Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µs, 50 GHz in 125µs, and 25 GHz in 50µs (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach–Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at two different wavelengths enables estimation of the frequency shift of all comb modes.

Kuse et al., Optical Society of America (2021)

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 components 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 confirm the requisite performance of the individual passive elements of the proposed interposer—octave-wide dichroics, multimode interferometers, and tunable ring filters, and implement the octave-spanning spectral filtering of a microcomb, central to the interposer, using silicon nitride photonics. 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. Finally, we numerically confirm the feasibility of operating the proposed interposer 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.

Rao et al., Light Science & Applications (2021)

Optical frequency division via optical frequency combs has enabled a leap in microwave metrology, leading to noise performance never explored before. Extending this method to the millimetre-wave and terahertz-wave domains is of great interest. Dissipative Kerr solitons in integrated photonic chips offer the unique feature of delivering optical frequency combs with ultrahigh repetition rates from 10 GHz to 1 THz, making them relevant gears for performing optical frequency division in the millimetre-wave and terahertz-wave domains. We experimentally demonstrate the optical frequency division of an optically carried 3.6 THz reference down to 300 GHz through a dissipative Kerr soliton, photodetected with an ultrafast uni-travelling-carrier photodiode. A new measurement system, based on the characterization of a microwave reference phase locked to the 300 GHz signal under test, yields attosecond-level timing-noise sensitivity, overcoming conventional technical limitations. This work places dissipative Kerr solitons as a leading technology in the millimetre-wave and terahertz-wave field, promising breakthroughs in fundamental and civilian applications.

Tetsumoto et al., Nature Photonics (2021)

Delicate engineering of integrated nonlinear structures is required for developing scalable sources of non-classical light to be deployed in quantum information processing systems. In this work, we demonstrate a photonic molecule composed of two coupled microring resonators on an integrated nanophotonic chip, designed to generate strongly squeezed light uncontaminated by noise from unwanted parasitic nonlinear processes. By tuning the photonic molecule to selectively couple and thus hybridize only the modes involved in the unwanted processes, suppression of parasitic parametric fluorescence is accomplished. This strategy enables the use of microring resonators for the efficient generation of degenerate squeezed light: without it, simple single-resonator structures cannot avoid contamination from nonlinear noise without significantly compromising pump power efficiency. We use this device to generate 8 dB of broadband degenerate squeezed light on-chip, with 1.65 dB directly measured.

Zhang et al., Nature Communications (2021)

Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, owing to their low nonlinearity, easy scalability, and low propagation loss. However, the development of SiN OPAs for light detection and ranging (LiDAR) applications has attracted less attention due to the low thermooptic (TO) effect of SiN. Moreover, SiN OPAs are considered to be undesirable for transmitters in LiDAR systems because of the limited longitudinal tuning efficiency of SiN grating antenna. We present a hybrid integrated SiNpolymer OPA for realizing an efficient LiDAR. Owing to the large TO effect of polymer, the proposed OPA with polymeric phase modulators resolves the power dissipation drawback of the SiN devices. The polymeric modulators are integrated with a SiN power splitter and grating antenna, and the coupling loss is alleviated via a spot-size converter. Additionally, a steering angle magnifying lens is used to enhance the tuning efficiency of the antenna, expanding the longitudinal beam steering range. The prepared OPA transmitter equipped with the lens exhibits decent two-dimensional beam steering, featuring stable emission characteristics over a 12 30 field of view. With the transmitter, efficient time-of-flight based LiDAR is demonstrated to provide a detection range of 10 m under a peak optical power of 55 dBm. The proposed SiNpolymer OPA is highly anticipated to circumvent the challenges of conventional SiN counterparts and potentially spearhead the development of a prominent OPA platform for advanced LiDAR schemes.

Im et al., IEEE (2021)

Integrated tunable lasers based on the co-integration of InP-based SOAs with low-loss Si3N4 dielectric waveguides have emerged as promising solutions in applications where the control of light phase is fundamental. Μicrowave photonics, coherent communications and LIDARs are only some of the applications where sub-KHz linewidths have already been achieved. Nevertheless, the majority of these demonstrations are based on Si3N4 platforms featuring thicknesses lower than 300nm and providing modes with effective indices below 1.6 imposing a major restriction on the achievable FSR values and devices’ footprint. In this work, we present the design of Vernier ring-inspired reflectors based on an 800nm- thick Si3N4 platform providing a TE fundamental mode with an effective index close to 1.71 for a width of 800nm, a group index close to 2.08 at λ=1550nm wavelength, and propagation losses as low as 0.2dB/cm. The proposed thick- Si3N4 designs are based on a simple dual ring Vernier configuration achieving an experimental FSR near 38nm and a 15dB side-mode suppression. These results are in close agreement with the ones obtained theoretically through a detailed Transfer Matrix Formulation verifying the accuracy of the presented semi-analytical model. This simulation model is then employed for the prediction of the performance of more advanced structures such as triple cascaded and high-order Vernier Ring designs, towards extending the achievable FSR and SMSR metrics.

