M had been maintained about 1.6 during the process of amplification. With the increase of pumping power, the output power stopped increasing at 2278 W. Figure 4b shows the temporal signals and corresponding FFT spectra at the maximum power a periodic fluctuation of the time domain signal was observed, reasonably indicating TMI occurred [24], which resulted in the power stagflation. However, the beam quality shows no sign of deterioration. The final output was limited by TMI. These results that with GDF length of 3 m are referred to as the case of without CTFBG for the following comparison with the results with CTFBG. Photonics 2021 Photonics , 8, x FO 2021R P , 8, 532 EER REVIEW 6 of 10 6 of 10 0.025 2.2 2.2 2.0 2.0 0.020 -40 1.8 1.8 -60 0.015 1780W 2278W -80 1.6 1.6 -100 2047W 0.010 1361W -120 1.4 1.4 -140 0.005 -160 1.2 1.2 0 2500 5000 7500 10000 12500 Frequency(Hz) 0.000 1.0 1.0 -1.0 -0.5 0.0 0.5 1.0 1250 1500 1750 2000 2250 Time(s) Output power (W) (a) (b) Figure 4. (a) The beam quality at several output power. (b) Temporal signals and corresponding Fourier spectra at the Figure 4. (a) The beam quality at several output power. (b) Temporal signals and corresponding Fourier spectra at the output power of 2278 W. output power of 2278 W. 3.2. Laser Performance with Amplifier Configuration 2 3.2. Laser Performance with Amplifier Configuration 2 In order to further weaken the SRS, experiments were carried out in amplifier con- In order to further weaken the SRS, experiments were carried out in amplifier con- figuration 2. A BPF was inserted between the seed laser and MFA. Not it can filter out figuration 2. A BPF was inserted between the seed laser and MFA. Not it can filter out the the background spectral noise of the seed, but prevent the backward SRS affecting the seed performance. Furthermore, a specially designed CTFBG with high power capability background spectral noise of the seed, but prevent the backward SRS affecting the seed provided an effective method to directly filtering forward SRS at the output of multi-kW performance. Furthermore, a specially designed CTFBG with high power capability pro- level fiber laser. Figure 5a shows the output spectrum at different output power with vided an effective method to directly filtering forward SRS at the output of multi-kW level CTFBG. The maximum output power came to 2576 W at the pump power of 3346 W with fiber laser. Figure 5a shows the output spectrum at different output power with CTFBG. the CTFBG included, 300 W higher than that without CTFBG. The difference between The maximum output power came to 2576 W at the pump power of 3346 W with the the signal and Raman light was 40 dB at the maximum output power. The spectrum at CTFBG included, their highest power 300 W h leveliis gher than compared that with in Figure out CTFBG. The 5b. It can be seen that difference betwe the forward Raman en the sig- light was greatly filtered out, the suppression ratio is about 12.4 dB under the same output nal and Raman light was 40 dB at the maximum output power. The spectrum at their power level. More importantly, it brought an increase in output power with a lower Raman highest power level is compared in Figure 5b. It can be seen that the forward Raman light intensity. Figure 5c shows the Signal to Raman ratio evolution. If the Signal to Raman ratio was greatly filtered out, the suppression ratio is about 12.4 dB under the same output of 50 dB is set as the SRS threshold, the green curve could demonstrate the SRS threshold is power level. More importantly, it brought an increase in output power with a lower Ra- improved by 800 W with CTFBG operated. When the signal power increased to 2.58 kW, man intensity. Figure 5c shows the Signal to Raman ratio evolution. If the Signal to Raman the Signal to Raman ratio came to 40 dB. This value is 850 W higher than no CTFBG ap- plied, as illustrated by the blue curve in Figure 5c. During the process of amplification, the ratio of 50 dB is set as the SRS threshold, the green curve could demonstrate the SRS backward spectrum of the seed be measured. Figure 5d compared the backward spectrum threshold is improved by 800 W with CTFBG operated. When the signal power