Soliton microcomb generation by cavity polygon modes

Soliton microcombs,which require the hosting cavity to operate in an anomalous dispersion regime,are essential to integrate photonic systems.In the past,soliton microcombs were generated on cavity whispering gallery modes(WGMs),and the anomalous dispersion requirement of the cavity made by normal dispersion material was achieved through structural dispersion engineering.This inevitably degrades the cavity optical quality factor(Q)and increases pump threshold power for soliton comb generation.To overcome the challenges,here,we report a soliton microcomb excited by cavity polygon modes.These modes display anomalous dispersion at near-infrared while optical Q factors exceeding 4×10^(6) are maintained.Consequently,a soliton comb spanning from 1450 nm to 1620 nm with a record low pump power of 11 m W is demonstrated,a three-fold improvement compared to the state of the art on the same material platform.

Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching

Optical true delay lines(OTDLs)of low propagation losses,small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications.Here,we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching.Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay.The fabricated OTLDs show an ultra-low propagation loss of^0.03dB/cm for waveguide lengths well above 100 cm.

大规模铌酸锂光子集成系统的超快激光光刻研究进展

飞秒光刻辅助化学机械刻蚀技术(photolithography assisted chemo-mechanical etching,PLACE)实现了在薄膜铌酸锂(thin-film lithium niobate,TFLN)上高品质大规模光子集成电路(photonic integrated circuit,PIC)的制造,并推动了光子集成电路重大应用领域的持续发展,产生了一系列高性能PIC应用,诸如高Q微谐振器、低损耗波导、波导放大器、阵列波导光栅(arrayed waveguide grating,AWG)和电光(electro-optic,EO)可调谐/可编程光子器件等.针对PIC器件和光刻系统的大规模生产,本文介绍了一种超高速高分辨率激光光刻制造系统,采用高重复频率飞秒激光器和高速多边形激光扫描仪,可实现200 nm分辨率下4.8 cm2/h的光刻制造效率,并且展示了基于TFLN光子器件的晶圆级制造、晶圆级微电极结构等的应用.

Simplified Cluster Voltage Balancing Control Based on Zero-sequence Voltage Injection for Star-connected Cascaded H-bridge STATCOM

The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-wire system.However,as the zero-sequence voltage is expressed in trigonometric form,traditional control methods involve many complicated operations,such as the square-root,trigonometric operations,and inverse tangent operations.To simplify cluster voltage balancing control,this paper converts the zero-sequence voltage to the dq frame in a DC representation by introducing a virtually orthogonal variable,and the DC components of the zero-sequence voltage in the dq frame are regulated linearly by proportional integral regulators,rather than being calculated from uneven active powers in traditional controls.This removes all complicated operations.Finally,this paper presents simulation and experimental results for a 400 V±7.5 kvar star-connected STATCOM,in balanced and unbalanced scenarios,thereby verifying the effectiveness of the proposed control.

Erbium-ytterbium co-doped lithium niobate single-mode microdisk laser with an ultralow threshold of 1μW

We demonstrate single-mode microdisk lasers in the telecom band with ultralow thresholds on erbium-ytterbium co-doped thin-film lithium niobate(TFLN).The active microdisk was fabricated with high-Q factors by photolithography-assisted chemomechanical etching.Thanks to the erbium-ytterbium co-doping providing high optical gain,the ultralow loss nanostructuring,and the excitation of high-Q coherent polygon modes,which suppresses multimode lasing and allows high spatial mode overlap between pump and lasing modes,single-mode laser emission operating at 1530 nm wavelength was observed with an ultralow threshold,under a 980-nm-band optical pump.The threshold was measured as low as 1μW,which is one order of magnitude smaller than the best results previously reported in single-mode active TFLN microlasers.The conversion efficiency reaches 4.06×10^(-3),which is also the highest value reported in single-mode active TFLN microlasers.