Chatzitheocharis et al., SPIE (2021)

Growing interest in quantum computing for practical applications has led to a surge in the availability of programmable machines for executing quantum algorithms. Present-day photonic quantum computers have been limited either to non-deterministic operation, low photon numbers and rates, or fixed random gate sequences. Here we introduce a full-stack hardware−software system for executing many-photon quantum circuit operations using integrated nanophotonics: a programmable chip, operating at room temperature and interfaced with a fully automated control system. The system enables remote users to execute quantum algorithms that require up to eight modes of strongly squeezed vacuum initialized as two-mode squeezed states in single temporal modes, a fully general and programmable four-mode interferometer, and photon number-resolving readout on all outputs. Detection of multi-photon events with photon numbers and rates exceeding any previous programmable quantum optical demonstration is made possible by strong squeezing and high sampling rates. We verify the non-classicality of the device output, and use the platform to carry out proof-of-principle demonstrations of three quantum algorithms: Gaussian boson sampling, molecular vibronic spectra and graph similarity. These demonstrations validate the platform as a launchpad for scaling photonic technologies for quantum information processing.

Arrazola et al., Nature (2021)

The implementation of a polarization beam splitter (PBS) on a silicon nitride platform remains challenging owing to its relatively low index. We therefore propose a silicon nitride PBS that exploits serially cascaded asymmetric directional couplers (ADCs), leading to a high polarization extinction ratio (PER) over a broad bandwidth. The ADC spatially routes incident light through polarization-selective mode coupling under a small footprint of 112 µm. The proposed PBS does not require an active phase control. It is thus effectively realized via a single-step lithography process. The measured transverse-electric and transverse-magnetic PERs were determined to be above 23 dB and 10 dB over an 80-nm bandwidth, respectively, spanning 𝜆=1520−1600nm. The proposed device is thus anticipated to play a key role in providing polarization diversity in photonic-integrated circuits.

Bhandari et al., Optics Letters (2020)

Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, due to their potential use in enhanced solid-state light detection and ranging. Grating antennas based on a SiN waveguide suffer from a limited beam steering efficiency, owing to the relatively lower effective refractive index of the waveguide. To mitigate this limitation, we propose and demonstrate a backward-emitting SiN OPA incorporating a reinforced grating vector, which provides efficient wavelength-tuned beam steering along the longitudinal direction. Two backward-emitting OPAs were primarily characterized by their steering efficiency and the spectral emission for the main lobe, and they were compared with a forward-emitting device using a weakened grating vector. The results indicated that strengthening the grating vector for the antenna improved the beam steering efficiency but adversely affected the dependence of its directionality on the wavelength. The directionality was particularly inspected, with respect to major design parameters for the grating antennas, including the pitch and etch depth of a SiN grating, as well as the thickness of a buried oxide and SiN core layer. The proposed backward-emitting OPA scheme is expected to significantly promote its feasibility as an advanced beam scanning device.

Im et al., IEEE (2020)

Low-loss, fabrication-tolerant Si₃N₄ CWDM lattice filters and AWGs are demonstrated for 990 – 1065nm bottom-emitting VCSELs. Channel separation of 25 nm, XT <; -35 dB and -20 dB are reported with temperature shift of 14.5 pm/°C.

Stanley Cheung and Michael R.T Tan, IEEE (2020)

The availability of nonlinear parametric processes, such as frequency conversion in photonic integrated circuits is essential. In this contribution, we demonstrate a highly tunable second-harmonic generation in a fully complementary metal–oxide–semiconductor (CMOS)-fabrication-compatible silicon nitride integrated photonic platform. We induce the second-order nonlinearity using an all-optical poling technique with the second-harmonic light generated in the fundamental mode, and a narrow quasi-phase matching (QPM) spectrum by avoiding higher-order mode mixing. We are then able to broadly tune the phase-matched pump wavelength over the entire C-band (1540 nm to 1560 nm) by varying the poling conditions. Fine-tuning of QPM is enabled by thermo-optic effect with the tuning slope  Δ𝜆 / Δ𝑇  in our device being 113.8 pm/°C. In addition, we exploit the measurable variation of the 3 dB QPM bandwidth to confirm how the length of the all-optically inscribed grating varies with exposure time.

