arXiv subjects

Jijun He

Publications and source records attributed to Jijun He.

At least 19 recordsLinked to original sources

arXiv2026

Integrated Microcomb-Driven Vortex Electromagnetic Waves for Broadband Forward-looking Sensing

Microwave sensing is a critical enabler for all-weather perception, yet its resolution is fundamentally capped by the diffraction limit of the physical antenna aperture. While vortex electromagnetic (EM) waves offer a route to bypass this barrier, practical deployment is constrained by the trade-off between bandwidth, mode purity, and hardware complexity. Here, we propose a microwave photonic architecture enabled by a chip-scale dissipative Kerr soliton (DKS) microcomb that resolves these constraints. The microcomb provides a grid of over 270 optical lines with linewidths below 30 kHz, which are modulated and optically processed to synthesize vortex waves covering 8 GHz (18-26 GHz) with 15 programmable orbital angular momentum (OAM) modes. In contrast to conventional parallel-laser systems, our approach reduces phase error and improves OAM mode purity, while condensing the multi-wavelength source onto a monolithic chip. We demonstrate superior forward-looking imaging performance, clearly resolving both point targets and complex scenes. This work establishes a scalable framework bridging integrated soliton physics with broadband microwave processing, paving the way for next-generation compact smart sensors.

physics.optics↗

arXiv2025

Visualization of nonlinear optics in a microresonator

A precise understanding of nonlinear optical phenomena in whispering gallery mode (WGM) microresonators is crucial for developing next-generation integrated photonic devices. Applications include on-chip sensors for biomedical use, optical memories for all-optical networks and frequency combs for optical clocks. However, our ability to spatially localize nonlinear optical processes within microresonators has been limited because optical feedback is often only collected through a bus waveguide. In this study, we present the direct visualization of nonlinear optical processes using scattering patterns captured by a short-wave infrared (SWIR) camera. Through systematic analysis of these scattering patterns, we can distinguish between different nonlinear effects occurring within the microresonator. Direct imaging of nonlinear processes in microresonators can significantly impact many applications, including the optimization of soliton frequency combs, real-time debugging of photonic circuits, microresonator-based memories, and chip-based data switching in telecom circuits.

physics.optics↗

arXiv2025

A chip-based optoelectronic-oscillator frequency comb

Microresonator-based Kerr frequency combs ("Kerr microcombs") constitute chip-scale frequency combs of broad spectral bandwidth and repetition rate ranging from gigahertz to terahertz. An appealing application exploiting microcombs' coherence and large repetition rate is microwave and millimeter-wave generation. Latest endeavor applying two-point optical frequency division (OFD) on photonic-chip-based microcombs has created microwaves with exceptionally low phase noise. Nevertheless, microcomb-based OFD still requires extensive active locking, additional lasers, and external RF or microwave sources, as well as sophisticated initiation. Here we demonstrate a simple and entirely passive (no active locking) architecture, which incorporates an optoelectronic oscillator (OEO) and symphonizes a coherent microcomb and a low-noise microwave spontaneously. Our OEO microcomb leverages state-of-the-art integrated chip devices including a high-power DFB laser, a broadband silicon Mach-Zehnder modulator, an ultralow-loss silicon nitride microresonator, and a high-speed photodetector. Each can be manufactured in large volume with low cost and high yield using established CMOS and III-V foundries. Our system synergizes a microcomb of 10.7 GHz repetition rate and an X-band microwave with phase noise of $-$97/$-$126/$-$130 dBc/Hz at 1/10/100 kHz Fourier frequency offset, yet does not demand active locking, additional lasers, and external RF or microwave sources. With potential to be fully integrated, our OEO microcomb can become an invaluable technology and building block for microwave photonics, radio-over-fiber, and optical communication.

