[论文解读] Mode interaction aided soft excitation of dark solitons in normal dispersion microresonators and offset-frequency tunable Kerr combs

Xiaoxiao Xue, Yi Xuan+7 more|arXiv (Cornell University)|Apr 10, 2014

Advanced Fiber Laser Technologies参考文献 37被引用 7

一句话总结

该论文首次通过模式相互作用辅助激发，在正常色散微腔中实现了暗孤子梳的软激发，利用微加热器实现了稳定、低噪声、模式锁定的运行，并可调谐梳状线偏移频率。该方法在以往尝试失败的条件下实现了相干暗脉冲生成，并将动力学特性与双稳态稳态之间的前缘相互作用联系起来。

ABSTRACT

Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Soft excitation of dark pulse solutions is difficult and has been demonstrated neither experimentally nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided soft excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. A micro-heater actuated tuning scheme provides a new degree of freedom for adjusting the laser-resonator detuning; as a result we are able to substantially tune the comb offset frequency while maintaining low noise, highly coherent mode-locked operation. Through simulation we show that the observed dark pulses can be related to interactions of fronts which connect the bistable steady-state solutions of the nonlinear cavity. Microresonator-based optical frequency combs, also termed Kerr combs, are generated through conversion of a single pump frequency to a broadband frequency comb inside a high-quality-factor microresonator via the third-order Kerr nonlinearity [1]-[10]. For comb generation, the optical pump frequency is tuned close to a resonance of the microresonator. When the intracavity power exceeds some threshold, frequency combs grow up and are expanded due to modulational instability gain and cascaded four-wave mixing. The advantages of Kerr combs include very compact size, high repetition rate, and capability of generating ultra-broad combs. The dynamics of Kerr comb generation have attracted intense investigations since the first demonstration of the method [11]-[34]. It has been found that Kerr combs are not always coherent [11-12] and may be characterized by high intensity noise [13]-[14]; furthermore, lack of coherence and high intensity noise are generally correlated. Experiments have revealed transitions from low coherence, high noise states to highly coherent mode-locked state accompanied by a sudden drop in intensity noise [14]-[16], [18], [19]. It has been found in simulations and experiments that the mode locking of broadband Kerr combs is usually related to soliton formation in the cavity [15], [18]-[31]. These dissipative cavity solitons are localized structures stabilized by a balance between Kerr nonlinearity and dispersion. In time domain they exist as bright or dark pulses, depending on whether the cavity dispersion is anomalous or normal, respectively. Bright microresonator solitons

研究动机与目标

• 在以往实验和仿真尝试均失败的正常色散微腔中，实现暗孤子梳的软激发。
• 在正常色散区域首次实验观测到模式锁定转变，此前尚未实现。
• 在保持低强度噪声和高相干性的前提下，实现克尔梳的可调谐偏频。
• 将观测到的暗脉冲动力学与非线性腔中双稳态稳态之间的前缘相互作用联系起来。

提出的方法

• 利用模式相互作用辅助激发，在正常色散微腔中启动暗脉冲形成，克服了软激发的挑战。
• 采用微加热器驱动的调谐方案，动态控制激光-微腔失谐，为梳状结构工程提供了新的自由度。
• 进行仿真以模拟腔体动力学，表明暗脉冲源于连接双稳态稳态的前缘之间的相互作用。
• 利用高品质因子微腔中的三阶克尔非线性，通过级联四波混频和调制不稳定性产生宽带频率梳。
• 监测强度噪声和相干性，以识别从非相干、高噪声状态向相干、低噪声模式锁定运行的转变。

实验结果

研究问题

• RQ1 能否在缺乏调制不稳定性的情况下，通过软激发在正常色散微腔中稳定激发暗孤子梳？
• RQ2 在缺乏反常色散的条件下，模式相互作用在暗脉冲形成中起到何种作用？
• RQ3 在正常色散区域中，能否在保持低噪声和高相干性的同时，主动调谐克尔梳的偏频？
• RQ4 观测到的暗脉冲动力学与底层非线性腔物理（特别是双稳态之间的前缘相互作用）有何关联？

主要发现

• 首次通过模式相互作用辅助软激发，在微腔的正常色散区域实现了模式锁定转变的实验观测。
• 成功在正常色散微腔中生成了暗孤子梳，尽管缺乏反常色散，仍表现出相干、低噪声的运行特性。
• 微加热器调谐方案实现了梳状线偏频的大幅且连续调谐，同时保持了高相干性和低强度噪声。
• 仿真结果证实，暗脉冲动力学源于连接非线性腔双稳态稳态解的前缘之间的相互作用。
• 实验观测到从高噪声、非相干状态向低噪声、相干模式锁定运行的转变，并与暗孤子的形成相关联。