Yin Ke, Zhu Rongzhen, Zhang Bin, Jiang Tian, Chen Shengping, Hou Jing
Opt Express. 2016 Sep 5;24(18):20010-20. doi: 10.1364/OE.24.020010.
Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.
输出光谱覆盖红外大气窗口的基于光纤的超连续谱（SC）源在远程大气应用中非常有用。事实证明，石英光纤能够支持产生从可见光到短波红外区域的宽带SC源。在本文中，我们通过数值模拟和实验展示了在包层泵浦掺铥光纤放大器（TDFA）中产生超高亮度、光谱平坦的2 - 2.5μm SC源的过程。首先利用一个包含光纤增益和损耗、色散和非线性效应的数值模型研究了SC产生过程背后的物理机制。模拟结果表明，在TDFA中产生了丰富的孤子脉冲，它们通过拉曼孤子自频移（SSFS）的非线性效应很快向长波长方向移动，最终由于石英光纤的红外损耗，拉曼SSFS过程停止。由于这些丰富的孤子脉冲的光谱叠加，能够产生光谱平坦的2 - 2.5μm SC源。这些模拟结果与以下实验结果在定性上很好地对应。然后，在实验中，搭建了一个包层泵浦的大模面积TDFA以获得高功率的2 - 2.5μm SC源。通过增强泵浦强度，输出的SC光谱逐渐向长波长方向展宽。在最高泵浦功率下，获得的SC源最大平均功率为203.4W，功率转换效率为38.7%。它在1990至2535nm范围内具有545nm的3dB光谱带宽，表明功率谱密度超过370mW/nm。同时，输出的SC源具有良好的光束轮廓。据我们所知，这个SC源是迄今为止报道的最亮的光谱平坦的2 - 2.5μm光源。它在许多远程大气应用中，如广谱激光雷达、自由空间通信和高光谱成像等方面将非常有价值。