physics.optics↗

arXiv2024

Vector spectrometer with Hertz-level resolution and super-recognition capability

High-resolution optical spectrometers are crucial in revealing intricate characteristics of signals, determining laser frequencies, measuring physical constants, identifying substances, and advancing biosensing applications. Conventional spectrometers, however, often grapple with inherent trade-offs among spectral resolution, wavelength range, and accuracy. Furthermore, even at high resolution, resolving overlapping spectral lines during spectroscopic analyses remains a huge challenge. Here, we propose a vector spectrometer with ultrahigh resolution, combining broadband optical frequency hopping, ultrafine microwave-photonic scanning, and vector detection. A programmable frequency-hopping laser was developed, facilitating a sub-Hz linewidth and Hz-level frequency stability, an improvement of four and six orders of magnitude, respectively, compared to those of state-of-the-art tunable lasers. We also designed an asymmetric optical transmitter and receiver to eliminate measurement errors arising from modulation nonlinearity and multi-channel crosstalk. The resultant vector spectrometer exhibits an unprecedented frequency resolution of 2 Hz, surpassing the state-of-the-art by four orders of magnitude, over a 33-nm range. Through high-resolution vector analysis, we observed that group delay information enhances the separation capability of overlapping spectral lines by over 47%, significantly streamlining the real-time identification of diverse substances. Our technique fills the gap in optical spectrometers with resolutions below 10 kHz and enables vector measurement to embrace revolution in functionality.

physics.optics↗

arXiv2023

Microresonator soliton frequency combs via cascaded Brillouin scattering

We demonstrate Kerr soliton frequency comb generation that is seeded by a cascaded Brillouin scattering process. In this process, a pump laser is used to generate multiple orders of Brillouin sidebands in a microresonator, which in turn generate the soliton. In such a process, even orders of Brillouin scattering sidebands are co-propagating with respect to the pump laser while odd orders of Brillouin scattering are backwards propagating. In this work we present the generation of forward propagating Kerr solitons via a forward propagating second order Brillouin scattering process in a fused silica rod resonator. Importantly, we show that the Brillouin scattering process can bridge the gap between different microresonator mode families, such that the repetition rate of the Kerr soliton is independent from the Brillouin gain frequency shift (about 10 GHz in fused silica). In our work we demonstrate this by generating soliton pulse trains with a repetition rate of 107 GHz. Our work opens up a new way for using cascaded Brillouin lasing as a seed for microresonator frequency comb generation. This can be of particular interest for the realization of soliton frequency combs with low noise properties from Brillouin lasing while still having arbitrary repetition rates that are determined by the resonator size. Applications range from optical communication to LIDAR systems and photonic signal generation.

physics.optics↗

arXiv2023

Foundry manufacturing of tight-confinement, dispersion-engineered, ultralow-loss silicon nitride photonic integrated circuit

The foundry development of integrated photonics has revolutionized today's optical interconnect and datacenters. Over the last decade, we have witnessed the rising of silicon nitride (Si$_3$N$_4$) integrated photonics, which is currently transferring from laboratory research to foundry manufacturing. The development and transition are triggered by the ultimate need of low optical loss offered by Si$_3$N$_4$, which is beyond the reach of silicon and III-V semiconductors. Combined with modest Kerr nonlinearity, tight optical confinement and dispersion engineering, Si$_3$N$_4$ has today become the leading platform for linear and Kerr nonlinear photonics, and has enabled chip-scale lasers featuring ultralow noise on par with table-top fiber lasers. However, so far all the reported fabrication processes of tight-confinement, dispersion-engineered Si$_3$N$_4$ photonic integrated circuit (PIC) with optical loss down to few dB/m have only been developed on 4-inch or smaller wafers. Yet, to transfer these processes to established CMOS foundries that typically operate 6-inch or even larger wafers, challenges remain. In this work, we demonstrate the first foundry-standard fabrication process of Si$_3$N$_4$ PIC with only 2.6 dB/m loss, thickness above 800 nm, and near 100% fabrication yield on 6-inch wafers. Such thick and ultralow-loss Si$_3$N$_4$ PIC enables low-threshold generation of soliton frequency combs. Merging with advanced heterogeneous integration, active ultralow-loss Si$_3$N$_4$ integrated photonics could pave an avenue to addressing future demands in our increasingly information-driven society.