Nitiss et al., Optics Letters (2020)

Integrated photonic devices based on Si₃N₄ waveguides allow for the exploitation of nonlinear frequency conversion, exhibit low propagation loss, and have led to advances in compact atomic clocks, ultrafast ranging, and spectroscopy. Yet, the lack of Pockels effect presents a major challenge to achieve high-speed modulation of Si₃N₄. Here, microwave-frequency acousto-optic modulation is realized by exciting high-overtone bulk acoustic wave resonances (HBAR) in the photonic stack. Although HBAR is ubiquitously used in modern communication and superconducting circuits, this is the first time it has been incorporated on a photonic integrated chip. The tight vertical acoustic confinement releases the lateral design of freedom, and enables negligible cross-talk and preserving low optical loss. This hybrid HBAR nanophotonic platform can find immediate applications in topological photonics with synthetic dimensions, compact opto-electronic oscillators, and microwave-to-optical converters. As an application, a Si₃N₄-based optical isolator is demonstrated by spatiotemporal modulation, with over 17 dB isolation achieved.

Tian et al., Nature Communications (2020)

We discuss the design and demonstration of 4-channel coarse wavelength-division (de-)multiplexers based on cascaded Mach-Zehnder interferometer (MZI) lattice filters and arrayed waveguide gratings (AWG) on a 150 nm silicon nitride ( Si3N4 ) platform. The 1 × 4 (de-)multiplexers are designed for a channel separation of 25 nm and operate within 990 — 1065 nm for bottom emitting vertical cavity surface emitting lasers (VCSEL) based optical links. For Si3N4 Gaussian AWGs, we demonstrate crosstalk < -35 dB at the peak transmission band with insertion loss < 0.5 dB. Measurements were performed over many dies, and pass-band standard deviations for channel 1 × 4 are 0.49, 0.66, 0.42, and 0.37 nm respectively. Results for flat-top AWGs indicate crosstalk < -20 dB with insertion loss < 3 dB for the best devices. Flat-top cascaded 2nd order and 3rd order MZI lattice filters show a minimum of crosstalk < -15 dB and < – 20 dB respectively. The pass-band temperature shift was determined to be 14.5 pm/ °C, which is lower than reported values for silicon. The device footprint of the Gaussian and flat-top AWGs are both ~ 670 x 200 μm. The device footprint of the flat-top cascaded 2nd order and 3rd order lattice filters are 1160 x 470 μm and 1570 x 470 μm, respectively. We believe the Si3N4 platform has potential for its use in CWDM and possibly DWDM transceiver/optical-modules for data/computer.

Stanley Cheung and Michael R.T Tan, Journal of Lightwave Technology (2020)

Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si₃N₄) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si₃N₄ waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.

Yu et al., Optics Express (2020)

Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) are promising to realize chip-scale integration of full soliton comb systems providing high precision, broad spectral coverage, and a coherent link to the micro/mm/THz domain with diverse applications coming on line all the time. However, the large soliton comb spacing hampers some applications. For example, for spectroscopic applications, there are simply not enough comb lines available to sufficiently cover almost any relevant absorption features. Here, we overcome this limitation by scanning the comb mode spacing by employing Pound–Drever–Hall locking and a microheater on the microresonator, showing continuous scanning of the soliton comb modes across nearly the full free-spectral range of the microresonator without losing soliton operation, while spectral features with a bandwidth as small as 5 MHz are resolved.

Kuse et al., Optics Letters (2020)

We report a comprehensive study of low-power, octave-bandwidth, single-soliton microresonator frequency combs in both the 1550 nm and 1064 nm bands. Our experiments utilize fully integrated silicon-nitride Kerr microresonators, and we demonstrate direct soliton generation with widely available distributed-Bragg-reflector lasers that provide less than 40 mW of chip-coupled laser power. We report measurements of soliton thermal dynamics and demonstrate how rapid laser-frequency control, consistent with the thermal timescale of a microresonator, facilitates stabilization of octavebandwidth soliton combs. Moreover, since soliton combs are completely described by fundamental linear and nonlinear dynamics of the intraresonator field, we demonstrate the close connection between modeling and generation of octave-bandwidth combs. Our experiments advance the development of self-referenced frequency combs with integrated-photonics technology, and comb-laser sources with tens of terahertz pulse bandwidth across the near-infrared.

