arXiv subjects

Junqiu Liu

Publications and source records attributed to Junqiu Liu.

At least 37 records · Page 2Linked to original sources

arXiv2025

Photonic chip-based high-efficiency soliton microcombs via electroopitc-Kerr synergy

Temporal soliton mode-locking in coherently pumped microcavities provides a promising platform for miniaturized frequency comb systems. While significant progress has been made, achieving high conversion efficiency in such microcombs remains a critical challenge. Soliton generation through pulse pumping has emerged as an effective strategy to improve conversion efficiency. However, the on-chip integration of pulse generation with dissipative Kerr soliton (DKS) formation within the photonic chip has not yet been realized. In this work, we demonstrate a photonic chip-based soliton microcomb with high conversion efficiency, achieved by integrating on-chip pulse generation and DKS generation. The pulsed laser, fabricated on a lithium niobate-on-insulator (LNOI) platform, delivers a 35.5GHz repetition rate with broadly tunable center frequencies. By coupling these on-chip pulses to a silicon nitride microresonator, we achieve stable DKS generation with a pump-to-soliton conversion efficiency of 43.9% under steady-state conditions. This integrated architecture establishes a viable pathway toward chip-scale soliton microcombs with unprecedented efficiency, opening up new possibilities for optical communications, precision spectroscopy, and photonic sensing.

physics.optics↗

arXiv2025

Laser transfer and retrieval via nanophotonic supercontinuum process

The nature of optical metrology is to perform efficient transfer and precise retrieval for lasers and optical signals, which is beneficial for a variety of applications ranging from optical clocking, spectroscopy, to telecommunications and quantum optics. While efforts have been made to promote the detection accuracy of optical frequencies, retrieval on optical waveforms remains on the autocorrelation scheme with limited performances. Here, we demonstrate a novel scheme for optical metrology, particularly on direct retrieval of optical waveform in terms of the field amplitude profile. The scheme is based on massive four-wave-mixings underlying a nanophotonic supercontinuum process, which enables arbitrary transfer of an additive laser to modulational sidebands of the broadened continuum. Detection of the transferred signals is then flexible to be within the whole span of the supercontinuum from visible to the mid-infrared range. We demonstrate such a transfer scheme for both CW lasers and pulsed lasers. For the latter, the temporal amplitude profile of the optical wave can be retrieved, which reveals high-order dynamics of solitary pulses including the self-steepening, self-compression, and the soliton splitting, and shows a remarkable square-fold increase of signal-to-noise ratio in the power spectrum. Our results may contribute to advance optical metrology particularly towards chip scale optical waveform detection, and more fundamentally, they reveal insights of massive ultrafast nonlinear interactions underlying the soliton-based supercontinuum process.

physics.optics↗

arXiv2025

Hybrid-integrated dark-pulse microcombs towards visible light spectrum

Leveraging hybrid integration, we demonstrate dark-pulse formation at 780-nm wavelength band in integrated Si$_3$N$_4$ microresonators driven by high-power AlGaAs-based chip-scale lasers. The device outputs coherent frequency combs with electronically detectable repetition rates down to 20 GHz, paving a route to efficient and compact atom-chip interfaces for spectroscopy, metrology and sensing.

physics.optics↗

arXiv2025

A high-resolution microresonator-frequency-comb spectrometer

Spectral analysis is one of the most powerful technologies for studying and understanding matter. As the devices for spectral analysis, spectrometers are widely used in material detection, isotope analysis, trace gas detection, and the study of atomic and molecular hyperfine structures. While high resolution, wide bandwidth and fast speed are essential factors, they are always trade-offs for conventional spectrometers. Here, we present a soliton-microcomb-based spectrometer that overcomes these challenges by integrating dissipative Kerr solitons (DKSs) with double-sideband modulation and parallelized detection. Leveraging a high-quality silicon nitride microresonator, we generate a broadband, fully stabilized soliton microcomb and employ radio-frequency-modulated double sidebands to scan the optical spectrum with the resolution constrained only by the comb-line linewidth. By projecting the comb lines onto a two-dimensional charge-coupled device (CCD) via a virtually imaged phased array (VIPA)-grating system, we enable parallel processing of all spectral components, circumventing sequential scanning delays. The resulting spectrometer achieves 200-kHz resolution across a 4-THz bandwidth with minutes-level processing time while maintaining robustness against environmental fluctuations. Being promising for miniaturization, this work bridges the gap between laboratory-grade performance and field-deployable practicality, unlocking new possibilities for spectroscopy in astronomy, metrology, and integrated photonics.

