• Chinese Journal of Lasers
  • Vol. 49, Issue 17, 1701002 (2022)
Nanyu Chen1、2, Xiaolong Chen1, Wansheng Liu1、2, Yibo Liu1, He Wang1、2, Bing He1、2、*, and Jun Zhou1、2
Author Affiliations
  • 1Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China
  • 2College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
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    DOI: 10.3788/CJL202249.1701002 Cite this Article Set citation alerts
    Nanyu Chen, Xiaolong Chen, Wansheng Liu, Yibo Liu, He Wang, Bing He, Jun Zhou. High Efficiency and Stability 1018 nm Single Fiber Oscillator[J]. Chinese Journal of Lasers, 2022, 49(17): 1701002 Copy Citation Text show less

    Abstract

    Objective

    The laser power requirement is becoming higher and higher in the industrial and medical fields. It is an effective way to improve the output power of the 1018 nm fiber laser using the tandem-pumped technology. Compared with the conventional direct pumping technology for the 976 nm semiconductor laser, this technology has the advantages of high brightness and low quantum loss, resulting in the high output of pump power and the low influence of thermal effect from fiber and device. However, the main difficulty in the 1018 nm fiber laser is that the absorption cross-section of Yb3+ at 1018 nm is larger than that at a longer wavelength. This leads to fierce competition between lasing at 1018 nm and amplified spontaneous emission (ASE) at 1030 nm. Based on the continuous wave rate equation model, the effects of gain fiber length, output grating reflectivity, and bandwidth on the output characteristics of a 1018 nm laser are analyzed. We hope that the methods and results of our research can be helpful to the design of stable and efficient tandem-pumped sources.

    Methods

    In this paper, first, we divide the ASE spectrum into the K equally spaced regions based on the rate equation model of a laser. The central wavelength of each region is λi,and the bandwidth is Δλi,where i =1, 2, 3, …, K. A rate equation model with an ASE term is established for a continuous wave. Second, the above model is used to simulate the output powers and output spectra of the laser under different pumping modes, and the appropriate pumping mode is selected. Third, the output characteristics of the output couplers with different reflectivity and bandwidths are simulated to screen out the appropriate range of reflectivity and bandwidth for the output coupler. Fourth, the effect of gain fiber length on the output characteristics of the laser is quantitatively described. According to the results of numerical simulation, we optimize the parameters of the laser system and conduct the experiments. Finally, the experimental results are compared with the simulated results, and an efficient and stable laser scheme is founded.

    Results and Discussions

    According to the physical model established above, it is found by calculation that the output power under forward pump is 2.1% higher than that under backward pump when the total pump power is the same [Fig. 2(a)]. Therefore, the forward pump is more reasonable. When we adopt forward pump and use a 3 m gain fiber, only the reflectivity and bandwidth of the output coupler are changed. The results of numerical simulation are shown in Fig. 3(a). The bandwidth range of the output coupler is 0.2-1.0 nm, and the reflectivity is 0.05-0.15, which are a reasonable choice for the laser. We increase the ytterbium doped fiber (YDF) length from 3.0 m to 4.0 m, and the laser output power grows slowly [Fig. 4 (a)]. At the same time, the power proportion of the 1018 nm laser in the output power decreases, while the power proportion of ASE increases rapidly from 0.2% to 7.5%. Finally, we use a 2.7 m 30 μm/250 μm double-clad Yb3+ fiber with an output grating reflectivity of 9.5% and a 3 dB bandwidth of 1 nm. A 447 W 1018 nm laser output with a central wavelength of 1018.38 nm and a 3 dB bandwidth of 0.19 nm is achieved with a 531 W pump input. The maximum optical to optical efficiency is 84.2% and the slope efficiency is 82.8%. The beam quality factor (M2) indicating beam quality is 1.85[Fig. 5(c)].

    Conclusions

    In this paper, a 1018 nm high power fiber laser is reported. Based on the continuous wave rate equation model, the influence of gain fiber length on the output characteristics of the 1018 nm laser is analyzed. It is found that when the gain fiber length reaches 4 m or more, the power of ASE is up to 40 W and accounts for 7.5% of the total output power. Therefore, in order to suppress the strong ASE, a 2.7 m gain fiber is used after theoretical calculation and experimental optimization. Meanwhile, as the absorption peak of Yb3+ at 976 nm is very narrow, a wave-locked semiconductor pump source is used to maximize the absorption of pump light. The maximum power output of 520 W is obtained. It has the maximum optical to optical efficiency of 84.2% at 447 W. To our knowledge, it is one of the highest efficiencies for the 1018 nm fiber laser oscillators with a core diameter larger than 30 μm. In order to make the laser stable, the structure of the laser is further optimized. The pump source with less power fluctuation and the water cooler with high stability are used. In addition, the laser operation at 21 ℃ is kept. The power instability is 0.7% at 520 W, which indicates that it can be used as a reliable pump source for tandem pumping of high power fiber lasers.

    Nanyu Chen, Xiaolong Chen, Wansheng Liu, Yibo Liu, He Wang, Bing He, Jun Zhou. High Efficiency and Stability 1018 nm Single Fiber Oscillator[J]. Chinese Journal of Lasers, 2022, 49(17): 1701002
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