increased at the output power of 1600 W with and without BPF. The backward SRS was filtered out to 2.58 kW, the Signal to Raman ratio came to 40 dB. This value is 850 W higher than no clean and after adding BPF, there was no difference between the forward spectrum and the CTFBG applied, as illustrated by the blue curve in Figure 5c. During the process of ampli- original one. fication, the backward spectrum of the seed be measured. Figure 5d compared the back- ward spectrum at the output power of 1600 W with and without BPF. The backward SRS was filtered out clean and after adding BPF, there was no difference between the forward spectrum and the original one. 2 2 M M Vlotage(V) Amplitude(dB) Photonics 2021, 8, 532 7 of 10 Photonics 2021, 8, x FOR PEER REVIEW 7 of 10 -10 -10 With CTFBG 1003W Without CTFBG 2278W 32dB -20 1474W -20 With CTFBG 2231W 44.4dB 1838W With CTFBG 2576W 42dB -30 2231W -30 2576W 40dB -40 -40 -50 -50 -60 -60 -70 -70 -80 -80 1050 1100 1150 1200 1050 1100 1150 1200 Wavelength(nm) Wavelength (nm) (a) (b) -20 without CTFBG without BPF Backward with CTFBG with BPF -30 -40 40 -50 -60 -70 -80 1000 1500 2000 2500 1050 1100 1150 1200 Output power(W) Wavelength (nm) (c) (d) Figure 5. (a) The output spectrum versus output power. (b) The forward spectrum comparison with and without CTFBG Figure 5. (a) The output spectrum versus output power. (b) The forward spectrum comparison with and without CTFBG at at their highest power. (c) The signal to Raman ratio versus output power. (d) The backward spectrum comparison with their highest power. (c) The signal to Raman ratio versus output power. (d) The backward spectrum comparison with and and without BPF. without BPF. Figure 6a shows the output power and efficiency versus pump power. The slope ef- Figure 6a shows the output power and efficiency versus pump power. The slope ficiency of the system is 77.1%, which is slightly lower than before due to the insertion efficiency of the system is 77.1%, which is slightly lower than before due to the insertion loss of CTFBG. The inserted picture shows the 3 dB and 20 dB signal bandwidth of 0.87 loss of CTFBG. The inserted picture shows the 3 dB and 20 dB signal bandwidth of 0.87 nm nm and 4.62 nm. Figure 6b compared the 3 dB and 20 dB bandwidth of the signal laser and 4.62 nm. Figure 6b compared the 3 dB and 20 dB bandwidth of the signal laser versus versus output laser power between with and without CTFBG. From the results of band- output laser power between with and without CTFBG. From the results of bandwidth width comparison, with the increase of 0.7 m GDF introduced by CTFBG, the linewidth comparison, with the increase of 0.7 m GDF introduced by CTFBG, the linewidth was was wider than that without CTFBG, especially for 20 dB linewidth, resulting in a decrease wider than that without CTFBG, especially for 20 dB linewidth, resulting in a decrease of linewidth increasing rate obviously. The beam quality with different output powers is of linewidth increasing rate obviously. The beam quality with different output powers is illustrated in Figure 6b. When the output power reached to 2576 W, the beam quality was illustrated in Figure 6b. When the output power reached to 2576 W, the beam quality was maintained about 1.6 and didn’t deteriorate. Figure 6d shows the temporal signals and maintained about 1.6 and didn’t deteriorate. Figure 6d shows the temporal signals and corresponding FFT spectra at the maximum power. When the maximum output power corresponding FFT spectra at the maximum power. When the maximum output power was reached, TMI didn’t appear. This means the threshold of TMI was improved with the was reached, TMI didn’t appear. This means the threshold of TMI was improved with the suppression on SRS. By analyzing, the reason may be that the CTFBG coupled the forward suppression on SRS. By analyzing, the reason may be that the CTFBG coupled the forward Raman light into backward-propagating cladding, which decreased the heat deposition Raman light into backward-propagating cladding, which decreased the heat deposition in i the n the fi fiber ber, so the TMI threshol , so the TMI threshold d woul would d i incr ncrea ease se w with ith the