physics.optics↗

arXiv2025

Space compatibility of emerging, wide-bandgap, ultralow-loss integrated photonics

Integrated photonics has revolutionized optical communication, sensing, and computation, offering miniaturized and lightweight solutions for spacecraft with limited size and payload. Novel chip-scale instruments based on ultralow-loss integrated photonic platforms, including lasers, frequency combs and atomic traps, have been developed for space applications. Therefore, quantifying the space compatibility of ultralow-loss photonic integrated circuits (PICs), particularly their radiation resistance, is critical. This study experimentally evaluates the radiation resistance of ultralow-loss Si$_3$N$_4$, 4H-SiC, and LiNbO$_3$ PICs under intense $γ$-ray and high-energy proton irradiation. Results show that proton irradiation with $1.1 \times 10^{10}$ $\mathrm{p/cm^2}$ total flux does not significantly increase optical loss or alter the refractive index of these PICs, while $γ$-ray irradiation with 1.2 Mrad accumulated dose only marginally increases their optical loss. These findings provide preliminary evidence of the excellent space compatibility of ultralow-loss Si$_3$N$_4$, 4H-SiC, and LiNbO$_3$ PICs, highlighting their potential for compact and lightweight space systems.

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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.

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arXiv2024

Nanometric dual-comb ranging using photon-level microcavity solitons

Absolute distance measurement with low return power, fast measurement speed, high precision, and immunity to intensity fluctuations is highly demanded in nanotechnology. However, achieving all these objectives simultaneously remains a significant challenge for miniaturized systems. Here, we demonstrate dual-comb ranging (DCR) that encompasses all these capabilities by using counter-propagating (CP) solitons generated in an integrated Si$_3$N$_4$ microresonator. We derive equations linking the DCR precision with comb line powers, revealing the advantage of microcomb's large line spacing in precise ranging. Leveraging the advantage, our system reaches 1-nm-precision and measures nm-scale vibration at frequencies up to 0.9 MHz. We also show that precise DCR is possible even in the presence of strong intensity noise and loss, using a mean received photon number as low as 5.5$\times$10$^{-4}$ per pulse. Our work establishes an optimization principle for dual-comb systems and bridges high performance ranging with foundry-manufactured photonic chips.

physics.optics↗

arXiv2024

Piezoelectric actuation for integrated photonics

Recent decades have seen significant advancements in integrated photonics, driven by improvements in nanofabrication technology. This field has developed from integrated semiconductor lasers and low-loss waveguides to optical modulators, enabling the creation of sophisticated optical systems on a chip scale capable of performing complex functions like optical sensing, signal processing, and metrology. The tight confinement of optical modes in photonic waveguides further enhances the optical nonlinearity, leading to a variety of nonlinear optical phenomena such as optical frequency combs, second-harmonic generation, and supercontinuum generation. Active tuning of photonic circuits is crucial not only for offsetting variations caused by fabrication in large-scale integration, but also serves as a fundamental component in programmable photonic circuits. Piezoelectric actuation in photonic devices offers a low-power, high-speed solution and is essential in the design of future photonic circuits due to its compatibility with materials like Si and Si3N4, which do not exhibit electro-optic effects. Here, we provide a detailed review of the latest developments in piezoelectric tuning and modulation, by examining various piezoelectric materials, actuator designs tailored to specific applications, and the capabilities and limitations of current technologies. Additionally, we explore the extensive applications enabled by piezoelectric actuators, including tunable lasers, frequency combs, quantum transducers, and optical isolators. These innovative ways of managing photon propagation and frequency on-chip are expected to be highly sought after in the future advancements of advanced photonic chips for both classical and quantum optical information processing and computing.

