Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap

Optical parametric oscillation(OPO)in Kerr microresonators can efficiently transfer near-infrared laser light into the visible spectrum.To date,however,chromatic dispersion has mostly limited output wavelengths to>560 nm,and robust access to the whole green light spectrum has not been demonstrated.In fact,wavelengths between 532 nm and 633 nm,commonly referred to as the“green gap”,are especially challenging to produce with conventional laser gain.Hence,there is motivation to extend the Kerr OPO wavelength range and develop reliable device designs.Here,we experimentally show how to robustly access the entire green gap with Kerr OPO in silicon nitride microrings pumped near 780 nm.Our microring geometries are optimized for green-gap emission;in particular,we introduce a dispersion engineering technique,based on partially undercutting the microring,which not only expands wavelength access but also proves robust to variations in resonator dimensions.Using just four devices,we generate>150 wavelengths evenly distributed throughout the green gap,as predicted by our dispersion simulations.Moreover,we establish the usefulness of Kerr OPO to coherent applications by demonstrating continuous frequency tuning(>50 GHz)and narrow optical linewidths(<1 MHz).Our work represents an important step in the quest to bring nonlinear nanophotonics and its advantages to the visible spectrum.

Multi-mode microcavity frequency engineering through a shifted grating in a photonic crystal ring

Frequency engineering of whispering-gallery resonances is essential in microcavity nonlinear optics.The key is to control the frequencies of the cavity modes involved in the underlying nonlinear optical process to satisfy its energy conservation criterion.Compared to the conventional method that tailors dispersion by cross-sectional geometry,thereby impacting all cavity mode frequencies,grating-assisted microring cavities,often termed as photonic crystal microrings,provide more enabling capabilities through mode-selective frequency control.For example,a simple single period grating added to a microring has been used for single frequency engineering in Kerr optical parametric oscillation(OPO)and frequency combs.Recently,this approach has been extended to multifrequency engineering by using multi-period grating functions,but at the cost of increasingly complex grating profiles that require challenging fabrication.Here,we demonstrate a simple approach,which we term as shifted grating multiple mode splitting(SGMMS),where spatial displacement of a single period grating imprinted on the inner boundary of the microring creates a rotational asymmetry that frequency splits multiple adjacent cavity modes.This approach is easy to implement and presents no additional fabrication challenges compared to an unshifted grating,and yet is very powerful in providing multi-frequency engineering functionality for nonlinear optics.We showcase an example where SGMMS enables OPO across a wide range of pump wavelengths in a normal-dispersion device that otherwise would not support OPO.

Universal frequency engineering tool for microcavity nonlinear optics:multiple selective mode splitting of whispering-gallery resonances

Whispering-gallery microcavities have been used to realize a variety of efficient parametric nonlinear optical processes through the enhanced light–matter interaction brought about by supporting multiple high quality factor and small modal volume resonances.Critical to such studies is the ability to control the relative frequencies of the cavity modes,so that frequency matching is achieved to satisfy energy conservation.Typically this is done by tailoring the resonator cross section.Doing so modifies the frequencies of all of the cavity modes,that is,the global dispersion profile,which may be undesired,for example,in introducing competing nonlinear processes.Here,we demonstrate a frequency engineering tool,termed multiple selective mode splitting(MSMS),that is independent of the global dispersion and instead allows targeted and independent control of the frequencies of multiple cavity modes.In particular,we show controllable frequency shifts up to 0.8 nm,independent control of the splitting of up to five cavity modes with optical quality factors≳10^5,and strongly suppressed frequency shifts for untargeted modes.The MSMS technique can be broadly applied to a wide variety of nonlinear optical processes across different material platforms and can be used to both selectively enhance processes of interest and suppress competing unwanted processes.

Towards integrated photonic interposers for processing octave-spanning microresonator frequency combs

Microcombs-optical frequency combs generated in microresonators-have advanced tremendously in the past decade,and are advantageous for applications in frequency metrology,navigation,spectroscopy,telecommunications,and microwave photonics.Crucially,microcombs promise fully integrated miniaturized optical systems with unprecedented reductions in cost,size,weight,and power.However,the use of bulk free-space and fiber-optic comp on ents to process microcombs has restricted form factors to the table-top.Taking microcomb-based optical frequency synthesis around 1550 nm as our target application,here,we address this challenge by proposing an integrated photonics interposer architecture to replace discrete components by collecting,routing,and interfacing octave-wide microcomb-based optical signals between photonic chiplets and heterogeneously integrated devices.Experimentally,we con firm the requisite performa nee of the individual passive elements of the proposed interposer一octave-wide dichroics,multimode interferometers,and tunable ring filters,and implement the octave-spanning spectral filteri ng of a microcomb,central to the in terposer,using silicon n itride phot onics.Moreover,we show that the thick silicon nitride needed for bright dissipative Kerr soliton generation can be integrated with the comparatively thin silicon nitride interposer layer through octave-bandwidth adiabatic evanescent coupling,indicating a path towards future system-level consolidation.Fin ally,we numerically confirm the feasibility of operating the proposed in terposer synthesizer as a fully assembled system.Our interposer architecture addresses the immediate need for on-chip microcomb processing to successfully miniaturize microcomb systems and can be readily adapted to other metrology-grade applications based on optical atomic clocks and high-precision navigation and spectroscopy.