基于色散傅里叶变换的孤子脉冲建立与放大研究
0 引 言
锁模光纤激光器是产生高峰值功率、窄脉宽、极稳频率光脉冲的重要方法。因其独特的物理特性, 锁模光纤激光器在精密材料加工、医学治疗、生物光子学、光学频率梳、光学频率转换等领域有着不可替代的应用[1-5]。锁模光纤激光腔内的光孤子在各种激光腔参数下包含着丰富的非线性现象, 研究这些在脉冲周期时间量级里的现象对优化锁模光纤激光器参数、研究锁模脉冲行为机制、探究非线性光学有重要意义。传统光谱仪的扫描时间无法做到对单个脉冲光谱的测量。近年来, 作为一种新颖又强大的实时光谱测量技术, 色散傅里叶变换 (DFT) 技术被广泛应用于瞬态光孤子动力学, 获得单脉冲的光谱信息。该技术的原理是: 窄带脉冲的色散传输服从与一维近轴衍射相似的抛物型微分方程[6-8], 类比于夫琅禾费衍射, 在被大色散介质拉伸后, 脉冲频域光谱被映射到时域波形, 拉伸后对应光谱形状的脉冲经光电探测器后被数字示波器实时测量。许多在锁模光纤激光器中基于DFT技术的孤子瞬态现象 (如: 不同类型的孤子建立过程[9-12]、非平衡激光腔态引起的孤子爆炸[13-16]、孤子分子的相互作用[17-19]、呼吸孤子[20-22]、光学怪波[23-25]等) 被报道。
净反常色散的锁模光纤激光器产生的传统光孤子脉冲形状的保持源于色散和非线性效应的相互平衡。而产生于净正常色散锁模光纤激光腔中平衡了非线性、色散、增益和损耗[26]的耗散孤子脉冲不受传统孤子的单脉冲能量限制[27], 具有更大的脉宽和单脉冲能量。稳态输出情况下, 这两类光孤子由于脉宽、峰值功率、光谱、脉宽、啁啾特性的不同, 在经光放大器放大后有不同的光谱变化。动态情况下, 这两类锁模孤子脉冲在建立过程中有各自独特的孤子动力学过程。
本文基于DFT技术并结合掺铒锁模光纤激光器和掺铒光纤放大器, 研究了净反常色散区的传统孤子和净正常色散区的耗散孤子建立过程中的光谱瞬态演化; 同时在高能量分辨率状态下, 研究了脉冲光谱随放大器泵浦能量上升过程中传统孤子的谱宽展宽现象和耗散孤子光谱尖峰的形成机理, 并在模拟中进行了验证。
1 实验方案与装置
本研究所用的实验装置如
图 1. 被动锁模光纤激光器实验装置图。(a) 传统孤子激光器; (b) 耗散孤子激光器;(c) 掺铒光纤激光放大器和 DFT 脉冲测量装置
Fig. 1. Schematic illustration of the passive erbium-doped mode-locked fiber laser. (a) Conventional soliton fiber laser;(b) Dissipative soliton fiber laser; (c) Setup of the erbium-doped fiber laser amplifier and DFT pulse measurement
如
2 实验结果与讨论
泵浦功率为163 mW时, 传统孤子锁模激光器可以输出稳定的脉冲序列, 如
图 2. 两类孤子参数。上排: 传统孤子; 下排: 耗散孤子。(a), (d) 脉冲序列; (b), (e) 基频射频谱( 分辨率带宽: 100 Hz, 插图: 1 GHz范围 ); (c), (f) 自相关迹
Fig. 2. Pulse parameters of mode-locked seed source. Top row: Conventional soliton; Bottom row: Dissipative soliton. (a), (d) Pulse train; (b), (e) Fundamental repetition RF spectrum (Bandwidth resolution: 100 Hz; Insert: 1 GHz span); (c), (f) Autocorrelation trace
耗散孤子锁模激光器在泵浦功率369 mW下产生稳定的耗散孤子脉冲, 如
传统孤子光谱如
图 3. 脉冲光谱。(a) 传统孤子种子; (b) 传统孤子放大后; (c) 耗散孤子种子; (d) 耗散孤子放大后。黑线: 对数坐标(光谱仪); 红线: 线性坐标 (光谱仪); 蓝线: 线性坐标 (DFT)
