Experimental study of supercontinuum generation in Yb<sup>3+</sup>-doped microstructure fiber pumped by femtosecond pulses
Authors: Wang Wei , Zuo Yu-Ting , Dong Ting-Ting , Zhu Wei-Zhen , Lin Tian-Xu , Xu Hai-Dong , Qing Yuan , Han Ying , Qi Yue-Feng , Hou Lan-Tian ,   School of Information Science and Engineering, Yanshan University, Qinhuangdao 066004, China,   Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Qinhuangdao 066004, China,   Jiangsu Key Laboratory of Meteorological Observation and Information Processing, Nanjing University of Information Science and Technology, Nanjing 210044, China
Published: December 2020

The nonlinear effects and supercontinuum generation by the concept of wavelength conversion and amplification are experimentally studied in two Yb<sup>3+</sup>-doped microstructure fibers (Yb<sup>3+</sup>-MSFs), with the Ti: sapphire femtosecond pulses used as pump. Firstly, two Yb<sup>3+</sup>-MSFs are pumped by continuous wave separately to obtain the emission spectrum. The relationship between the luminous efficiency and the deviation of pump light from the Yb<sup>3+</sup> absorption peak is studied for each of the two fibers. The experimental results indicate that the luminous efficiency decreases as the deviation increases. However, both fibers still have high luminous efficiency even when the deviation reaches to 85 nm. Secondly, the supercontinuum spectrum is generated by the femtosecond laser pumping the cores of the two fibers. The influence of the pump power, relative position between emission light and zero-dispersion wavelength <i>λ</i><sub>0</sub>, pump wavelength and fiber length on the supercontinuum generation are studied. The results demonstrate that the amplified emission light at 1035 nm is first captured by the pump light to evolve into ultrashort pulse, and nonlinear effects are subsequently generated. As the pump power increases, for Yb<sup>3+</sup>-MSF1 whose <i>λ</i><sub>0</sub> is located near the emission light of Yb<sup>3+</sup> irons, the fundamental soliton is generated and further shifts toward red region under Raman effect. Compared with Yb<sup>3+</sup>-MSF1, the Yb<sup>3+</sup>-MSF2 has a small core, which means that its <i>λ</i><sub>0</sub> is short and the emission light is located in its anomalous dispersion region far from the <i>λ</i><sub>0</sub>. Experimental results reveal that higher-order soliton and soliton fission are more likely to happen and supercontinuum spectrum can be formed. However, the further broadening of the supercontinuum spectrum is limited by OH- absorption at 1380 nm. Either increasing the deviation of pump light from the Yb<sup>3+</sup> absorption peak or shortening the fiber length reduces the accumulated power of the emission light, so the experimental results show that red-shift of Raman soliton is reduced and the supercontinuum spectrum is narrowed for both fibers. The supercontinuum generation efficiency in the output spectrum can reach 98% when the effect of pump light coupling efficiency and microstructure fiber loss are neglected. It means that almost all the residual pump light and emission light of Yb<sup>3+</sup> contribute to the generation of supercontinuum. Finally, the Yb<sup>3+</sup>-MSF2s are tapered to different taper lengths to study their influence on supercontinuum generation. The results indicate that the leakage after tapering weakens the energy of the Raman soliton, which further results in the decrease of red-shift. Eventually, the red edge of supercontinuum spectrum shrinks seriously with theincrease of the taper length. However, the decreasing of <i>λ</i><sub>0</sub> at the taper waist leads to blue-shift of dispersive wave that satisfies the phase matching condition with Raman soliton. This contributes to the blue-shift of the short wavelength boundary and widens the range of supercontinuum spectrum at short wavelength. Therefore, tapering is a promising method of expanding supercontinuum spectrum towards short wavelength. In conclusion, the supercontinuum spectrum is generated in Yb<sup>3+</sup>-doped microstructure fiber pumped by the Ti: sapphire femtosecond laser. The output spectrum can be adjusted flexibly by combining the merit of high peak power and wavelength tunability of Ti: sapphire femtosecond laser and the characteristics of wavelength conversion and amplification of Yb<sup>3+</sup> irons. Thus, the method presented in the paper provides a promising way to obtain tunable supercontinuum spectrum.