摘要
采用熔融芯法制备出高增益的Yb∶YAG晶体衍生光纤,纤芯内Yb_(2)O_(3)的掺杂浓度(质量分数)达到5.25%。光纤在976 nm处的增益系数为12.6 dB/cm,在1550 nm处的传输损耗为1.29 dB/m。采用DBR线性腔结构,将8 mm长的Yb∶YAG晶体衍生光纤作为增益光纤,实现了17.8 mW的976 nm单频激光输出,对应的斜效率为12.1%,激光的信噪比大于45 dB,线宽小于41 kHz。
Objective Single-frequency fiber lasers(SFFLs)are widely used in areas of coherent beam combination,gravitational wave detection,lidar,and nonlinear frequency conversion because of their excellent performance.In particular,SFFLs operating at 976 nm are highly demanded for nonlinear wavelength conversion to generate coherent blue light.SFFLs use either a ring-or linear-cavity configuration.The ring-cavity setup is complicated because many additional components must be inserted to enable a single-frequency output,which unavoidably introduces insertion loss.In addition,the stable single-frequency operation of a ring-cavity fiber laser is susceptible to environmental changes and vibrations,thereby resulting in mode hopping.In comparison,linear-cavity construction,such as the distributed Bragg reflector(DBR)scheme,is more compact,which creates a large longitudinal mode spacing,helping to maintain lasing on a stable single longitudinal and hop-free mode.The cavity length of DBR SFFL is limited to only a few centimeters.Therefore,high-gain fibers are demanded to enable sufficiently high gain.A novel Yb∶YAG crystal-derived fiber(YDSF)that exhibits some unique properties in fiber lasers has been developed.The YDSF was fabricated based on a molten core method(MCM)and shows advantages such as high doping levels and high stimulated Brillouin scattering threshold.In addition,the pure silica cladding of the YDSF makes it highly compatible with commercially available silica fiber devices.All the above mentioned characteristics make the YDSF suitable for high-power single-frequency lasers.Based on these fibers,singlefrequency lasers emitting at 1μm have been demonstrated recently.In 2019,we demonstrated a 110-mW singlefrequency YDSF laser at 1064nm.However,to the best of our knowledge,single-frequency YDSF lasers below 1μm have never been reported.Methods A commercially available 10%(atomic number fraction)Yb∶YAG crystal was used to prepare a YDSF.In the experiment,the entire preparation process was divided into two steps to maintain the uniformity of the optical fiber.First,a rod fiber having a diameter of~1.7mm was fabricated using a 1.6-mm YAG crystal and pure silica tube(Dinner=2mm,Dexternal=10 mm).The drawing temperature was controlled at~2000℃.Second,the YDSF was fabricated based on the rod fiber.A short piece of rod fiber was inserted into a different silica tube with the same specification to constitute a new preform,which was drawn into the fiber at 1940℃.Next,the physical and optical properties of the YDSF were measured using some devices and methods,such as an optical microscope,energy dispersive spectrometer,fiber refractometer,and cut-back method.Afterward,a homemade all-fiber amplifier was used to measure the gain coefficient of the YDSF at 976nm.Then,the laser performance of the YDSF was investigated by optimizing the gain-fiber length and reflectivity of fiber Bragg grating(FBG).In addition,a DBR SFFL based on an 8-mm-long YDSF was built to further verify the performance of the YDSF.Results and Discussions The mass fraction of SiO2and Yb2O3in the core region of the YDSF were measured to be58.83%and 5.25%,respectively(Fig.1).As expected,interdiffusion occurred between the Yb∶YAG core and silica cladding during the drawing process.The refractive index profile of the fiber cross section was measured;the numerical aperture(NA)of the core with a diameter of 8.7μm was 0.5(Fig.1),indicating that the YDSF was a multimode fiber.The absorption peaks of the YDSF were located at 915nm and 976nm,corresponding to the transitions from the ground state 2 F7/2to higher states of 2 F5/2 of Yb3+.The peak absorption coefficients were6dB/cm and 30dB/cm for 915nm and 976nm,respectively(Fig.1).For a signal power of 0dBm and pump power of 181mW,the net gain coefficient of the YDSF reached 12.6dB/cm(Fig.2),which indicated that the YDSF could be used as a gain medium for a 976-nm laser.By optimizing the gain-fiber length and reflectivity of FBG,a maximum output power of 37.2mW was obtained with a slope efficiency of 24.3%(Fig.3).In addition,using the 8-mm-long YDSF as the gain medium,a 976-nm DBR SFFL was demonstrated.A maximum output power of 17.8mW with a signal-to-noise ratio(SNR)of>45dB was obtained at a launched pump power of 203mW,and no output power saturation was observed.The corresponding slope efficiency was 15.1%(Fig.5),which was low because of the mode mismatch.More efforts should be made for reducing the NA and improving Yb3+doping concentration.The linewidth of the laser was measured to be less than 41kHz,which was limited by the measurement setup(Fig.6).The beam quality of the laser output was also measured using a charge-coupled device(Thorlabs,BC106N-VIS);the beam quality factor was measured to be 1.01and 1.02in the horizontal and vertical directions,respectively(Fig.5).Conclusions A YDSF with 5.25%Yb2O3 doping concentration(mass fraction)was fabricated using MCM.The transmission loss of the YDSF with a core diameter of 8.7μm was measured to be 1.29dB/m at 1550nm.The gain coefficient of the YDSF was 12.6dB/cm at 976nm with a pump absorption coefficient of 6dB/m at 915nm.Using the DBR linear cavity,a 17.8-mW single-frequency laser at 976nm was achieved with an 8-mm-long YDSF,exhibiting a slope efficiency of 18.5%.To the best of our knowledge,this is the first demonstration of a singlefrequency YDSF laser below 1μm.The SNR was measured to be>45dB with a linewidth of less than 41kHz.Results indicate that the YDSF is a promising candidate material for the SFFL operating in the 976-nm wavelength region.
作者
谢永耀
丛振华
赵智刚
张行愚
赵显
邵贤彬
赵微
刘兆军
Xie Yongyao;Cong Zhenhua;Zhao Zhigang;Zhang Xingyu;Zhao Xian;Shao Xianbin;Zhao Wei;Liu Zhaojun(School of Information Science and Engineering,Shandong University,Qingdao,Shandong 266237,China;Shandong Provincial Key Laboratory of Laser Technology and Application,Qingdao,Shandong 266237,China;Center for Optics Research and Engineering,Shandong University,Qingdao,Shandong 266237,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2021年第12期150-158,共9页
Chinese Journal of Lasers
基金
国家自然科学基金(62075117,62075116)
教育部联合基金(6141A02022430)
山东省重点研发计划(2019JMRH0111)
山东省自然科学基金(ZR2019MF039)
山东省自然科学基金青年基金(ZR2020QF095)
山东大学卓越团队基金
山东大学杰出中青年基金
山东大学齐鲁青年启动基金。
