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Abstract
In this paper, we demonstrate experimental investigations on kilowatt-level Yb-Raman fiber amplifiers (YRFAs) employing a superfluorescent fiber source (SFS) or a multi-longitudinal mode fiber oscillator (OSC) as the Raman-pump laser. Through comparing the output properties of the two YRFAs, the experimental results reveal that the YRFA employing the SFS is superior to the YRFA employing the OSC in the performances of power scalability and narrow-linewidth operation. Meanwhile, about 1.16 kW Raman-signal laser at 1120 nm is obtained through the YRFA employing the SFS as the Raman-pump laser. Overall, the presentation could provide an effective solution for the design of high-power narrow linewidth YRFAs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power fiber lasers at 1.1-1.2 μm region have been highly desired in the application of nonlinear frequency conversion, such as frequency doubling to obtain visible lasers [1,2] and optical pump to generate near-infrared lasers [3,4]. Both ytterbium-doped fiber lasers (YDFLs) and Raman fiber lasers (RFLs) could be applied to achieve laser sources at 1.1-1.2 μm region [5,6]. However, power scaling of YDFLs at 1.1-1.2 μm region is strongly restricted by the strong amplified spontaneous emission (ASE) [7], and the reported output power of the YDFLs at 1.1-1.2 μm region is only several hundred watts [8,9]. Accordingly, most of the high-power fiber lasers at 1.1-1.2 μm are realized through Raman fiber amplifiers (RFAs) [10–17]. In particular, RFAs employing both ytterbium ion gain and Raman gain, which are commonly called as Yb-Raman fiber amplifiers (YRFAs), have drawn extra attention benefitted from the excellent performances in beam quality and narrow-linewidth operation [16,17].
YRFAs could take advantages of both ytterbium ion gain and Raman gain, in which the initial pump laser is converted into the Raman-signal laser mainly through two steps. First, the initial pump laser is converted efficiently into the transitional Raman-pump laser through ytterbium ion gain, and then the transitional Raman-pump laser is converted quickly into the Raman-signal laser through Raman gain [12]. Nevertheless, similar to other high-power fiber amplifiers [18], the performances of YRFAs are also restricted by the nonlinear behaviors, especially the spectral broadening phenomenon and the occurrence of higher order Raman Stokes light [12–15]. Those two phenomena could be effectively suppressed through replacing the commonly-used multi-longitudinal mode fiber oscillators (OSCs) by the temporally stable lasers [19,20], such as the phase-modulated single-frequency fiber lasers (PM SFFLs) [21]. Apart from PM SFFLs, superfluorescent fiber sources (SFSs) have also been proved to be an available laser source with relatively stable intensity for optical pump in fiber lasers [22–26].
Recently, we built a 200-watt level YRFA and showed that the SFS could provide a compact and low-cost laser source for the Raman-pump laser in narrow-linewidth YRFAs [27]. In this work, we further investigate the applicability of the SFS in high-power YRFAs experimentally through comparisons of two kilowatt-level YRFAs employing the OSC or the SFS as the Raman-pump laser. The performances of power conversion, second order Raman Stokes light, and spectral broadening properties in the two YRFAs are demonstrated and compared in details. The results reveal that the performances of the YRFA employing the SFS are superior to those of the YRFA employing the OSC, and about 1.16 kW Raman-signal laser at 1120 nm is obtained through the YRFA employing the SFS as the Raman-pump laser.
2. Experimental setup
The experimental setup of the YRFA is shown schematically in Fig. 1. The Raman-pump seed (an SFS or OSC) at 1070 nm and the Raman-signal seed (a PM SFFL) at 1120 nm are first combined through a 1070/1120 nm wavelength division multiplexer (WDM). Then, the two seed lasers are coupled into the main amplifier through a mode field adaptor (MFA) and a (6 + 1)×1 pump combiner, together with initial pump laser provided by laser diodes (LDs) at 976 nm. The active fiber in the main amplifier is a commercial double clad Yb-doped fiber (YDF). The core and cladding diameters of the YDF are 20 μm and 400 μm, respectively, and the cladding absorption coefficient of the YDF is about 1.2 dB/m at 976 nm. To ensure enough power conversion efficiency from the initial pump laser to the Raman-signal laser and investigate the properties of the second order Raman stokes light, the length of the YDF is designed to be 40 m in this YRFA. A cladding power stripper (CPS) is spliced after the YDF to mitigate the residual cladding light, and a quartz block holder (QBH) is connected for power delivering. The output laser of the YRFA is separated by a dichroic mirror (DM), and then the output properties of the Raman-signal laser could be measured independently.
