孤子分子相位演化的“可视化”追踪

孤子是普遍存在于流体力学、凝聚态物理、化学、神经生物学、光学等研究领域的非线性局域波。飞秒激光器中由色散和非线性,增益和损耗的双重平衡而形成光孤子,其在非线性介质中能够稳定传输。类似于化学分子,光孤子具有粒子特性,多个孤子通过相互作用可以形成各种各样的束缚态,通常被称为光学孤子分子。光学孤子分子因其可提高通讯容量和类似物质孤子分子的特性而受到广泛关注。

长期以来,科研人员在各类激光器中通过光学光谱观测到具有恒定的时间间隔和相对相位的静态飞秒孤子分子。然而,光谱分析仪受到探测速度的限制,很难获取非重复性的瞬态信息。近年来,时间展宽色散傅里叶变换(TS-DFT)技术的出现克服了传统光谱仪的速度限制,实现了各种稳态、振荡以及振动孤子分子的快速实时光谱测量。但是,由于受到实时示波器存储能力的限制,其观测时间被限制在数百微秒。

为此,天津大学胡明列课题组提出了一种新的探测手段——轨道角动量(OAM)解析法,能够将孤子分子内部相位演化映射为光学涡旋的旋转运动,从而在CCD上直接观测。这一工作发表在Photonics Research 2020年第10期上(Yuwei Zhao, Jintao Fan, Youjian Song, Uwe Morgner, Minglie Hu. Extraction of internal phase motions in femtosecond soliton molecules using an orbital-angular-momentum-resolved method[J]. Photonics Research, 2020, 8(10): 10001580.),论文第一作者为赵雨薇博士,通讯作者为胡明列教授和宋有建副教授。

轨道角动量(OAM)解析法探测孤子分子内部相对相位的演化

OAM解析法的核心是将孤子分子内部时域相位转移到两个具有相反拓扑电荷的光学涡旋的空间相位。因此,利用快速CCD监测两个光学涡旋叠加后产生的干涉模式的旋转运动,便可以实现对孤子分子内部复杂的相对相位运动的长期监测。通过采用该OAM解析方法,该研究团队测量了飞秒激光器产生的不同时间间隔的双孤子以及三孤子分子内部的相对相位的长期演化过程。

该方法简单直观、成本低,可以实现多孤子结构中的复杂相位动力学的可视化,为探索光学非线性系统内部孤子动力学提供了新思路。此外,由于OAM解析方法能够将孤子对的时间相位转移到矢量光束的空间相位,因此有可能通过操纵孤子分子相对相位来捕获和控制纳米粒子的旋转,从而将该方法推广到纳米光子学领域。

Visualizing phase evolution within optical soliton molecules

Solitons, non-spreading wave structures, are universal in a variety of research fields that ranging from fluid and condensed matter physics to chemistry and neurobiology. Optical solitons can be formed and propagated in laser cavities with remarkable stability thanks to a double balance between loss and gain and between dispersion and nonlinearity. Optical solitons show particle-like interactions and can form various bound states akin to chemical molecules, which are frequently referred as soliton molecules. Soliton molecules have attracted tremendous attention due to their potential to upgrade the transmission capability of optical communication and to demonstrate complex soliton interaction behaviors in dynamical nonlinear systems.

Stationary femtosecond soliton molecules with constant binding separation and relative phase have long been proved to exist in various laser configurations based on optical spectral analysis, while the time-averaged measurements based on optical spectral analyzers hinder the observation of more complex soliton molecular dynamics. Recently, a time-stretch dispersive Fourier transform (DFT) technique has been frequently utilized to unveil the shot-to-shot evolution of soliton molecular spectra, enabling the identification of different kinds of vibrating and oscillating modes in soliton molecules.

Prof. Minglie Hu and Prof. Youjian Song from the Ultrafast Laser Laboratory at Tianjin University notice that while the DFT technique is very appealing for probing the transient properties within soliton molecules, the observation time frame is limited to hundreds of microseconds, mainly confined by the storage capability of the real time oscilloscopes. "A long-term phase evolution description of soliton molecules is still missing to date and calls for advanced ultrafast characterization approaches." says Prof. Hu.

Orbital angular momentum (OAM)-resolved method to unveil the internal phase motions within soliton molecule

Prof. Hu and his team proposed a new diagnostic, orbital angular momentum (OAM)-resolved method, which is capable of visualizing the complex internal phase motion of soliton molecules over a long period. Related research results are reported in Photonics Research, Vol. 8, Issue 10, 2020 (Yuwei Zhao, Jintao Fan, Youjian Song, Uwe Morgner, Minglie Hu. Extraction of internal phase motions in femtosecond soliton molecules using an orbital-angular-momentum-resolved method[J]. Photonics Research, 2020, 8(10): 10001580.).

Temporal varying phases within soliton molecules were transformed to the spatial phase difference between two optical vortices with opposite topological charges. As a consequence, by using a fast frame rate CCD camera to monitor the rotational movement of the combined vortices, the complex internal motion of soliton molecules in terms of relative phase evolution can be monitored over a long period.

Prof. Song explains, "By means of the OAM-resolved method, the long-term relative phase evolution of doublet and triplet soliton molecules with different bind separations produced by a femtosecond Er-doped mode-locked fiber laser can be successfully resolved." Prof. Hu points out that various soliton molecule states with stationary and monotonically evolving phase can be easily characterized by overlapping the solitons properly.

Prof. Hu and his team expect that such a practical, low-cost and simple diagnostic method provides a new idea for visualizing the complex internal phase dynamics in dissipative systems. Moreover, the OAM-resolved method is capable to transfer the temporal phase of soliton pairs to the spatial phase of vector beams, thus they anticipate that the proposed setup holds the potential to trap and control the rotation of nanoparticles via manipulating soliton-molecular relative phases.

The method is novel, interesting and timely. It will provide new direct visualization tool to study phase dynamics in various transient phenomena, thus the paper will attract wide attention.