Fig. 3. Pulse spectra. (a) Conventional soliton seed; (b) Amplified conventional soliton; (c) Dissipative soliton seed; (d) Amplified dissipative soliton. Black line: Logarithmic coordinate (OSA); Red line: Linear coordinate (OSA); Blue line: Linear coordinate (DFT)
如
通过种子源泵浦光的开关和DFT技术, 可以观测到传统孤子建立过程腔内脉冲光谱的实时演化过程。如
图 4. 传统孤子光谱 (上) 和单脉冲能量演化 (下)。(a) 建立过程; (b) 放大过程
Fig. 4. Spectral (up row) and single pulse energy (down row) evolution of conventional soliton.(a) Buildup process; (b) Amplification process
在传统孤子稳定输入时, 通过放大器泵浦光的开关可以测得随放大器泵浦功率从0上升至60 mW输出脉冲的光谱演化。如
如
图 5. 耗散孤子光谱 (上) 和单脉冲能量演化 (下)。(a) 建立过程; (b) 放大过程
Fig. 5. Spectral (up row) and single pulse energy (down row) evolution of dissipative soliton.(a) Buildup process; (b) Amplification process
3 数值模拟
采用光纤中扩展的非线性薛定谔方程 (ENLSE) 对传统孤子和耗散孤子的腔内产生以及放大进行模拟。光纤中脉冲演变由
描绘[31], 式中:
表 1. 锁模激光器与光放大器模拟参数设置
Table 1. Parameter set of mode-locked laser and optical amplifiers
|
模拟所得传统孤子与耗散孤子脉冲参数如
图 6. 模拟所得孤子光谱与啁啾参数。(a), (b) 传统孤子; (c), (d) 耗散孤子; (e), (f) 放大后传统孤子;(g), (h) 放大后耗散孤子
Fig. 6. Simulated spectra and chirp parameters. (a), (b) Conventional soliton; (c), (d) Dissipative soliton;(e), (f) Amplified conventional soliton; (g), (h) Amplified dissipative soliton
图 7. 模拟所得孤子光谱随放大功率上升的演化。(a) 传统孤子; (b) 耗散孤子
Fig. 7. Simulated evolution of soliton spectra versus amplification power. (a) Conventional soliton; (b) Dissipative soliton
若不考虑放大器的铒纤对脉冲的重吸收所致短波分量减弱, 传统孤子在放大后光谱中心会逐渐凹陷, 整体光谱被展宽并包含振荡边带。而耗散孤子的光谱能保持稳定的谱宽, 并随放大功率上升在光谱边沿出现逐渐明显的尖峰。
4 结 论
研究了基于色散傅里叶变换技术的传统孤子和耗散孤子锁模掺铒光纤激光器的瞬态光谱演变, 结果表明脉宽388 fs、中心波长1569.88 nm的传统孤子放大后光谱被展宽, 形状发生了改变, 而脉宽16.34 ps、中心波长1565 nm的耗散孤子放大后光谱除了长波分量相对更强外, 形状保持较好。在孤子建立阶段, 传统孤子和耗散孤子的光谱会经历一段振荡过程。在实验与模拟中, 孤子放大阶段传统孤子在放大后光谱中心会逐渐凹陷, 整体光谱被展宽并包含振荡边带; 耗散孤子的光谱则能保持稳定的谱宽, 并随放大功率上升在光谱边沿出现逐渐明显的尖峰。这些结果为预测锁模腔和放大器中的脉冲行为、提高锁模激光器的稳定性和优化锁模激光器和光放大器的开关性能提供了可能, 也有助于孤子动力学和被动锁模光纤激光器的研究。
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