非集成微腔中的光学频率梳研究进展(特邀)

Progress in Optical Frequency Combs Based on Non-integrated Microresonators(Invited)

光学频率梳在频域上是一系列离散的、等间隔的、具有稳定相位关系的谱线,光谱覆盖范围可达百太赫兹量级,被称为光学标尺,它的出现显著提高了时间与频率的测量精度,其重要性得到了广泛认可,该领域科学家也因此获得了2005年诺贝尔物理学奖。早期的光频梳系统由固态锁模激光器构成,系统复杂且体积庞大。近年来,新兴的微腔光频梳以小型化、低功耗、相对简易的产生系统等优势极大促进了光频梳技术的发展。由于制备相对简单,且具有超高的品质因子和丰富的模式族,非片上集成的光学微腔为克尔光频梳提供了更为便利的研究平台,进一步拓展了克尔光频梳的应用场景。本文介绍了微腔光频梳的理论模型,描述了微腔光频梳的产生过程与产生方式,梳理了克尔光频梳在各类非集成微腔中的研究进展,展示了非集成微腔光梳的应用场景,总结了目前非集成微腔光梳研究所面临的挑战,并对该领域未来的研究趋势进行了展望。

An Optical Frequency Comb(OFC)consists of a series of discrete,equidistant,and phasecoherent lines in the frequency domain,spanning a spectrum that can reach hundreds of terahertz.These frequency combs are often referred to as“optical rulers”,and have revolutionized the accuracy of optical frequency and temporal measurements.The importance of OFCs has been widely recognized,leading to the awarding of 2005 Nobel Prize in Physics to two scientists in this field.The early OFC systems were based on mode-locked lasers,which were complicated and bulky.In recent years,the development of OFCs has been greatly advanced by the emergency of microresonator OFCs(optical microcombs),which are characterized by compact footprint,low power consumption,and relatively simple generation systems.Due to the relatively straightforward fabrication and the presence of a rich mode family with ultrahigh quality factors,non-integrated optical microresonators have provided a more convenient research platform for Kerr OFCs.This has further expanded the range of applications for Kerr OFCs.The generation of optical microcombs are based on the four-wave mixing effect.There are two mainstream theoretical models that can describe the evolution of optical microcombs:the nonlinear coupling mode theory and the Lugiato-Lefever equation.The experimental microcomb generation requires the precise coupling of the pump wavelength into a cavity resonance.There are four states during the microcomb formation when the pump wavelength is forward tuned over the resonance:(Ⅰ)No comb,the pump wavelength is blue-detuned;(Ⅱ)Chaotic comb state in the regime of modulation instability;(Ⅲ)Soliton state;(Ⅳ)No comb,the pump wavelength is red-detuned.Dissipative solitons balance nonlinearity and dispersion as well as parametric gain and loss.In the experiments,thermal effect plays an important role during the soliton formation.Several methods have been developed to achieve soliton states in non-integrated microresonators,including but not limited to fast frequency scanning,auxiliary laser heating,and power kicking.Non-integrated optical microresonators include Fabry-Pérot(F-P)resonators,crystalline resonators,microbottle resonators,microsphere resonators,microrod resonators,and microbubble resonators.Benefitting from the simple structure,F-P resonators effectively reduce the difficulty of coating and doping,which facilitate the generation and stabilization of solitons.Microbottle resonators have enormous potential for dispersion engineering by changing the cavity length,maximum radius and taper,which enables the development of OFCs tailored to specific requirements.The axial modes in microbottle resonators opens up possibilities for OFCs in applications such as spectral measurement and frequency conversion,offering versatile regimes for OFC generation.The first soliton in microresonators was generated in a crystalline resonator.Because of the remarkable properties,many physical phenomena have been realized in crystalline resonators,including mid-infrared OFCs,octave-spanning OFCs,and so on.Crystalline-resonator-based microcombs have impressive performances in microwave frequency synthesis and optical communications.They can generate low-noise microwave signals with a phase noise level of−135 dBc/Hz@104 Hz,and are capable of optical communications with a maximum data transmission rate of 1.45 Tbps exceeding 40 km,which highlights significant potential of crystalline-resonator-based microcombs in modern communication systems.Microsphere resonators feature the straightforward fabrication while possessing extremely high quality factors which brings numerous modes enabling OFC generation,motivating diverse applications.Utilizing the beat frequencies of multiple Stokes solitons in a microsphere resonator enables single molecule detection and the identification of mixed gases with sub-Hz spectral resolution.Microrod resonators possess ultrahigh quality factors and can be engineered in the intrinsic dispersion through adjustments to the sidewall curvature.Microrod-based OFCs were applied to frequency synthesis,precision spectral detection,and frequency measurement.Particularly,microrod-based OFCs can down-convert signals to the microwave domain within kHz precision in the wavelength measurement.Microbubble resonators are named for the unique hollow structure,enabling OFCs in visible wavelengths.However,non-integrated microcombs still face onerous challenges,such as unstable comb state and insufficient comb power.We also discuss potential solutions for these challenges,then envision the future depending on current research status.There are convincing reasons to believe non-integrated microcombs can lighten bright prospect under the efforts of the whole community.