physics.optics↗

arXiv2022

A photonic integrated circuit based erbium-doped amplifier

Erbium-doped fiber amplifiers have revolutionized long-haul optical communications and laser technology. Erbium ions could equally provide a basis for efficient optical amplification in photonic integrated circuits. However, this approach has thus far remained impractical due to insufficient output power. Here, we demonstrate a photonic integrated circuit based erbium amplifier reaching 145 mW output power and more than 30 dB small-signal gain -- on par with commercial fiber amplifiers and beyond state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We achieve this by applying ion implantation to recently emerged ultralow-loss Si3N4 photonic integrated circuits with meter-scale-length waveguides. We utilize the device to increase by 100-fold the output power of soliton microcombs, required for low-noise photonic microwave generation or as a source for wavelength-division multiplexed optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of a wide range of fiber-based devices such as high-pulse-energy femtosecond mode-locked lasers.

physics.optics↗

arXiv2022

Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators

Silicon nitride (Si$_3$N$_4$) is an ever-maturing integrated platform for nonlinear optics. Yet, due to the absence of second-order ($χ^{(2)}$) nonlinearity, Si$_3$N$_4$ is mostly considered for third-order ($χ^{(3)}$) nonlinear interactions. Recently, this limitation was overcome by optical poling in both Si$_3$N$_4$ waveguides and microresonators via the photogalvanic effect, resulting in the inscription of quasi-phase-matched $χ^{(2)}$ gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si$_3$N$_4$ microresonator with combined $χ^{(2)}$ and $χ^{(3)}$ nonlinearities. We demonstrate that the photo-induced $χ^{(2)}$ grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs at pump wavelength. Additionally, the doubly resonant pump and second-harmonic fields allow for cascaded third-harmonic generation, where a secondary optically written $χ^{(2)}$ grating is identified. Finally, we reach a low-noise, broadband microcomb state evolved from the sum-frequency coupled primary comb. These results expand the scope of cascaded effects in $χ^{(2)}$ and $χ^{(3)}$ microresonators.

physics.optics↗

arXiv2021

Compact, spatial-mode-interaction-free, ultralow-loss, nonlinear photonic integrated circuits

Nonlinear photonics based on integrated circuits has enabled applications such as parametric amplifiers, soliton frequency combs, supercontinua, and non-reciprocal devices. Ultralow optical loss and the capability for dispersion engineering are essential, which necessitate the use of multi-mode waveguides. Despite that rich interaction among different spatial waveguide eigenmodes can give rise to novel nonlinear phenomena, spatial mode interaction is typically undesired as it increases optical loss, perturbs local dispersion profile, and impedes soliton formation. Adiabatic bends, such as Euler bends whose curvature varies linearly with their path length, have been favoured to suppress spatial mode interaction. Adiabatic bends can essentially connect any two waveguide segments with adiabatic mode conversion, thus efficiently avoid mode mixing due to mode mismatch. However, previous works lack quantitative measurement data and analysis to fairly evaluate the adiabaticity, and are not based on photonic integrated circuits with tight optical confinement and optical losses of few dB/m. Here, we adapt, optimize, and implement Euler bends to build compact racetrack microresonators based on ultralow-loss, multi-mode, silicon nitride photonic integrated circuits. The racetrack microresonators feature a small footprint of only 0.21 mm^2 for 19.8 GHz FSR. We quantitatively investigate the suppression of spatial mode interaction in the racetrack microresonators with Euler bends. We show that the optical loss rate (15.5 MHz) is preserved, on par with the mode interaction strength (25 MHz). This results in an unperturbed microresonator dispersion profile. We further demonstrate single soliton of 19.8 GHz repetition rate. The optimized Euler bends and racetrack microresonators can be key building blocks for integrated nonlinear photonic systems, programmable processors and photonic quantum computing.