the suppr suppres ession sion on on SRS. SRS. Th The e further further power power improvement wa improvement was s limited limited by pump power. by pump power. Intensity (dBm) Signal Raman ratio(dB) Intensity (dBm) Intensity(dBm) Photonics 2021, 8, 532 8 of 10 Photonics 2021, 8, x FOR PEER REVIEW 8 of 10 With CTFBG Without CTFBG 3dB 2500 5 slope efficiency: 77.1% With CTFBG 3dB Without CTFBG 20dB 2000 4 With CTFBG 20dB 1500 3 -1 0 3 dB: 0.87nm 60 2576W 20dB: 4.62nm -2 0 1000 2 -3 0 -4 0 10 76 1 07 8 10 80 10 82 Wavelen gth(nm) 0 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 3000 3500 Output power(W) Pump power (W) (a) (b) 2.2 2.2 0.025 2.0 2.0 0.020 -40 1.8 1.8 -60 0.015 2000W -80 990W 2576W 1.6 1.6 -100 2300W 1530W 0.010 -120 1.4 1.4 -140 0.005 -160 1.2 1.2 0 2500 5000 7500 10000 12500 Frequency(Hz) 0.000 1.0 1.0 -1.0 -0.5 0.0 0.5 1.0 750 1000 1250 1500 1750 2000 2250 2500 2750 Output power (W) Time(s) (c) (d) Figure 6. (a) The output power and optical-optical efficiency versus pump power with CTFBG. (b) The 3-dB and 20-dB Figure 6. (a) The output power and optical-optical efficiency versus pump power with CTFBG. (b) The 3-dB and 20-dB bandwidths of the signal laser versus output laser power. (c) The beam quality at several output power. (d) Temporal bandwidths of the signal laser versus output laser power. (c) The beam quality at several output power. (d) Temporal signals and corresponding Fourier spectra at the output power of 2576 W. signals and corresponding Fourier spectra at the output power of 2576 W. Next, when using CTFBG to suppress SRS in NLFL, the influence of the system fiber Next, when using CTFBG to suppress SRS in NLFL, the influence of the system length on the output linewidth needs to be considered. Other methods to suppress spec- fiber length on the output linewidth needs to be considered. Other methods to suppress trum broadening and TMI also need to be taken. Furthermore, the experimental design spectrum broadening and TMI also need to be taken. Furthermore, the experimental design strategies for higher output power should mainly focus on the character of injected seed strategies for higher output power should mainly focus on the character of injected seed laser, active fiber, and the co/counter pumping power ratios to achieve a comprehensive laser, active fiber, and the co/counter pumping power ratios to achieve a comprehensive suppression for both SRS and TMI. suppression for both SRS and TMI. 4. Con 4. Conclusions clusions In summary, we have presented an all-fiber narrow linewidth fiber amplifier seeded In summary, we have presented an all-fiber narrow linewidth fiber amplifier seeded by a narrow reflection FBG-based oscillator, and a CTFBG was used to suppress the SRS by a narrow reflection FBG-based oscillator, and a CTFBG was used to suppress the SRS ffor or llaser aser power i power incr ncreasi easing. ng. Wi Without thout the the CT CTFBG, FBG, the the ma maximum ximum output laser power wa output laser power was s 2276 W, which was mainly limited by TMI. With the CTFBG being inserted between the 2276 W, which was mainly limited by TMI. With the CTFBG being inserted between the backward combiner and the output passive fiber, the increasing of both the threshold of backward combiner and the output passive fiber, the increasing of both the threshold of SRS and TMI were observed, and the maximum laser power was improved to 2576 W SRS and TMI were observed, and the maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. At the maximum power, no TMI was observed and the beam quality factor M 2 0.87 nm. At the maximum power, no TMI was observed and the beam quality factor M maintained about 1.6. By further optimizing the system parameters, such as the power and maintained about 1.6. By further optimizing the system parameters, such as the power the linewidth of the seed, the active fiber length and its coiled way in the amplifier, the and the linewidth of the seed, the active fiber length and its coiled way in the amplifier, position of the CTFBG, and so on, this system could be expected to reach 5 kilowatts level the position of the CTFBG, and so on, this system could be expected to reach 5 kilowatts in the future. This work could provide good reference for obtaining compact high-power level in the future. This work could provide good reference for obtaining compact high- narrow-linewidth fiber lasers with high signal to Raman ratio. power narrow-linewidth fiber lasers with high signal to Raman ratio. 