[论文解读] Fundamental scaling limits and bandwidth shaping of frequency-modulated combs

M. Roy, Zhifeng Xiao+2 more|arXiv (Cornell University)|Sep 29, 2023

Spectroscopy and Laser ApplicationsChemistry被引用 3

一句话总结

本文提出扩散损耗整形技术——在太赫兹量子级联激光器中采用共振耦合的损耗结构——以克服增益曲率的影响，实现接近本征增益带宽极限（单模激光器带宽的80%）的调频光梳。该方法同时稳定光梳并实现类孤子的脉冲态，为有源腔光梳工程提供了新的自由度。

ABSTRACT

Frequency-modulated (FM) combs based on active cavities like quantum cascade lasers have recently emerged as promising light sources in many spectral regions. Unlike passive modelocking, which uses amplitude modulation to generate amplitude modulation, FM combs use phase modulation to generate phase modulation. They can therefore be regarded as a phase-domain version of passive modelocking. However, while the ultimate scaling laws of passive modelocking have long been known -- Haus showed in 1975 that pulses have a bandwidth proportional to effective gain bandwidth -- the limits of FM combs have been much less clear. Here, we show that FM combs are governed by the same fundamental limits, producing combs whose bandwidths are linear in the effective gain bandwidth. Not only do we show theoretically that the diffusive effect of gain curvature limits comb bandwidth, we also show experimentally how this limit can be increased. By adding carefully designed resonant-loss structures that are evanescently coupled to the cavity of a terahertz laser, we reduce the curvature and increase the effective gain bandwidth of the laser, demonstrating bandwidth enhancement. Our results give a new degree of freedom for the creation of active chip-scale combs and can be applied to a wide array of cavity geometries.

研究动机与目标

• 解决由于增益曲率导致的调频量子级联激光器光梳的根本带宽限制问题。
• 克服因增益变化引起的强度波动导致的调频光梳不稳定问题。
• 展示一种新型方法，用于在有源腔光梳中同时调控色散与扩散，以提升性能。
• 实现更宽、更相干的光梳，并探索QCL中脉冲态的出现。
• 为集成频率梳提供可扩展的、与腔体无关的带宽增强策略。

提出的方法

• 设计与QCL腔体倏逝耦合的共振损耗结构，以塑造有效增益轮廓。
• 通过控制这些损耗结构的尺寸和位置，使激光光谱范围内的增益曲率得以平坦化。
• 利用有源腔平均场理论和广义Lugiato-Lefever方程对系统动力学进行建模。
• 采用扩散损耗整形技术，抑制由增益曲率引起的幅度波动，从而稳定调频光梳工作状态。
• 利用SWIFTS和超导热释电探测器表征时间域和频谱域的相干性及带宽。
• 将器件偏置在不稳定性阈值附近，以观察从调频光梳向脉冲态的转变。

Figure 1: a. Effect of gain curvature on FM comb formation. For an FM comb with gain curvature, frequency modulation gives rise to intensity modulation that destabilizes the comb. b. By introducing diffusive loss shaping, in which resonant structures near the cavity reduce the gain curvature and mit

实验结果

研究问题

• RQ1 增益曲率如何限制量子级联激光器中调频光梳的带宽？
• RQ2 共振扩散损耗整形是否能有效抵消QCL光梳中增益曲率的不稳定影响？
• RQ3 利用该方法，光梳带宽能在多大程度上逼近本征增益带宽极限？
• RQ4 色散与扩散之间的相互作用在实现稳定、宽带光梳工作状态中起到何种作用？
• RQ5 扩散损耗整形是否也能促进QCL中类孤子脉冲态的形成？

主要发现

• 扩散损耗整形使QCL光梳的带宽达到单模激光器最大振荡带宽的80%，接近理论极限。
• 该方法抑制了由增益曲率引起的强度波动，稳定了调频光梳的工作状态。
• 通过SWIFTS验证了全光梳带宽范围内的相干性，实现了超过700 GHz的自参考相干性。
• 高动态范围光谱证实了使用超导热释电探测器实现的700 GHz同时带宽光梳。
• 在接近不稳定性阈值的偏置点，系统从调频光梳转变为脉冲态，脉冲强度为连续波部分的3至4倍。
• 该策略兼容多种腔体结构，包括法布里-珀罗腔和环形腔，并可通过偏置控制实现动态调谐。

Figure 2: Gain curvature theory. a. Comb bandwidths that can be achieved given a gain bandwidth, provided the optimum dispersion is chosen. The linear relationship demonstrates the necessity of low gain curvature. b. Effect of gain inhomogeneity on FM comb spectra. Even if a coherent comb can form,