To better distinguish the two cases when the OSC or the SFS is employed as the Raman-pump laser in the YRFA, the two YRFAs are called as the OSC-pumped YRFA and the SFS-pumped YRFA for short, respectively, in the following analysis.
The detailed structures of the self-constructed seed lasers (the SFS and the PM SFFL) are described in our previous work [27], and the output powers of the two seeds are adjusted to be about 20 W here. In addition, the output power of the OSC is also about 20 W. Figures 2(a) and 2(b) illustrate the normalized spectra of the Raman-pump seed and Raman-signal seed in the two YRFAs. As shown in Fig. 2(a), the center wavelengths of the two Raman-pump seeds are around 1070 nm, and the corresponding 3 dB linewidths are about 0.27 nm (OSC) and 14.8 nm (SFS), respectively. As shown in Fig. 2(b), the central wavelength of the Raman-signal seed is around 1120 nm, and the 3 dB spectral linewidth is about 0.42 nm.
3. Results and discussion
Figures 3(a) and 3(b) illustrate the output power and power ratio of the Raman-signal laser at different pump powers in the two YRFAs, respectively. The pump power corresponds to the power of the initial pump laser at 976 nm here and in the following analysis. The power ratio of the Raman-signal laser is calculated through dividing the output power of the Raman-signal laser by the total output power of the fiber amplifier. As shown in Fig. 3(a), the output power of the Raman-signal laser grows with the pump power in the two cases. Specifically, a maximum output power of about 0.81 kW is obtained at the pump power of 1.13 kW with an optical-to-optical efficiency of about 71.7% in the OSC-pumped YRFA, and further power scaling of this YRFA is restricted by the strong second order Raman Stokes light. Meanwhile, a maximum output power of about 1.16 kW is obtained at the pump power of 1.60 kW with an optical-to-optical efficiency of about 72.5% in the SFS-pumped YRFA, and further power scaling of this YRFA is only limited by available pump power here. As shown in Fig. 3(b), the power ratio of the Raman-signal laser grows quickly from about 73.4% at a pump power of 0.19 kW to about 96.4% at a pump power of 0.75 kW, and drops to about 93.5% at a pump power of 1.13 kW in the OSC-pumped YRFA. Meanwhile, the power ratio of the Raman-signal laser increases gradually from about 54.0% at a pump power of 0.19 kW to about 96.6% at the maximum pump power in the SFS-pumped YRFA. Thus, although the power in the Raman-pump laser could be converted into the Raman-signal laser much quickly in the OSC-pumped YRFA than the SFS-pumped YRFA, the ultimate power ratio of the Raman-signal laser could be similar to each other in the two YRFAs.
Fig. 3. The output power properties of the Raman-signal laser in the two YRFAs at different pump powers: (a) Output power; (b) Power ratio.
To clarify the differences between the output power properties of the Raman-signal laser shown in Fig. 3, we measure and compare the total output spectra of the two YRFAs. Figure 4 illustrates the normalized total output spectra of the two YRFAs at the corresponding maximum output powers. The total spectra (from 950 nm to 1250 nm) of the YRFA are measured via an optical spectrum analyzer (OSA) with the spectral resolution of 0.1 nm when the DM is removed. As shown in Fig. 4, obvious second order Raman Stokes light is observed and the peak value of the spectral component around 1178 nm is as high as -23.4 dB at an output power of 0.81 kW in the OSC-pumped YRFA. Accordingly, further power scaling of this OSC-pumped YRFA is restricted by the second order Raman Stokes light. In contrast, the spectral component of the second order Raman stokes light around 1178 nm is negligible at an output power of 1.16 kW in the SFS-pumped YRFA, thus the second order Raman stokes light is effectively suppressed here.
