[论文解读] Multi-Octave Frequency Comb from an Ultra-Low-Threshold Nanophotonic Parametric Oscillator

Ryoto Sekine, Robert M. Gray+4 more|arXiv (Cornell University)|Sep 8, 2023

Advanced Fiber Laser Technologies被引用 6

一句话总结

该论文展示了一种在纳米光子铌酸锂光学参量振荡器（OPO）中实现多倍频程频率梳生成，仅使用飞焦量级泵浦能量。通过利用一种时间自清洁机制，该机制在远高于阈值且偏离线性同步的情况下稳定了相干性，OPO实现了超宽带、相干的光谱展宽，而无需高Q因子腔体或窄带谱宽，与以往集成方法相比，能量需求降低了数个数量级。

ABSTRACT

Ultrabroadband frequency combs coherently unite distant portions of the electromagnetic spectrum. They underpin discoveries in ultrafast science and serve as the building blocks of modern photonic technologies. Despite tremendous progress in integrated sources of frequency combs, achieving multi-octave operation on chip has remained elusive mainly because of the energy demand of typical spectral broadening processes. Here we break this barrier and demonstrate multi-octave frequency comb generation using an optical parametric oscillator (OPO) in nanophotonic lithium niobate with only femtojoules of pump energy. The energy-efficient and robust coherent spectral broadening occurs far above the oscillation threshold of the OPO and detuned from its linear synchrony with the pump. We show that the OPO can undergo a temporal self-cleaning mechanism by transitioning from an incoherent operation regime, which is typical for operation far above threshold, to an ultrabroad coherent regime, corresponding to the nonlinear phase compensating the OPO cavity detuning. Such a temporal self-cleaning mechanism and the subsequent multi-octave coherent spectrum has not been explored in previous OPO designs and features a relaxed requirement for the quality factor and relatively narrow spectral coverage of the cavity. We achieve orders of magnitude reduction in the energy requirement compared to the other techniques, confirm the coherence of the comb, and present a path towards more efficient and wider spectral broadening. Our results pave the way for ultrashort-pulse and ultrabroadband on-chip nonlinear photonic systems for numerous applications.

研究动机与目标

• 在芯片上实现多倍频程频率梳生成，使用极低泵浦能量，克服以往集成频率梳源因高能量阈值而受限的问题。
• 在紧凑的集成平台上，利用薄膜铌酸锂中的二次非线性，实现相干光谱展宽。
• 在OPO中展示一种新颖的时间自清洁机制，可在高增益、远高于阈值的运行条件下稳定相干性。
• 与现有集成和体材料系统相比，将多倍频程光谱展宽的能量需求降低数个数量级。
• 通过高效节能的宽带相干光源，实现双梳光谱学、光频合成和超快光子学等实际片上应用。

提出的方法

• OPO在周期性极化周期为5.075 µm的薄膜铌酸锂上制造，实现准相位匹配，以高效生成二次谐波。
• 该器件作为同步泵浦简并OPO运行，使用100 fs脉冲作为泵浦源，波长为1045 nm，重复频率为250.1775 MHz。
• 通过色散工程实现接近零群速度色散（GVD）和群速度失配（GVM），以实现宽带参量增益和光谱展宽。
• OPO在远高于阈值且偏离线性同步条件下运行，通过非线性相位补偿机制实现从非相干到相干输出的转变。
• 数值模拟通过Lumerical软件求解非线性包络方程，仅建模χ(2)非线性且无恒定相位偏移，以预测光谱和时间域动力学。
• 实验表征采用高阻抗光纤和氟化物光纤进行宽带光谱收集（350–3400 nm），射频谱通过高速光电二极管和频谱分析仪测量。

Figure 1: Principle and design of the multi-octave nanophotonic OPO . a, Illustration of the sync-pumped OPO on thin-film lithium niobate with key features highlighted. b, Microscope image of several devices when the one in the center is pumped at 1 $\mu$ m. The chip glows green due to second harmon

实验结果

研究问题

• RQ1 是否可以在片上集成纳米光子平台中，使用飞焦量级泵浦能量实现多倍频程频率梳生成？
• RQ2 在远高于阈值且偏离线性同步运行的OPO中，何种机制可实现相干光谱展宽？
• RQ3 OPO中的时间自清洁机制如何在低精细度和中等Q因子腔体条件下稳定相干性并实现超宽带输出？
• RQ4 在色散工程的OPO中，利用二次非线性可将多倍频程光谱展宽的能量需求降低到何种程度？
• RQ5 纳米光子OPO是否可在不依赖高Q谐振器或窄带组件的情况下实现八度或更宽的相干光谱？

主要发现

• OPO使用仅飞焦量级的泵浦能量，生成从350 nm到3400 nm的多倍频程相干频率梳，覆盖超过三个八度，实现超宽带相干输出。
• 器件总通光损耗为43.4 dB，其中输入耦合损耗为35.7 dB，输出耦合损耗为7.7 dB，表明片上光管理效率高。
• OPO在0.45 pJ的泵浦脉冲能量下运行，远低于集成系统中通常所需的10–100 pJ，实现数量级的能量降低。
• 时间自清洁机制通过非线性补偿腔体失谐，实现从非相干到相干输出的转变，且在远高于阈值条件下保持稳定。
• 模拟结果表明，在522 nm（二次谐波）与2090 nm（半谐波）之间GVM为721 fs/mm，解释了光谱走离现象，并说明了模拟中对短波长分量的低估。
• 通过射频谱分析确认了测量到的三倍频程光谱具有相干性，重复频率为250 MHz，边带间隔等于基频重复频率。

Figure 2: OPO characterization. a, Oscillation peaks of the OPO as the pump repetition rate is modulated by a piezoelectric transducer (PZT) in the pump laser cavity at 600 Hz. b, Signal spectrum at 35 fJ of pump energy for three different roundtrip detunings and c, the corresponding OPO signal grow