Briles et al., arXiv (2020)

We report the generation of broadband single-mode (degenerate) quadrature squeezed vacuum from an integrated nanophotonic device based on two coupled silicon nitride microring resonators. Dual-pump spontaneous four-wave mixing in one microring resonator is exploited to generate squeezed light, while unwanted single-pump parametric fluorescence and Bragg-scattering four-wave mixing processes are suppressed by selectively corrupting individual resonances using a second, auxiliary resonator. Balanced homodyne detection is employed for quadrature measurements, with 1.5 dB of quadrature squeezing directly measured. We infer that approximately 6 dB of squeezing is present on-chip, before collection and detection losses.

Zhang et al., arXiv (2020)

We experimentally demonstrate stimulated four-wave mixing in two linearly uncoupled integrated Si3N4 micro-resonators. In our structure the resonance combs of each resonator can be tuned independently, with the energy transfer from one resonator to the other occurring in the presence of a nonlinear interaction. This method allows flexible and efficient on-chip control of the nonlinear interaction, and is readily applicable to other third-order nonlinear phenomena.

Tan et al., arXiv (2019)

A broadband visible (VIS) blue-to-red, 10 GHz repetition rate frequency comb is generated by combined spectral broadening and triple-sum-frequency generation in an on-chip silicon nitride waveguide. Ultra-short pulses of 150 pJ pulse energy, generated via electro-optic modulation of a 1560 nm continuous-wave laser (CW), are coupled to a silicon nitride waveguide giving rise to a broadband near-infrared (NIR) supercontinuum. Modal phase matching inside the waveguide allows direct triple-sum-frequency transfer of the NIR supercontinuum into the VIS wavelength range covering more than 250 THz from below 400 to above 600 nm wavelength. This scheme directly links the mature optical telecommunication band technology to the VIS wavelength band and can find application in astronomical spectrograph calibration, as well as referencing of CW lasers.

Obrzud et al., Optics Letters (2019)

We describe research on an optical-frequency synthesizer in a low-power chip-scale package that provides a laser output with a programmable optical frequency across 50 nm of bandwidth centered at 1550 nm, with a resolution smaller than one part in 1014.

Bowers et al., Gomatech (2019)

Earth-like planets, dark energy and variability of fundamental physical constants can be discovered by observing wavelength shifts in the optical spectra of astronomical objects. These wavelength shifts are so tiny that exquisitely accurate and precise wavelength calibration of astronomical spectrometers is required. Laser frequency combs, broadband spectra of laser lines with absolutely known optical frequencies, are uniquely suited for this purpose, provided their lines are resolved by the spectrometer. Generating such astronomical laser frequency combs (‘astrocombs’) remains challenging. Here, a microphotonic astrocomb is demonstrated via temporal dissipative Kerr solitons in photonic-chip-based silicon nitride microresonators, directly providing a spurious-free spectrum of resolvable calibration lines. Sub-harmonically driven by temporally structured light, the astrocomb is stabilized to a frequency standard, resulting in absolute calibration with a precision of 25 cm s^–1 (radial velocity equivalent), relevant for Earth-like planet detection and cosmological research. The microphotonic technology can be extended in spectral span, further boosting the calibration precision.

Obrzud et al., Nature Photonics (2019)

We report the first demonstrations of both quadrature squeezed vacuum and photon number difference squeezing generated in an integrated nanophotonic device. Squeezed light is generated via strongly driven spontaneous four-wave mixing below threshold in silicon nitride microring resonators. The generated light is characterized with both homodyne detection and direct measurements of photon statistics using photon number-resolving transition edge sensors. We measure 1.0(1) dB of broadband quadrature squeezing (∼4 dB inferred on-chip) and 0.70(1) dB of photon number difference squeezing (∼6 dB inferred on-chip). Multi-photon events of over 10 photons are detected with rates exceeding any previous quantum optical demonstration using integrated nanophotonics. These results will have an enabling impact on scaling continuous variable quantum technology.

Vaidya et al., arXiv (2019)

Frequency combs based on nonlinear-optical phenomena in integrated photonics are a versatile light source that can explore new applications, including frequency metrology, optical communications, and sensing. We demonstrate robust frequency-control strategies for near-infrared, octavebandwidth soliton frequency combs, created with nanofabricated silicon-nitride ring resonators. Group-velocity-dispersion engineering allows operation with a 1064 nm pump laser and generation of dual-dispersive-wave frequency combs linking wavelengths between approximately 767 nm and 1556 nm. To tune the mode frequencies of the comb, which are spaced by 1 THz, we design a photonic chip containing 75 ring resonators with systematically varying dimensions and we use 50 ◦C of thermo-optic tuning. This single-chip frequency comb source provides access to every wavelength including those critical for near-infrared atomic spectroscopy of rubidium, potassium, and cesium. To make this possible, solitons are generated consistently from device-to-device across a single chip, using rapid pump frequency sweeps that are provided by an optical modulator.

