2.3 W 3.5 m Fiber Laser Based on Bidirectional Pumping

doi:10.3788/CJL202249.1801001

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	2.3 W 3.5 m Fiber Laser Based on Bidirectional Pumping
	X. Zhang; C. Tong; K. Cai; Y. Wang; L. Wang and S. Tian
	2022
发表期刊	Zhongguo Jiguang/Chinese Journal of Lasers
ISSN	2587025
卷号	49 期号:18
摘要	Objective There is an increasing scientific interest in developing mid-infrared laser sources to fulfill various application requirements in the biomedical, defense and security, and material processing. In particular, the 3.5 m fiber laser has become a research hotspot due to its good beam quality, high brightness, and being in an atmospheric window. Limited by the nature of fiber materials and lack of fiber optical devices, the power of a mid-infrared fiber laser is two to three orders of magnitude less than that of a near-infrared laser. However, the low power of a mid-infrared fiber laser is a key factor limiting its practical applications. It is great significance for expanding its application fields to improve the power of a mid-infrared fiber laser. Methods The method of cascade pumping is proposed to achieve a high efficiency 3.5 m fiber laser. The 1973 nm and 976 nm dual-wavelength laser source is used to pump the Er-doped fluoride fiber (Er: ZBLAN). The ground-state particles are pumped by the 976 nm laser to the energy level of 4I11/2, and the particles at the 4L11/2 energy level are pumped by the 1973 nm laser to the upper energy level to achieve a 3. 5 m laser. However, the end of a fiber would be damaged under a high pump power. Considering that the main factor of the 3. 5 m laser power is the 1973 nm pump power, the bidirectional pumping scheme is proposed to improve the output power here. Double homemade 1973 nm pump laser sources are coupled into the 4. 8 m long Er: ZBLAN fiber. The core diameter and numerical aperture of the fiber are 17.8 m and 0. 12, the cladding diameter and numerical aperture are 250 m and 0. 5, and the doping concentration (mole fraction) is 1%. Both ends of the optical fiber are cut into a 0 angle and closely attached to the cavity mirror. The reflectivity of the output coupling mirror is about 60% at 3500 nm (high transmission @ 1973 nm & high transmission @976 nm). And a 45 dichroic mirror DM2 (high reflection@ 3500 nm & high transmission @ 1973 nm & high transmission @976 nm) is used to output a 3.5 m laser from the system. Results and Discussions First, the 3. 5 m lasers are realized by forward and backward pumping, respectively. And the output powers are measured (Fig. 2). It can be seen that the output power for backward pumping is slightly higher than that for forward pumping. When the 976 nm pump power is fixed, the output power increases with the increase of the 1973 nm power and then reaches saturation. At this time, the output power continues to increase with the increase of the 1973 nm pump power when the 976 nm pump power increases. The reason can be analyzed by the energy level structure of the 3. 5 m Er: ZBLAN fiber laser (Fig. 3). The ground-state particles are pumped by the 976 nm laser to the energy level of 4I11/2 and subsequently these particles are pumped by the 1973 nm laser to the upper energy level to achieve a 3.5 m laser. When the 976 nm pump power is low, the number of particles at the 4I11/2 energy level is small. The output laser power will be saturated under the high 1973 nm pump power. And the output power continues to increase with the increase of the 1973 nm pump power by enhancing the 976 nm pump power, because the number of particles at the 4I11/2 energy level increases under the high 976 nm pump power. Of course, the number of particles pumped to the 4I11/2 energy level should be matched with the 1973 nm pump power. The excessively high 976 nm pump power would not contribute to improve the laser power, but increase the heat dissipation pressure on the fiber end face. By fully optimizing the 3. 5 m laser output power in both forward and backward pumping, the bidirectional pumping is conducted. When the backward pumping is turned on and the maximum output power is realized, the forward pumping is then turned on. With the increase of the 1973 nm pump power, the 976 nm power is also increased complementarily. The power curve for bidirectional pumping is recorded (Fig. 4). The laser threshold is 1.5 W, the maximum output power is 2.32 W, and th total light-to-light conversion efficiency is 10.33%. The output spectra under different pump powers are measured by the mid-infrared Fourier spectrometer (Fig. 5). When the 1973 nm pump power is 2 W, the central wavelength of the lasing spectrum is 3457 nm. As the pump power exceeds 2 W, the central wavelength is switched to 3539 nm. The beam envelope and diameter of the 3.5 m output laser are measured by pyroelectric camera. The beam quality factor (M2) is calculated according to the laser beam quality test method (Fig. 6). The output laser beam presents a fundamental mode Gaussian distribution and its beam quality factor M2 is less than 1.5. Conclusions Based on the homemade 1973 nm pump sources, a 3.5 m Er: ZBLAN fiber laser based on bidirectional pumping is achieved at room temperature. The maximum output power is 2.32 W, and the total light-to-light conversion efficiency is 10. 33%. And there is no damage on the fiber end under the maximum pump power. When the 1973 nm pump power is below 2 W, the laser central wavelength is 3457 nm. The central wavelength is switched to 3540 nm as the 1973 nm pump power exceeds 2 W. The envelope of the laser beam presents a fundamental mode Gaussian distribution and the beam quality factor M2 is less than 1.5. By optimizing the pumping structure and fiber parameters, the laser power can be further improved. 2022 Science Press. All rights reserved.
DOI	10.3788/CJL202249.1801001
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收录类别	ei
引用统计
文献类型	期刊论文
条目标识符	http://ir.ciomp.ac.cn/handle/181722/66364
专题	中国科学院长春光学精密机械与物理研究所
推荐引用方式 GB/T 7714	X. Zhang,C. Tong,K. Cai,et al. 2.3 W 3.5 m Fiber Laser Based on Bidirectional Pumping[J]. Zhongguo Jiguang/Chinese Journal of Lasers,2022,49(18).
APA	X. Zhang,C. Tong,K. Cai,Y. Wang,&L. Wang and S. Tian.(2022).2.3 W 3.5 m Fiber Laser Based on Bidirectional Pumping.Zhongguo Jiguang/Chinese Journal of Lasers,49(18).
MLA	X. Zhang,et al."2.3 W 3.5 m Fiber Laser Based on Bidirectional Pumping".Zhongguo Jiguang/Chinese Journal of Lasers 49.18(2022).

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