physics.optics↗

arXiv2024

A microcomb-empowered Fourier domain mode-locked LiDAR

Light detection and ranging (LiDAR) has emerged as an indispensable tool in autonomous technology. Among its various techniques, frequency modulated continuous wave (FMCW) LiDAR stands out due to its capability to operate with ultralow return power, immunity to unwanted light, and simultaneous acquisition of distance and velocity. However, achieving a rapid update rate with sub-micron precision remains a challenge for FMCW LiDARs. Here, we present such a LiDAR with a sub-10 nm precision and a 24.6 kHz update rate by combining a broadband Fourier domain mode-locked (FDML) laser with a silicon nitride soliton microcomb. An ultrahigh frequency chirp rate up to 320 PHz/s is linearized by a 50 GHz microcomb to reach this performance. Our theoretical analysis also contributes to resolving the challenge of FMCW velocity measurements with nonlinear frequency sweeps and enables us to realize velocity measurement with an uncertainty below 0.4 mm/s. Our work shows how nanophotonic microcombs can unlock the potential of ultrafast frequency sweeping lasers for applications including LiDAR, optical coherence tomography and sensing.

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arXiv2024

Arbitrary-phase locking of fiber Mach-Zehnder interferometers

Optical interferometers are extensively used in fundamental physics test, gravitational wave detection, quantum metrology, topological photonics, and quantum information processing. Fiber-based interferometers are compact, robust and cheap, thus are ubiquitously deployed. However, the optical phase in fiber interferometers is sensitive to ambient perturbation, resulting in compromised phase sensing precision. Therefore, phase control, shifting and stabilization of fiber interferometers is essential. Methods to create stable interference patterns and to lock a fiber interferometer at arbitrary phase have been shown, which however are sophisticated, bulky and delicate, preventing wider application in harsh environment outside laboratories or in space. Here we demonstrate a new method for arbitrary-phase locking of fiber unbalanced Mach-Zehnder interferometers. Compared to existing method, our method is simpler, more robust and more compact. We showcase the preparation and characterization of narrow-band energy-time-entanglement photon state generated in integrated nonlinear microresonators, where two-photon interference visibility reaching 0.993(6) is enabled. Our method constitutes a critical building block for photonic quantum network, and is useful to emerging single-photon interference in curved space-time that facilitates exploration of the interface of quantum mechanics and general relativity.

physics.optics↗

arXiv2024

An alkali-referenced vector spectrum analyzer for visible-light integrated photonics

Integrated photonics has reformed our information society by offering on-chip optical signal synthesis, processing and detection with reduced size, weight and power consumption. As such, it has been successfully established in the near-infrared (NIR) telecommunication bands. With the soaring demand in miniaturized systems for biosensing, quantum information and transportable atomic clocks, extensive endeavors have been stacked on translating integrated photonics into the visible spectrum, i.e. visible-light integrated photonics. Various innovative visible-light integrated devices have been demonstrated, such as lasers, frequency combs, and atom traps, highlighting the capacity and prospect to create chip-based optical atomic clocks that can make timing and frequency metrology ubiquitous. A pillar to the development of visible-light integrated photonics is characterization techniques featuring high frequency resolution and wide spectral coverage, which however remain elusive. Here, we demonstrate a vector spectrum analyzer (VSA) for visible-light integrated photonics, offering spectral bandwidth from 766 to 795 nm and frequency resolution of 415 kHz. The VSA is rooted on a widely chirping, high-power, narrow-linewidth, mode-hop-free laser around 780 nm, which is frequency-doubled from the near-infrared via an efficient, broadband CPLN waveguide. The VSA is further referenced to hyperfine structures of rubidium and potassium atoms, enabling 8.1 MHz frequency accuracy. We apply our VSA to showcase the characterization of loss, dispersion and phase response of passive integrated devices, as well as densely spaced spectra of mode-locked lasers. Combining operation in the NIR and visible spectra, our VSA allows characterization bandwidth exceeding an octave and can be an invaluable diagnostic tool for spectroscopy, nonlinear optical processing, imaging and quantum interfaces to atomic devices.