Electro-optic tuning of a single-frequencyultranarrow linewidth microdisk laser

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication,light detection and ranging,to metrology and sensing.Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality(Q)factor and supports only a single lasing frequency simultaneously.Here,we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate(LN)microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths.As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart,while their Q factors(above 10 million)are even higher due to the significantly reduced scattering on their propagation paths,single-frequency lasing with a linewidth as narrow as 322 Hz is observed.The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers.Finally,enabled by the strong linear electro-optic effect of LN,real-time electro-optical tuning of the microlaser with a high tuning efficiency of∼50 pm∕100 V is demonstrated.

Advances in on-chip photonic devices based on lithium niobate on insulator

Crystalline lithium niobate(LN)is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients.Furthermore,the recent development and commercialization of LN-on-insulator(LNOI)technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss,optical nonlinearity,and electro-optic tunability.This review begins with a brief introduction of the history and current status of LNOI photonics.We then discuss the fabrication techniques of LNOI-based photonic structures and devices.The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI,which enables the production of building block structures,such as optical microresonators and waveguides of unprecedented optical qualities.The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits.Some conclusions and future perspectives are provided.

Lithium niobate microring with ultra-high Q factor above 10^(8)

We demonstrate integrated lithium niobate(LN) microring resonators with Q factors close to the intrinsic material absorption limit of LN.The microrings are fabricated on pristine LN thin-film wafers thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching.A record-high Q factor up to 10^(8)at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage,indicating an ultra-low waveguide propagation loss of ~0.0034 dB/cm.The ultra-high Q microrings will pave the way for integrated quantum light source,frequency comb generation,and nonlinear optical processes.

Optimization of control parameters of droplet density in citrus trees using UAVs and the Taguchi method

The control parameters of the unmanned aerial vehicle(UAV)should be carefully designed to improve UAV spraying performance on citrus trees.The present study investigated the optimal droplet distribution control parameters in citrus trees using a UAV and the Taguchi method,of which optimal results were observed with an inverted triangle citrus tree canopy shape,a spraying height of 1.40 m,and a flight speed of 1.0 m/s.Among the discussed control parameters,the flight speed presented the most significant effect with a contribution percentage of 74.0%.The established multiple regression model predicted an optimal spraying height of 1.27 m and a maximum droplet density of 35.39 droplets/cm2.In addition,the effects of individual control parameter on the droplet density of the lower layer of citrus trees were systematically analyzed,of which inverted triangle shape more significantly affected the droplet density of the lower layer and presented an 82.0%increase in droplet density as compared to the triangle shape.An improvement of 59.6%in the lower layer droplet density was observed at a spraying height of 1.40 m.In addition,the other spraying heights did not present significant differences in their coefficient of variation(CV)values.

High-quality-factor optical microresonators fabricated on lithium niobate thin film with an electro-optical tuning range spanning over one free spectral range [Invited]

We demonstrate high-quality(intrinsic Q factor∼2.8×106)racetrack microresonators fabricated on lithium niobate thin film with a free spectral range(FSR)of∼86 pm.By integrating microelectrodes alongside the two straight arms of the racetrack resonator,the resonance wavelength around 1550 nm can be red shifted by 92 pm when the electric voltage is raised from−100 V to 100 V.The microresonators with the tuning range spanning over a full FSR are fabricated using photolithography assisted chemo-mechanical etching.

An effective spray drift-reducing method for a plant-protection unmanned aerial vehicle

A spraying system for a plant-protection unmanned aerial vehicle(UAV)was designed to reduce spray drift.A custom low-speed wind tunnel was constructed to generate a wind speed ranging from 0 to 5.92 m/s.The results showed that the wind speed was attenuated with an increase in distance.To compensate for the attenuation,a linear-fitting model was adopted.Then,the relationship between the spraying pressure and atomization rate was analyzed,and a fuzzy algorithm was adopted to adjust the spraying angle and pressure according to the wind speed and its changing rate.Finally,an evaluation of the proposed system in the compensated wind tunnel was conducted,and the drift distance was reduced by 33.7%compared with the system without adjustment of the spraying angle and pressure.