physics.optics↗

arXiv2021

Platicon microcomb generation using laser self-injection locking

The past decade has witnessed major advances in the development of microresonator-based frequency combs (microcombs) that are broadband optical frequency combs with repetition rates in the millimeter-wave to microwave domain. Integrated microcombs can be manufactured using wafer-scale process and have been applied in numerous applications. Most of these advances are based on the harnessing of dissipative Kerr solitons (DKS) in optical microresonators with anomalous group velocity dispersion (GVD). However, microcombs can also be generated with normal GVD using dissipative localized structures that are referred to as "dark pulse", "switching wave" or "platicon". Importantly, as most materials feature intrinsic normal GVD, the requirement of dispersion engineering is significantly relaxed for platicon generation. Therefore while DKS microcombs require particular designs and fabrication processes, platicon microcombs can be readily built using standard CMOS-compatible platforms such as thin-film (i.e. typically below 300 nm) Si3N4. Yet laser self-injection locking that has been recently used to create highly compact integrated DKS microcomb modules has not been demonstrated for platicons. Here we report the first fully integrated platicon microcomb operating at a microwave-K-band repetition rate. Using laser self-injection locking of a DFB laser edge-coupled to a Si3N4 microresonator, platicons are electrically initiated and stably maintained, enabling a compact microcomb module without any complex control. We further characterize the phase noise of the platicon repetition rate and the pumping laser. The observation of self-injection-locked platicons facilitates future wide adoption of microcombs as a build-in block in standard photonic integrated architectures via commercial foundry service.

physics.optics↗

arXiv2021

Photonic chip-based continuous-travelling-wave parametric amplifier

The ability to amplify optical signals is of pivotal importance across science and technology. The development of optical amplifiers has revolutionized optical communications, which are today pervasively used in virtually all sensing and communication applications of coherent laser sources. In the telecommunication bands, optical amplifiers typically utilize gain media based on III-V semiconductors or rare-earth-doped fibers. Another way to amplify optical signals is to utilize the Kerr nonlinearity of optical fibers or waveguides via parametric processes. Such parametric amplifiers of travelling continuous wave have been originally developed in the microwave domain, and enable quantum-limited signal amplification with high peak gain, broadband gain spectrum tailored via dispersion control, and ability to enable phase sensitive amplification. Despite these advantages, optical amplifiers based on parametric gain have proven impractical in silica fibers due to the low Kerr nonlinearity. Recent advances in photonic integrated circuits have revived interest in parametric amplifiers due to the significantly increased nonlinearity in various integrated platforms. Yet, despite major progress, continuous-wave-pumped parametric amplifiers built on photonic chips have to date remained out of reach. Here we demonstrate a chip-based travelling-wave optical parametric amplifier with net signal gain in the continuous-wave regime. Using ultralow-loss, dispersion-engineered, meter-long, silicon nitride photonic integrated circuits that are tightly coiled on a photonic chip, we achieve a continuous parametric gain of 12 dB that exceeds both the on-chip optical propagation loss and fiber-chip-fiber coupling losses in the optical C-band.

physics.optics↗

arXiv2021

Ultralow-noise frequency-agile photonic integrated lasers

Low-noise lasers are of central importance in a wide variety of applications, including high spectral-efficiency coherent communication protocols, distributed fibre sensing, and long distance coherent LiDAR. In addition to low phase noise, frequency agility, that is, the ability to achieve high-bandwidth actuation of the laser frequency, is imperative for triangular chirping in frequency-modulated continuous-wave (FMCW) based ranging or any optical phase locking as routinely used in metrology. While integrated silicon-based lasers have experienced major advances and are now employed on a commercial scale in data centers, integrated lasers with sub-100 Hz-level intrinsic linewidth are based on optical feedback from photonic circuits that lack frequency agility. Here, we demonstrate a wafer-scale-manufacturing-compatible hybrid photonic integrated laser that exhibits ultralow intrinsic linewidth of 25 Hz while offering unsurpassed megahertz actuation bandwidth, with a tuning range larger than 1 GHz. Our approach uses ultralow-loss (1 dB/m) Si$_3$N$_4$ photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise. By utilizing difference drive and apodization of the photonic chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm.