physics.optics↗

arXiv2024

Ultralow-loss integrated photonics enables bright, narrow-band, photon-pair sources

Photon-pair sources are critical building blocks for photonic quantum systems. Leveraging Kerr nonlinearity and cavity-enhanced spontaneous four-wave mixing, chip-scale photon-pair sources can be created using microresonators built on photonic integrated circuit. For practical applications, a high microresonator quality factor $Q$ is mandatory to magnify photon-pair sources' brightness and reduce their linewidth. The former is proportional to $Q^4$, while the latter is inversely proportional to $Q$. Here, we demonstrate an integrated, microresonator-based, narrow-band photon-pair source. The integrated microresonator, made of silicon nitride and fabricated using a standard CMOS foundry process, features ultralow loss down to $3$ dB/m and intrinsic $Q$ factor exceeding $10^7$. The photon-pair source has brightness of $1.17\times10^9$ Hz/mW$^2$/GHz and linewidth of $25.9$ MHz, both of which are record values for silicon-photonics-based quantum light source. It further enables a heralded single-photon source with heralded second-order correlation $g^{(2)}_\mathrm{h}(0)=0.0037(5)$, as well as a time-bin entanglement source with a raw visibility of $0.973(9)$. Our work evidences the global potential of ultralow-loss integrated photonics to create novel quantum light sources and circuits, catalyzing efficient, compact and robust interfaces to quantum communication and networks.

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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↗

arXiv2024

A chip-integrated comb-based microwave oscillator

Low-noise microwave oscillators are cornerstones for wireless communication, radar and clocks. Optical frequency combs have enabled photonic microwaves with unrivalled noise performance and bandwidth. Emerging interest is to generate microwaves using chip-based frequency combs, namely microcombs. Here, we demonstrate the first, fully integrated, microcomb-based, microwave oscillator chip. The chip, powered by a microelectronic circuit, leverages hybrid integration of a DFB laser, a nonlinear microresonator, and a high-speed photodetector. Each component represents the best of its own class, yet allows large-volume manufacturing with low cost in CMOS foundries. The hybrid chip outputs an ultralow-noise laser of 6.9 Hz linewidth, a microcomb of 10.7 GHz repetition rate, and a 10.7 GHz microwave of 6.3 mHz linewidth -- all three in one entity of 76 mm$^2$ size.The microwave phase noise reaches -75/-105/-130 dBc/Hz at 1/10/100 kHz Fourier offset frequency. Our results can reinvigorate our information society for communication, sensing, timing and precision measurement.

physics.optics↗

arXiv2024

Rhythmic soliton interactions for integrated dual-microcomb spectroscopy

Rotation symmetry of microresonators supports the generation of phase-locked counter-propagating (CP) solitons that can potentially miniaturize dual-comb systems. Realization of these dual-comb compatible solitons in photonic integrated circuits remains a challenge. Here, we synthesized such CP solitons in an integrated silicon nitride microresonator and observed forced soliton oscillation due to rhythmic, time-varying soliton interactions. The interactions result in seconds mutual-coherence passively. Temporal motion in the soliton streams is discerned by measuring a quadratic-scaling frequency noise peaks and an inverse quadratic-scaling microcomb sidebands. By generating a CP soliton trimer to have two synchronized solitons in one of the orbiting directions, we resolve the incapability of measuring two unsynchronized CP soliton dimer pulses by optical cross-correlation, and show CP solitons undergo complex motion trajectory. We further prove that precise dual-comb spectroscopy with an acquisition time as short as 0.6 $μ$s is feasible using these solitons, although the temporal motion limits the dynamic range. Besides revealing soliton interactions with different group velocities, our work propels the realization of photonic integrated dual-comb spectrometers with high passive coherence.

