Intermodal group velocity engineering for broadband nonlinear optics
Schematic of intermodal four-wave mixing: The intense pump pulse (green), guided in a superposition of the LP0,4 and LP0,5 modes (mode image inset), is converted to broadband anti-Stokes (blue) and Stokes (red) pulses along the length of the optical fiber.
Nonlinear frequency conversion is a crucial technology for operating high power pulsed laser systems at the arbitrary wavelengths required by applications ranging from biological imaging to undersea communications, among many others. A stable and efficient solution based on guided waves, for instance using optical fibers, is in high demand; however, efforts to generate such a device have been limited by the need to conserve momentum, or phase match, in fibers.
Multimode fibers provide the ability to examine nonlinear interactions between guided modes—with the idea multiple modes allow for multiple possible combinations to achieve phase matching. However, in practice, typical intermodal four-wave mixing interactions are only phase-matched over narrow wavelength ranges, leading to impractically narrowband nonlinear gain, inefficiencies in conversion, and limitations in the ability to spectrally and temporally tailor the converted light pulses.
To solve this limitation, the research group from the Nanostructured Fibers and Nonlinear Optics laboratory of Boston University, led by Professor Siddharth Ramachandran, has demonstrated that tailoring of the relative intermodal group velocity, in addition to phase matching, is the key to unlocking extended nonlinear gain bandwidths. This new work highlights an intermodal four-wave mixing process where a pump pulse guided in a superposition of the LP0,4 and LP0,5 modes is converted to two group-velocity-matched pulses in the LP0,4 and LP0,5 modes at wavelengths shorter and longer than that of the pump, respectively. By matching the group velocities of these output pulses, the phase-matched bandwidth is increased by more than order of magnitude compared with typical intermodal processes, leading to broadband gain regions separated by nearly an octave (63 nm centered at 1553 nm, and 17 nm centered at 791 nm). By seeding this process, the authors demonstrate an efficient, quasi-CW, high power and wavelength tunable all-fiber analogue of the ubiquitous Ti:Sapphire laser. This work is published in Photonics Research, Volume 7, Issue 1, 2019 (Jeff Demas, et al., Intermodal group-velocity engineering for broadband nonlinear optics).
In general, these results represent a new parameter space in which to design and implement intermodal parametric nonlinearities—analogous to group velocity dispersion engineering in photonic crystal fibers. Future work will explore using the design flexibility inherent to these multimode systems to target specific wavelength bands for applications underserved by conventional frequency conversion systems, as well as exploiting the bandwidth and group-velocity-matching in these systems to explore fiber-based four-wave mixing in the ultrafast pulse regime.
PR封面故事：宽带非线性光学的模间群速度工程
多模四波混频示意图：在LP0,4和LP0,5模式叠加的情况下引导强抽运脉冲（绿色）转换为宽带反斯托克斯脉冲（蓝色）和斯托克斯脉冲（红色）。
非线性频率转换是在任意波长下操作高功率脉冲激光系统的关键技术，广泛应用于生物成像、海底通信等领域。人们对类似光纤这种基于导波且稳定有效的方案非常有兴趣，然而前提是需要保持光纤中的动量或相位匹配。
多模光纤具有检查导模之间非线性相互作用的能力，即对多模进行各种可能的组合来实现相位匹配。然而在实践中发现，典型的多模四波混频相互作用仅能够在非常窄的波长范围内实现相位匹配，这导致窄带非线性增益难以实现，转换效率低，以及很难在光谱和时间上对转换光脉冲进行调制。
为了解决这一难题，由美国波士顿大学纳米结构光纤和非线性光学实验室的Siddharth Ramachandran教授领导的研究团队证明，除了相位匹配之外，调制相对模间群速度是增加非线性增益带宽的关键。他们着重介绍了一种模间四波混频过程，以LP0,4和LP0,5模式叠加状态导入的抽运脉冲分别转换为两个群速度匹配的脉冲，一个波长小于抽运脉冲，一个波长大于抽运脉冲。通过匹配这些输出脉冲的群速度，相位匹配带宽比典型的模间过程大了一个数量级以上，导致宽带增益区域相隔几乎一个倍频程（63 nm以1553 nm为中心，17 nm以791 nm为中心）。通过这一过程，研究人员展示了普遍存在的钛蓝宝石激光器的高效、准连续、高功率和波长可调谐全光纤模拟。这项工作发表在Photonics Research 2019年第7卷第1期上(Jeff Demas, et al., Intermodal group-velocity engineering for broadband nonlinear optics)。
总的来说，这些结果代表了一个新的参数空间，可以在其中设计和实现类似于光子晶体光纤中群速度色散工程的模间参数非线性。未来的工作将着眼于探索利用这些多模系统的设计灵活性在传统频率转换系统应用中实现特定波段，并在研究这些系统中的带宽和群速度匹配的过程中探索基于光纤的超快脉冲四波混频。