2 2 M M Output power (W) Intensity(dB) Efficiency (%) Vlotage(V) Bandwidth(nm) Amplitude(dB) Photonics 2021, 8, 532 9 of 10 Author Contributions: Conceptualization, Z.W.; methodology, X.X. and M.W.; validation, X.T., X.Z., X.X. and M.W.; formal analysis, X.T. and X.X.; investigation, X.T., C.G. and C.W.; resources, X.X. and X.T.; data curation, X.Z., C.W. and C.G.; writing—original draft preparation, X.T.; writing—review and editing, Z.W.; visualization, X.T. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Outstanding Youth Science Fund Project of Hunan Province Natural Science Foundation (2019JJ20023), National Natural Science Foundation of China (NSFC) (11974427, 12004431), and State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR05, SKL2021ZR01). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zervas, M.N.; Codemard, C.A. High power fiber lasers: A review. IEEE J. Sel. Top. Quantum Electron. 2014, 20, 219–241. [CrossRef] 2. Stiles, E. New developments in IPG fiber laser technology. In Proceedings of the 5th International Workshop on Fiber Lasers, Dresden, Germany, 30 September 2009. 3. Shiner, B. The Impact of Fiber Laser Technology on the World Wide Material Processing Market; CLEO: Applications and Technology: San Jose, CA, USA, 2013; AF2J.1. 4. Shcherbakov, E.A.; Fomin, V.V.; Abramov, A.A.; Ferin, A.A.; Mochalov, D.V.; Gapontsev, V.P. Industrial grade 100 kW power CW fiber laser. In Advanced Solid-State Lasers Congress; Huber, G., Moulton, P., Eds.; OSA Technical Digest (online); Optical Society of America: Paris, France, 2013; ATh4A. 2. 5. Wirth, C.; Schmidt, O.; Tsybin, I.; Schreiber, T.; Eberhardt, R.; Limpert, J.; Tünnermann, A.; Ludewigt, K.; Gowin, M.; Ten Have, E.; et al. High average power spectral beam combining of four fiber amplifiers to 8.2 kW. Opt. Lett. 2011, 36, 3118–3120. [CrossRef] [PubMed] 6. Zheng, Y.; Yang, Y.; Wang, J.; Hu, M.; Liu, G.; Zhao, X.; Chen, X.; Liu, K.; Zhao, C.; He, B.; et al. 10.8 kW spectral beam combination of eight all-fiber super-fluorescent sources and their dispersion compensation. Opt. Express 2016, 24, 12063–12071. [CrossRef] [PubMed] 7. Yu, C.; Shatrovoy, O.; Fan, T.; Taunay, T. Diode-pumped narrow linewidth multi-kilowatt metalized Yb fiber amplifier. Opt. Lett. 2016, 41, 5202–5205. [CrossRef] 8. Ma, P.; Tao, R.; Su, R.; Wang, X.; Zhou, P.; Liu, Z. 1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality. Opt. Express 2016, 24, 4187–4195. [CrossRef] 9. Su, R.; Liu, Y.; Yang, B.; Ma, P.; Wang, X.; Zhou, P.; Xu, X. Active polarization control of a 1.43 kW narrow linewidth fiber amplifier based on spgd algorithm. J. Opt. 2017, 19, 045802. [CrossRef] 10. Li, T.; Zha, C.; Sun, Y.; Ma, Y.; Ke, W.; Peng, W. 3.5 kW bidirectionally pumped narrow-linewidth fiber amplifier seeded by white-noise-source phase-modulated laser. Laser Phys. 2018, 28, 105101. [CrossRef] 11. Huang, Z.; Liang, X.; Li, C.; Lin, H.; Li, Q.; Wang, J.; Jing, F. Spectral broadening in high-power yb-doped fiber lasers employing narrow-linewidth multilongitudinal-mode oscillators. Appl.Opt. 2016, 55, 297–302. [CrossRef] 12. Qi, Y.; Yang, Y.; Shen, H.; He, B.; Zhou, J. 2.7 kW CW Narrow Linewidth Yb-Doped All-Fiber Amplifiers for Beam Combining Application. Advanced Solid-State Lasers: ATu3A.1; Optical Society of America: Paris, France, 2017. [CrossRef] 13. Su, R.; Tao, R.; Wang, X.; Zhang, H.; Ma, P.; Zhou, P.; Xu, X. 2.43 kW narrow linewidth linearly polarized all-fiber amplifier based on mode instability suppression. Laser Phys. Lett. 2017, 14, 085102. [CrossRef] 14. Ma, P.; Xiao, H.; Meng, D.; Liu, W.; Tao, R. High power all-fiberized and narrow-bandwidth MOPA system by tandem pumping strategy for thermally induced mode instability suppression. High Power Laser Sci. Eng. 2018, 6, e57. [CrossRef] 15. Lin, H.; Tao, R.; Li, C.; Wang, B.; Guo, C.; Shu, Q.; Zhao, P.; Xu, L.; Wang, J.; Jing, F.; et al. 3.7 kW monolithic