physics.optics↗

arXiv2021

Coherent terahertz-to-microwave link using electro-optic-modulated Turing rolls

Arising from modulation instability, Turing rolls in optical Kerr microresonators have been used in the generation of optical frequency combs and the synthesis of microwave and terahertz frequencies. In this work, by applying electro-optic modulation on terahertz-frequency Turing rolls, we implement electro-optic frequency division with a microcomb to synthesize variable low-noise microwave signals. We also actively stabilize the terahertz oscillations to a microwave reference via intracavity power modulation, obtaining fractional frequency instabilities that are better than those of the free-running situation by up to six orders of magnitude. This study not only highlights the extraordinary spectral purity of Turing roll oscillations but also opens the way for bidirectional terahertz-to-microwave links with hybrid optical frequency comb techniques.

physics.optics↗

arXiv2021

Magnetic-Free Silicon Nitride Integrated Optical Isolator

Integrated photonics has enabled signal synthesis, modulation and conversion using photonic integrated circuits (PIC). Many materials have been developed, among which silicon nitride (Si$_3$N$_4$) has emerged as a leading platform particularly for nonlinear photonics. Low-loss Si$_3$N$_4$ PIC has been widely used for frequency comb generation, narrow-linewidth lasers, microwave photonics, photonic computing networks, and even surface-electrode ion traps. Yet, among all demonstrated functionalities for Si$_3$N$_4$ integrated photonics, optical non-reciprocal devices, such as isolators and circulators, have not been achieved. Conventionally, they are realized based on Faraday effect of magneto-optic materials under external magnetic field. However, it has been challenging to integrate magneto-optic materials that are not CMOS-compatible and that require bulky external magnet. Here, we demonstrate a magnetic-free optical isolator based on aluminum nitride (AlN) piezoelectric modulators monolithically integrated on ultralow-loss Si$_3$N$_4$ PIC. The transmission reciprocity is broken by spatio-temporal modulation of a Si$_3$N$_4$ microring resonator with three AlN bulk acoustic wave resonators that are driven with a rotational phase. This design creates an effective rotating acoustic wave that allows indirect interband transition in only one direction among a pair of strongly coupled optical modes. Maximum of 10 dB isolation is achieved under 100 mW RF power applied to each actuator, with minimum insertion loss of 0.1 dB. The isolation remains constant over nearly 30 dB dynamic range of optical input power, showing excellent optical linearity. Our integrated, linear, magnetic-free, electrically driven optical isolator could become key building blocks for integrated lasers, chip-scale LiDAR engines, as well as optical interfaces for superconducting circuits.

physics.optics↗

arXiv2021

Gain-switched semiconductor laser driven soliton microcombs

Dissipative Kerr solitons (DKSs) have been generated via injection locking of chipscale microresonators to continuous-wave (CW) III-V lasers. This advance has enabled fully integrated hybrid microcomb systems that operate in turnkey mode and can access microwave repetition rates. Yet, CW-driven DKS exhibits low energy conversion efficiency and high optical power threshold, especially when the repetition rate is within the microwave range that is convenient for direct detection with off-the-shelf electronics. Efficient DKS can be generated by spatiotemporally structured light (i.e., pulsed pumping), which to date however has required complex cascaded modulators for pulse synthesis. Here we demonstrate a photonic integrated approach to pulsed pumping. By actively switching the bias current of injection-locked III-V semiconductor lasers with switching frequencies in the X-band and K-band microwave ranges, we pump a crystalline and integrated microresonators with coherent picosecond laser pulses, achieving DKS generation with stable repetition rates and lowering the required average pumping power by one order of magnitude to a record-setting level of a few milliwatts. In addition, we unveil the critical role of the phase profile of the pumping pulses, and for the first time implement phase engineering on the pulsed pumping scheme by either accessing a multimode lasing regime in the gain-switching mode or applying external chirping to support robust single-soliton generation. Our work leverages the advantages of gain switching technique and pulse pumping technique, and establishes the merits of combining distinct compact frequency comb platforms that enhance the potential of energy-efficient chipscale microcombs.