The possible reason for the results shown in Fig. 3 and Fig. 4 is that the Raman-pump laser could have weaker intensity fluctuation when the broadband SFS is employed, which benefits from the original relatively stable intensity [28] or the broadband spectrum of the SFS. On the one hand, higher intensity fluctuation in the Raman-pump laser means higher effective Raman gain in the YRFA, and then the Raman-pump laser could be converted into the Raman-signal laser more quickly, which is consistent with the results shown in Fig. 3(b). On the other hand, previous study has shown that the four-wave mixing (FWM) among the Raman-pump laser, Raman-signal laser, and second order Raman Stokes light, regardless of the large wave-vector mismatch, could occur and lead to the enhancement of the second order Raman Stokes light in high-power RFAs [20]. Meanwhile, this FWM effect could be effectively suppressed through applying temporal stable laser as the Raman-pump laser, which is consistent with the results shown in Fig. 4. Therefore, the SFS-pumped YRFA is superior to the OSC-pumped YRFA in the performance of power scalability restricted by the second order Raman Stokes light.
We also notice that the output spectrum of the Raman-signal laser in the SFS-pumped YRFA is narrower than that of the OSC-pumped YRFA. To demonstrate the detailed spectral property of the Raman-signal laser, we measure the output spectra of the Raman-signal laser at different output powers through an OSA with the spectral resolution of 0.02 nm. Figures 5(a) and 5(b) illustrate the normalized output spectra of the Raman-signal laser at different output powers in the two YRFAs. As shown in Fig. 5, the output spectra of the Raman-signal laser broaden with the increasing output power in the two cases while the spectral broadening law is different in the two YRFAs. Specifically, the overall spectra of the Raman-signal laser broaden uniformly with the increasing output power in the OSC-pumped YRFA, and the central part of the spectra keeps almost unchanged while the spectral wing gradually broadens with the increasing output power in the SFS-pumped YRFA.
Fig. 5. Normalized output spectra of the Raman-signal laser in: (a) OSC-pumped YRFA; (b) SFS-pumped YRFA.
To give a quantitative description of the spectral broadening phenomenon, we also have calculated the spectral linewidth of the Raman-signal laser in the two YRFAs. Figures 6(a) and 6(b) illustrate the corresponding 3 dB, 10 dB, and 20 dB spectral linewidths of the Raman-signal lasers at different output powers in the two YRFAs. The 3 dB, 10 dB, and 20 dB spectral linewidths of the Raman-signal seed are about 0.40 nm, 0.80 nm, and 1.15 nm, respectively. As shown in Fig. 6(a), the 3 dB, 10 dB, and 20 dB spectral linewidths of the Raman-signal laser in the OSC-pumped YRFA grow quickly to about 2.29 nm, 7.05 nm and 13.58 nm, and the corresponding spectral broadening factors are 5.7, 8.8, and 11.8, respectively. As shown in Fig. 6(b), the 3 dB, 10 dB, and 20 dB spectral linewidths of the Raman-signal laser in the SFS-pumped YRFA grow gradually to about 0.93 nm, 1.94 nm and 4.77 nm, and the corresponding spectral broadening factors are 2.3, 2.4, and 4.1, respectively. Accordingly, despite that the maximum output power of the SFS-pumped YRFA is higher than that of the OSC-pumped YRFA, the output spectral linewidth and corresponding spectral broadening factor of the SFS-pumped YRFA is much smaller than those of the OSC-pumped YRFA at the maximum output powers. Therefore, the SFS-pumped YRFA is also superior to the OSC-pumped YRFA in the performance of narrow-linewidth operation.
4. Conclusion
In conclusion, we construct two kilowatt-level YRFAs employing the OSC or the SFS as the Raman-pump laser, and compare their output powers and spectral properties experimentally. Specifically, further power scaling of the SFS-pumped YRFA and the OSC-pumped YRFA are limited by available pump power and the strong second order Raman Stokes light, respectively. In addition, the output spectrum of the Raman-signal laser in the SFS-pumped YRFA is also much narrower than that in the OSC-pumped YRFA. The experimental results reveal that the YRFA employing the SFS as the Raman-pump laser is superior to the YRFA employing the OSC as the Raman-pump laser in the performances of power scalability restricted by the second order Raman Stokes light and narrow-linewidth operation. In particular, about 1.16 kW Raman-signal laser at 1120 nm is obtained through the YRFA employing the SFS as the Raman-pump laser. The presentation could provide a well reference for high-power narrow linewidth YRFAs.
Funding
National Natural Science Foundation of China (62005313, 62061136013); Innovative Research Team in Natural Science Foundation of Hunan Province (2019JJ10005); Guangdong Key Research and Development Program (2018B090904001); Postgraduate Scientific Research Innovation Project of Hunan Province (CX20200018).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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