narrow linewidth single mode fiber laser through simultaneously suppressing nonlinear effects and mode instability. Opt. Express 2019, 27, 9716–9724. [CrossRef] 16. Ma, P.; Xiao, H.; Liu, W.; Zhang, H.; Wang, X.; Leng, J.; Zhou, P. All-fiberized and narrow-linewidth 5 kW power-level fiber amplifier based on a bidirectional pumping configuration. High Power Laser Sci. Eng. 2021, 9, e45. [CrossRef] 17. Huang, Z.M.; Shu, Q.; Tao, R.M.; Chu, Q.H.; Luo, Y.; Yan, D.L.; Feng, X.; Liu, Y.; Wu, W.J.; Zhang, H.Y.; et al. >5 kW Record high power narrow linewidth laser from traditional step-index monolithic fiber amplifier. IEEE Photonics Technol. Lett. 2021, 33, 1181–1184. [CrossRef] 18. Huang, Y.; Yan, P.; Wang, Z.; Tian, J.; Li, D.; Xiao, Q.; Gong, M. 2.19 kW narrow linewidth fbg-based mopa onfiguration fiber laser. Opt. Express 2019, 27, 3136–3145. [CrossRef] Photonics 2021, 8, 532 10 of 10 19. Wang, Y.; Ma, Y.; Peng, W.; Ke, W.; Chang, Z.; Sun, Y.; Zhu, R.; Tang, C. 2.4 kW, narrow linewidth, near-diffraction-limited all-fiber laser based on a one-stage master oscillator power amplifier. Laser Phys. Lett. 2019, 17, 015102. [CrossRef] 20. Lee, J.; Lee, K.H.; Jeong, H.; Park, M.; Seung, J.H.; Lee, J.H. 2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg grating-stabilized laser diode seed source. Appl. Opt. 2019, 58, 6251–6256. [CrossRef] 21. Zhang, L.; Zhang, K.; Zhao, H.; Li, Y.; Zhu, C.; Zhang, D.; Gao, P.; Liu, Y.; Chen, N.; Zhou, S. 2 kW narrow linewidth all fiber laser. In Proceedings of the SPIE 11717, 24th National Laser Conference & Fifteenth National Conference on Laser Technology and Optoelectronics, Shanghai, China, 2 December 2020. 117173H. 22. Wang, Y.; Ke, W.; Peng, W.; Chang, Z.; Feng, Y.; Sun, Y.; Gao, Q.; Ma, Y.; Zhu, R.; Tang, C. 3 kW, 0.2 nm narrow linewidth linearly polarized all-fiber laser based on a compact mopa structure. Laser Phys. Lett. 2020, 17, 075101. [CrossRef] 23. Huang, Y.; Xiao, Q.; Li, D.; Xin, J.; Wang, Z.; Tian, J.; Wu, Y.; Gong, M.; Zhu, L.; Yan, P. 3 kW narrow linewidth high spectral density continuous wave fiber laser based on fiber bragg grating. Opt. Laser Technol. 2021, 133, 106538. [CrossRef] 24. Tao, R.; Xiao, H.; Zhang, H.; Leng, J.; Wang, X.; Zhou, P.; Xu, X. Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode instabilities. Opt. Express 2018, 26, 25098–25110. [CrossRef] 25. Chu, Q.; Shu, Q.; Chen, Z.; Li, F.; Yan, D.; Guo, C.; Lin, H.; Wang, J.; Jing, F.; Tang, C.; et al. Experimental study of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers. Photonics Res. 2020, 8, 595–600. [CrossRef] 26. Wang, M.; Zhang, Y.; Wang, Z.; Sun, J.; Cao, J.; Leng, J.; Gu, X.; Xu, X. Fabrication of chirped and tilted fiber Bragg gratings and suppression of stimulated Raman scattering in fiber amplifiers. Opt. Express 2017, 25, 1529–1534. [CrossRef] 27. Wang, M.; Wang, Z.; Liu, L.; Hu, Q.; Xiao, H.; Xu, X. Effective suppression of stimulated Raman scattering in half 10 kW tandem pumping fiber lasers using chirped and tilted fiber Bragg gratings. Photonics Res. 2019, 7, 167–171. [CrossRef] 28. Jiao, K.; Shu, J.; Shen, H.; Guan, Z.; Yang, F.; Zhu, R. Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators. High Power Laser Sci. Eng. 2019, 7, 76–82. [CrossRef] 29. Tian, X.; Zhao, X.; Wang, M.; Hu, Q.; Li, H.; Rao, B.; Xiao, H.; Wang, Z. Influence of Bragg reflection of chirped tilted fiber Bragg grating on Raman suppression in high-power tandem pumping fiber amplifiers. Opt. Express 2020, 28, 19508–19517. [CrossRef] 30. Tian, X.; Zhao, X.; Wang, M.; Wang, Z. Effective suppression of stimulated Raman scattering in direct laser diode pumped 5 kilowatt fiber amplifier using chirped and tilted fiber Bragg gratings. Laser Phys. Lett. 2020, 17, 085104. [CrossRef] 31. Song, H.; Yan, D.; Wu, W.; Shen, B.; Feng, X.; Li, L.; Chu, Q.; Li, M.; Wang, J.; Tao, R. SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG. Opt. Express 2021, 29, 20535–20544. [CrossRef]

Journal

Photonics – Multidisciplinary Digital Publishing Institute

Published: Nov 25, 2021

Keywords: narrow linewidth; fiber lasers; Stimulated Raman scattering; chirped and tilted fiber Bragg gratings

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