physics.optics↗

arXiv2023

A chip-scale second-harmonic source via injection-locked all-optical poling

Second-harmonic generation allows for coherently bridging distant regions of the optical spectrum, with applications ranging from laser technology to self-referencing of frequency combs. However, accessing the nonlinear response of a medium typically requires high-power bulk sources, specific nonlinear crystals, and complex optical setups, hindering the path toward large-scale integration. Here we address all of these issues by engineering a chip-scale second-harmonic (SH) source based on the frequency doubling of a semiconductor laser self-injection-locked to a silicon nitride microresonator. The injection-locking mechanism, combined with a high-Q microresonator, results in an ultra-narrow intrinsic linewidth at the fundamental harmonic frequency as small as 57 Hz. Owing to the extreme resonant field enhancement, quasi-phase-matched second-order nonlinearity is photoinduced through the coherent photogalvanic effect and the high coherence is mapped on the generated SH field. We show how such optical poling technique can be engineered to provide efficient SH generation across the whole C and L telecom bands, in a reconfigurable fashion, overcoming the need for poling electrodes. Our device operates with milliwatt-level pumping and outputs SH power exceeding 2 mW, for an efficiency as high as 280%/W under electrical driving. Our findings suggest that standalone, highly-coherent, and efficient SH sources can be integrated in current silicon nitride photonics, unlocking the potential of $χ^{(2)}$ processes in the next generation of integrated photonic devices.

physics.optics↗

arXiv2023

A wideband, high-resolution vector spectrum analyzer for integrated photonics

The analysis of optical spectra - emission or absorption -- has been arguably the most powerful approach for discovering and understanding matters. The invention and development of many kinds of spectrometers have equipped us with versatile yet ultra-sensitive diagnostic tools for trace gas detection, isotope analysis, and resolving hyperfine structures of atoms and molecules. With proliferating data and information, urgent and demanding requirements have been placed today on spectrum analysis with ever-increasing spectral bandwidth and frequency resolution. These requirements are especially stringent for broadband laser sources that carry massive information, and for dispersive devices used in information processing systems. In addition, spectrum analyzers are expected to probe the device's phase response where extra information is encoded. Here we demonstrate a novel vector spectrum analyzer (VSA) that is capable of characterizing passive devices and active laser sources in one setup. Such a dual-mode VSA can measure loss, phase response and dispersion properties of passive devices. It also can coherently map a broadband laser spectrum into the RF domain. The VSA features a bandwidth of 55.1 THz (1260 to 1640 nm), frequency resolution of 471 kHz, and dynamic range of 56 dB. Meanwhile, our fiber-based VSA is compact and robust. It requires neither high-speed modulators and photodetectors, nor any active feedback control. Finally, we successfully employ our VSA for applications including characterization of integrated dispersive waveguides, mapping frequency comb spectra, and coherent light detection and ranging (LiDAR). Our VSA presents an innovative approach for device analysis and laser spectroscopy, and can play a critical role in future photonic systems and applications for sensing, communication, imaging, and quantum information processing.

physics.optics↗

arXiv2023

Programmable access to microresonator solitons with modulational sideband heating

Dissipative Kerr solitons formed in high-$Q$ optical microresonators provide a route to miniaturized optical frequency combs that can revolutionize precision measurements, spectroscopy, sensing, and communication. In the last decade, a myriad of integrated material platforms have been extensively studied and developed to create photonic-chip-based soliton combs. However, the photo-thermal effect in integrated optical microresonators has been a major issue preventing simple and reliable soliton generation. Several sophisticated techniques to circumvent the photo-thermal effect have been developed. In addition, instead of the single-soliton state, emerging applications in microwave photonics and frequency metrology prefer multi-soliton states. Here we demonstrate an approach to manage the photo-thermal effect and facilitate soliton generation. The approach is based on a single phase-modulated pump, where the generated blue-detuned sideband synergizes with the carrier and thermally stabilizes the microresonator. We apply this technique and demonstrate deterministic soliton generation of 19.97 GHz repetition rate in an integrated silicon nitride microresonator. Furthermore, we develop a program to automatically address to target $N-$soliton state, in addition to the single-soliton state, with near 100% success rate and as short as 10 s time consumption. Our method is valuable for soliton generation in essentially any platforms even with strong photo-thermal effect, and can promote wider applications of soliton frequency comb systems for microwave photonics, telecommunication and frequency metrology.

physics.optics↗