physics.optics↗

arXiv2020

Low-loss integrated nanophotonic circuits with layered semiconductor materials

Monolayer transition metal dichalcogenides with direct bandgaps are emerging candidates for microelectronics, nano-photonics, and optoelectronics. Transferred onto photonic integrated circuits (PICs), these semiconductor materials have enabled new classes of light-emitting diodes, modulators and photodetectors, that could be amenable to wafer-scale manufacturing. For integrated photonic devices, the optical losses of the PICs are critical. In contrast to silicon, silicon nitride (Si3N4) has emerged as a low-loss integrated platform with a wide transparency window from ultraviolet to mid-infrared and absence of two-photon absorption at telecommunication bands. Moreover, it is suitable for nonlinear integrated photonics due to its high Kerr nonlinearity and high-power handing capability. These features of Si3N4 are intrinsically beneficial for nanophotonics and optoelectronics applications. Here we report a low-loss integrated platform incorporating monolayer molybdenum ditelluride (1L-MoTe2) with Si3N4 photonic microresonators. We show that, with the 1L-MoTe2, microresonator quality factors exceeding 3 million in the telecommunication O-band to E-band are maintained. We further investigate the change of microresonator dispersion and resonance shift due to the presence of 1L-MoTe2, and extrapolate the optical loss introduced by 1L-MoTe2 in the telecommunication bands, out of the excitonic transition region. Our work presents a key step for low-loss, hybrid PICs with layered semiconductors without using heterogeneous wafer bonding.

physics.optics↗

arXiv2020

High-yield wafer-scale fabrication of ultralow-loss, dispersion-engineered silicon nitride photonic circuits

Low-loss photonic integrated circuits (PIC) and microresonators have enabled novel applications ranging from narrow-linewidth lasers, microwave photonics, to chip-scale optical frequency combs and quantum frequency conversion. To translate these results into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in fabrication of integrated Si3N4 photonics have shown that ultralow-loss, dispersion-engineered microresonators can be attained at die-level throughput. For emerging nonlinear applications such as integrated travelling-wave parametric amplifiers and mode-locked lasers, PICs of length scales of up to a meter are required, placing stringent demands on yield and performance that have not been met with current fabrication techniques. Here we overcome these challenges and demonstrate a fabrication technology which meets all these requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30 million, corresponding to a linear propagation loss of 1.0 dB/m, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances and cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dB/m loss in dies of only 5x5 mm size. Using a modulation response measurement self-calibrated via the Kerr nonlinearity, we reveal that, strikingly, the intrinsic absorption-limited Q factor of our Si3N4 microresonators exceeds a billion. Transferring the present Si3N4 photonics technology to standard commercial foundries, and merging it with silicon photonics using heterogeneous integration technology, will significantly expand the scope of today's integrated photonics and seed new applications.

physics.app-ph↗

arXiv2020

Reconfigurable radiofrequency filters based on versatile soliton microcombs

The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb based radiofrequency (RF) filters have required programmable pulse shapers, which inevitably increase the system cost, footprint, and complexity. Here, by leveraging the smooth spectral envelope of single solitons, we demonstrate for the first time microcomb based RF filters free from any additional pulse shaping. More importantly, we achieve all-optical reconfiguration of the RF filters by exploiting the intrinsically rich soliton configurations. Specifically, we harness the perfect soliton crystals to multiply the comb spacing thereby dividing the filter passband frequencies. Also, a completely novel approach based on the versatile interference patterns of two solitons within one round-trip, enables wide reconfigurability of RF passband frequencies according to their relative azimuthal angles. The proposed schemes demand neither an interferometric setup nor another pulse shaper for filter reconfiguration, providing a practical route towards chip-scale, widely reconfigurable microcomb based RF filters.

physics.optics↗