Yu et al., Physical Review Applied (2018)

In this article, we provide a review of fabrication platforms and state of the art developments in the area of silicon nitride photonic integrated circuits (PICs) for the visible wavelength range, mostly employed in bio-medical and chemical sensing applications. Additionally, we introduce PIX4life, the European pilot line for silicon nitride PICs for visible applications. PIX4life provides a unified framework for the development of such PICs, including design and software, fabrication, characterization and packaging.

Porcel et al., Science Direct (2018)

Spencer et al., Nature (2018)

Manurkar et al., OSA Continuum (2018)

Controlling femtosecond optical pulses with temporal precision better than one cycle of the carrier field has a profound impact on measuring and manipulating interactions between light and matter. We explore pulses that are carved from a continuous-wave laser via electro-optic modulation and realize the regime of sub-cycle optical control without a mode-locked resonator. Our ultrafast source, with a repetition rate of 10 GHz, is derived from an optical-cavity-stabilized laser and a microwave-cavity-stabilized electronic oscillator. Sub-cycle timing jitter of the pulse train is achieved by coherently linking the laser and oscillator through carrier–envelope phase stabilization enabled by a photonic-chip supercontinuum that spans up to 1.9 octaves across the near infrared. Moreover, the techniques we report are relevant for other ultrafast lasers with repetition rates up to 30 GHz and may allow stable few-cycle pulses to be produced by a wider range of sources.

Carlson et al., Science (2018)

Supercontinuum generation in integrated photonic waveguides is a versatile source of broadband light, and the generated spectrum is largely determined by the phase-matching conditions. Here we show that quasi-phase-matching via periodic modulations of the waveguide structure provides a useful mechanism to control the evolution of ultrafast pulses during supercontinuum generation. We experimentally demonstrate quasi-phase-matched supercontinuum to the TE20 and TE00 waveguide modes, which enhances the intensity of the SCG in specific spectral regions by as much as 20 dB. We utilize higher-order quasi-phase-matching (up to the 16th order) to enhance the intensity in numerous locations across the spectrum. Quasi-phase-matching adds a unique dimension to the design-space for SCG waveguides, allowing the spectrum to be engineered for specific applications.

Hickstein et al., Physical Review Letters (2017)

Dissipative solitons formed in Kerr microresonators may enable chip-scale frequency combs for precision optical metrology. Here we explore the creation of an octave-spanning, 15-GHz repetition-rate microcomb suitable for both f–2f self-referencing and optical-frequency comparisons across the near infrared. This is achieved through a simple and reliable approach to deterministically generate, and subsequently frequency stabilize, soliton pulse trains in a silica-disk resonator. Efficient silicon-nitride waveguides provide a supercontinuum spanning 700 to 2100 nm, enabling both offset-frequency stabilization and optical-frequency measurements with >100 nW per mode. We demonstrate the stabilized comb by performing a microcomb-mediated comparison of two ultrastable optical-reference cavities.

Lamb et al., Physical Review Applied (2017)

Integrated optics has distinct advantages for applications in space because it integrates many elements onto a monolithic, robust chip. As the development of different building blocks for integrated optics advances, it is of interest to answer the important question of their resistance with respect to ionizing radiation. Here we investigate effects of proton radiation on high-Q (O(106 )) silicon nitride microresonators formed by a waveguide ring. We show that the irradiation with high-energy protons has no lasting effect on the linear optical losses of the microresonators.

Brasch et al., Optics Express (2014)

Optical solitons are propagating pulses of light that retain their shape because nonlinearity and dispersion balance each other. In the presence of higher-order dispersion, optical solitons can emit dispersive waves via the process of soliton Cherenkov radiation. This process underlies supercontinuum generation and is of critical importance in frequency metrology. Using a continuous wave–pumped, dispersion-engineered, integrated silicon nitride microresonator, we generated continuously circulating temporal dissipative Kerr solitons. The presence of higher-order dispersion led to the emission of red-shifted soliton Cherenkov radiation. The output corresponds to a fully coherent optical frequency comb that spans two-thirds of an octave and whose phase we were able to stabilize to the sub-Hertz level. By preserving coherence over a broad spectral bandwidth, our device offers the opportunity to develop compact on-chip frequency combs for frequency metrology or spectroscopy.

Brasch et al., Science (2016)

Chang et al., Optics Letters (2017)